Section 5 - Antennas & Feeders

5.4 - 5.6 The Yagi

The yagi antenna is a directional antenna, often described as a beam antenna. That means that it is able to focus the energy radiated from it in one specific direction. Also this means that it will receive radio signals from only one direction. The most common example of a yagi antenna is the familiar standard TV aerial.

In the picture, the thick black line is a half-wave dipole. It is mounted on a boom of some sort to support it and the other elements. These other elements consist of a reflector which is longer than the dipole and several directors, shorter than the dipole. The maximum signal from the antenna is towards the directors. The energy is focussed by the directors to form a beam of RF energy. The more directors that are fitted, the greater the focussing effect, the narrower the beam width, and the more intense the RF energy beam becomes. It is similar to the action of the reflector fitted inside a headlight or torch which, by directing the light energy and focussing it into a beam, produces an apparently much more intense light than the bare bulb alone.

The focussing ability of the antenna allows us to project our RF signal much further than we would be able to do with an omni-directional antenna like a vertical. Obviously we can only do this in one direction at a time, so most yagi antennas are used with a rotator in order that we can easily select the desired direction of our beam.

The yagi is usually mounted horizontally or vertically. This is known as "polarisation" because of the effect it has on the radio waves being transmitted from it. For two stations to communicate using yagi antennas, they need to have them pointing at each other, and should ideally both have the same polarisation. Significant signal losses will occur if the polarisation is not the same.

5.7 Antenna Gain

The yagi, and other antennas, have the ability to focus the energy they radiate in a given direction. Thus a higher signal strength will be recorded in that direction than would be present if an omni-directional antenna was used. From the point of view of a receiving station, it will appear that the transmitting station is transmitting with more power when using such an antenna.

This apparent increase in power is called "antenna gain", and is usually provided by the manufacturer. Gain is usually quoted in decibels (dB), a conversion chart is available here.

The effective radiated power (erp) of a transmitting station is the product of actual transmitter power (measured at the antenna) and antenna gain.

erp = transmit power X antenna gain

What does erp really mean? Basically, erp is the amount of power that you would need to feed to an omni-directional antenna to get the same signal strength at the receiving station.