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BASIC ELECTRONICS
Part 4
A Course of Training Developed for
THE UNITED STATES NAVYby the New York firm of
Management Consultants and Graphiological Engineers
VAN VALKENBURGH, NOOGER & NEVILLE, INC.
W1GANICENTRAL]JJBRARY
Adapted to British and Commonwealth Usageby a Special Electronics Training Investigation Team of
the Royal Electrical & Mechanical Engineers
LONDON
THE TECHNICAL PRESS, LTDNEW YORK
THE BROLET PRESS
British and Commonwealth Edition first published 1959
©Copyright 1959 by
VAN VALKENBURGH, NOOGER & NEVILLE, INC.
New York, U.S.A.
All rights reserved
American Edition first published 1955
©Copyright 1955 by
VAN VALKENBURGH, NOOGER & NEVILLE, INC.
New York, U.S.A.
U S. Library of Congress Catalog Card No. 55-6984
All rights reserved
W1GANPUBLIC
LIBRARIES
7a 13
1
AT ub^i 3*1
Made and printed by Offset in Great Britain by
William Clowes and Sons, Limited, London and Beccles
PREFACE
IN THESE six Manuals on BASIC ELECTRONICS and the five which have pre-
ceded them on BASIC ELECTRICITY, there lies the core of an illustrated
Course of Technician Training—carefully planned, brilliantly simplified, and radi-
cally new—which was developed some years ago at the request of the United States
Navy by a distinguished New York firm of management consultants and graphio-logical engineers, Messrs. VAN VALKEN BURGH, NOOGER & NEVILLE, INC.The Course has since become standard in U.S. Navy Training Schools. More than
50,000 men have taken it as ah essential part of their training to technician level in
14 different Navy trades; their average training time has been cut by half; andsupplies of Course materials are now held as part of the Navy's official War Mobiliza-tion Stores.
The text of the Course was subsequently released in a condensed form to the
general public in the United States, where it has proved an outstanding success. In
addition to large sales to individuals, to schools and to technical institutions of all
kinds, more than a score of world-famous companies have taken the publishedManuals for use in their Apprentice Training Schemes, and have found that theyenable them to turn out qualified technicians both faster and at less cost than didthe old methods of text-book and lecture. Several American trade unions (who takea keen interest in the "up-grading" of their members to more skilled and better-paid
jobs) have chosen the Manuals as the best available training materials for their
purpose.
This notable Series is now being made available, in a revised, reset, and suitablyre-worded edition, to users in Britain and the Commonwealth.
While negotiations with the American authors were still in progress, word reachedthe British publishers that there had recently been set up, under command of Train-ing Headquarters, Royal Electrical and Mechanical Engineers, at Arborfield in
Berkshire, a special "Electronics Training Investigation Team" whose task was to
devise solutions for some of the training problems which would face the British
Army when National Service ended, and when the Army's increasingly elaborateelectrical and electronics gear would have to be manned and serviced by recruits
entering the Army with none of the technical knowledge which many NationalServicemen had hitherto brought with them into the Forces.
It seemed possible that most of the REME requirements for a new-style, yettechnically sound, instructional approach could be met by a suitably edited British
version of the VVN&N Manuals. A visit to Arborfield was accordingly arranged,where the reception given to the Manuals, with their attractive appearance andproved record of success, was enthusiastic; and after a careful evaluation of. their
merits and potential suitability had been made, War Office consent was secured toa proposal that the work of adapting text and illustrations to British notation andterminology should be undertaken by the Electronics Team at Arborfield.
Later, while this work was still proceeding, a decision was reached to adopt therevised Manuals as basic texts for the training of future REME technicians, and anorder for large numbers of complete sets of the Manuals was placed. Early interest
was also shown by several other branches of the Armed Forces, notably the Royal
Corps of Signals and the Royal Air Force. Military Advisers to the High Com-missioners of at least six leading Member Nations of the Commonwealth submitted
early proofs of the English edition to their respective Ministries of Defence.
The original U.S. Navy Course was based on a novel technique of teaching
developed by the Authors after extensive research and practical experience with
thousands of students. Immense pains were taken to identify and present only the
essential facts about each new concept or piece of equipment. These facts were
then explained in the simplest possible language, one at a time; and each was illus-
trated by a cartoon-type drawing. Nearly every page in every one of the Manuals
carries one or more of these brilliantly simple "visualizations" of the concept
described.
The approach throughout is non-mathematical. Only the simplest equations
needed for working with the fundamental laws of electricity are employed. Yet
there has been no shirking of essentials, even when they are difficult; and students
with higher qualifications and educational background find nothing in the Manuals
to irritate or slow them down. They merely pass on to the next subject quicker
than the rest.
Despite their Services background, the Manuals have been proved suitable for
civilian use. Their purpose, however, is limited to the training of technicians, not
of engineers. They aim to turn out men capable of operating, maintaining, and
carrying out routine repairs to the equipment described—not men capable of invent-
ing or improving it.
They present a unique simplification of an ordinarily complex set of subjects—so
planned, written and illustrated as to become the best and quickest way to teach or
learn BASIC ELECTRICITY and BASIC ELECTRONICS that has ever been
devised.
In these Manuals, first things come first—and only the essentials come anywhere.
Their accuracy and thoroughness, combined with their extreme lucidity, will make
their publication a landmark in technical education in Britain and the Common-
wealth.
Page
TABLE OF CONTENTS
Section
1 The Role of the Transmitter 4.3
2 Class C Amplifiers 4.9
3 The Basic Three-stage Transmitter 4,17
4 Frequency Multipliers 4.35
5 Transmission Lines 4.40
6 Aerials 4.59
7 Continuous-wave (CW) Transmission 4.72
8 Amplitude Modulation 4.77
9 General Review of Transmitters 4.92
10 Introduction to Receivers 4.95
Index 496
This Course in
BASIC ELECTRONICS
comprises 6 Parts
This is PART 4
It is preceded by a Course in
BASIC ELECTRICITY
comprising 5 Parts
all uniform with this volume.
Part 1 explained the General Principles of Electricity.
Part 2 described and discussed D.C. and D.C. Circuits.
Parts 3 and 4 described and discussed A.C. and A.C. Circuits.
Part 5 described and discussed A.C. and D.C. Machines.
an&/uwibm4lfei&
§1: THE ROLE OF THE TRANSMITTER 4.3
Introduction
Probably few of you have had any direct experience with transmitters. The word
itself may even be unfamiliar. But you have undoubtedly talked often enough about
one particular type of transmitter—a radio station
!
When you listen to a radio, the sounds you hear are the result of signals travelling
to the radio receiver through the air. If someone were to ask you how those signals
happened to be in the air, you would probably say, "A radio station broadcasts
them."
Different transmittersoperate on different
FREQUENCIES
There are other things you already know about transmitters from your experience
with radio sets. You know that "changing stations" is also called "tuning." Fromthis you realize that different transmitters operate at different frequencies. Youselect the station you want to listen to by tuning your radio to the frequency of that
station.
TyH >ADifferent transmitters
have different
POWER OUTPUTS
You have also noticed that some stations come in stronger than others. If differ-
ent transmitters at equal distances away have different power outputs, the station
whose transmitter has the largest power output will be heard the loudest.
If there are two stations whose transmitters have the same power output, you will
hear more loudly the station which is the closer to your radio set.
You see you really knew a few things about transmitters all the time—even if the
word itself came as a new one to you
!
4.4 [§ I
A Simple Transmitter
The simplest possible form of transmitter would consist of an oscillator generating
a high-frequency signal—the type of oscillator doesn't matter—connected to an
aerial. This aerial would then radiate a signal constant in amplitude, and of the
same frequency as the oscillator.
A simple transmitterconsists of •
If your home radio set picked up the constant-amplitude signal from such a trans-
mitter, you would hear nothing at all. And though there are some special com-
munications receivers used by the G.P.O., the Army, etc., which would produce a
constant audio tone if they received it, in neither case could any message be "read"
from the incoming signal.
Such a signal is said to contain "no intelligence."
To get intelligence into the signal, the oscillator would have to be turned on and
off with a key, so as to produce dots and dashes such as are used in the morse code.
Putting the transmitter to work —
dit"
A signal of this type contains intelligence, since a message can be obtained from
it. Special communications receivers would produce a sound somewhat like "dit-
dah-dit"—which a radio operator who knew the morse code would interpret as the
letter "R."
§ I] 4.5
A Simple Transmitter (continued)
Almost every transmitter, however, contains a good deal more than just an oscil-
lator and an aerial.
There are two main drawbacks to connecting the oscillator directly to the aerial.
The first is that the power output would be limited, because there are no stages of
r.f. amplification between the oscillator and the aerial to build up the strength of the
r.f. signal. And power output is important because it determines the distance over
which the transmitted signal can be picked up by a receiver.
The second consideration is frequency stability. An oscillator from which a large
amount of power is drawn has a tendency to drift in frequency. And any drift in
the frequency of a transmitted signal means that a portion of the message will belost by an operator trying to receive it.
For these reasons—low power output and poor frequency stability—oscillators are
not usually connected directly to an aerial.
result in poor reception ....
4.6 [§A Simple Transmitter (continued)
To overcome the drawbacks of connecting an oscillator directly to the transmitting
aerial, one or more stages of amplification are connected between the oscillator and
the aerial.
The stage which is connected to the aerial itself is usually called the "final poweramplifier." The other stages of amplification are known by several names. Some-
times they are referred to as the "first and second power amplifiers"; sometimes as
"intermediate power amplifiers."
The first power amplifier, since it serves to isolate the oscillator from variations
of load, is also called a "buffer" amplifier.
Oscillator
Signal
c=> c=> =>
1st Power2nd power
Amplifier . A ,.,. . 1-*,^*
(Buffer)AmP llfier Amplifier
AmplifiedSignal
The r.f. signal is generated in the oscillator circuit and is amplified by the first and
second power amplifiers, which drive the final power amplifier. The powerful
signal from the final power amplifier is fed to the aerial, which in turn radiates the
signal into space.
As has been said, the r.f. signal by itself does not contain any intelligence, though
several things can be done to it which will make it able to contain or carry a message.
For this reason, the r.f. signal is commonly referred to as "the carrier wave." It
is not, of itself, the message; but it can carry a message to some distant point.
§1] 4 -7
Modulation, and Keyed Transmission
The process by which the carrier wave is changed so that it can carry a message is
called "modulation." Every communications transmitter needs modulation, because
the carrier by itself (i.e., unmodulated) cannot convey intelligence.
In most transmitters the message is transmitted either in code (radio telegraphy)
or by voice (radio telephony).
The most common types of code transmission are "continuous wave" (CW) and
"modulated continuous wave" (MCW). In CW transmission the r.f. to the aerial is
interrupted, or turned on and off, with a hand key; so that the carrier is radiated as
dots and dashes. CW is used primarily for long-distance communication. Aspecial receiver is needed to receive it.
^CW\TRANSMISSION
Phones
In MCW transmission, a constant-amplitude audio frequency is superimposed on
the carrier. The carrier is then turned on and off with a key, just as in CW trans-
mission. Any receiver with the proper frequency range can receive MCW.
MCW Keyed
TRANSMISSION ,M°<"^tedRF
^
v
>.RF
Transmitter
AudioOscillator
DIT -Drr.
Receiver
Phones
M#
4.8[§ |
Radio Telephony
Radio telephony, or voice transmission, is also of two types. In the most commontype used, the amplitude of the carrier is varied at a rate dependent on the frequency
of the voice signal, and to an extent dependent on the amplitude of the voice signal.
This is called "amplitude modulation" (AM). It is the type of transmission used in
the standard radio broadcast.
AMVoice modulated RF
UnmodulatedRF Carrier
-A*.
vVoicewaves
v Phones
V
RFTransmitter
1Amplitudemodulator
AMTRANSMISSION
Receiver
Microphone **?
The other type of radio telephony, which is being used more and more, is called
"frequency modulation" (FM). Here the frequency of the carrier is shifted back and
forth at a rate equal to the frequency of the voice signal. FM transmission is com-
paratively free from "static" interference, and is used in place of AM when the latter
may be difficult to receive. It will be fully explained in Part 6.
FMVoice modulated RF
UnmodulatedRF Carrier
A.
V
RFTransmitter
Voicewaves
Microphone ^
Frequencymodulator
Note that there are also some other types of modulation which are used in special
communications equipment. They will not be covered by this basic course, since
they are not widely used.
§2: CLASS C AMPLIFIERS 4.9
The Three Classes of Operation—A Reminder
The type of amplifier most commonly used in transmitter circuits is the tuned
Class C amplifier.
You remember from your study of amplifiers that there are three main bias con-
ditions for amplifiers—Class A, Class B, and Class C.
In Class A operation, the grid is biased near the midpoint of the linear portion of
the anode current-grid voltage curve. The a.c. signal on the grid causes the grid
voltage to vary above and below the bias value. The current variations are propor-
tional to the grid voltage, since the grid voltage swing does not go beyond the linear
portion of the curve. Anode current flows throughout the entire a.c. cycle, since
the grid voltage does not drive the valve into cut-off.
In Class B operation, the grid is biased at or near its cut-off value. The a.c. signal
drives the valve into cut-off for approximately half the cycle. Thus the valve con-
ducts for about 180 degrees of the cycle and is cut off during the other 180 degrees of
the cycle.
In Class C operation—the type of operation with which you will be most con-
cerned in your study of transmitters—the grid is biased considerably beyond cut-off.
The valve remains cut off for most of each a.c. cycle, and current flows in the valve
only when the a.c. signal increases the grid voltage above cut-off. The anode current
therefore flows in the kind of pulses shown below.
OPERATION OF
CLASS
?4tHfeU£ten6
-8 -6 -4
No Anodecurrent flowing
Cut-off
TwiceCut-off
Current flowingall the time
Class B Current flowshalf the time
Class CCurrent flows
less thanhalf the time
^
4.10
Tuned Class C Amplifiers
IS 2
OHT+
The operation of a Class C amplifier will become clear when you analyse whathappens in a tuned amplifier such as the one shown in the schematic diagram above.
An a.c. signal is developed across the tuned-circuit in the anode of the previous stage.
This voltage also appears across the r.f . choke (RFC) in the grid circuit of the tuned
Class C amplifier stage. The d.c. bias provided by the bias battery causes the valve
to operate in Class C.
The pulses of anode current
which flow as a result of this type
of operation deliver a "kick" to the
tuned-circuit in the anode. This
"kick" makes the tuned-circuit
oscillate, so that the part of the
cycle during which anode current
has stopped is filled in. (For a
review of how oscillations are kept
going in a tuned-circuit, refer to
the Section on oscillators in Part 3.)
The anode voltage is the differ-
ence between the H.T. voltage and
the a.c. voltage across the tuned-
circuit. When the pulse of anode
current flows, the voltage at the
anode end of the tuned-circuit goes
negative, and therefore subtracts
from the H.T. voltage. When the
voltage across the tuned-circuit
reverses and goes positive at the
anode end, it adds to the H.T.
voltage.
As a result, the anode voltage
waveform varies above and below
the H.T. voltage level, as shown in
diagram 4 opposite.
HT+
GridVoltage
AnodeCurrent
Voltage
acrosstuned
circuit
Total
Plate
Voltage
§2]
Tuned Class C Amplifiers (continued)
The reason why tuned Class Camplifiers are universally used in
high-powered transmitters is be-
cause of their high efficiency of
operation, which results in a maxi-
mum of radiated power.
The power supplied to an ampli-
fier is always greater than the
power which can be got out of it.
The reason is that some of the
power put into an amplifier is used
up by the valve; only the re-
mainder can appear as useful out-
put in the load. The power used
up by the valve equals its anode
voltage times its anode current.
Since the anode current of a
Class C amplifier flows during less
than half the cycle, the average
anode current is less than it is in
Class A or B operation. There-
fore less power is used up by the
valve, and more power can get to
the output.
This makes the Class C ampli-
fier more efficient, and therefore
more desirable for use in a trans-
mitter.
4.11
ANODE VOLTAGE
If the tuned-circuit in the anode is not tuned to the frequency of the input signal,
then the voltage across it will be lower—in proportion to the extent to which it is
mistuned. The further "off" it is tuned, the less power will appear across it, and the
more power will be dissipated by the valve. Then the efficiency of the amplifier is
lower, the valve heats up more, and the power output is lower.
VARIATION OF ANODE VOLTAGE
AS TUNING VARIES • ••
Well belowsignal
frequency
Approachingsignal
frequency
Slightly lessthan signal
frequency
At signal
frequency3
4.12 [§ 2
Fixed Bias
The term "fixed bias" describes any method of obtaining bias in which the bias
remains fixed as the strength of the input signal varies.
Fixed bias may be obtained from a negative power supply, from a motor-generator
set with a negative d.c. output, or from a battery. Each of these methods will keep
the grid at a constant negative d.c. voltage which will not vary whatever the strength
of the signal input.
Fixed Bias may be obtained from...
12 H>| 1 7J « 4J *•
One of the advantages of fixed bias is that the valve remains cut off under no-signal
conditions.
The disadvantage of fixed bias is that the gain of the amplifier remains constant;
so that if the grid signal varies in amplitude, the output will similarly vary. This is
not desirable in a transmitter, because the output to the aerial must remain constant
in amplitude if the radiated signal strength is to remain constant.
If the bias could be made to vary as the signal input to the amplifier varies, the
amplifier output could be maintained practically constant.
§ 2] 4.13
Self-bias
The term "self-bias" describes any grid bias which results from the current flow
in the valve which is being biased. You are already familiar with the two methodscommonly used to provide it.
In the first method, a resistor placed in the cathode circuit makes the cathodepositive with respect to earth, and therefore makes the grid more negative than the
cathode. The bias voltage developed across this resistor is equal to the average
current multiplied by the size of the resistor.
If a large cathode resistor is used, the bias voltage will thus also be large; and the
resistor can in fact be made sufficiently large to cause the bias to approach cut-off
when there is no signal on the grid.
GRID VOLTAGE CATHODE CURRENT
NoSignal
No signal
current
Cut-off
When a signal is applied to the grid, the cathode current will increase on the posi-tive half-cycles; and will become zero (cut-off) on the negative half-cycles. Theaverage current will thus be increased; and the bias will also increase.
cut-off -/- J
\J \J
Small/
• Bias SiKna]
Averagecurrent
If a larger signal is applied to the grid, the current will be larger during the positive
half-cycles of voltage, but will remain zero during the negative halves. Thus, theaverage valve current increases as the grid signal becomes larger, resulting in in-
creased bias for larger signals.
A ACut-off J-
„. LargeBias signal
Averagecurrent
This effect of bias varying with signal strength tends to stabilize the amplitude ofthat portion of the grid signal above the cut-off level. As a result, the amplitude ofthe current pulses in the anode will not vary as much as their corresponding gridsignals vary.
Because of the above-mentioned effect, self-bias tends to produce amplitudestability of the anode signal, and is therefore sometimes called "automatic bias."
The cathode bias method just described, however, is not common in high-poweredtransmitter circuits.
4.14 [§2
Self-bias (continued)
The other, and very common, type of self-bias arrangement found in transmitters
makes use of the current which flows from the cathode to the grid at the positive
peaks of the signal output. This is called "grid-leak bias."
Class C Amplifier with
Grid-leak Bias
HT+
To Anode OfPrevious Stage
Whenever the signal drives the grid positive, the grid draws current; and in doing
so charges up capacitor C-l to make the grid negative again. Resistor R-l provides
a path for C-l to discharge slightly between the pulses of grid current flow.
The main advantage of this type of bias is that it develops a voltage whose ampli-
tude depends on the strength of the input signal. If this input signal increases, the
grid will draw more current, and the bias will become more negative. After the new
value of bias has become established, the peaks of the larger input signal will not
drive the grid very much more positive than did the weaker signal.
Thus, the peaks of the larger signal will only cause about the same amount of
anode current to flow as did the peaks of a smaller signal. In this way, grid-leak
bias provides for amplitude stability.
The main disadvantage of grid-leak bias is that it depends entirely on the presence
of a signal in order to develop any bias voltage at all; it does not therefore protect
the valve when there is no signal on the grid.
If the oscillator of a transmitter stopped oscillating for any reason, the grid-leak
arrangement in the amplifiers would not develop any bias; since the grid would not,
under these conditions, be driven positive. The transmitting valve would then draw
a very large current with zero bias, and might therefore be seriously damaged.
§ 2] 4.15
Combination Bias
The most common bias arrangement in transmitters is a combination of fixed bias
and grid-leak bias.
The fixed bias is sufficient to limit the current to a low value, or even to cut-off in
the absence of a signal. And when a large enough signal is present to drive the grid
positive, then the grid-leak bias which is developed stabilizes the amplitude of the
output.
In this way, combination bias both protects the valve and stabilizes the output.
{^dutaiam Bias
4.16
REVIEW
[§2
Class C Operation. The grid of the
valve is biased well below cut-off, so
that anode current flows only in pulses.
1 -8
6 A A
•1c
r\•4 1/
1
-
No anoda 9}current flowing
*<*
i -6 - i 1-2 : o
> ^0cr
Tuned Class C Amplifiers. Used in
transmitters because they are very
efficient when tuned to the frequency
of the input signal.
Grid-leak Bias. Depends on grid
current, and varies as the strength of
the input signal changes.
Combination Bias. A combination
of fixed and grid-leak bias most
commonly used in transmitters.
TOANOOEOFPREVIOUSSTAGE
§3: THE BASIC THREE-STAGE TRANSMITTER 4.17
The Three Basic Circuits
A block diagram of a basic three-stage transmitter is shown below. All three
stages are operated in Class C for high efficiency. The ECO master oscillator (MO)generates the r.f. signal, which can be varied (for example) between 2 and 4 mega-
cycles.
The intermediate power amplifier (IPA) both amplifies the r.f. signal and isolates
the master oscillator from the final power amplifier, so as to improve frequency
stability. The IPA is therefore called a "buffer amplifier."
The IPA may also act as a frequency doubler to double the oscillator frequency.
(The way in which it works in this role will be explained later.) The output
frequency of the IPA can therefore vary between 2 and 4, or between 4 and 8 mega-
cycles.
The final power amplifier (PA) generates a large amount of power output and
delivers it to the aerial, usually at the same frequency as its grid signal.
Basic
THRU STAGE
\Transmitter
-GeneratesRF Signal
IPA
(Class C)2 to 4 Mc/s
4 to 8 Mc/s
Intermediate PowerAmplifier
Buffer Amplifier
Frequency Doubler
v
PA
(Class C)2 to 4 Mc/s4 to 8 Mc/s
Final PowerAmplifier
4.18 [§3
The Oscillator
The purpose of the electron-coupled master oscillator is to generate a stable r.f.
signal, the frequency of which can be varied over a given range.
The ECO operates as follows:
—
The oscillator section of the ECO is composed of the grid and screen circuits,
and is a Colpitts oscillator. The oscillator frequency is determined by the grid
tuned-circuit, consisting of L-l, C-l, C-2 and C-3. The screen, which acts as the
anode of the oscillator section, is coupled to the tuned-circuit through the r.f. de-
coupling capacitor, C-5. Grid-leak bias is developed across R-l by the discharge of
C-4.
The r.f. choke in the cathode circuit provides a low resistance d.c. path to earth
for the cathode. But because the high reactance of the choke does not allow r.f. to
flow through it, the r.f. must flow through C-3 (the feedback capacitor) to the
cathode. The screen-dropping resistor, R-2, drops the screen voltage to the correct
value. The r.f. oscillations generated in the oscillator section of the ECO are
electron-coupled to the anode through the flow of anode current.
The r.f. choke in the anode lead acts as a high impedance for the r.f. signal, and
serves the same purpose as the anode load resistor in an audio amplifier. The r.f.
coupling capacitor, C-6, passes the signal to the grid of the IPA.
^^^g Master Oscillator(ECO) 9|5Ki. . . Generates RF Signal
RFC
MO(ECO)
2 to 4 Mc/s
C-4ll-^IPA
1 I 1"1-i:£ c-2i iR-i
L-l«
RFC=:c-3
^HT+
§ 3]41»
The Intermediate Power Amplifier
Two functions of the intermediate power amplifier are to isolate the oscillator for
improved frequency stability, and to amplify the r.f. signal in order to drive the
power amplifier more efficiently.
The IPA also serves to increase the tuning range, if desired, by doubling or tripling
the generated frequency in its anode tuned-circuit.
The operation of the IPA is as follows:
—
A combination of grid-leak and cathode bias is provided by R-3 : C-6, and by
R-4 : C-7 respectively. Resistor R-5 drops the screen voltage to the correct value.
The screen decoupling capacitor, C-8, is returned directly to the cathode, rather
than to earth. This provides a more direct path back to the cathode for any r.f.
variations on the screen.
The r.f. choke in the anode lead acts as a high impedance for the r.f. signal, and
serves the same purpose as does the anode load resistor in an audio amplifier. C-9
is a coupling capacitor which passes the r.f. to the tuned-circuit, and at the same
time blocks the d.c.
The anode tuned-circuit, C-10 and L-2, can either be tuned to the IPA grid signal,
or it can be tuned to twice the grid signal frequency—in which case the IPA is called
a "doubler." When the IPA doubles, the isolation between the grid and anode
circuits is improved, and there is less chance of the IPA breaking into oscillation.
Doubling has another advantage in that it raises the carrier frequency while still
permitting the oscillator to operate at a lower frequency, at which it will be more
stable.
Capacitor C-ll couples the r.f. to the grid of the power amplifier.
§f|§ Intermediate Power Amplifier (IPA) liH. . . Intermediate Power Amplifier
. . . Buffer Amplifier. . . Frequency Doubler
IPA(Class C)2 to 8 Me/s
HT+
4.20B ,
The Power Amplifier
The purpose of the power amplifier is to increase the power of the r.f. signal sothat it can be radiated by the aerial.
The PA usually operates at the same frequency as does its preceding stage; andonly in unusual cases does the PA act as a doubler.
The PA operates as follows:
—
Capacitor C-ll couples the r.f. from the output of the IPA to the grid of the PA.Here, as in the IPA, there is a combination of grid-leak and cathode bias providedby R-6 and C-ll, and by R-7 and C-12, respectively.
The r.f. choke, while providing a d.c. path from anode to H.T.(+), also acts as ahigh impedance anode load for the r.f. signal.
C-13 couples the r.f. to the tuned-circuit, and blocks the d.c.
The anode tuned-circuit C-15 : L-3 (note, by the way, that the anode tuned-circuitis sometimes called the "tank circuit" in a final power amplifier circuit) is tuned tothe grid signal frequency; and a high r.f. voltage is developed across it. The high-powered r.f. signal in the anode tuned-circuit is coupled by coil L-5 to the aerial for
radiation.
Coil L-4 couples some energy back to the grid through capacitor C-14, which is
called a "neutralizing capacitor." The purpose of this neutralizing circuit will
become apparent to you a little later on.
§i§ltl The final Power Amplifier(PA) WMm,
PA(Class C)2 to 8M</s
L-5 TOAER1
§3]
Diagram of a Complete Three-stage Transmitter
4.21
Q X
8'8
4.22
The Transmitting Valve Filament Circuit
In the transmitter circuit shown on the
previous page, the valves (other than
the rectifier) were indirectly heated. In
many transmitters, however, the trans-
mitting valves used have directly heated
cathodes, which are capable of supplying
the large current requirements. Tungsten
cathodes are commonly used because of
their relatively long life.
The use of directly heated valves,
however, complicates the wiring of the
cathode circuit slightly, as shownopposite.
The filament is connected across a
secondary winding of a filament trans-
former. This secondary winding is
centre-tapped in order to prevent the
50 c/s filament current from affecting
the anode current of the valve.
The centre tap of the transformer is
connected to earth through the r.f. chokein order to prevent the r.f. current fromflowing in the transformer winding.
The r.f. current gets to the filament
through C-l and C-2.
The d.c. valve current flows throughthe r.f. choke, divides in going through
the filament transformer winding, andarrives at the filament.
Because the d.c. current divides, both
ends of the filament are at the same d.c.
potential. If one side were less positive
than the other, more anode current
would be drawn from that side. Butsince the two sides of the filament are
at the same potential, equal currents are
drawn from each, with the result that
the valve has a longer life.
[§3
Typical cathodecircuit of
transmitting valve
II t IIC-l ^ C-2
r TflRP—
I
RFC
pWOOOOOOOtn Filament* A Transformer
Path for
50-cycleheater current
,1.
Path for
RF current
/"5W555S55W
Path for
DC valve
current
.0 RFC
''655559S5705';
§ 3] 4.23
The Purpose of Tuning
If a Class C amplifier is to operate efficiently, the anode tuned-circuit must
resonate at the same frequency as the grid signal.
If the tuning capacitor is variable, the anode circuit will be either on or off
resonance, depending on the setting of the variable capacitor.
Adjustment of the variable capacitor so as to make the anode tuned-circuit
resonate to the grid signal is called "tuning."
When a transmitter is de-tuned, a weak signal will be radiated—so weak that even
receivers tuned to the transmitter frequency may not pick up the signal.
When a transmitter is tuned to a given frequency, all the tuned-circuits in the
transmitter are tuned to resonate at this given frequency. The transmitter then
radiates a stable signal at maximum efficiency and with maximum power output.
The tuning of a transmitter is therefore the most important procedure in its opera-
tion.
OriginalSignal
Detuned. ••
transmitter.
0««llll««Oi"«0
I 1st Power I , ~Oscillator Amplifier
2"d ^°f
wer
(Buffer)AmPWl"
Final
Power
F>W TransmittedvSignal
Original
Signal III<=> IIIc>
111
1 1 xst Power n , „~. . ,, . I . ..... 2nd PowerOscillator . Amplifier . , ,
1 1 Ir, tc AmplifierI
(Buffer) 1
K
oFinal
Poweramplifier
f^>
TransmittedSignal
4.24 [§3
Tuning Methods
A tuned-circuit in series with the anode of a Class C amplifier can be compared to
a rheostat in series with the anode.
When the anode circuit is completely de-tuned, it acts just as a very small resistance
in the anode. As a result, the anode voltage will always be nearly equal to H.T.,
and the pulses of current (when grid is driven above cut-off) will be large. The d.c.
meter (M-l) which measures the average of the current pulses will therefore read
high.
Tuned Class CAMPLIFIER
CIRCUIT
\ ' HT+M-l
As the tuning is varied so that the resonant frequency of the anode tuned-circuit
comes closer to the grid signal frequency, so the impedance of the anode tuned-
circuit rises.
Now a signal voltage appears across this impedance. Just as in an ordinary
amplifier, when the grid signal is positive, the anode voltage drops because of the
voltage drop across the anode load. Since the anode voltage is now lower than it
was before, during the time the grid is driven above cut-off, the pulses of anode
current will be lower in amplitude; and therefore their average value will be less.
When the anode tuned-circuit is tuned to the grid signal, the anode impedance
is at its highest point; and the voltage drop across this impedance is therefore at its
highest point also. In consequence, the anode voltage (i.e., the difference between
H.T. and the voltage drop across the load) is at its lowest, point. The anode cur-
rent pulses (and therefore the average anode current) will also be at their lowest
value.
A minimum d.c. anode current reading is therefore an indication that the anode
tuned-circuit is tuned to the grid signal frequency.
When an anode tuned-circuit is tuned for a minimum reading on the anode current
meter, it is called "tuning for a dip."
Variations of Anode Voltages and
Currents as tuning varies
HT+-
Anode Voltage
Anode Currenti
i
l ; • • »
I II III »
i »
AVERAGE ': I
Well belowsignal
frequency
Approachingsignal
frequency
V\7
Slightly less
than signal
frequency
Atsignal
frequency
|Slightly more
|than signal
!frequency
§ 3] 4.25
Tuning Methods (continued)
The first step in tuning a transmitter is to set its oscillator to the desired frequency.
This may be done by adjusting the frequency of its output to be the same as the
frequency of an oscillator which is accurately calibrated, and which has been set to
the desired frequency. This comparison is made with an instrument called a wave-
meter or frequency meter.
TUNING THE TRANSMITTER
The next stage to be tuned is the stage
which follows the master oscillator. This is
done by rotating its tuning control until the
milliammeter indicating the anode current
of this stage gives a minimum reading—at
which point the anode circuit will be tuned
to the frequency of the master oscillator.
Initially, this stage is likely to be de-tuned,
and the anode current will be at a fairly high
value. For a time, as the tuning control is
rotated, no change in the milliammeter read-
ing will be noticed—until the tuned-circuit
frequency gets near the oscillator frequency.
At this point the reading on the milliammeter
will start to "dip," and the control should be
rotated slowly.
The .current will continue to decrease as
the tuning control is rotated, until a mini-
mum value occurs. This is the dip reading;
and rotation of the control should now stop.
For if rotation be continued in the samedirection, the circuit will be de-tuned once
more, and the current will again rise.
When this is seen to be happening, the control should be turned in the opposite
direction until the milliammeter again registers minimum current. At this point,
the tuned-circuit is at the same frequency as the signal frequency, and the output of
the stage is maximum.The anode tuned-circuits of the other stages can be tuned in exactly the same way.
4.26 [§3
Tuning Methods {continued)
In addition to the anode current meter, there is another meter which indicates
correct tuning of the anode circuit. This meter is in the grid circuit of the following
stage, and is labelled M-2 in the diagram below.
^ » HT+When the anode circuit is tuned to
the frequency of the input signal, the
voltage developed across the circuit
is at its highest—and so is the out-
put from that amplifier stage. Thelarger the output from that stage,
the greater is the signal to the grid
of the following stage.
The grid of the following stage will draw current whenever the input signal drives
the grid positive. The larger the signal input, the greater will be the flow of current
from the cathode to the grid. Since the signal input to the grid will be greatest
when the anode tuned-circuit of the previous stage is accurately tuned, the grid will
at that moment be drawing maximum current, and milliammeter M-2 (which
measures the average grid current) will be giving a maximum reading.
Thus when the anode tuned-circuit is accurately tuned, the anode current meter
indicates a dip, and the grid current meter of the following stage simultaneously
registers a rise known as a "peak reading."
• •Anode Current • •Grid Current
If the grid circuit has fixed bias or combination bias, no grid current will be
drawn until the signal is fairly large. This will happen some time after the anode
current meter has started to dip. For this reason, the rise in grid current indication
is sharper than is the decrease in anode current indication.
The normal procedure for tuning a stage which has an anode current meter and
which is followed by a stage having a grid current meter, is to tune first for a mini-
mum anode current. * This indication is broader, and less likely to be overlooked
as you vary the tuning. Once you have seen the anode current start to decrease,
watch the grid current meter for a rise. Tune to the point of maximum grid current.
Watching for maximum grid current is the preferred method of tuning.
Jjp 4.27
Tuning Methods {continued)
When an anode tuned-circuit is tuned to the same frequency as the grid signal, the
voltage across the tuned-circuit is at its maximum. And if another coil is trans-
former-coupled to the coil of the tuned-circuit, the voltage induced in this coil will
also be a maximum.This second coil can be connected to a pilot lamp, which will glow if the induced
voltage is large enough. If the anode tuned-circuit is de-tuned from the grid signal,
the induced voltage in the lamp circuit will drop, and the lamp will go out.
The transformer-coupled lamp is therefore a convenient means of tuning a circuit;
for the lamp glows at its brightest when the circuit is tuned to the signal frequency.
This method of tuning is not as accurate as the current meter indications, how-ever, because the lamp circuit loads down the tuned-circuit and de-tunes it slightly.
When using this method for tuning indication, therefore, the coupling must be kept
as loose as possible in order to minimize the de-tuning effects on the tuned-circuit.
The lamp method of tuning can be conveniently used on experimental transmitters
in which the anode coils are accessible. But in many transmitters it cannot be used,
since the tuning coils are out of sight; and tuning is therefore done exclusively bycurrent meter indications.
In high-power transmitters, it is essential to follow the maker's instructions onsetting the "tuning controls before switching on. Failure to do so may result in
insufficient r.f. being fed to the power amplifier stage, causing reduced bias and the
possibility of damage to a valve.
Using a lamp for tuning
At resonance
4.28 [§ 3
Neutralization
Sometimes a tuned Class C amplifier will act as a "tuned-anode tuned-grid"
(TATG) oscillator at the resonant frequency of the tuned-circuits. When it does,
the inter-electrode capacitance between anode and grid will be large enough to pro-
vide an amount of feedback sufficient to cause sustained oscillation.
This type of unwanted oscillation is most often encountered with triodes, because
of their large inter-electrode capacitance. It is seldom encountered with tetrodes
and pentodes, because their inter-electrode capacitance is very low.
When triodes are used as r.f. amplifiers, however, it is possible to eliminate the
oscillations by a process called "neutralization." In this process, a circuit is in-
cluded in the amplifier which counteracts the feedback effect of the inter-electrode
grid to anode capacitance.
There are two circuits which can be used to neutralize the grid-to-anode capaci-
tance, and so to reduce the possibility of oscillations. Both achieve neutralization
by feeding back a signal from the anode to the grid through a neutralizing capacitor.
This signal is opposite in phase, and equal in magnitude, to the signal fed back
through the grid-to-anode capacitance.
These circuits are called "anode neutralization" and "grid neutralization" respec-
tively. They get their names from the part of the circuit in which the feedback
voltage is developed.
Above is shown the circuit for anode neutralization. Cga is the grid-to-anode
capacitance, represented in the schematic as a capacitor external to the valve. Cn is
the neutralizing capacitor—that is, the capacitor through which the neutralizing
signal is brought to the grid.
The tuning coil, L-l, is centre-tapped at point C, which is placed at r.f. earth by
the r.f. decoupling capacitor CB . Since points A and B are at opposite ends of
coil L-l, they must be 180 degrees out of phase. Therefore the r.f. voltages measured
at points A and B with respect to earth are 180 degrees out of phase, and equal in
amplitude if the centre-tap at C be assumed to be in the exact centre of the tuning
coil.
The neutralizing capacitor, Cn , is connected between point A and the grid, while
the inter-electrode ca'pacitance, Cga , is between point B and the grid. Therefore the
phase of the voltage fed from the anode to the grid through Cn is opposite to the
phase of the voltage fed through the grid-to-anode capacitance; and the voltages
therefore cancel each other out.
Cn is made variable, so that the amplitude of the signal fed back through it can be
made to balance out exactly that fed back through Cga .
§ 3] 4.29
Neutralization (continued)
In the particular anode neutralization circuit just considered, both plates of the
tuning capacitor and one plate of the neutralizing capacitor are at a high d.c. poten-
tial with respect to earth. Therefore, the rotor of the tuning capacitor must be in-
sulated from earth.
But in many common types of tuning capacitor, the rotor is common to the
capacitor frame; an insulated mounting must therefore be provided to keep the
capacitor frame insulated from the chassis.
The anode neutralization circuit can, however, be modified in such a way that nod.c. voltage will be present on the rotor plate. Insulated mountings need not beused in such circuits.
In the schematic on the left (below), the rotor of the tuning capacitor is earthed.
The tap on the coil is earthed for r.f. through the 0-05 fxF r.f. decoupling capacitor.
The tap is also connected to H.T. through a radio frequency choke.
Observe that only part of the coil from A to B is in the tuned-circuit. The re-
mainder of the coil from B to C is transformer-coupled to the A-B portion of the
coil, and thus picks up r.f. for the neutralizing circuit.
In the other schematic, the tuned-circuit is capacity-coupled to the anode, so that
the d.c. anode current flows only through the radio frequency choke. One side of
the tuning coil and tuning capacitor connect directly to earth; and the tuning andneutralizing circuits are thus completely isolated from d.c.
HT+
ANODE NEUTRALIZATIONRFC
HT+
4.30
Neutralization {continued)
[§3
Grid Neutralization Circuit
•:&tJt:o*+:*ltH!*Ji!iSj&
The other type of circuit which provides a means of neutralizing the grid-to-anode
capacitance is the grid neutralization circuit.
In this circuit, the neutralizing voltage is applied to end B of the centre-tapped
coil L-l, while the anode-to-grid feedback voltage appears at end A of coil L-l.
If these two voltages are equal and of the same polarity, they will cause currents to
flow in the balanced grid tuned-circuit whose effects will caftcel each other out.
If, therefore, steps be taken to adjust Cn so that it is equal to Cga , the voltages
coupled through these capacitors will cancel each other, and the stage will not
oscillate.
Once a neutralizing capacitor is adjusted for a particular valve, it will require
only an occasional check. But if the valve be changed for a new one, the neutralizing
capacitor will need adjustment; for the new valve will have a slightly different value
of Cga .
The neutralizing capacitor may also need adjustment when the operating fre-
quency of a transmitter is changed by a large amount.
§3] 4.31
Neutralization Procedures
The procedures for neutralization are almost independent of the type of neutral-
izing circuit used.
The first step is to remove the anode voltage from the stage to be neutralized, the
object being to ensure that any signal present in the anode circuit is due to the inter-
electrode capacitance coupling between the grid and anode.
Then the master oscillator, and those amplifier stages which precede the un-
neutralized stage, are tuned. This will provide a strong signal to the grid of the
un-neutralized stage.
The next step depends on the indicator used; but it always involves the adjust-
ment of the neutralizing capacitor until there is a minimum amount of energy trans-
ferred to the anode circuit.
If there be a grid current meter, the grid current can be used to indicate the correct
adjustment of the neutralizing capacitor. When this capacitor is not properly
adjusted, the grid current will dip as the anode circuit is tuned through resonance.
When the circuit is properly neutralized, there will be no dip in the grid current whenthe anode circuit is tuned to resonance.
INPUT
Gridmtcurrent vwn
meter
Effect of varying anode tuning on grid current
LARGE DIPwhen stage is
poorly neutralized
JHK^^'p^»
SMALwhen neut
is imp
LDIPralization
roved
/
NO DIPwhen properlyneutralized
4.3215 *
Neutralization Procedures (continued)
Other methods used to adjust the neutralizing capacitor make use of devices
which can indicate the presence of r.f. energy in the de-energized anode circuit.
Devices which can be used for this purpose include the oscilloscope, a neon lamp, a
small flashlight bulb, or a sensitive d.c. milliammeter.
The device chosen affects the accuracy of neutralization, but not the method of
adjusting the neutralizing capacitor.
indicators used in neutralizing
As before, the circuits in the transmitter which precede the un-neutralized stage
are tuned to provide a strong signal to that stage. The anode supply voltage is dis-
connected from the anode of the stage; and when the anode is tuned to resonance,
the indicator will show either a maximum current flowing in the tuned-circuit, or a
maximum voltage across the tuned-circuit.
The anode circuit remains tuned to resonance; and the neutralizing capacitor is
adjusted until the voltage across (or the current in) the tuned-circuit appears on the
indicating device as a minimum.
§ 3] 4.33
Parasitic Oscillations
In a transmitter which is operating correctly, the tuned Class C amplifiers serve
only to amplify the ri. generated by the master oscillator. Sometimes, however,
the inductance of wires in the circuit combine with stray capacitance to form tuned-
circuits which are resonant to frequencies much higher than the desired transmitted
frequency.
These stray tuned-circuits will often cause the amplifiers to oscillate at very high
frequencies; and these oscillations (called "parasitic oscillations") are then trans-
mitted together with the desired frequency.
Parasitic oscillations are undesirable because they cause undue power losses and
so reduce the efficiency of the transmitter; and also because they cause interference
with other transmitters.
One way to eliminate parasitic oscillations is to improve the wiring by shortening
leads and by re-locating any components which may be in the parasitic oscillatory
circuit.
If this does not help, low-value resistors, or chokes of a few turns of wire, should
be connected directly to the grid and anode leads. These added components will
have very little effect on the amplification of the desired frequency; but they will
isolate the grid from the stray tuned-circuits sufficiently for the parasitic oscillations
to be eliminated.
Components which are placed in a circuit to eliminate parasitic oscillations are
called "parasitic suppressors."
Sometimes parasitic oscillations can be eliminated only by completely re-wiring
the circuit.
4.34
REVIEW
The Three Stages. The
master oscillator, the inter-
mediate power amplifier, and
the final power amplifier make
up the basic three-stage trans-
mitter.
PA
MO
IPA
I Sk>
Tuning. For efficient opera-
tion, the anode tuned-circuit of
the amplifier must resonate at
oscillator frequency. Adjust-
ment of the variable capacitor
to bring this about is called
"tuning."
P>
Tuning Methods. The anode
circuit of each transmitter stage
may be tuned by adjusting the
variable capacitor either for
minimum d.c. anode current, or
for maximum grid current in the
following stage.
Original [-^ |||j|j|r—N Hi r-N
signal '
—
yI llillli
1—y DHIII!
—
v
TransmittedSignal
Neutralization. Anode or
grid neutralization circuits may
be used to counteract the feed-
back effect of the grid-to-anode
capacitance in amplifiers using
triodes.
§4: FREQUENCY MULTIPLIERS 4.35
The Purpose of Frequency Multiplication
Until now, it has always been assumed that the anode tuned-circuit of an amplifier
stage in a transmitter can be tuned only to the grid signal frequency, whatever that
may be. For example, if the grid signal frequency is 1 Mc/s, the anode circuit is
also tuned to 1 Mc/s.If the grid signal is a pure sine wave, the anode circuit can indeed be tuned only
to the frequency of this sine wave (called the fundamental), and to no other. But it
so happens that generated frequencies are very seldom pure; they usually contain
harmonics of the fundamental frequency.
This is especially true in transmitters, where Class C amplifiers introduce manyharmonics into the generated signal. For example, if the master oscillator (oper-
ating in Class C) generates a 1 Mc/s sine wave, that sine wave is rich in harmonics—it contains not only the fundamental (1 Mc/s), but also the second harmonic(2 Mc/s), the third harmonic (3 Mc/s), etc.
If a signal rich in harmonics is applied to the grid of a tuned amplifier, the anodecircuit can be tuned to any one of the harmonics present in the original grid signal
—
with a consequent stepping-up of the frequency of the output signal. In this way, if
the output of an oscillator be only 1,000 kc/s, the output of the buffer amplifier maybe made 2,000 kc/s by tuning its anode to the second harmonic of the signal putinto it; the output of the next amplifier may be made 4,000 kc/s in the same way;and so on.
This process of converting the input frequency to the grid to a higher frequencyat the anode by tuning to a harmonic of the fundamental is called "frequency multi-plication."
tynequencif MuttifJicatfonM/ 4 Mc/s
Masteroscillator
Buffer(Tnpler
2nd I. P. A.Tripler)
F P. A,
The reason why frequency multiplier circuits are useful in transmitters is becauseoscillators operate more satisfactorily at low frequencies. By making use of fre-
quency multiplication when a high frequency is required, the oscillator can be left tooperate at a low frequency, while the multiplier circuits step up the oscillatorfrequency to the one desired.
For very high frequencies, crystal oscillators are often used in order to obtain goodfrequency stability. But it is impractical to manufacture a crystal to vibrate atsuch high frequencies. Therefore, the crystal oscillator is operated at a much lowerfrequency, and the desired output frequency is obtained by frequency multiplication.
4.36 [§4
The Final Power Amplifier
The maximum power which can be radiated from a transmitting aerial depends
on the power output of the final power amplifier (FPA). If the final power ampli-
fier has a power output of 100 watts, the aerial can radiate 100 watts, and no more.
A frequency multiplier has a lower output than has the same stage used as an
amplifier at the fundamental frequency.
If a final power amplifier which is capable of an output of 100 watts at the funda-
mental frequency is used as a doubler, its power output will be only about 65 watts
—if as a tripler, 40 watts—as a quadrupler, 30 watts—and so on. As the multi-
plication of the frequency increases, so the power output decreases.
And so, because the power output of a transmitter depends to a great extent on
the output of the final power amplifier, the FPA is not usually operated as a fre-
quency multiplier. All the multiplication of. the oscillator frequency has to take
place in the intermediate power amplifiers.
If you had this circuit.
and wanted this output
. 12Mc/s
_\/_ 100 W
__ __ __
F.P.A.MasterOscillator
1st
I. P. A.2nd
I. P. A.
you could
use this.
4McM Jl2Mc/fe\ Jl2Mc/A
X4 ~" X3 "F.P.A."
12 Me/a100 W
12 Mc/j
— X4h
12Mc/<
-F.P.A.
12 Mc/s
100 W
lMc/s>
|M.O.
but. .Not this12 Mc/s65 W
becausedoubling in the F.P.A.would result in
lower power output.
§4 4 -37
Frequency Doubling
Let us now examine a typical doubler circuit—that is, a circuit in which the out-
put frequency is twice the input frequency—and see how it works.
The circuit of a frequency doubler is not very different from that of an amplifier
which operates at the input frequency. The only differences are that the anode
circuit will be tuned to twice the input frequency; and that no neutralization is
required, since the input and output operate at different frequencies, so reducing
the possibility of self-excited oscillations.
WAVE FORMS IN A TYPICAL DOUBLER CIRCUIT
The doubler circuit is operated in
Class C, with the anode tuned-circuit
resonant at twice the grid signal fre-
quency. The pulses of current at
the same frequency as the input
signal flow from the cathode to the
anode, energizing the anode tuned-
circuit and causing it to oscillate at
twice the grid signal frequency.
Between pulses of anode current,
the tuned-circuit continues to oscil-
late. This is because the pulses of
"current always arrive at the same
time during alternate cycles of the
Cut-off
Bias
IWWHT+
doubled frequency, thus energizing the tuned-circuit at the right moment.
When accurately tuned, the voltage across the doubler tuned-circuit is at a maxi-
mum, and the voltage at the anode at a minimum, when current flows. The indica-
tions for tuning to twice the frequency are therefore the same as they were for tuning
to the input frequency.
The anode current meter will indicate a dip as the anode circuit is tuned to twice
the input frequency. At the same time, the grid current meter will indicate a rise.
4.38 [§4
Frequency Tripling
A frequency-tripling circuit (more briefly known as a tripler) has an output fre-
quency three times the input frequency. The appearance of the circuit is the sameas that of a doubler, or of an ordinary amplifier. Frequency tripling is accomplishedby tuning the anode circuit of the tripler to the third harmonic of its input frequency.
Input
voltageCut-off
Bias
HT+
Pulses of current flow from cathode
to anode—one pulse per cycle of
applied signal. These pulses arrive at
the tuned-circuit during every third
cycle of output voltage, and deliver
enough energy to the tuned-circuit to
sustain oscillations during those cycles ___________________«»_«.when no pulses occur.
The same tuning indications hold for frequency doubling and tripling as for funda-
mental frequency amplification. When the circuit is tuned accurately to the third
harmonic of the applied frequency, the voltage across the tuned-circuit will be larger
than if the circuit were poorly tuned. This will cause the voltage fed to the next
stage to be larger, which results in more grid current.
The larger voltage across the accurately tuned -circuit causes the anode voltage to
be at a low value when the valve conducts. This results in decreased anode current.
Therefore the correct tuning of the anode circuit—whether it be tuned to the input
frequency or to the second or third harmonic of the input frequency—is always in-
dicated as a dip on the anode current meter, or as a rise on the grid current meter.
The grid current meter in the following stage will also show a rise.
INPUT
GRD ,
CURRENT(MAMETER
FREQUENCY
CIRCUIT
§ 4]4.39
Tuning Indications
At this point the question arises, "How can you tell to which frequency—the
fundamental, or one of its harmonics—the anode tuned-circuit is tuned when the
anode current meter indicates a dip reading?"
The only way is to use a frequency indicator such as a wavemeter, or a calibrated
dial if the tuned-circuit has been previously tuned.
If you are working with an uncalibrated transmitter, the thing to do is to tune a
stage, starting with the tuning capacitor fully meshed. The first dip indicates that
the tuned-circuit is tuned to the fundamental. This can be checked with the wave-
meter.
As you continue decreasing the capacitance, you come to a second dip (not as
pronounced as the first), which is the second harmonic. Again you can check the
frequency with the wavemeter.
Continue decreasing capacitance, and you may come to a third dip (provided the
circuit constants are correct), which in turn is not as pronounced as were either the
first or the second dips. This dip indicates that the anode tuned-circuit is tuned to
the third harmonic. Here, too, you can check the resonant frequency by using the
wavemeter. :.„....„..............................._
'
=
-lllliliilAnode current ijlj.
'illmeieT dip II.
Third ®Harmonic(Triple r)
4.40 §5: TRANSMISSION LINES
Introduction
The function of a transmitter is the radiation of r.f. energy into space, so that
signals may be picked up by receiving aerials situated at various distances from the
transmitter.
You have studied oscillator and Class C amplifier circuits, whose function it is to
generate and amplify r.f. energy. Now other circuits are needed, in addition to the
ones just mentioned, to transfer the amplified r.f. from the anode circuit of the final
power amplifier into surrounding space.
These additional circuits are coupling circuits, transmission lines and aerials.
Just as a speaker in audio work transfers audio energy from electronic circuits into
the air, so the aerial is the means of transferring r.f. energy from the electronic cir-
cuits into space. The transmission line is the conveyor, or link, between the trans-
mitter and the aerial; and the coupling circuit connects the final power amplifier
tank circuit to the transmission line.
HOW RF IS DELIVERED FROM TRANSMITTER TO SPACE
AERIAL \ TRANSMISSION LINERadiates RF Supplies aerial with RF
COUPLING CIRCUITCouples RF from tank circuit
to transmission line
Totransmission
line
Final PA
In this Section you will learn about coupling circuits and transmission lines
—
what they are like and how they do their job. Aerials will be discussed separately
in Section 6.
§ 5]4.41
Coupling Circuits
A coupling circuit is used to transfer energy from the output of the transmitter to
the transmission line which feeds the aerial.
In addition to doing its job of transferring energy, the coupling circuit isolates the
aerial system from the high d.c. potentials present in the anode of the final power
amplifier. The coupling circuit also determines the amount of power transferred
from the tank circuit of the power amplifier to the transmission line input.
The simplest coupling circuit is direct coupling from the tank circuit to a single-
wire transmission line. A small capacitor is always placed at the input to the line
to block the d.c. from the aerial. The coupling is adjusted by varying the tap on
the anode tank coil.
DIRECT COUPLING4HT +
HI-*Blockingcapacitor
To single
wire i:n*
Another simple coupling circuit is inductive coupling to the tank circuit with an
untuned coil of a few turns. This type of coupling is used principally with un-
tuned transmission lines (to be discussed later).
INDUCTIVE COUPLING
A system of untuned coupling called "Link Coupling" is used when the aerial
coupling is remote from the tank circuit. The link consists of two pick-up coils of
about two or three turns, connected by wires and coupled to the tank and the aerial
coupling circuit respectively.
LINK COUPLING
IS 54.42
Tuned Coupling Circuits
A more commonly-used type of coupling is tuned coupling, in which the couplingcircuit is tuned to the operating frequency.
The advantage of tuned coupling is that it is frequency-selective, and so mini-mizes the possibility of undesired frequencies being radiated. In addition, since thetuned coupler is almost always variable-tuned, it can compensate for changes in theimpedance of the transmission line, and thus ensure maximum power transfer fromthe final power amplifier to the line at all times.
When the transmission line has a low input impedance, a series-tuned couplingcircuit is used. Series tuning is called "current feed," and can match the final PAto the low line impedance.
Se^,iHT+
PUHGTo line
with UflViinput
impedance
When a transmission line has a high input impedance, parallel tuning, called
"voltage feed," is used. Here the high impedance of the parallel tuned coupling
circuit matches the high input impedance of the line, and maximum power transfer
is effected.
To line
with lMriil
input
impedance
If the input impedance of the line is other than purely resistive, either of the above
two tuned coupling circuits can be adjusted so that the reactance of the line is can-
celled by the reactance of the tank circuit. This results in a pure resistive load andgives maximum power transfer.
§5] 4.43
Transmission Lines
A transmission line provides a means of transferring electrical energy from one
point to another. You know of at least one use of a transmission line in carrying
50 c/s power from the generator to a point of application.
In transmitters, transmission lines are similarly used to convey r.f. power fromone point to another. For example, a transmission line is used to carry r.f. powerfrom the transmitter to the aerial when the latter is some distance from the trans-
mitter.
7%cuuw£^iatt ^,irte&
50%flO€U&l
4.44 [§5
Frequency and Wavelength
Before you learn the theory of transmission lines, you should understand some-
thing about the properties of a radiated wave—its velocity of propagation (that is
to say, how fast it travels), its frequency, and its wavelength.
For purposes of simplicity, consider an a.c. generator sending 50 c/s energy
along a transmission line. Assume that the rate of travel of the a.c. is the same
as the velocity of electro-magnetic radiation in free space, which is constant at
186,000 miles per second (or 300,000,000 metres per second) regardless of the
frequency.
DISTANCE TRAVELLED
IN VlOO Of A SECOND
50cps
&t*<a
Hi
I860'miles
.ilk
DISTANCE TRAVELLED
IN 1/50 OF A SECOND
If the generator starts its generating action at the zero voltage point on the sine
wave, after a half-cycle has elapsed (1/ 100th of a second in time), the zero voltage
point will have travelled a distance which can be determined by 'multiplying the
velocity of the wave by the time duration for a half-cycle. This distance is about
1860 miles (186,000 x 1/100) which is approximately the distance from London to
Tobruk.
When another half-cycle (making a total of one full cycle) has elapsed (l/50th
of a second), the zero voltage point will have travelled a distance of 3720 miles
(1 86,000 X 1 /50), which is the approximate distance from London to Nairobi. This
distance of 3720 miles is the wavelength of the 50-cycle a.c, or the distance which
the wave travels during the time interval for one complete cycle.
§5] 4.45
Frequency and Wavelength (continued)
The wavelength of any radiated wave can be determined by multiplying the velocity
of the wave by the time taken for one full cycle of the wave. It is usual to express
wavelengths in metres ; so the units used in such calculations are : for velocity, metres
per second ; and for time, fractions of a second.
Now the time taken for one full cycle is 1 divided by the frequency (1//), so the
wavelength of a radiated wave is the constant velocity of propagation (v) divided by
the frequency (/). This is expressed algebraically as X=v/f, the Greek letter
"lambda" being the symbol generally used for wavelength.
This formula can also be written in the form v=f\; and when you look at it ex-
pressed thus, you can see that (since v is constant) the higher the frequency (/) the
shorter is the wavelength (A), and vice versa.
The wavelength of a signal whose frequency is 10 Mc/s can be found from the
formula A=v//as follows:
, 300,000,000 „ ,A=10,000,000
= 30metres
So the wavelength of the 10 Mc/s signal is 30 metres.
Similarly, the wavelength of a signal whose frequency is 15 Mc/s is:
, 300,000,00 1Q .
Q ,A=15,400,000
=1948metres
And the wavelength of a signal whose frequency is 1439 kc/s is:
. 300,000,000 ____tA=
1,439,000=208>5metres
You will find later that aerials and transmission lines are always defined in terms ofthe wavelength of the signals they radiate, rather than in frequencies.
In the table below are listed a few familiar transmitters, and their broadcasting
frequencies and wavelengths.
Transmitter Frequency Wavelengthin metres
BBC Light ProgrammeLong Waveband 200 kc/s 1500
Wrotham (VHF) 891 Mc/s 3-36
BBC Home Service
Medium Waveband 908 kc/s 330-4
Wrotham (VHF) 93-5 Mc/s 3-208
BBC Third ProgrammeMedium Waveband 647 kc/s 463-7
Wrotham (VHF) 91-3 Mc/s 3-285
Radio Luxembourg 1439 kc/s 208-5
BBC Television Channel 1
Vision 45 Mc/s 6-66
Sound 41-5 Mc/s 7-23
ITA Channel 9
Vision 194-75 Mc/s 1-54
Sound 191-25 Mc/s 1-57
4.46 [§5
The "Equivalent Circuit" of a Transmission Line
A typical transmission line used to convey r.f. energy from one point to another
may consist of two parallel lengths of wire held apart by insulating spacers, as illus-
trated below.
An r.f. transmission line will have a certain amount of resistance, capacitance,
and inductance along its length. The resistance is simply the resistance of the wire;
the inductive effect is caused by the expansion and collapse of the magnetic field
(generated by current flow) along the entire length of the line; and the capacitance
exists because the two conductors of the line act as plates of a capacitor separated
by a dielectric (which in this case is air).
Since the line illustrated can be theoretically "broken up" into a number of small
segments, each having equal amounts of inductance, capacitance, and resistance, the
entire line can be represented as consisting of a series of L, C, R networks connected
as shown below.
Transmission line and its equivalent circuit.
§ 5] 4.47
Characteristic Impedance
Suppose an r.f. generator is connected across a transmission line. The generator
will set up a voltage across the line, which forces a current to flow. The amplitude
of this current will be determined by the resistance, the inductance, and the capaci-
tance of the line, which together make up the line's impedance.
If the magnitude of the input current be now measured, and then divided into the
input voltage, the input impedance (Zin) of the line will be found.
If the line be now assumed to have infinite length, the input impedance of such a
line is termed the "characteristic impedance" of the line.
The symbol for characteristic impedance is Z .
You know that, when a pure resistance loads a generator, all the power generated
will be dissipated by this resistance. In the same way, if a generator could be madeto send electrical energy down an infinitely long transmission line, the energy wouldtravel down the line indefinitely, and would all be dissipated by the infinitely long
line.
In other words, the infinite line would act as a resistance equal in value to its
characteristic impedance, Z .
The infinite line can therefore be replaced by a resistance equal to its characteristic
impedance; and the generator will send the same amount of power into the resistance
as it did (theoretically) into the infinite line.
*t$?
*'l.l.
4.48 [§5
Line Termination in Characteristic Impedance
If a transmission line is terminated in a resistive load equal to its characteristic im-
pedance, this load will absorb all the energy from the line which has been applied
to the input by the generator. This is the ideal condition for maximum powertransfer.
• • . FOR MAXIMUM POWER
Tload I
Take as an example the case of a transmission line feeding an aerial. If a certain
type of aerial, called a "half-wave dipole," is used, the impedance at its centre feed-
point is 73 ohms. It follows that, in order to get maximum power transfer from
the transmission line to the aerial, the characteristic impedance of the line should
be 73 ohms, or close to it.
When this is the case, the line is said to be "matched" to the aerial.
• . . MATCHING LINE TO AERIAL
lf...ZAE
=730. .
for Maximum Power Output
Z should equal 73
A
§5] 4.49
Non-resonant and Resonant Lines
When a transmission line is matched to a load (Zioad = Z ), the a.c. voltage
measured across the line at any point will be the same, if you discount the slight
voltage drops in the line caused by its resistance. The current measured at any
point in the line will also be the same. This condition is shown in the illustration
by equal readings on the r.f. voltmeters and ammeters placed along the length of the
line.
The effective voltage and current distribution along the line can be shown graphi-
cally by two straight lines, indicating that the effective r.f. voltages and currents are
equal all along the length of line.
Such a line is called an untuned, or non-resonant, line. A transmission line will
always be non-resonant if it is terminated in its characteristic impedance—which
you will remember is the condition required for maximum power transfer.
distribution along an Untuned Line
^-^a$-^&-r-Qi
© @ E R = Zq
If a line is not terminated in its characteristic impedance, it is said to be "mis-
matched"; and not all the r.f. energy travelling down the line will be absorbed at
the load end. The amount of energy that will be absorbed depends on how close
the value of the load impedance is to the characteristic impedance of the line.
Since the load of a mis-matched line does not absorb all the energy coming downthe line, that part of the energy which is not absorbed must be reflected back up the
line. This energy is called the "reflected wave."
A mis-matched line therefore has two waves flowing through it—the forward waveand the reflected wave. These two waves combine all along the line (now called a
"resonant line") to form a resultant wave called a "standing wave."
4.50 [§5
"Standing Waves" on a Rope
It will help you to understand better how energy travels down a transmission line,
and how reflected waves generate standing waves on the line, to consider a rope, oneof whose ends is fastened to a wall while the other end is held in the hand.
When the hand flicks the rope once, a vibration starts to travel down the rope.
If the rope were infinitely long, the vibration would continue down the rope for ever.
(This is what would happen with an infinite length of transmission line, or an un-
tuned line, in that the energy put into the line is completely absorbed.)
The Long Transmission Line
and the Long Rope
Toinfinity
*•
But when the vibration travelling down the rope reaches the end attached to the
wall, it is in fact reflected back towards the hand. Similarly, when a transmission
line is mis-matched, the electrical energy is reflected back towards the generator.
If the hand vibrates the rope at a constant rate, the reflected vibrations combine
with the oncoming vibrations to produce standing waves along the rope.
At some points along the rope, the forward and reflected vibrations will be in
phase, reinforcing each other to produce vibration of large amplitude. At other
points they will be out of phase, thereby cancelling each other; and the rope will
appear to be motionless at these points.
In a similar manner, standing waves of voltage and current are formed on a trans-
mission line when it is mis-matched.
STANDING WAVES on a rope
§ 5]4.51
Open and Shorted Transmission Lines
When a transmission line is open at its end, the forward and reflected waves com-
bine along the line to form points of varying effective voltage and current.
At the open end, the effective voltage is a maximum, and the effective current is
zero. (It is easy to see that the current must be zero at all times at the open end,
because it is an open circuit. And, since charges build up on the open ends, a large
voltage difference will always exist there.)
At every half-wavelength distance from the open end, these conditions of voltage
and current will repeat themselves; and between these half-wave points the effective
voltage and current readings will vary as a sine wave varies.
The meter reading in the illustration below shows the variations in the effective
voltage and current along the length of the line at quarter-wavelength distances from
the open end to the input. The waveforms shown are actually a plot of these voltage
and current readings at different points along the line.
Observe that the waveforms (called, you will remember, "standing waves") cause
the voltage and current to be zero at certain definite points along the line. Notice,
too, that when the current is zero, the voltage is maximum; and when the voltage
is zero, the current is maximum.
Standing waves on
OPEN CIRCUITEDLINE
When the transmission line is shorted at its terminating end, however, the voltage
at that end must be zero; because no voltage can exist across a short. But the
current at the short will be a maximum; because the short provides a zero resistance
path through which current can flow.
Just as in the open-circuited line, these voltage and current conditions at the
terminating end will repeat themselves at half-wavelength intervals back from the
short circuit.
Observe that the standing waves on the short-circuited line are displaced a distance
equivalent to a quarter of a wavelength (90 degrees) compared to waves on the open-
circuited line.
Standing waves onSHORT CIRCUITED
LINE SHORTED
[§54.52
The Input Impedance of a Line
In a transmission line terminated in its characteristic impedance, the voltage andcurrent readings are the same all along the line. Therefore, the impedance anywherealong the line is constant, and equal to its characteristic impedance.
In other words, if you were to break off the line anywhere along its length, andmeasure the impedance (Zin) looking in towards the load end, the impedance value
measured would always be the same, and would be equal to the characteristic
impedance, Z —which is resistive.
When a transmission line is terminated in any other than its characteristic im-
pedance, it becomes resonant, and develops standing waves. The input impedance
then varies with the length of the line, because the effective values of the current and
voltage vary along the length of the line.
The reactance of the input impedance also varies—being sometimes resistive,
sometimes capacitive, and sometimes inductive. A resonant line has, therefore, the
characteristics of a resonant circuit, which presents a resistive load at the resonant
frequency, and inductive or capacitive reactance on either side of the resonant fre-
quency.
resohnMl\HE
§5] 4.53
The Input Impedance of a Short-circuited Line
A short-circuited line offers very low resistance at its shorted end, since the voltage
is minimum and the current is maximum. This low resistance is repeated every
half-wavelength back from the shorted end.
Since the line is called "resonant," it is convenient to think of the low resistance
points along the line as series-resonant circuits. For example, the input impedance
at a half-wavelength section of shorted line is that of a series-resonant circuit.
A quarter-wavelength back from the shorted end, however, the current is minimumand the voltage is maximum. This is therefore a point of high resistance.
These points of high resistance are repeated every half-wavelength back from the
first high resistance point. They can be considered to be parallel-resonant circuits,
just as the low resistance points can be considered series-resonant circuits.
Between the high and the low resistance points, the input impedance is either a
capacitive reactance or an inductive reactance.
From the shorted-end to a quarter-wavelength back from the terminating short
circuit, the input impedance is inductive. The inductive reactance is low in the
vicinity of the short circuit, but increases in magnitude as you approach the quarter-
wave point. Exactly at the quarter-wave point, the impedance is a pure high
resistance.
Between a quarter-wavelength and a half-wavelength, the input impedance is
capacitive reactance. This capacitive reactance decreases as the half-wavelength
point is approached; until, at the half-wavelength point, the impedance is a pure
low resistance.
The type and magnitude of the input impedance, as seen at different points along
the short-circuited line, is illustrated below.
INPUT IMPEDANCE ALONG A SHORT CIRCUITED LINE
4.54 [§5
The Input Impedance of an Open-circuited Line
In an open-circuited line, the terminating impedance (open circuit) is a high
resistance, and therefore acts like a parallel circuit.
A quarter-wavelength back, the input impedance is a low resistance; it therefore
has the characteristics of a series-resonant circuit. Between the open-circuited endand a quarter-wavelength back from the open circuit, the input impedance is capaci-
tive; and between a quarter and a half-Wavelength, the input impedance is inductive.
If you compare the open- and short-circuited lines, you will observe that, for a
given wavelength back from the end, the reactances are opposite to one another.
Where one is capacitive, the other is inductive, and vice versa.
INPUT IMPEDANCE ALONG AN OPEN-CIRCUITED LINE
*hMil? 1 TT TU^
ToGenerator
The following diagrams illustrate different lengths of open and shorted lines, and
the input impedance they present to a generator.
It is obvious from the above diagrams that the terminating conditions at the end
of the line determine the type and magnitude of the input impedance at any point
along the line.
§5] ".55
Some Applications of Transmission Line Principles
Suppose you now learn about a few of the many applications of transmission lines
in electronic equipment.
A shorted quarter-wave transmission line, known as a "stub," will, for instance,
offer a very high impedance at its input. It can therefore be used as a metallic
insulator to support a two-wire transmission line without shorting the line.
A/4 STUB USED AS A METALLIC INSULATOR
Two-wiretransmission
line
Supportingmetal base
The shorted quarter-wave stub also makes a very effective filter for harmonic
frequencies of the fundamental which it is not desired to transmit.
For the fundamental frequency, the stub is a high impedance, as was shown above.
For the second harmonic, the stub becomes a half-wavelength in length. It will
therefore act as a short circuit across the transmission line, shorting out the un-
desirable harmonic and preventing it from getting to the aerial.
/ 2 STUB ASHARMONIC FILTER
LOWIMPEDANCE
4.56 B s
Some Applications of Transmission Line Principles (continued)
An important application of a short transmission line—or "tuned line section,"as it is called—is to tune out the reactance of a load on a transmission line, thusleaving the load resistive.
Suppose, for example, that a 300-ohm line is feeding a load which looks like a300-ohm resistance in parallel with a capacitance. Since the load is not completelyresistive, standing waves will exist on the line, and maximum power transfer to theload will not be realized.
But if an inductance could be placed in parallel with the capacitance, so as to
effect a parallel-resonant circuit, the transmission line would look into the 300-ohmresistive component in parallel with the high resistance of the parallel-resonant cir-
cuit. And since the high resistance of the parallel-resonant circuit is so much greater
than 300 ohms, the transmission line would effectively "see" only the 300-ohmresistance.
The effect of the capacitance in preventing full power transfer to the load wouldthus be cancelled out.
The way to introduce an inductance across the load is to place a quarter-wave
shorted stub, with a movable shorting arm, across the load terminals. Then, bymoving the short so that the stub becomes less than a quarter-wavelength long, the
input reactance of the stub becomes inductive.
The value of this inductance can be varied by means of the movable short until it
exactly cancels the capacitance of the load, thus leaving the load resistive.
K A/4H ___=A 300 i i High
MATCHED LINE...No standing wavesMaximum power transfer
Quarter-wave sections of line are also used as transformers, or as matching devices
to connect circuits of unequal impedances. If a low-impedance input circuit is to
be connected to a high-impedance grid circuit, the input circuit may be tapped downon the coil of a tuned-circuit, as shown below.
If a tuned line is used, the input circuit can similarly be tapped down on the
tuned line. This is an example of a tuned line used as a step-up transformer.
Lowimpedance
input
circuit
A quarter-wave stub can be used as a step-down transformer to match a high-
impedance line to a low-impedance dipole aerial. The line is connected to the high-
impedance input of the stub, while the aerial is connected near the low-impedance
shorted end of the same stub.
High impedance I
line -r-
Stub used as 1 High imp. 1
a step-downtransformer
line -•_ ^4 -»
1 l
§5] 4.57
Types of Transmission Lines
Many different types of transmission lines are used in electronic applications.
Each has a certain characteristic impedance, current-carrying capacity, insulation,
or physical shape fitting it to meet a particular requirement.
Shown below are some of the most frequently used transmission lines.
SingleA simple method of feeding an aerial /Wire
from a transmitter is to use a single-wire
transmission line, with an earth return com-
pleting the circuit.
Another type of transmission line consists of two parallel wires, maintained at
a fixed distance from one another by insulated spacers. Since this type of line is
not screened, losses of power occur in it by reason of radiation, and of absorption
by metallic objects. The use of the line is therefore restricted to comparatively low-
frequency transmission; and it should be strung only in places where it will be away
from metallic objects and out in the open.
TWO WIREOPEN LINE Insulated spacer^ Parallel wires'
Some of the disadvantages of the two-wire open line are overcome in the concen-
tric line, which is made of a cylindrical copper tube with a thin conductor running
full length through the centre. The inner conductor is kept centred by spacers, and
the outer conductor is earthed in order to screen the inner conductor.
Since the concentric line is mechanically rigid, however, it can be used only for
permanent installations.
CONCENTRIC
LINE Outer conducto Inner conductor
The inflexibility of the concentric line is overcome in the coaxial cable, which
consists of an inner conductor embedded in an insulating material, which is in turn
covered with copper braid. The coaxial cable incurs, however, much higher powerlosses than does the concentric line.
COAXIAL
CABLE
Insulation Rubber covering-.
Innerconductor Copper braiding
At very high frequencies, the losses in any of the above-mentioned lines becomeexcessive; and wave guides must be used instead. Wave guides are made of roundor rectangular hollow tubes.
WAVEGUIDE Rectangular hollow tube
[§54.58
REVIEW
Transmission Lines. The purpose of a
transmission line in a transmitter is to con-
vey r.f. energy from the transmitter to the
aerial. The characteristic impedance of
the transmission line must match the input
impedance of the aerial, if maximum power
transfer to the aerial, and therefore maxi-
mum radiated power, is to be realized.
Characteristic Impedance. A transmission line has a characteristic impedance (Z ).
If it is terminated in a load equal to its characteristic impedance, maximum power is
transferred to the load, and no standing waves will be created on the line.
. . . MATCHING LINE TO AERIAL
lf...ZAE
= 73 n...
for Maximum Power Output
Z should equal 73H
Standing Waves. When a transmission line is terminated in a load other than its
characteristic impedance, some of the energy is reflected from the end of the line back
towards the generator. The forward and reflected waves combine along the line to form
standing waves.
The voltage and current distribution along an open and a shorted line respectively are
shown again below.
OPEN SHORTED
§6: AERIALS 4.59
The Function of an Aerial
The purpose of a transmitting aerial is to convert the power delivered by the
transmission line into a wave called an "electro-magnetic wave." This electro-
magnetic wave is then radiated through space.
All aerials work on the same principle—the aerial current generates an electro-
magnetic field, which leaves the aerial and radiates outwards as an electro-magnetic
wave.
The aerials you will be concerned with now are those which have been designed
as transmitting aerials. But before you begin on them, you may be interested to
hear of an example of aerial action in which planning and design has played no part
at all.
THE AERIALCONVERTS ELECTRICALPOWER INTO ELECTRO-MAGNETIC WAVES
Electro-Magneticwaves
Try touching your finger to the Y-plate terminal of an oscilloscope. You will
see a 50-cycle waveform on the 'scope screen, which obviously must come from your
body.
What is actually happening is that your body is picking up 50-cycle electro-
magnetic waves which are being radiated from the mains wires which carry 50-cycle
current. The wires are in fact acting as transmitting aerials, although they were
obviously not designed for that purpose.
Body picking up50 cvcle
radiation
tiom mams
[§64.60
How an Aerial Works
If the wires of an open-ended transmission line are bent back at right angles tothe line, at a point one quarter-wavelength back from the open end, a simple aerial is
formed which is called either, a "half-wave dipole," a "doublet," or a "Hertz aerial."
The voltage and current distribution on this aerial are the same as on the originaltransmission line.
Bend transmission line here
If at any point on a transmission line, the voltage on one wire is (+) 10 volts with
respect to the other wire, then you could also say that the voltage on the second wire
is (— ) 10 volts with respect to the first wire. So also on the half-wave dipole aerial
wires, the voltages at any two corresponding points on the two wires are equal, but
opposite in polarity. The same thing is true of the currents.
Therefore, to indicate polarity as well as amplitude on the wires Which comprise
the transmission line and aerial, the waveforms must be re-drawn as shown.
WAVE FORMS SHOWING POLARITY AND AMPLITUDE
Transmission Line Aerial
Observe that the standing waves of voltage and current indicate that the aerial
ends are points of maximum voltage and minimum current; whereas the centre of
the aerial is a point of maximum current and minimum voltage.
§6] 4.61
How an Aerial Works (continued)
You know that, whenever there is a difference in voltage between two points, an
electrical field is set up between these points. You also learnt in Basic Electricity
that when a capacitor is charged, one plate will be positive and the other negative.
The consequence of these two facts is that an electrical field, having a direction
towards the positively-charged plate, is built up between the capacitor plates—as
shown in the first diagram below.
In the same way, the voltage difference between the two wires of an aerial also
generates an electrical field, which has a pattern and direction that you can see in
the second diagram.
Electric Field of aCharged Condenser Electric Field Surrounding
an Amort'h^^x^M^MM^M^M^WM^Mm -̂
Besides this electrical field, there is also a magnetic field, which is generated by
the aerial current. The plane of this magnetic field is at right angles to the direction
of the current flow; and therefore is at right angles to the aerial (see below).
The electrical and magnetic fields are therefore at right angles to each other.
Magnetic Field Surrounding
<ut/hria£
These electrical and magnetic fields alternate about the aerial—building up, reach-
ing a peak, collapsing, and building up again in the opposite direction—at the same
frequency as the aerial current.
In the process of building up and collapsing, a portion of these fields escape from
the aerial, and become the electro-magnetic waves which radiate through space,
conveying the transmitted intelligence to distant receivers.
IS 64.62
Basic Aerials
The half-wave dipole, or Hertz, aerial is one type of basic aerial which finds wideapplication in many kinds of transmitting and receiving equipment.
Another basic aerial is a vertical quarter-wave grounded aerial sometimes called a
"Marconi aerial." If one of the elements of a Hertz aerial be removed, and the wire
which went to that element earthed or grounded, the result will be a Marconiaerial. The earth actually takes the place of one of the quarter-wave elements, so
that the earth and the remaining quarter-wave element form an effective half-wave
dipole.
The current maximum and voltage minimum points are at the base of the aerial,
as you can see below.
8ASf€A ILS
VERTICAL A/4 GROUNDEDAERIAL (Marconi)
When a Marconi aerial is used, the earth directly beneath the aerial must be a
good electrical conductor. Sometimes, in order to improve the ground conductivity
at this point, a length of copper tubing is driven into the ground at the base of the
aerial.
An earth can be simulated by placing metal rods or wire mesh at the base of an
aerial. This arrangement is called a "counterpoise earth."
Since a quarter-wavelength dipole aerial is only half as long, physically, as is a
half-wave dipole, it is often preferred at low frequencies (large wavelength), especi-
ally when there are space restrictions on aerial mountings.
At high frequencies the half-wavelength dipole is extensively used; because, even
though it is longer than the quarter-wave aerial, its overall length is still small—and
it can be made of metal tubing, which makes it self-supporting.
§t]4 -63
Radiation Resistance
In a half-wave dipole aerial, the voltage at the centre is minimum (in fact it is
practically zero), whereas the current is maximum. If you recall the characteristics
of a series-resonant circuit, you will remember that the voltage across it is also
minimum, and the current through it also maximum. At its centre, therefore, a
half-wave dipole is equivalent to a series-resonant circuit when it is operated at the
frequency for which it is designed.
A generator supplying power to a series-resonant circuit works into a pure resistance
(since XL and Xc cancel each other out)—the resistance into which it works being
principally the resistance of the coil. In the same way, a transmission line works
into a pure resistance when a half-wave dipole is connected to it.
This resistance is made up of the resistance of the wire, and another resistance
called the "radiation resistance." Since the resistance of the wire is in practice
negligible, only the radiation resistance need be taken into consideration at this point.
THE INPUT IMPEDANCE OF A DOUBLET LOOKSUKL.
series resonant circuU
The radiation resistance, however, is not a physical resistance. It is, rather, an
"equivalent" resistance—a resistance which, if it were connected in place of the
aerial, would dissipate the same amount of power as the aerial radiates into space.
The value of this radiation resistance can be determined from the power formula,
R = P/I2, where P is the power radiated from the aerial, and / is equal to the aerial
current at the centre of the aerial.
For a half-wave dipole the radiation resistance is about 73 ohms, measured at the
centre of the aerial. This value is fairly constant for half-wave dipoles working at
any frequency.
73A
M^#k-V4-j e/gpfl*A '" -Wr-
.the radiation resistance
(S 64.64
Aerial Impedance
Since a half-wave dipole acts like a series-resonant circuit, it will exhibit eitherinductive or capacitive properties as the r.f. frequency applied to the aerial is varied.When the frequency of the r.f. is just right, the dipole is exactly a half-wavelength
long, and is series-resonant—with its impedance resistive, and equal to the radiationresistance. It is always desirable that a transmitting aerial shall present a resistiveload to the transmission line; for in this way a maximum amount of power will beabsorbed by the aerial, and so radiated.
Optimum conditionm^^^SSESfeexactly x/2 730
[-^gggMh-VWri = -m—The radiationresistance
If the frequency of the transmitter goes up, the aerial will be greater in length thanis the new half-wave. The series circuit will then be operating at a frequency whichis above its resonant frequency. At this applied frequency, the inductive reactancewill be larger than the capacitive resistance, and the aerial will appear inductive
to the transmitter.
Dipole LONGER than ty2 appears INDUCTIVE
= —nppp--*VWV—
/2
If the frequency of the transmitter goes down, however, the aerial will now beslightly shorter than a half-wavelength. The series circuit will be operating at a
frequency which is below its resonant frequency. The capacitive reactance will belarger than the inductive reactance, and the aerial will appear capacitive to the
transmitter.
Dipole SHORTER than A/2 appears CAPACITIVE
B" — —1|—WW—A/2
Tuning the Aerial
You have seen that, when the frequency of the transmitter is varied, the electrical
length of the aerial varies as does the impedance at its input. You also know that
it is desirable to have the aerial impedance resistive for all transmitter frequencies,
in order to obtain maximum radiated power.
The electrical length of an aerial can be effectively increased or diminished (and
the aerial can thus be resonated) by the insertion into the circuit of appropriate in-
ductors or capacitors.
Suppose, for example, that a vertical quarter-wave grounded aerial be less than a
quarter-wavelength long. The input impedance at its base will be both resistive and
capacitive. This aerial can be electrically lengthened (and so resonated) by adding
an inductor of the correct size to cancel the capacitance, thus leaving the aerial
resistive.
The inductor must be placed in series with the aerial at its base, as shown below.
RESONATING A MARCONI AERIAL
O -
looks like looks like —WWW—If a vertical quarter-wave grounded aerial be more than a quarter-wavelength long,
the input impedance at its base will be both resistive and inductive. The aerial can
be electrically shortened by adding a capacitor of the right size to cancel the induc-
tance, thus leaving the aerial resistive.
looks like o-W/VWr—/0OT^-o looks like —WUk-
[§64.66
Radiation Pattern
When an aerial radiates electro-magnetic waves, the radiation will be stronger in
some directions than in others. The aerial is said to be "directional" along its line
of strongest radiation.
In the case of the half-wave dipole, maximum radiation occurs in the plane at
right-angles to the aerial conductors which passes through the point of maximumcurrent in the conductors.
The amount of radiation from a dipole is shown graphically in the diagramsbelow. Maximum radiation takes place along the lines OP, OP', OP", etc.
Dipole\^ t t
Point of maximum*" ~
'
—current
Radiation pattern asviewed in direction of
arrow A'
A radiation tester, called a "field strength meter," can be used to measure the field
strength at all points around the aerial. The radiation patterns for a half-wavedipole mounted both horizontally and vertically are illustrated below.
r —-«.
rf>5$*£«d»
i
MaximumRadiationin this plane
Horizontal
aerial MinimumRadiation
MaximumRadiationin this plane
All of the above diagrams assume that the aerial is isolated in space. In actual
practice, however, the aerial will certainly be located near ground surfaces, so that
the radiation patterns will be appreciably different from those shown above.
4.67
§6]
Wave Propagation
You know that the function of an aerial is to radiate electro-magnetic energy into
space. Once this energy is released from the aerial, it will travel through space until
it is picked up by a receiving aerial-or until it strikes an object and is reflected off
it as is the case with radar transmission.
'it is therefore important to you to know what happens to a radiated wave in space
-what its path is, if it is absorbed by the earth, if it is reflected by the sky, and so
on-in order that you may be able to tell how far the wave will travel before it can
be picked up. . .
The subject of what happens to a radiated electro-magnetic wave once it leaves
the aerial is called the theory of "wave propagation."
When a radiated wave leaves the aerial, part of its energy travels along the earth,
following the curvature of the earth. This is called the "ground wave."
Other waves which strike the ground between the transmitter and the horizon are
called "space waves"; and those which leave the aerial at an angle greater than that
between the aerial and the horizon are called "sky waves."
The ground wave, the space waves, and the sky waves all carry the transmitted
intelligence. But at certain frequencies one of the waves will be much more effective
in transmitting the intelligence than will the others.
At comparatively low transmitted frequencies, most of the radiated energy is in the
ground wave. Since the earth is a poor conductor, the ground wave is rapidly
reduced, or "attenuated," by absorption and is therefore not effective for transmission
over great distances unless large amounts of transmitted power are used.
The medium and long wave-band broadcast frequencies are examples of trans-
missions using ground waves. At these frequencies the effective radiating area
usually lies within 200 miles radius from the transmitter. Stations more than
400 miles away from each other can therefore transmit on the same low frequencies,
and yet not interfere with each other.
(S 64.68
Sky Waves and Ground WavesAt first sight, one would think that sky waves can serve no useful purposes, since
they will only travel straight out into space and get lost.For very high frequencies, this actually happens, and the sky wave is useless But
below a certain critical frequency the sky wave does not travel straight our it isbent back to earth in the upper layers of the atmosphere.
This returning wave is not sharply reflected, as is light from a mirror. It is bentback slowly, as if it were going round a curve; it is therefore called a "refractedwave.
This refracted wave, once it returns to earth, is reflected back to the sky again—where it is once again refracted back to earth. This process of refraction from thesky and reflection from the earth continues until the wave is completely attenuated—the energy of a radiated wave dropping as its distance from the transmitting aerialincreases. A receiving aerial will be able to pick up a signal at any point where therefracted wave hits the earth.
If the sky wave were radiated to the sky at one angle only, of course, no signalwould arrive at any point between those where the refracted wave hit the earth.Sky waves, however, are radiated at all angles to the sky; the earth's surface (beyonda certain minimum distance from the aerial) is therefore completely covered withradio signals.
As the angle of radiation of the sky wave gets steeper, a point is eventually reachedat which the wave is no longer refracted back to earth, but continues travelling oninto space. As a result, there is a zone around the aerial in which no refracted skywave hits the earth.
Since the ground wave itself is only effective over a short distance, there exists azone between the maximum effective radiating distance of the ground wave and thepoint where the first sky wave is refracted back to earth, which is an area of "radiosilence" in which no signals from this particular transmitter are received. This zoneis called the "skip distance" of the transmitter.
Lost SkyWaves
Limit of
Space Waves
First RefractedSky Wave
^'^^f^PMk-^
Limit of
Ground Wave
'::'#:?
rw.m
The critical frequency, which is the frequency above which no sky wave (whateverits angle of radiation) can return to earth, varies—depending on numerous factors
such as the time of day, the time of year, the weather, and others.
§4]
The Space Wave and Fading
At frequencies above the critical frequency, neither the ground wave nor the sky
wave can be used for transmission. At these high frequencies, the ground wave is
rapidly attenuated, and the sky wave is not refracted back to earth.
The only radiated wave which can be used for transmission at these frequencies
is one that travels in a direct line from the transmitting aerial to the receiving aerial.
This type of transmission is called "line-of-sight transmission"; and the radiated
wave is called a "space wave."
Line-of-sight transmission is used in radar for detecting enemy aircraft, and (for
instance) in ship-to-plane communication. The frequencies used are usually above
30 megacycles.
LINE OF SIGHT TRANSMISSION
..tuedtinXadax/
• • • • ^»}i2^BfJtiSi,l-ii;,'!-
Sometimes a receiving aerial picks up two signals which have travelled along dif-
ferent paths from the same transmitting aerial. For example, one signal may travel
direct from the aerial, and the other signal may have been reflected off an object.
Since the signal paths are constantly changing, the two signals will sometimes be
in phase, and at other times out of phase with each other—thus tending either to
cancel or to reinforce each other.
The result is a variation in signal strength at the receiver end which is called
"fading."
^mx^^i
FADING
ignals received
*3»
Frequency Spectrum
There follows now an outline of the components of a radiated wave which areused for transmission at various frequencies:
From 30 to 300 kilocycles (low-frequency band), the ground wave is largely usedfor medium-range communication, since its stability is not affected by seasonal orweather changes. For very long distance communication, the sky wave is used.
From 300 to 3000 kilocycles (medium-frequency band), the range of the groundwave varies from 15 to 400 miles. Sky wave transmission is excellent at night forranges up to 8000 miles. In the daytime, however, sky wave transmission becomeserratic, especially at the high end of the band.
From 3 to 30 megacycles (high-frequency band), the range of the ground wavedecreases rapidly, and sky wave transmission is highly erratic on account of theseasonal factors previously mentioned. Space wave transmission begins to becomeimportant.
From 30 to 300 megacycles (very high-frequency band—or VHF), neither theground wave nor the sky wave are usable; and space wave (Iine-of-sight) transmissionfinds major application.
From 300 to 3000 megacycles (ultra-high-frequency band—UHF), space wavetransmission is used exclusively.
TRANSMITTING FREQUENCIES.
.
.
^ppw^wv
illlilKi,,
$;:&&;;£I&x-Svl|\;X-.\vX;X;X*X\'X*X%P*j-'.X-Xv.*vXvX*XvX*X€*
r -x-x ::'•:•:'.£88$$30 Ke/8 300 Kc/s
Used for Communication3Mc/s
Radar.
5 6]
REVIEW—Aerials
Half-wave Dipole. A half-wave dipole aerial can be considered as a parallel-wire
transmission line whose wires have been bent at 90 degrees to the line a quarter wave-
length from the open end.
Bend transmission line here
Radiation. The voltage and current distributions along the aerial generate electric
and magnetic fields at right angles to each other, which are radiated into space as
electro-magnetic waves. These waves carry the intelligence of the modulating signal,
and can be detected by distant receivers.
Magnetic Field Surrounding
4* Aenai
Wave Propagation. The energy radiated from an aerial consists of sky waves,
space waves and ground waves. Each of these is used for transmission at those
frequencies for which it is best suited.
4 72 § 7: CONTINUOUS-WAVE (CW) TRANSMISSIONThe Advantages of CW Transmission
You will remember from the Section on "The Role of the Transmitter" that amessage can be transmitted either by code or by radio telephony. Code transmissionis either CW (continuous wave) or MCW (modulated continuous wave). In bothcases the r.f. radiated by the aerial is turned on and off, either by a hand-operatedkey, or by an automatic device, in sequences of dots and dashes.CW transmission is very widely used. When a transmitter is modulated by voice
or by MCW, it sends out not only the carrier frequency, but also the sum anddifference (beat) frequencies of the carrier and of the modulation signal. Theseadditional frequencies are called "sideband frequencies." Now in order to pick upa voice or MCW signal, a receiver must be sufficiently broadly tuned to pick up boththe carrier and the sidebands. As a result, it may also pick up a nearby signal inaddition to the one desired; and this interference may make it impossible to under-stand the desired signal.
CW transmission, on the other hand, has negligible sidebands. A receiver, there-fore, does not need to cover as wide a range of frequencies for a CW signal as it doesfor a voice signal. The receiver may be made highly selective, and there is thusless likelihood of interference when receiving a CW signal.
Carrier Wave Frequency
SGf 0F FHf WU
f^ptSOVTRANSMlW-
CW\
MCW
Carrier Wave Frequency
\ Sideband
L^V FrequenciesmCarrier Wave Frequency i
\Sidebands I Sidebands
There are many different circuits used to obtain CW transmission. They lookdifferent, and they operate differently; but each has the same purpose—namely, toturn the r.f . of the transmitter on and off.
You will learn about some of these circuits in the next few pages.
4.73§7]
Cathode Keying
Regardless of the circuit used, the CW output of a transmitter looks like a series
of pulses of r.f. separated by gaps of no r.f. The gaps between the ri. pulses occur
when the key is up; while the length of each ri. pulse is determined by the length of
time the operator holds the key down.
The simplest method of obtaining CW transmission is by "cathode keying." In
this type of circuit, the key may be connected as illustrated below. When the key
is opened, no current can flow, and no r.f. can be radiated from the aerial. When
the key is closed, the circuit operates normally.
The stage which is usually keyed in this manner is either the master oscillator
itself, or the master oscillator plus one or more of the following amplifier stages.
Seated,
\\v-tr\y\ ladiiectltf
Seated- -
l^M^H^MjB 1 1 *
j
^ Kevi
r^ 1
4.74[§7
Cathode Keying (continued)
The disadvantage of using direct cathode keying is that the operator will get ashock if he puts his fingers across the key contacts while the key is open. For whenthe key is up, the series circuit of key, valve, and H.T. is open at the key; and nocurrent can flow. But with 'the operator's fingers across the contacts, the circuit iscompleted; and current flows—through the fingers!
What happens is that the anode resistance of the valve and the resistance of theoperator's fingers across the key contacts form a voltage divider circuit across H.T.The resistance of the operator's fingers will usually be large compared to the anoderesistance—with the result that most of the H.T. voltage (of about 300 volts) will beacross the key, and therefore across the fingers.
*/%& & efutvateHt ta t&U
HOW A VOLTAGE DEVELOPS ACROSSTHE OPEN KEY CONTACTS
To safeguard the operator, a slight variation can be made to this basic circuit.
The key is connected to a low voltage circuit containing the coil of a relay. Whenthe coil of the relay is energized, the contacts of the relay (which are in series withthe cathode circuit) close, and permit the stage to operate normally.
KEYING CIRCUITSUSING RELAY
Armature ContactsTo
circuit
The relay consists of a soft-iron core,
a coil and an armature. The armatureis attracted to the core when the coil is
energized, thereby closing the contacts.
A spring re-opens the contacts when the
coil is de-energized.
Electro-magnet
To voltage •*
source
To circuit
Insulation
Armature
RELAY Returnspring
Blocked-grid Keying
Keying can also be accomplished by changing the grid voltage of the stage being
keved.
When the key is open, the grid bias is well beyond cut-off; so that the r.f.grid
signal can never bring the valve into conduction, and no r.f. signal appears at the
anode. When the key is closed, the bias is the normal value for Class C operation,
and the stage operates normally. This type of keying is known as "blocked-grid
keying."
In the circuit shown on the left below, the key (or relay) controls the d.c. bias on
the grid of an intermediate power amplifier. With the key open, the voltage on the
grid is equal to the bias voltage, which is many times cut-off value. With the key
closed, the grid is connected to a voltage divider which provides normal operating
bias to the valve.
With the key down, therefore, the transmitter is sending out an r.f. signal. This
signal is interrupted each time the key is opened.
The same idea can be applied to the screen grid (see the circuit on the right
below).
In this circuit, the voltage varies from a positive operating voltage with the key
closed, to a negative blocking voltage with the key open. When the key is opened,
the screen is connected through resistor R to the bias voltage, which is sufficient to
cut off the stage completely. When the key is closed, the screen is connected
directly to H.T. The purpose of R is to limit the current flowing from the bias
terminal to H.T.(+) when the key is closed.
In this circuit, as in the last, a keying relay may be used to protect the radio
operator from high d.c. voltages.
BLOCKED GRID KEYING CIRCUITS
CONTROL GRID
RFInput
o *
SCREEN GRID
HT>
US
RFInput
o—
Bias Bias Bias
[§74.76
Keying Valve Circuits
Relay or key contacts cannot close or open circuits as quickly as a valve canstart or stop conducting. Therefore some applications use one or more valves tokey the r.f. circuits. These valves are called "keying valves."
There are several variations of keying valve circuits; but they all turn the trans-mitter on when the hand key is closed, and off when the key is opened.
In the circuit below, the keying valve is connected in series with the cathodeof the power amplifier valve. The transmitter will be on when the keying valve con-ducts, and will be off when the keying valve is cut off.
The keying valve can be keyed by either of the blocked-grid keying methodsdescribed previously.
' ' HT+
INPUTo©OUTPUT
A simplified schematic of another type of keying valve circuit is shown below.With the key open, current flows through R-l and R-2, producing a large voltagedrop across these resistors. Resistor R-l is the PA screen dropping resistor, andresistor R-2 is the PA anode dropping resistor. The keying valve current flowsthrough R-l and R-2, causing the power amplifier's screen and anode voltages todrop, thereby cutting off the power amplifier.
When the key is closed, bias is applied to the grid of the keying valve so that it
will be cut off. As a result, the screen and anode voltages of the power amplifierincrease to their normal values; the power amplifier conducts; and the transmittedpulse is radiated from the aerial.
VWVVfR-2
INPUTO—1|
HT+
O OUTPUT
§8*. AMPLITUDE MODULATION 4.77
What Amplitude Modulation Is
The type of voice transmission most commonly used is one in which the ampli-
tude of the carrier is varied in accordance with the amplitude of the voice signal.
This method of modulating the carrier is called "amplitude modulation.
MCW transmission is amplitude modulation in which a steady audio frequency is
used instead of voice, to vary the amplitude of the r.f.carrier.
In addition to the normal oscillator and power amplifiers, an AM transmitter con-
tains a modulator, whose job is to apply the audio-frequency signal to the PA, where
it is combined with the r.f. carrier wave.
A block diagram of a typical voice AM transmitter is shown below.
AmplitudeModulatedTransmitter
MasterOscillator
IPA 1^V
/sys^^\ M PA
AudioAmplifier
Modulator
#
When you are operating an AM transmitter, it is essential that the modulator
unit be operating during transmission; because the intelligence which is to be trans-
mitted must come through the modulator. If the modulator is either defective or
disconnected, only unmodulated r.f. will be transmitted; and a receiver at some
distant point will not receive any message.
MODULATOR
K8Sidebands
When an r.f. carrier is amplitude-modulated, the effect is to add new frequenciesto the transmitted signal, in addition to the original carrier frequencyFor example, if in MCW transmission a 500-kc/s carrier is modulated with a 2000-
cycle audio note, the frequencies radiated by the aerial will contain, in addition tothe carrier frequency, the sum (502 kc/s) and difference (498 kc/s) frequencies be-tween the carrier and the modulating audio frequency.These new frequencies are called "sidebands"-the higher frequency being known
as the upper sideband" and the lower frequency the "lower sideband." The rangeof frequencies transmitted from the lower sideband to the upper sideband is knownas the "bandwidth" of the transmission.
In the example above, the bandwidth is 4 kc/s—from 498 kc/s to 502 kc/s. Butif the modulating audio signal were reduced in frequency from 2000 to 1000 cyclesthe sidebands would be closer to the carrier frequency, and the bandwidth would beonly 2 kc/s.
It is the sideband frequencies, and not the carrier frequency, which contain theintelligence of the transmission. If, for example, an MCW receiver were to pickup only the carrier and exclude the sidebands, no intelligence would be received
MCW Sidebands— Bandwidth ^
§MA I
Si
..ower
deband"1
^ UpperSideband
W^Ww 498 500 502
2kc Frequency-Kc/s
modulation 1
In a voice transmission, the modulating signal contains many frequencies—someas high as 5000 c/s. As a result, voice transmissions contain many sidebands (onesideband for each frequency), which may be as much as 5 kc/s on either side ofthe carrier frequency.
This type of transmission, therefore, may have a bandwidth of as much as 10kc/s.
VOICE Sidebands
VoiceModulation
Carrier
Lower Sideband
1
Upper Sideband
-liiH mJJiiHlu :l'- n-':i!
|l ^'i|,,|; iIiii! ::l
,i l!;
:
i :,; ill;, , l!n, H I. ;:'),
495 500 505500
BandwidthlOKcM i
§ 8]4.79
How Modulation is Accomplished
In an unmodulated transmitter, the amplitudes of the anode current pulses in the
Class C amplifiers are the same, cycle after cycle.
These anode current pulses flow to an LC circuit, which is tuned to the r.f.fre-
quency or to a multiple of it. The pulses of current deliver a certain amount of
power to the tuned-circuit, and this power remains the same for each cycle. It
follows that the amplitude of r.f. voltage across the tuned-circuit must remain the
same for every cycle.
0—
J
JUUUUUUUL
When the transmitter is modulated, the amplitude of the anode current pulses is
made to vary according to the amplitude of the modulating signal.
Thus the amplitude of the r.f. current varies from one cycle to the next, and the
power delivered to the tuned-circuit also varies. This varying power, in turn, causes
the r f voltage across the tuned-circuit to vary.
These variations will follow the modulating signal in amplitude and frequency—
and in this way modulation is accomplished.
moduloTRANS* MCW VOICE
ModulatingSignal
Anode CurrentPulses
VoltageAcrossTunedCircuit
[§84.80
The Modulator
In MCW and voice amplitude modulation, a modulator is used to impress theaudio on to the r.f.
For voice, the modulator is nothing more than an ordinary audio amplifier whichprovides the voltage or power needed to vary the amplitude of the transmitter's r fFor MCW, the modulator contains an audio oscillator which drives the audio
amplifier. The output is a pure sine wave which varies the amplitude of the rfpulses at a rate equal to the frequency of the a.f. sine wave, and to an extent depen-dent on the amplitude of the a.f. sine wave.
Since the modulator is connected to the stage of the transmitter which is to bemodulated, its output must be of sufficient power to produce the necessary variationsof current m the modulated stage of the transmitter. For this reason, Class B push-pull amplifiers are often used as the final stage in the modulator unitThe following schematic illustrates a push-pull amplifier which can be used as a
modulator It is almost exactly the same as the push-pull amplifier described inPart 2 of Basic Electronics. The only difference lies in the modulation transformerwhich has a different turns ratio and a higher current capacity than had the output'transformer you met earlier on.
PUSH-PULL AMPLIFIER^ M • -***•
-r
—
mw
The modulating voltage may be applied in series with any of the electrodes of thevalve to be modulated. And the name of the type of modulation used depends onthe electrode to which the secondary winding of the modulation transformer is con-nected.
For example, "anode modulation" is achieved by connecting the output of themodulator m series with the anode circuit.
mOther types of modulation used with triode valves are "grid modulation" and
cathode modulation"; while in pentode valves, "screen grid modulation" or "sup-pressor grid modulation" may be used in place of the other methods
14Anode Modulation
In the simplified circuit of the power amplifier shown below, the modulating audio
voltage is applied to the anode of the valve. The audio voltage, since it is in series
with the d c anode supply voltage, will cause the total applied anode voltage to vary
above and below H.T. by an amount equal to the peak audio voltage, and at a rate
equal to the frequency of the audio.
Simplified circuit for
fw#kmM****
While the applied anode voltage
is varying, a constant amplitude of
r.f. is being fed to the grid of the
valve from the output of the pre-
vious stage, the IPA.
During the positive half-cycles
of the audio, the anode voltage of
the PA is higher than H.T., and
more anode current therefore
flows. So on the positive half-
cycles of the a.f. modulating
voltage, a greater r.f. voltage is
developed across the tuned-circuit.
During the negative half-cycles
of the audio, the anode voltage is
lower than H.T., resulting in less
current flow and less voltage
developed.
The amplitude of the r.f. output
voltage thus varies as shown in the
bottom illustration opposite, which
is of an amplitude modulated wave.
The power available at the
anode of the modulated amplifier
is shared between the carrier wave
and its sidebands. The power in
the sidebands is derived from the
modulator, while the power in the
carrier is derived from the H.T.
supply. In a 100-watt transmitter
using anode modulation, the modu-
lator may have to supply about 70
watts.
Cut-off
Bias
300 V
Fixed grid
Signal
Modulating
HT+
- 300 V+800 V
+500V
4200VX
Varyinganode supph
voltage
Resultant
anode
[§84.82
Grid Modulation
If the audio voltage is applied in the grid circuit instead of in the anode circuityou get grid modulation.
The effect of the modulating voltage is to vary the grid bias at an audio rate Theanode current flowing during each r.f. cycle will then vary as the grid bias increasesand decreases.
Tht+
CIRCUIT FOR
INPUT ,j ifadMt***** t
ModulatingVoltage
AnodeCurrent
Grid Voltage_Grid_bigsi. JMODULATOR
In the accompanying waveforms yousee that the total grid voltage is the sumof three voltages—the r.f. input voltage,
the a.f. modulating voltage, and the d.c.
bias voltage.
During the positive half-cycles of themodulating voltage, the bias decreases;and during the negative half-cycles, thebias increases. Since the r.f. will
always vary about the bias level, thepositive half-cycles of r.f. are raisedduring positive modulation peaks, anddepressed during negative modulationpeaks. It follows that the anode cur-rent pulses are larger in amplitudeduring the positive half-cycles of theaudio voltage than they are during thenegative half-cycles.
Since the voltage developed across theanode tank circuit varies with the anodecurrent amplitude, the r.f. output voltagewill also vary according to the modulat-ing signal.
Grid modulation is used in compact or mobile transmitters, because this type ofmodulation does not require a modulator with a large power output.When the weight of the modulator is only a minor consideration, however anode
modulation is used instead, because (despite the larger modulator it requires) itproduces much better results.
Output
§8] 4.83
Other Methods of Modulation
Anode voltage has almost no effect on the anode current in a pentode or in a
tetrode; so in these valves anode modulation is never used. Instead, the audio
voltage is applied to the screen grid—and the results are almost identical to those
of anode modulation with a triode.
•-HT+
MODULATEDOUTPUT©
Screen Grid Modulation
The variations in screen voltage
cause the amplitude of the r.f.
pulses of anode current to vary;
and this causes the output to be
modulated by the audio signal.
Modulation can also take place when the audio output of the modulator is con-
nected in the circuit of the suppressor grid. With a negative voltage on it, the sup-
pressor can control anode current in the same way as a control grid can, except that
the valve is less sensitive to voltage changes on the suppressor. Of course, only
pentode valves which have external connections to the suppressor can use this type
of modulation.
The operation is very similar to control grid modulation, and the modulator does
not need a large power output.
Suppressor Grid ModulationMODULATED
OUTPUTo
Grid bias'
If the audio voltage is applied to the cathode of the valve, the cathode's voltage
will vary with respect to earth. This will have the same effect as applying the audio
voltage to every other electrode in the valve simultaneously; for applying the voltage
to the cathode causes the voltage on every other valve electrode to vary with respect
to the cathode.
Cathode modulation is, therefore, in effect, a combination of the other types of
modulation.HT+
Cathode Modulation MODULATEDOUTPUTc
*° li
4.84 [§ 8
The Modulation Pattern
The oscilloscope can be used to good advantage to indicate the extent to which the
output of a transmitter is modulated. It can also indicate the presence of distortion
in the modulation.
If a pick-up loop, connected to the 'scope input terminals, is brought close to the
anode tuned-circuit coil in the output circuit of a modulated transmitter, the 'scope
can be used to show what is called the "modulation pattern."
stffahd***"VAtUm
If the modulating voltage is a sine wave
(as in MCW), the pattern on the right is
obtained. This pattern is useful in deter-
mining the presence of distortion.
XAA/
A pattern such as this, for instance, would
indicate that the positive peaks of the modu-lating voltage are not causing corresponding
peaks in anode current. This could be due
to incorrect grid bias, to saturation due to
low emission, or to insufficient excitation of
the power amplifier stage.
When breaks appear in the modulation
pattern, the transmitter is said to be "over-
modulated." This is usually caused by
excessive modulating voltage; but it may also
be due to insufficient r.f . signal voltage on the
grid, or to excessive grid bias voltage on the
final power amplifier (or on the modulated
stage).
§8] 4.85
The Trapezoid Figure
The trapezoid figure is another oscilloscope pattern which is often used to deter-
mine the presence of distortion in the modulated signal, and to show how much the
signal is being modulated. It has the advantage of making possible the detection
of certain types of distortion which cannot be detected by means of the pattern
described on the previous page.
To produce the trapezoid figure, the modulating signal is used as an external
horizontal sweep signal, instead of the internal time-base voltage of the 'scope. The
vertical deflection is still the modulated r.f . output of the transmitter.
The advantage of using trapezoid figures over the other modulation pattern to
analyse the operation of a transmitter is that they are easier to interpret.
A typical arrangement for showing trapezoid figures is illustrated below. The
vertical input of the 'scope is coupled to the anode coil of the power amplifier, and
the horizontal input is coupled to the a.f. modulating source.
In order to understand how trapezoid figures are formed, you must first know
something about the action of the vertical and horizontal plates inside the cathode
ray tube.
Arrangement for observing Trapezoid figures
Tomodulator
Pickup loop
on PA coil
4.86 [§ 8
The Trapezoid Figure (continued)
The picture you see on an oscilloscope screen is the path followed by a beam of
electrons striking the inner surface of the cathode ray tube.
In the cathode ray tube there are two pairs of metal plates which deflect the
electron beam from its path. The top and bottom plates, or "Y" plates, move the
electron beam vertically. The "X" plates move it horizontally from left to right.
The "Y" plates are connected to the signal under observation. This signal dis-
places the electron beam in a vertical direction.
Under normal operating conditions, the "X" plates are connected to the output of
an oscillator built into the oscilloscope. This oscillator, called a "time-base gen-
erator," generates a saw-tooth voltage which sweeps the electron beam across the
face of the 'scope screen, from left to right, at a selected constant speed.
If the input signal to the vertical plates is the familiar sine wave of voltage, the
combined action of this signal and of the horizontal sweep acting on the electron
beam produce the sine wave picture.
Sometimes the internal time-base generator is disconnected, and an external signal
is used as the sweep voltage. This is what is done to produce the trapezoid figure.
Y-Input
Terminals I m
L
X-InputTerminals
«//
VerticalAmplifier
v*.s?
HorizontalAmplifier
VerticalDeflection Horizontal
Plates Deflection
Plates
Sweep Voltage
TimeBase
Generator
IfieSetpePeflfecefoit 7%ates
§8]
The Trapezoid Figure (continued)
4.87
ModulatingVoltage
The trapezoid figure is produced in the following manner. When the modulating
voltage is at its most negative value, the 'scope sweep (which is produced by the
modulating voltage) will be at the left of the 'scope screen (Point C).
As the modulating voltage increases to its most positive value, the electron beam
will swing over to the right-hand side of the screen (Point A).
If the modulating voltage were a perfect sine wave, the electron beam would be
midway between the sides of the trapezoid figure (Point B) when the modulating
voltage is zero.
At any instant, therefore, the horizontal position of the electron beam is a measure
of how negative or how positive the modulating voltage is.
At the same time as the electron beam is moving from one side of the screen to
the other under the influence of the modulating voltage, so this modulating voltage
is causing the transmitter output to increase and decrease.
The transmitter output is applied to the
'scope to produce vertical deflections.
When the modulating voltage is at its posi-
tive peak, the transmitter output (and the
height of the 'scope picture) are greatest.
Thus the right-hand side of the trapezoid
figure shows the largest amplitude.
When the modulating voltage is at its
negative peak, the transmitter output (and
the height of the 'scope picture) are at their
minimum. This occurs when the electron
beam is at the left-hand side of the screen.
ModulatingVoltage
TransmitterOutput
Y Input
X Input o
Because of the way in which trapezoid figures are obtained, they represent a
graph of the output voltage as compared to the modulating voltage.
If the output voltage is always proportional to the modulating voltage—as it will
be when the modulation is linear—there will be a straight line along the top and on
the bottom of the trapezoid.
[§8
The Trapezoid Figure (continued)
FIGURE USING INTERNAL TIME-BASE TRAPEZOID FIGURE
The two 'scope presentations shown above are for the same condition of modula-tion. You could determine the maximum height (peak) and the minimum height(trough or valley) of the r.f. from either figure.
You could also determine the linearity of the modulation from either presentation;but it is easier to do so from the trapezoid.
If the modulating voltage is decreased in amplitude, the peak and trough points onthe wave-pattern come closer together. The same effect is displayed in the trapezoidpattern as a change in the horizontal and vertical dimensions.
The following illustrations show, one above the other, both types of waveformpresentation for three different modulating voltage amplitudes.
EFFECT OF VARYING AMPLITUDE OF MODULATING VOLTAGE
Small modulatingvoltage
WWVLarge modulating
voltageLarger modulating
voltage
§8] 4.89
Percentage Modulation
The "percentage modulation" is a measure of the extent to which the carrier is
modulated.
If it is modulated 100%, the maximum height of the modulated wave is twice that
of the unmodulated wave, and the minimum height is zero.
For this modulating condition, the trapezoid figure is a triangle.
For radio telephony, it would seem to be ideal if signals could be transmitted
with 100% modulation, because the modulated r.f. would then be transmitted at
maximum power. In practice, however, over-modulation would always tend to
occur. Moreover, the diode detector of a receiver cannot handle 100% -modulated
signals without introducing some distortion.
Equipments are normally set up, therefore, to give 60% to 80% modulation only.
^SJw**-For maximum power in transmission,
modulate 100%
Unmodulated carrier
If the maximum height of the modulated wave is more than twice that of the un-
modulated wave, and the minimum height is zero for more than an instant during
the cycle, the carrier is over-modulated.
This condition produces gaps in the wave figure, and a line extending from the
left-hand side of the triangle in the trapezoid figure. The more the wave is over-
modulated, the longer are the gaps, and the longer the line.
^^- distorts the signal and interferes
with other carrier frequencies
4.90 §8]
Percentage Modulation {continued)
Since over-modulation is undesirable, both because it distorts the signal andbecause it generates unwanted sidebands which may interfere with adjacent carrier
frequencies, you will have to find out the exact percentage of modulation whichmay be present in any transmitter you are checking.
The exact percentage modulation can be calculated by using the following formula:
modulationH max. —H min.
H max. +H min.xlOO
"H max." is the maximum height of the modulated wave, and "# min." is its
minimum height. These values can be measured from the 'scope pictures—the
trapezoid figure is more convenient for this purpose, but the wave figure itself gives
sufficiently accurate results.
In the figures below, H max. is 8 divisions and H min. is 2 divisions. The per-
centage modulation is therefore 60.
60j£ modulation
%modulation =
If H max. is 9 divisions and H min. is 1 division* the percentage modulation is
therefore 80.
80% modulation
-x 100 = 80%
max.9 divisions
§8]
REVIEW—Amplitude Modulation
Amplitude Modulation. A method
which uses either voice or an audio
signal to vary the amplitude of an r.f.
carrier wave. The modulator is that
part of the AM transmitter which com-
bines the audio and r.f. signals.
RF AUDIO
4.91
MCW
RF
+ AAA "I •
VOICE VOICE
+ WvA**"* mm
Sidebands. Frequencies contained in
the transmitted signal in addition to the
r.f. carrier frequency. Sidebands are
equal to the sum of, and to the difference
between, the carrier and modulating
signals. In MCW there are two side-
band frequencies; in radio telephony
there are many.
—! Bandwidth !—
Lower|
Sideband
UpperSideband
498 500 502
Carrier
Lower Sideband I Upper Sideband
uiu 4iidiyiiiiuiiiLiL
Anode Modulation. The arrange-
ment whereby the modulating signal
varies the PA anode voltage, thus modu-
lating its output in response to the
audio signal.
Grid Modulation. The modulating
signal is applied to the grid of the PAvalve. Varying grid voltage in this
manner controls PA valve anode cur-
rent, and so modulates output voltage.
Trapezoid Figure. An oscilloscope
pattern obtained by using the trans-
mitter output voltage as the Y input of
the 'scope, and the modulating signal
as the X input.
Percentage Modulation. The
measure of the extent to which the r.f.
carrier is modulated. Over-modulation
produces a distorted signal, and intro-
duces unwanted sidebands.
60X modulation
4.92 §9: GENERAL REVIEW OF TRANSMITTERSCW Transmission. An r.f. signal is
generated in the transmitter by an r.f.
oscillator, and radiated into space. In-
telligence is imparted by turning the
transmitter on and off with a telegraph
key. CW is generally used in long-
distance communication. It can only
transmit code (e.g. morse).
KeyedRFSo—r a
Key rf.RF
Transmitter
MCW Transmission. A constant ampli-
tude audio frequency signal is superimposed
on the r.f. carrier wave. Transmitter
is switched on and off by means of a
key as in CW transmission. Also for
code only.
Radio Telephony. In amplitude modu-lation, a voice signal varies the amplitude
of the r.f. carrier. Transmission is con-
tinuous; and is the type used for standard
radio broadcasting.
KeyedModulated RF
S~ ^\
&H Hif- 1 II
RFTransmitter
AudioOscillator
AMVoice modulated RF
UnmodulatedRF Carrier
* MM
RFTransmitter
\ Ampli\ modui
Amplitudemodulator
Microphone ±
Grid-leak Bias. A resistor and capaci-
tor are included in the grid circuit of an
amplifier valve to make the amplifier
operate in Class C. The amount of bias
depends on the grid current, and varies as
the strength of the input signal changes.
Combination Bias. A combination of
fixed and grid-leak bias most commonlyused in transmitters. TO ANODE OF
PREVIOUSSTAGE
§9]4.93
GENERAL REVIEW of Transmitters (continued)
Three-stage Transmitter. The master oscillator (MO), the intermediate power
amplifier (IPA), and the final power amplifier (PA) make up the basic three-stage
transmitter.
Tuning. For efficient operation, the anode tuned-circuits of the amplifiers must
resonate either at oscillator frequency or at one of its harmonics. Adjustment
of the variable capacitor to reach this condition is called "tuning." Anode voltage is
maximum, and current minimum, at the correct frequency.
ANOOENEUTRALIZATION
QROUTT
§ jfotl
Neutralization. Anode or grid
neutralization circuits may be used to
counteract the feedback effect of the
grid-to-anode capacitance of triodes
used in transmitter amplifiers.
4.94
GENERAL REVIEW of Transmitters (continued)
Transmission Line. Used to con-
vey the r.f. signal from the transmitter
to the aerial. For maximum power
output the characteristic impedance of
the line should equal the input
impedance of the aerial. Coupling
circuits are used to match the
transmission line to the transmitter.
AERIALRadiates RFJ
r—
[§*
TRANSMISSION LINESupplies «*ri»l i with RF
* COUPLING CIRCUITCouples RF from tank circuit
to transmission line
Aerial. Radiates the energy
received from the transmission line
into space. Electric and magnetic
fields generated by current and voltage
waves on the aerial expand and col-
lapse as the transmitter signal varies.
Sidebands. Frequencies contained
in the transmitted signal in addition
to the r.f. carrier frequency. Side-
bands are equal to the sum of, and to
the difference between, the carrier and
modulating signals. In MCW there
are two sideband frequencies; in radio
telephony there are many.
— | Bandwidth jo-
LowerSideband Sideband
488 500 502
Lower Sideband
Can ler
Upper Sideband
ililll iililliiilii am .;ili .:!»,;; II. Illl llii^j. iiMfcllfii 1,455 5(K) 505
Anode Modulation. The arrange-
ment whereby the modulating signal
varies the PA anode voltage, thus
modulating its output in response to
the audio signal.
Grid Modulation. The modulating
signal is applied to the grid of the PAvalve. Varying grid voltage in this
manner controls PA valve anode
current, and so modulates output
voltage.
§10: INTRODUCTION TO RECEIVERS 4.95
You have now learnt how a signal is generated in the transmitter by a radio-
frequency oscillator; how it is amplified; and how its energy is then transferred by
a coupling circuit to the transmission line. You have learnt that the aerial converts
the power delivered to it by the transmission line into electro-magnetic waves.
You have found out that these electro-magnetic waves are radiated into space;
and some of the ways in which they can be modulated so as to carry "intelligence"
with them as they speed to their destination.
The next thing, obviously, is to find out what happens to them when they get
there, and how the message which they carry is taken from them and made either
intelligible or usable at the other end.
Ih Part 5 of Basic Electronics, therefore, you will study Receivers. You will see
how the almost inconceivably weak currents generated in the receiving aerial by the
incoming signals are amplified until they become manageable; how the messages
they carry are detected (or, as it were, "taken out of the envelope" in which they
have been travelling); and how these messages, in turn, are amplified until they can
be used to operate a loudspeaker, or otherwise put to useful work.
Now let's go on and see how messages are received
!
^,w\\\\\\\\U4£
INDEX TO PART 4
(Note: A cumulative index covering all six Parts will be found at the end of Part 6)
Aerials, 4.59, 4.71
dipole, 4.60
radiation pattern, 4.66
radiation resistance, 4.63
tuning, 4.65
Amplifiers, 4.9, 4.16
classes of operation, 4.9
class C amplifier, 4.10
Amplitude modulation, 4.77, 4.91
modulation patterns, 4.84
percentage modulation, 4.89
Anode modulation, 4.81
Class C amplifiers, 4.10, 4.16
bias arrangements, 4.12
Coupling circuits, 4.41
tuned, 4.42
CW transmission, 4.72, 4.92
keying circuits, 4.73
Dipole, 4.60
Frequency, 4.44
Frequency multipliers, 4.35
doubling, 4.37
tripling, 4.38
tuning indicators, 4.39
Frequency spectrum, 4.70
Grid modulation, 4.82
Keying, 4.73
Modulation (AM), 4.7, 4.91
anode, 4.81
grid, 4.82
other methods of, 4.83
Neutralization, 4.28, 4.34
Radio telephony, 4.8
Sidebands, 4.78, 4.94
Three-stage transmitter, 4.17, 4.34, 4.93
complete diagram for, 4.21
FPA, 4.20
IPA, 4.19
oscillator, 4.18
tuning methods, 4.24
valve filament circuit, 4.22
Transmission
AM, 4.8
FM, 4.8
keyed, 4.7
voice, 4.8
Transmitter lines, 4.40, 4.94
applications, 4.55
characteristics impedance, 4.47
non-resonant and resonant lines, 4.49
open-circuited line, 4.51, 4.54
short-circuited line, 4.51, 4.53
Transmitter, 4.3
three-stage, 4.17, 4.34
Wavelength, 4.44
Wave propagation, 4.67
ground wave, 4.68
space wave, 4.68, 4.69
WIGANCENTRALLIBRARY<-- i n. 1—WWW8
621•381