Bredhurst Receiving and Transmitting Society

Part 2

5. Feeder and Antenna

5f Antennas

The sections of the syllabus has been re-ordered as it is considered that the order shown helps you to best understand the principles outlined.

5f.2 Recall that the gain of an antenna is measured in dB, and understand how to calculate the ERP for a known RF power and antenna gain (in multiples of 3 dB and 10dB).

Well here is some more maths for you and it does require some learning. The table below shows the relative comparison between dB and the linear unit. This table needs to be learned and is not provided in the examination. You will see that the table goes up in 3dB jumps and each time the gain is double the previous, except for 10dB which is ten times the original power input figure.

The dB has the same basic values as given for feeder losses and thus you should be able to quickly get you head around the figures for antenna gain.

ERP (effective radiated power) as you learned in the FL course is :-

ERP = Power into antenna x gain of the antenna

Example 1

If the power into the antenna is 50 watts and it has a gain of 10dB what is the ERP ?

power in = 50

gain = 10dB = x10

therefore ERP = 50 x 10 = 500 watts ERP

Example 2

If the power into the antenna is 10 watts and it has a gain of 9dB what is the ERP ?

power in = 10

gain = 9dB = x8

therefore ERP = 10 x 8 = 80 watts ERP

5f.3 Recall that a three-element yagi has a half-wave driven element, a reflector that is slightly longer than the driven element and a director that is slightly shorter than the driven element.

Recall that Yagi antennas may have more than one director.

The yagi antenna is a beam antenna which due to its construction concentrates the RF signal generally one direction.

This is achieved in its simplest form by three elements -

the driven element which is a di-pole constructed to be resonant for the frequency of transmission,

a reflector element which set behind the driven element and is a little longer than the driven element and

a director element which is set in front of the driven element and is a little shorter than the driven element.

Some students find understanding this section difficult and confusing. Can we suggest that you ask your tutor to show you a yagi antenna and explain what each part does and not just to reply upon this description.

With the construction of the bigger reflector and smaller directors than the driven element the focused direction of most of the radiation from the driven element is towards the director element and onwards.

5f.4 Recall that electromagnetic radiation comprises both an electrical field and a magnetic field.

Recall that the two fields are at right-angles to each other and that the direction of propagation is at right-angles to both fields.

Recall that it is the plane of polarisation of the electric field that defines polarisation of the wave.

This part is a new concept to you and has not been touched upon in the FL course.

It was James Clerk Maxwell theories of what he called Electromagnetism which explained the relationship between electricity and magnetism. His theory is based upon the idea of an electromagnetic field.

Let's take one step backwards. It is hoped that you have all at one time of another played with iron filings on a piece of paper and moved around a magnet beneath and have been able to move the iron filings around. The movement was caused by a magnetic field affecting the iron filings and the movement of the magnet causing the magnetic field.

It has been explained elsewhere 3e.1 that it an electric current when passed through a wire generates and magnetic field. This is what is called a stationary electromagnetic field as it stays bound to item that is causing the field. Examples of stationary fields are: the magnetic field around a wire carrying current or the magnetic field around the magnet.

Now if the current through the wire is changing something special happens if the changes are rapid enough.

The changing electromagnetic field propagates away from its origin in the form of a wave. We know these as radio waves.

The electromagnetic radiation has actually two components parts - an electrical field and a magnetic field. The two fields are located at right angles to each other and the direction of radiation is at right angles to both those fields. This is the concept that you have to know. Why they are at right angles to each other and which on goes where is of no consequence to you at this stage.

Vertical electrical field the vertical polarization

It is the electrical field which determines or defines what is called the polarisation of the radiation wave.

5f.5 Recall that VHF and UHF signals will normally be received best when the transmitter and the receiver have the same antenna polarisation and that this is less important at HF because the polarisation may change during ionospheric reflection.

The terms used at this exam level are horizontal and vertical polarization.

VHF and UHF signals

When using VHF and UHF the signals the best signals will normally be received if the same polarisation is used for the transmitter and receiver - else significant attenuation occurs.

The poorest reception of a signal at VHF or UHF would be when the transmitter is say vertical and the receiver is say horizontal

If the transmitting antenna is set vertically and the receiving antenna horizontally then the antennas are said to be "cross polarised" and the poorest received signal is likely.

HF signals

With HF, the signals are themselves twisted due to the effects of the ionospheric reflections that the effect of "cross polarisation" of antennas is less marked.

5f.1 Understand the concept of an antenna polar diagram.

Identify the polar diagrams for the half-wave dipole and Yagi antennas.

Identify the directions of maximum and minimum radiation.

So what is the concept of a polar diagram. Well it has nothing to do with the north or south pole.

As you are aware antennas radiate, the polar diagram is a representation of the radiation pattern as a simple drawing. The diagram below shows a set of arrows which represents the direction and radiation strength.

The direction is obvious but the radiation strength is proportional to length of the line - the greater the strength longer line.

The diagram is NOT representative of any particular antenna, it is the principle of the direction of the arrow and length of the arrow (showing strength) establishes the polar outline when the arrow head are linked up.

Dipole

Let's start with the polar diagram of the dipole.

It is hoped that you gather from the animation that wire hanging down in the middle is the feeder and the horizontal wire are the legs of the dipole.

The radiation is then a bit like a doughnut that has been slipped on one end and slid along to the middle.

Thus :-

• The maximum direction of radiation is then always out away from the wire of the dipole and

• minimum radiation is along the line of the dipoles there is minimal radiation off the ends of the wire.

Yagi

The yagi diagram develops to show the direction of the maximum radiation in the direction of the arrows. Remember the driven part of the yagi is in fact a dipole and thus the reflector and the directors do a good job of changing the radiation pattern but some of the signal still goes out at the sides and rear of the yagi. The pattern shows the field strength at a particular distance from the antenna and is not intended to show the range of the signal else you would not reach too far if the signal only reached the end to the antenna!!

The longer the arrow the bigger the radiated signal so on the yagi the minimum signal is to the rear next less is to the sides.

5g Dummy loads

5g.1 Understand the use of a dummy load and its construction.

The dummy load is a substitute for an aerial and as such is also called an artificial antenna and as far as transmitters or receivers are concerned it electrically resembles an aerial (usually 50 ohms impedance) and thus can stand totally in place of an aerial. The dummy load is a piece of test equipment which all radio amateurs should own.

The dummy load enables a transmitter to be tested without radiating a signal (or at worst a very low radiated signal). When connected to a receiver it does not pick up external RF signals and RF noise.

When testing a transmitter the Dummy Load must be capable of safely dissipating , as heat, the output power of the transmitter.

Carbon Resistor NOT wire wound resistor

The dummy load is a large resistor of usually 50 ohms (or a number of resistors to make up the required wattage dissipation). The resistor is set inside a suitable screening enclosure and has a connector so that it may be linked by a coaxial feeder to the transmitter or receiver or even the end of a coaxial cable run which is under test.

Above are three examples of dummy loads