Chapter 4

Chapter 4

Propagation, Antennas and Feed Lines

Chapter 4Propagation, Antennas & Feed Lines

Today’s agenda

• How radio signals travel from place to place

• Basic concepts of antennas

• How feed lines are constructed and used

• What SWR is and what it means to you

• Practical antenna system construction

04/20/23 2Technician -- 1 July 2010 - 30 June 2014

Chapter 4Propagation

Radio waves travel in many ways and are affected by several factors.

Like light waves, radio waves are affected by the phenomena of reflection, refraction, diffraction, absorption, polarization and scattering.

Radio propagation is also affected by the daily changes of water vapor in the troposphere and ionization in the upper atmosphere, due to the sun.



Fortunately, you don’t have to know or understand most of the stuff on the previous slide to get your Tech license.

Generally speaking, radio waves travel in a straight line unless they are reflected or diffracted along the way.

The strength of a radio wave decreases as it travels from the transmitting antenna and eventually becomes too weak to be received.

The distance over which a radio transmission can be received is called “range”.



The curvature of the Earth sets an effective range limit for many signals, creating a “radio horizon”.

“Line-of-sight” propagation occurs between transmitting and receiving antennas are within direct sight of each other.

Most propagation at VHF and higher frequencies is “line-of- sight”.

You can increase the range of line-of-sight propagation by increasing the height of the antenna or your transmitter’s power.



Radio waves at HF and lower frequencies can also travel along the surface of the earth as “ground wave” propagation.

Radio waves can be reflected by buildings, hills and even weather-related activity.

Radio waves can be “diffracted” (bent or spread) as they travel past the sharp edges of a building or other object. This is called “knife-edge” propagation.




When VHF/UHF signals are diffracted around a solid object, some signals appear behind the object as they are bent in different ways. The result is “shadow zones” where signals can be found.


Diffraction also makes the Earth seem less curved to VHF and UHF radio waves than light waves. This allows VHF and UHF signals to travel somewhat farther than the visual “line-of-sight” horizon.

Certain radio waves can penetrate solid objects. UHF signals are more effective in propagating into and out of buildings in urban areas.



Radio signals can arrive at a receiver after taking different paths from the transmitter. When this happens they can be out of phase and interfere with each other or even cancel each other completely.

This phenomenon is known as “multipath” and can cause a signal to become weak and distorted.

Simply moving your antenna just a few feet may avoid the location where the cancellation is occurring.



Multipath propagation of signals from distant stations results in irregular fading even when reception is generally good.

VHF or UHF signals from mobile stations moving through an area where multipath is present produce a characteristic rapidly changing signal strength known as “mobile flutter” or “picket fencing”.

Variations in signal strength from multipath can also cause digital data signals to be received with higher error rates, especially at VHF and UHF.



Propagation at or above VHF is sometimes “assisted” by atmospheric phenomenon such as weather fronts or temperature inversions. This phenomenon is called “tropospheric” propagation or just plain “tropo”.

Layers in the air form structures called “ducts” that can allow even microwave signals to be heard over long distances.

Tropo is regularly use by radio amateurs to make normally impossible long distance contacts at VHF and UHF. It is not uncommon for these contacts to exceed over 300 miles.



Radio signals are also conducted by conductive things in the sky. For example: Airplanes can reflect 2-meter and 70-centimeter signals over hundreds of miles.

About 30 miles above the surface of the Earth begins an area the extends to about 270 miles above the Earth. This area is called the “ionosphere” and it plays a critical role in amateur radio communications.

The ionosphere consists of four layers: D, E, F1 and F2. The D-layer is closest to Earth and the F1 and F2 combine at night to form the F-layer.



Depending on the time of day and the intensity of solar radiation, these layers (E, F1 and F2) can diffract or absorb (D and E) radio waves.

Radio waves at HF (and sometimes VHF) can be completely bent back toward the Earth by diffraction in the E and F layers of the ionosphere as if they were reflected.

This is called “skywave” propagation or “skip”.



Since the Earth’s surface is also conductive, it too can reflect radio waves. And yes, this means a radio wave can be reflected between the ionosphere and ground multiple times.

Each reflection from the ionosphere is called a “hop” and allows radio waves to be received hundreds or even thousands of miles away.

This is the most common way for hams to make long-distance contacts on the HF bands.



When sky-wave propagation on an amateur band is possible between two points, the band is said to be “open”.

If not, then the band is “closed”.

Because the Sun plays a huge role in HF propagation, that means that propagation may not be supported in all directions all the time. The changing seasons and time of day and the frequency are all important factors that affect propagation as well.



Higher frequencies are bent less than lower frequencies.

At VHF and higher frequencies, the waves usually pass through the ionosphere and are lost to space. VHF and UHF signals from beyond the “radio horizon” are rarely heard without being relayed by a repeater.

The highest frequency that can be reflected back to Earth is the “maximum useable frequency” or “MUF”. The lowest frequency that can travel between those points without being absorbed is the “lowest useable frequency” or “LUF”.



The MUF rises as the sun illuminates the ionosphere.

The upper HF bands such as 10 meters (28-29 MHz) are more likely to be open during the day.

VHF and UHF fans also experience ionospheric propagation, especially during the peak of the 11-year sunspot cycle when VHF signals can be bent back to Earth.

Under these circumstances long-distance communications are possible.



At all points in the solar cycle, portions of the E-layer can become sufficiently ionized to reflect VHF and UHF signals back to Earth. This is called “sporadic-E” or “ES” propagation and it is most common during early summer and mid-winter months on 10, 6 and 2 meters.




Looking for a challenge?

Try bouncing your signal off the Aurora Borealis or “Northern Lights”.

It’s a challenge because the aurora is constantly changing and the reflected signals change strength quickly and they are often distorted. Instructor: Click here for audioG6DER KI6IJ



Here’s another challenge:

Bouncing signals off the tail of a meteor.

It called “Meteor scatter”

Best band is 6 meters

Contacts with stations 1200 to 1500 miles away are possible.

Instructor: Click here for audioCQ Italy 4 X-ray Charlie Charlie

Chapter 4Antenna Fundamentals


There are three important rules about antennas…

1.There is no perfect antenna (except mine)2.Any antenna is better than no antenna3.Higher is better

Any electrical conductor can act as an antenna for radio signal. You do not have to purchase an antenna – why not build one instead? They’re not that complicated.



A “feed line” is used to deliver radio signals to or from the antenna.

The connection of the antenna and the feed line is called the “feed point” of the antenna.

The ratio of the radio frequency voltage to current at the feed point is the antenna’s “feed point impedance”.

The conducting portions of an antenna are called elements.

“I have a 15 element tri-bander on 10, 15 and 20-meters”



An antenna with more than one element is called an “array”.

The element connected to the feed line is called the “driven element”. If all elements are connected to a feed line that’s a “driven array”.

Elements not directly connected to the feed line but influence the performance of the antenna, are called “parasitic elements”.



RF current in the antenna element creates radio waves that travel away from the antenna.

The antenna wave consists of electrical and magnetic energy as a result of electrons moving back and forth in the antenna.

The wave is a combination of an electric and a magnetic field, just like those in a capacitor and an inductor, but spreading out into space like ripples on the surface of water.



Because a radio wave is made up of electric and magnetic fields, it is called an “electromagnetic wave”.

The wave’s electric and magnetic fields oscillate at the same frequency as the RF current in the antenna.

“Polarization” refers to the direction in which the electric field of a radio wave is oriented.

“Horizontally polarized” antennas radiate a radio wave whose electric field is oriented horizontally (parallel with Earth).



“Vertically polarized” antennas radiate a radio wave perpendicular to the surface of the Earth.

When the electric field of the radio wave and the antenna element have the same polarization, the maximum amount of signal is created in the antenna.

Hold that handheld radio upright!



Why else do we care about polarization?

1. When the polarization is mismatched, the received signal can be dramatically reduced by as much as 100 times!

2. When polarization is mismatched, less current is created in the antenna.

By the way, as radio waves travel through the ionosphere, their polarization can change randomly so it could be any polarization when it hits your antenna. Good luck!



Signal Strength

Antennas, propagation and electronic circuits change signal strengths by many factors of ten.

Radio signals vary in strength.

At the input to a receiver signals are often measured in one ten-billionth of a watt!

When they come out of the transmitter they’re often measured in kilowatts!



Signal Strength

So how do we level the playing field so to speak, with regard to signal strength?

a. We use very precise linear tool called the “Dessy Bell”

b. We wave a piece of paper with the French noun “d’Bee” written on it

c. We shout, “Give me S-units or give me Dessie Belle”d. We introduce the concept of the decibel (dB) at this

point.



Signal Strength

The “decibel” measures the ratio of two quantities as a power of 10. The formula for computing decibels is:

dB = 10 log (power ratio)

and

dB = 20 log (voltage ratio)

Positive values of dB mean the ratio is greater than 1

Negative values of dB mean the ratio is less than 1



Signal Strength

You have a choice

a. Memorize the answers to the three questions about decibels in the question pool – only one of which could appear in you exam.

b.Use the calculator that will be provided at the exam session.

Today, we’ll use the calculator on the laptop. In theory.



Signal Strength

What’s the approximate amount of change in decibels of a power increase from 5 watts to 10 watts?

10 Log (10/5) = ?

10 Log (2) = ?

Log 2 = 0.30102999 (type “2”; press “Log”; multiply by 10)

10 x 0.30102999 = 3.0102999 or 3 dB



Signal Strength

What’s the approximate amount of change in decibels of a power decrease from 12 watts to 3 watts?

10 Log (12/3) = ?

10 Log (4) = ?

Log 4 = 0.60205999 (type “4”; press “Log”; multiply by 10)

10 x 0.60205999 = 6.0205999 or 6 dB



Signal Strength

What’s the approximate amount of change in decibels of a power decrease from 200 watts to 20 watts?

10 Log (200/20) = ?

10 Log (10) = ?

Log 10 = 1 (type “1”; press “Log”; multiply by 10)

10 x 1 = 10 dB



Here’s another pesky concept: Antenna Gain

The concentration of radio signals in a specific direction is called “gain”. Antenna gain increases signal strength in a specified direction when compared to a reference antenna.

An antenna can create gain by radiating radio waves that add together in the preferred direction and cancel in others.

Gain only focuses power - it does not create power.



An “isotropic” antenna has no gain because it radiates equally in every possible direction.

In reality, isotropic antennas do not exist. They are used as imaginary references.

An “omnidirectional” antenna radiates a signal equally in every horizontal direction.

An antenna with gain in a specific direction is called a “beam” or “directional” antenna.



This is an “azimuthal pattern” of an antenna’s transmission pattern while looking from above.

There are two primary lobes and two secondary lobes with “nulls” between each.



A second “azimuthal pattern” of an antenna’s transmission pattern while looking from above.

There are two primary lobes and four very minor secondary lobes with “nulls” between each.



Looking at the side view we can see how the antenna radiates upwards.

There are four lobes going forwards and 3 minor lobes going to the back.

This is an example of a directional radiation pattern.



Here is an example of how the height of the antenna affects the radiation patterns of the antenna.

Chapter 4Feed Lines and SWR


Feed lines are what connect an antenna to a radio. We also use feed lines to transfer RF signals from one piece of equipment to another.

For example: From a radio to a tuner or to an amplifier.

Feed lines consist of two conductors separated by an insulating material.


Feed lines use special materials and construction methods to minimize the loss of power which is dissipated as heat and to prevent signals from leaking in or out.

When a loss occurs and there is always some loss, we call it “feed line loss”.

Feed line loss increases with frequency for all types of feed lines.



The most popular feed line used by hams to connect radios and antennas is “coaxial cable” or “coax” for short.

It’s easy to use and requires very few considerations for installation.



Coax consists of the following:

• A wire center conductor• An insulator or “dialectric”• A tubular shield of braided wire or foil• A rubber/plastic sheath called the “jacket”



Coax comes in a variety of types and sizes for various applications.


Type most commonly used


Coax comes in a variety of types and sizes for various applications.

One special type of coax is called “hard line” because its shield is made from a semi-flexible solid tube of aluminum or copper. This severely restricts the amount of bending the cable can do.

The advantage to this type of coax is that it has the lowest loss of any type of coaxial feed line.



Feed lines have a “characteristic impedance” which is a measurement of how energy is carried by the feed line and is abbreviated as Z0.

The dimensions of feed line conductors, the spacing between them, and the insulating material determine a feed line’s characteristic impedance.

Most coaxial cable used in ham radio has a characteristic impedance of 50 ohms.



Here is yet another pesky concept.

The power carried by the feed line is transferred to your antenna when the impedance of the antenna and the feed line are identical or “matched”.

If the impedances don’t match some of the power is “reflected” by the antenna. Power going toward the antenna is “forward power” and power reflected by the antenna is called “reflected power”.



The greater the difference between the antenna and feed line impedances, more power is reflected.

Reflected and forward power going in opposite directions create a stationary wave-like interference in the feed line called a “standing wave”.

The ratio of the maximum value to the minimum value of the interference is call the “standing wave ratio”. It is usually measured at the point where the feed line connects to the transmitter.



SWR in an antenna system is an indicator of how well the antenna and feed line impedances are matched.

When there is no reflected power the SWR is 1:1 which is called a “perfect match”.

A the amount of reflected power increases so does the SWR. SWR is always greater than or equal to 1:1.

SWR greater than 1:1 is called an “impedance mismatch” or “mismatch”.



An antenna’s feed point impedance changes with frequency which means the SWR will change.

So why do we care about SWR?

• Low SWR allows the efficient transfer of power from the feed line to the antenna.

• Low SWR reduces losses in the feed line.



High SWR can damage a transmitter’s output circuits.

Most amateur transmitting equipment is designed to work at full power with an SWR of 2:1 or lower. If the SWR is greater than 2:1 the transmitter may automatically reduce power in order to protect its output circuit.



What causes high SWR?

Antennas are too short or too long.

A faulty feed line connectors or connection

Erratic SWR is usually caused by a loose connection in the feed line or the antenna.

A “transmatch”, “impedance matcher” or an “antenna tuner” is used to correct high SWR not caused by fault.



The simplest antenna is a “dipole”.

Easy to make

Inexpensive

Work quite well

Most dipoles are horizontally oriented, particularly on the lower bands, and radiate a horizontally polarized signal.

Dipoles and be also be installed vertically, sloping or even drooping from a single support in the middle (inverted-Vee).

Chapter 4Practical Antenna Systems

To determine how many feet of wire you need for a half-wave dipole:

468 ÷ frequency (MHz)


When calculating the amount of wire needed for a half-wave dipole, add a few extra feet.

It’s a lot easier to cut wire off than to add more wire and it will let you compensate for the effects of ground or nearby conductors.

Use an SWR meter or antenna analyzer to determine the resonant frequency and then shorten the dipole until it is resonant at the desired frequency.



Dipoles radiate their signals strongest at right angles to the antenna and weakest off the end.




Looking down at the antenna’s radiation patterns from different heights.



½ wavelength by formula

Coax

Feed Point468

f (MHz)= amount of wire (in feet)





Another popular antenna is the “ground-plane” antenna.

One-half of a dipole with the missing portion made up by an “electrical mirror” called a “ground plane” which is made from sheet metal (e.g., car roof) or a screen of wires called “radials”.

Radials for a vertical antenna



Generally ¼ λ with the feed point at the base of the antenna.

One conductor connected to the antenna and the other to the ground plane.



The length of a ground-plane is half that of a dipole and is estimated in feet using the following formula:

234 ÷ frequency in MHz.

Ground plane antennas are often called “verticals” because it is easiest to mount them vertically or perpendicular to the surface of the Earth.

When mounted this way the radiation pattern of the single element ground plane is “omnidirectional”.



But are these antennas effective?

Remember the three rules at the beginning of this review?

Here’s a recording of an amateur using a vertical and you decide.

Instructor: Click here for audio

A station in New Zealand (ZL1BOS) talking to a station in Australia (VK5CC/2). VK5CC/2 is using the antenna outlined in red. The QSOwas recorded by F5VBY in France.



Even better than the 1/4 λ version of the vertical or ground-plane antenna is the 5/8 λ antenna.

Due to its extended length, the 5/8 λ vertical focuses more energy toward the horizon, thereby extending the range of the signal.



The antenna used with most hand-held radios is called a “rubber duck”. It’s a ground-plane antenna that is shortened by coiling the conductor inside a plastic or rubber coating.

The body of the radio and the operator form the antenna’s ground-plane.

While the rubber duck antenna is conveniently sized, it is not very efficient and does not transmit or receive as well a full-sized ground plane antenna.



Using a hand held radio with a rubber duck antenna from inside a car will result in a signal that is 10-20 times weaker than if you were outside.

The vehicle’s metal roof and doors act like shields and trap the signal inside.

Some signal does get out but as mentioned it is very weak.



Dipoles, ground-planes and loops work well but they have little gain. Their radiation patterns don’t have strongly preferred directions.

Many hams find it desirable to focus (or aim) transmitted power (and to optimize reception) in one direction. To achieve this, a directional beam antenna is used.



Beams are used to increase signal level at a distant station or to reject interference or noise.



When transmitting on VHF or UHF, signals can be blocked by buildings or other obstructions. A beam antenna can be used to aim the signal so that is reflected by an object and therefore is able to go around the obstruction.

Beams are created from arrays of multiple elements.

The most commonly used types of beam antennas are “Yagis” and “quads”.



The Yagi is named after one of it’s inventors, Dr Yagi and Dr Uda.



Reflector5% longer than

DE

Director5% shorter than DE

Driven Element1/2 wavelength

Boom

Direction of signal0.15 - 0.20

Wavelength



In a cubical quad, signals are transmitted towards the front of the antenna (opposite end

from the feed point) just like in a Yagi



Dual-band Quad

2-element tri-band (10-15-20m)

Yagi



Only the driven element is connected to the feed line. The remaining parasitic elements determine the radiation pattern of the antenna.

Yagis and quads are known as “parasitic arrays”.

Horizontally polarized Yagis and quads are usually used for long-distance communications, especially for weak signal SSB and CW contacts on VHF and UHF.

Horizontal polarization results in lower ground losses when the wave reflects from or travels along the ground.



As the frequency increases and the size of Yagi and quad elements decrease, it becomes more difficult to build practical antennas.

At frequencies above 1 GHz, a “dish” antenna becomes more practical.



Next to the characteristic impedance, feed line loss in coax is the most important characteristic. Feed line loss is specified in dB per 100 feet of cable at a certain frequency.

The chart on the following page shows the loss for several types of coax at different frequencies. Using such a chart as a guide, you can pick which cable is best for your situation.

Generally speaking, a large diameter cable such as RG-8, will have less loss than a small diameter cable such as RG-58.





In order for coax to be efficient and effective, it must be cared for. Periodic maintenance, visual and physical inspections will insure that the integrity of the outer coating (the jacket).

Nicks, cuts and abrasions can result in a breach of the jacket, allowing moisture inside which is the most common cause of coaxial cable failure.



Prolonged exposure to ultraviolet rays will cause the plastic in the jacket to degrade, resulting in cracks that allow water into the cable.

Coax should not be bent sharply. This could cause the center conductor to come in contact with the braid which would result in a short circuit.

Consider using open-wire when excessively long runs of cable are required. Remember to give this type of feed line tender loving care as well.



We use several different types of connectors to connect our feed lines to our antennas and radios.

UHF type connectors are used for HF and VHF. Above 400 MHz we use type-N connectors.

BNC connectors are typically used for VHF/UHF.



Coax connectors exposed to the weather must be carefully waterproofed. Water in the coax degrades the effectiveness of the braided shield and dramatically increases losses.

When using low-loss air-core coax, you need to pay extra attention to water-proofing the connectors. Special techniques are required to prevent water absorption in this type of cable.



Many hams prefer to use a wattmeter or better yet, a directional wattmeter than a SWR meter to measure forward and reflected power.

Directional wattmeters can measure power flowing toward the antenna and power reflected from the antenna.



If the SWR at the end of the feed line is to high for the radio to operate properly, devices called impedance matchers, transmatches or antenna tuners are connected to the output of the transmitter.



An antenna tuner is adjusted until the SWR measured at the transmitter output is acceptably close to 1:1. This means the antenna system’s impedance has been matched to that of the transmitter output.

Note: The antenna is not really tuned – the impedance at the output of the feed line has been adjusted to some other value.

All modern HF transceivers have built-in automatic antenna tuners although many hams use an external tuner.



An antenna analyzer is a great tool for antenna work. It uses a built-in low power signal source with an adjustable frequency and one or meters to show impedance and SWR. It is used to measure an antenna system without using a transmitter whose signal might cause interference.



Questions?

Read Chapter 5 (Amateur Radio Equipment)

for the next class.

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