A PROJECT REPORT ON

STUDY OF SWR METER

&

IT’S CALIBRATION FOR POWER MEASUREMENT

UNDER THE GUIDANCE OF: PREPARED BY:

SHRI O.P.N CALLA (FIE) KAPIL NEGI

& CO-GUIDANCE OF: (MEMBERSHIP NO: ST-433082-5)

SHRI DINESH BOHRA.

OBJECTIVES OF THE STUDY: -

The prime objective of calibrating SWR meter for measuring power is to design a

low cost power meter. Moreover, SWR has a number of implications that are directly applicable

to microwave use, thus by studying about SWR meter and related parameters, we can achieve the

following objectives: -

To gain knowledge about operating principle of SWR meter.

Understanding various microwave parameters such as standing waves, voltage standing

wave ratio, reflection co-efficient, return loss, and mismatch loss.

Methods of measuring VSWR and mismatch impedance.

Study of importance of SWR meter in transmission technology and its utility.

Relation between various parameters and their importance in microwave.

Relation between VSWR and transmitted power.

Calibration of SWR meter for measuring power.

Study of various implications of SWR meter.

METHODOLOGY OF THE STUDY:-

Before calibrating SWR meter for measuring power, first of all we will

understand the basic working principle of SWR meter, methods of taking measurements by

meter, depth knowledge of various transmission parameters such as voltage standing wave ratio,

reflection co-efficient, return loss, mismatch loss, relation among various parameters and

implications of SWR meter in microwave technology.

SWR: - In telecommunications, standing wave ratio is the ratio of the amplitude of a

partial standing wave at an antinode (maximum) to the amplitude at an adjacent node

(minimum).

The SWR is usually defined as a voltage ratio called the VSWR. It is

also possible to define SWR in terms of current resulting in the ISWR, which has the

same numerical value. The power standing wave ratio (PSWR) is defined as the square of

the SWR.

VOLTAGE STANDING WAVE RATIO: - is the ratio of voltage at the highest and

lowest point of standing wave. It is also called as ratio of cable impedance and load

impedance.

VMAX

VSWR = -------------- VMIN

Ei + Er OR VSWR = ------------------- Ei - Er

Where, VMAX = Maximum voltage on the standing wave.

VMIN = Minimum voltage on the standing wave.

EI = Incident voltage wave amplitude.

ER=Reflected voltage wave amplitude.

REFLECTION CO-EFFICIENT: - is the ratio of reflected voltage to the incident

voltage. It is always less than unity because reflected voltage cannot be greater than the

incident voltage.

VREFLECTED

Reflection co-efficient () = --------------------- VINCIDENT

VSWR - 1 Or = ------------------------ VSWR + 1

RETURN LOSS: -is a measure in dB of the ratio of power in incident wave to that in the

reflected wave and always have a positive value. For example if a load has a return loss

of 10dB, then 1/10 of the incident power is reflected. The higher the return loss, the less

power is actually lost.

Return loss= 10 log Pi Pr = -20log Er Ei = -20log (vswr – 1) (vswr + 1)

Where Pi = Incident power.

Pr = Reflected power.

VSWR= Voltage standing wave ratio.

INTRODUCTION OF SWR METER: -

SWR meter (Model VS-411DX) is a high gain tuned amplifier operating at a

fixed frequency of 1 KHz. It is designed primarily for use in making standing wave

measurements in conjunction with a suitable detector and slotted line or wave-guide section. This

may also be used for impedance measurements, relative power level measurements, as a null

detector in bridge circuits and in other applications requiring a sensitive fixed frequency

indicator. It is calibrated directly to indicate SWR directly or dB. When used with square law

devices such as crystal diodes and baratters.

Input circuit of SWR meter is arranged to match various external signal sources,

such as crystal diode, baratter or relatively high impedance devices.

Model VS-411DX has a provision of expanded scale for accurate measurement of

small variations in power levels. Coarse and fine controls are provided for fine adjustment of

amplifier gain to desired convenient value on the meter scale. Output of SWR meter can also be

recorded through a connector provided at the back panel of the instrument.

Model VS-411DX operates on 220vAC/50Hz mains supply.

SPECIFICATIONS: -

INPUT POWER SUPPLY 220VAC, +/- 10%, 50Hz

INPUT CONNECTOR BNC (F)

AMPLIFIER TYPE HIGH GAIN TUNED AT 1000Hz

BAND-WIDTH (3dB) 50Hz

INPUT SELECTOR XTAL HIGH (200K ohm) IMPEDANCE

XTAL LOW (200 ohm)

BOLO 200ohm, 4.5mA BIAS

BOLO 200ohm, 8.7mA BIAS

ACCURACY +/-0.2dB PER 10dB STEP

SENSITIVITY <1MICRO VOLT FOR FULL SCALE

AT XTAL LOW/BOLO POSITION

RANGE OVER 70dB

METER SCALES SWR 1-4, SWR 3-10, EXPANDED

SWR 1-1.13, dB 1-10.

MODE NORMAL/EXPANDED

GAIN CONTROL COARSE ADJUSTMENT 10dB

FINE ADJUSTMENT 2dB

RECORDER OUTPUT THROUGH BNC (F) CONNECTOR

RECORDER OUTPUT LEVEL 1V FOR FULL SCALE DEFLECTION

OF METER.

_________________________________________________________________________

DESCRIPTION OF BLOCK DIAGRAM: -

1 KHz signal from detector is fed to the SWR meter through a BNC female connector.

A four position input selector band switch selects the type of input coupling desired to

match various external signal sources such as a crystal diode, baratter, or a relatively high

impedance device to the input of SWR meter.

In Xtal high impedance position, the input signal is directly fed to 60dB step attenuator

stage via a coupling capacitor with reflected impedance of 200Kohms.

In Xtal low impedance & bolo 4.3ma/8.7ma positions, input is fed to attenuator stage

through a coupling transformer tuned at 1khz. This transformer provides a reflected

impedance of 200ohms to the input signal. It also provides over 30db of gain.

In bolo position a dc bias current of 4.3ma or 8.7ma is provided to the detected connected

at the input bnc connector.

The 0-60db gain control band switch controls the 60db step attenuator through a set of

eight relays. The input signal is amplified & band pass filtered in four amplifier cum filter

stages.

The overall gain of the system is also controlled by the coarse and fine gain

potentiometers provided on the front panel.

The output signal from amplifier stages is amplitude detected and fed to meter via output

meter circuit. An auxiliary output (through a bnc connector on the back panel of SWR

meter) is also provided for recording/monitoring the output of SWR meter on a

recorder/oscilloscope.

Normal/expand switch when set to the expand position, applies a dc bucking voltage to

the meter circuit forcing the meter needle to go backwards. The amplifier sensitivity must

then be increased to obtain an upscale reading, which can then be read on the expanded

meter scale.

Note: - It is important to note that 10db change shown on the meter scale actually

corresponds to a 20db change in the input. This has been done to obtain a “square law meter

calibration” on the meter.

In the expand position, a 2db change in meter scale corresponds to 4db change in

the input. The expand mode is provided to make the meter more sensitive to detect minute

changes in the input signal, i.e. While reading very low SWR readings. In this mode full

scale deflection of meter occurs with a step change of 4db in the input (corresponding to 2db

change on the meter reading); while in the normal mode full scale deflection of meter

corresponds to 20db change in the input (corresponding to 10db change on the meter

reading).

The low voltage regulated power supply is generated with the help of a step down

mains transformer and +/- 12 volts regulator circuit.

_________________________________________________________________________

SLOTTED SECTION: - The slotted section should cover the desired frequency and be

equipped with an accurate scale or indicator.

DETECTOR: - The detector should be a square law (output proportional to input RF

power) device such as a barratter or a crystal diode operated at low signal level. A

barratter is reasonable square-law detector when used at low signal levels but in general

this is not true in all cases with crystal diodes. However, the sensitivity of crystal

detectors is considerably better than that of barraters. For this reason crystal diodes

detectors are widely used for SWR measurements.

PRECAUTIONS WHEN USING CRYSTAL DETECTORS: - Whenever a crystal

detector with a matched load resistor is used, the input selector switch must be set at the

xtal-200kohm position to obtain an accurate square-law response. With an unloaded

crystal, select the input impedance, which gives maximum sensitivity. Usually the xtal-

200ohm position will give the best sensitivity. However, some crystal diodes may give

higher output in the xtal-200kohms position. Maximum sensitivity is desirable so that the

probe penetration in the slotted line can be kept to a minimum.

Crystal diodes exhibit a departure from the square-law response for which the

instrument is calibrated. This departure tends to occur when the RF power level exceeds a

few microwatts. This corresponds to a reading of approx, full-scale deflection on the 30db

range of the instrument with gain controls set to maximum.

PRINCIPLE OF OPERATION: -

Basically the measurement of SWR consists of setting the probe carriage at a

voltage maximum position and setting the gain of the SWR meter to obtain a reading of 1.0

marking on the SWR scale.

OPERATING PROCEDURE: -

The operating procedures for SWR meter are divided into two classifications: -

a) LOW SWR MEASUREMETNS (10 OR BELOW).

b) HIGH SWR MEASUREMENTS (ABOVE 10).

The step-by-step procedure for making these measurements are as given below: -

a) LOW SWR MEASUREMENTS (10 OR BELOW): -

1. Turn on the instrument; allot a few minutes warm up time for maximum stability.

2. Set the input selector switch for the type of detector that is to be used.

3. Connect the detector cable to the input.

4. Set gain control potentiometers at 12’o clock position.

5. Set the range switch at 30 or 40db position. Adjust the probe penetration to obtain

an up scale reading on the meter.

6. Peak the meter by adjusting the modulation frequency of the signal source. Reduce

probe penetration to keep the meter on the scale.

7. Peak the meter by tuning the probe detector, if tunable. Reduce probe penetration to

keep meter on scale.

8. Peak the meter reading by moving the probe carriage along the line. Reduce probe

penetration to keep meter on scale.

9. Adjust gain controls and or output power from the signal source to obtain exactly

full-scale reading.

10. Move the probe carriage along the line to obtain a minimum reading. Do not retune

probe or detector circuit.

11. Read SWR, which is indicated directly on the instrument scale.

b) HIGHER SWR MEASUREMENTS: -

The straightforward measurement of SWR with conventional methods is

generally applicable when measuring nominal SWR of up to 10, but at higher SWR, special

techniques are desirable.

When the SWR is high, probe coupling must be increased if a reading is to be

obtained at the voltage minimum. However, at the voltage maximum this high coupling may

result in a deformation of the pattern, with consequent error in reading. In addition to this

error caused by probe loading, there is also danger of error resulting from the change in

detector characteristics at higher RF levels.

DOUBLE MINIMUM METHOD: -

In the double minimum method, it is necessary to establish the electrical distance

between the points where the output is double the minimum, as shown below: -

1. Repeat steps 1 through 7 in the low SWR measurement procedure.

2. Move the probe carriage along the line to obtain a minimum reading and note the probe

carriage position.

3. For reference, adjust gain controls to obtain a reading of 3.0 on the db scale. If a linear

detector is being used, adjust gain controls for an indication of 1.5db on the db scale.

4. Move the probe carriage along the line to obtain a reading of full scale (‘0’) on the db

scale on each side of the minimum.

5. Record as d1 and d2, the probe carriage positions at the two equal readings obtained in

step 4.

6. Short the line and measure the distance between successive minima. Twice this distance

is l, the guide wavelength.

The SWR can then be obtained by substituting this distance into the expression: -

SWR= L / (d1 – d2).

Where l is the guide wavelength, d1 and d2 are the locations of the twice minimum

points.

This method overcomes the effect of probe loading since the probe is always set around a

voltage minimum where large probe loading can be tolerated. However, it does not

overcome the effect of detector characteristics.

CALIBRATED ATTENUATOR METHOD: -

Another method for measuring high SWR is to use a calibrated variable RF

attenuator between the signal source and the slotted line. Adjust the RF attenuator to keep the

rectified output of the crystal diode equal at the voltage minimum and voltage maximum

points. The SWR in db is the difference in the attenuator settings.

1. Repeat steps 1 through 7 in low SWR measurements procedure.

2. Move the probe carriage along the line for a voltage minimum, adjust the RF attenuator

to give a convenient indication on the meter and note the RF attenuator setting.

3. Move the probe carriage along the line for a voltage maximum, adjust the RF attenuator

to obtain the same indication on the meter as established in step 2, and note the RF

attenuator setting.

4. The SWR may be read directly in, db, as the difference between the first and second

readings.

While this method overcomes the effect of detector variations from a square-law

characteristic, the effect of probes loading still remains. Always use minimum probe

penetration.

________________________________________________________________________

CHECKING OF SQUARE-LAW RESPONSE: -

The square-law response of either a crystal diode or bolometer is easily checked

with slotted line equipment.

A simple method of calibrating a detector is by increasing the power level in the slotted line in

known steps and noting the detector response on the SWR meter.

Any new crystal being used for the first time should be checked, as there is often a significant

variation between crystals. Data should be taken in both Xtal positions, so that the meter setting

may be determined for any individual crystal diode.

LOCATION OF VOLTAGE MAXIMUM OR MINIMUM: -

It is more desirable to locate the voltage minimum than the voltage maximum

since the effect of probe loading is less at the minimum. However, the location of a voltage

minimum by a single measurement, particularly on low SWR, is usually inaccurate because of its

broadness, thus making the true minimum position hard to determine. An accurate method of

locating the voltage minimum is to obtain the position of the probe carriage at two equal output

readings on either side of the minimum and then averaging these two readings.

PRECAUTIONS WITH SIGNAL SOURCES: -

Signal sources can introduce at least three undesirable characteristics that will

affect slotted line measurements. These include presence of RF harmonics, frequency

modulation and spurious signals.

Signals sources used for standing wave measurements should have relatively low harmonic

content in their output. The standing wave ratio at a harmonic frequency may be considerably

higher than at the fundamental. Spurious frequencies in the signal source are also undesirable,

for, unless very slight, they will obscure the minimum points at high SWR values. The below

figure shows a plot of an SWR pattern made with signal source producing unwanted fm.

Instances are common where the presence of RF harmonics has led to very serious errors in

SWR measurements. Such harmonics are usually present to an excessive degree only in signal

sources that have coaxial outputs. Coaxial pickups of a broadband type will often pass harmonic

frequencies with greater efficiency than the fundamental. In wave guides systems, signal sources

such as internal cavity klystron have a more or less fixed coupling and in addition do not have

pickups extending into the tuned cavity to cause perturbations of the cavity fields. Consequently,

the harmonics problem is generally limited to coaxial systems. Harmonics become especially

troublesome when the reflection coefficient of a load at a harmonic frequency is much larger

than at the fundamental frequency-a common condition. When the harmonic content of the signal

source is high, the reflection coefficient of the load at the harmonic frequency can cause the

harmonic standing wave fields to be of the same order of magnitude as the fields at the

fundamental frequency. Thus, a device having a SWR of 2.0 at the fundamental frequency will

often have a SWR of 20 or more at the second harmonic frequency. If such a device is driven

from a signal source having, say, 15% second harmonic content will be about one fourth the

amplitude of the peaks at the fundamental frequency. Below figure shows a typical pattern

obtained when the RF signal contains harmonics.

STATEMENT OF THE PROBLEM: - To calibrate SWR meter for measuring power.

SETUP BLOCK DIAGRAM FOR CALIBRATING SWR METER: -

METHOD OF OPERATION: -

1. Connect all the equipments as shown in setup block diagram.

2. Set the frequency of signal generator says, at 8.0GHz.

3. Raise the power level of signal generator to maximum value.

4. Set fine & coarse gain controls of SWR meter to maximum so as to provide a gain of

10db.

5. Set the range input selector switch to 30db.

6. Peak the meter to obtain full scale deflection on the meter scale by adjusting modulation

frequency of the signal source, or by reducing probe penetration, so that meter scale

remains on the position marked ‘1’ on the meter scale.

7. Note the power meter reading in dBm for meter scale reading of SWR meter positioned

on marked ‘1’.

8. Reduce the power level and note the position of scale on SWR meter with respect to

power meter reading.

9. When the meter scale reading goes to left of marking ‘3’ on top scale, set the range

switch to next (40db) range and read the indication on the second SWR (3 to 10) scale.

10. Reduce power level in consequent steps and note power meter reading with meter scale

position.

11. Again, when meter scale goes to left of marking 10 on scale, then increase the gain to

50db position. Now read the SWR on the top scale. Note that the range switch is changed

in two steps, so use the top scale; however, all indications on this scale must be

multiplied by 10.

12. Convert SWR meter scale position reading in db.

13. Plot graph between SWR meter scale reading in db and power meter reading in dBm.

14. Similarly, set signal generator at different frequencies, i.e. 8.5GHz, 9.0GHz, 9.5GHz, and

so on.

15. Repeat the same procedure as above.

16. Note reading of the power meter with respect to position of indicator on SWR scale.

17. Plot the graph between SWR meter scale reading in db and power meter reading in dBm

at different frequencies.

________________________________________________________________________

TABLE SHOWING RELATIVE VALUES OF VSWR, REFLECTION CO-

EFFICIENT, RETURN LOSS, & MISMATCH LOSS: -

VSWR RETURN LOSS (dB)

% POWER LOSS

REFLECTION COEFFICIENT

MISMATCH LOSS (dB)

1 INFINITE 0 0 0.00

1.15 23.1 0.49 0.07 0.021

1.25 19.1 1.2 0.111 0.054

1.5 14.0 4.0 0.200 0.177

1.75 11.3 7.4 0.273 0.336

1.9 10.0 9.6 0.316 0.458

2.0 9.8 11.1 0.333 0.512

2.5 7.4 18.2 0.429 0.880

3.0 6.0 25.1 0.500 1.25

3.5 5.1 30.9 0.555 1.6

4.0 4.4 36.3 0.600 1.94

4.5 3.9 40.7 0.636 2.25

5.0 3.5 44.7 0.666 2.55

10 1,7 67.6 0.818 4.81

20 0.87 81.9 0.905 7.4

100 0.17 96.2 0.980 14.1

Readings taken at different frequencies and graphs plotted are shown on the next pages: -

Various implications of SWR meter: -

SWR meter or VSWR meter measures the standing wave ratio in a transmission line.

SWR meter is an item of radio equipment by which we can check the quality of the

match between the antenna and the transmission line.

SWR meter scale can be calibrated for measuring power but since transmitted power

varies a little bit at different frequencies so for calibration of meter scale, average can be

taken.

Graphs plotted between SWR meter scale reading (db) and power meter reading (dBm) at

different frequencies are linear.

VSWR meter should be connected in the line as close as possible to the antenna. This is

because all practical transmission lines have a certain amount of loss, causing the

reflected power to be attenuated as it travels back along the cable, and producing an

artificially low VSWR reading on the meter. If the meter is installed close to the antenna,

then the problem is minimized.

SWR meter does not measure the actual impedance of a load (i.e. resistance and

impedance), but only the mismatch ratio. To measure the actual impedance an antenna

analyzer or other similar RF measuring device is required. Note also that for accurate

readings, the SWR meter must be matched to line impedance.

_____________________________________________________________________