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.

VHF and UHF signals

5g Dummy loads

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