IAN LANG ELECTRONICS
No 8. Aerial
The aerial or antenna is the device by which the modulated wave is radiated into the air.
The aerial is often the least studied part of an RF system and yet it is one of the most important, for if the radiator is not good then neither will be the signal transmitted. Aerials can take many forms, the long wire, the dipole, the Yagi directional, loops, spearheads; the list goes on. As our model transmitter is supposed to be a high power AM transmitter we will follow the example of the very high power transmitters at Droitwich Transmitting Station and use a mast radiator. A common practice in low and medium frequency broadcasting, the mast radiator (aka radiating tower) is one where the whole structure is used as a transmitting aerial. To do this the height should be a quarter or half wavelength of the carrier frequency being transmitted (this is true for all aerials) and not less than one sixth or more than 5/8ths of the wavelength (see sidebar on the lower left). Now we have the problem of feeding it the modulated wave we have generated. There are three common options: Base feed, in which the mast is insulated from the ground and the transmitter is connected to the bottom, close to the insulator; Multiple feed, where the mast is divided into insulated sections and each is fed independently (this reduces sky-wave and enhances ground wave reception and reduces fading due to waves arriving together out of phase, especially at night); and shunt feed whereby the mast is grounded and fed part of the way up. Whichever method we use, we need a transmission line, and we must match the impedances of the output stage of the system with the aerial and the transmission line. A characteristic impedance will be found for the aerial and let us say that it is 50 ohms. It is unlikely that our output stage will match it exactly and so we must put in an antenna matching unit, which consists of a series of inductors that can be switched in and out, thus decreasing or increasing the impedance. However all of this is useless if the cable feeding the mast from the output stage is not matched to the same impedance. If this is the case, we get a standing wave, which is highly undesirable. A standing wave occurs because RF energy in a mismatched cable flows along the cable to the aerial (forward wave), and back in a reflection (reflected wave) to the transmitter. When waves of equal amplitude travel in opposite directions, on average no energy is propagated from the radiator. The standing wave ratio takes the antinode where an amplitude value is at a maximum, and compares it to an adjacent node where the value is at a minimum. Therefore we now need a meter to measure this, between the antenna matching unit and the feeder.
Hereto we have not discussed wavelength, it is simply the speed of light divided by the carrier frequency, and so for , let us say Talksport, on 1053 kHz the wavelength would be 285 metres, for Radio 5 on 693 kHz 433 metres. A longer wave can travel further round the curvature of the Earth and that's where AM scores over FM (remember because of the large bandwidth you cannot transmit FM on lower frequencies) in that you do not need as many transmitting stations or repeaters.
Aerial or Antenna?
These two terms are now synonymous but for the pedantic amongst us there is a difference. The term antenna used to be applied to a solid vertical pole, and the reason for this is that Marconi's earlier experiments used a metal cross-pole from a tent as the radiator, which in Italian was known as the antenna centrale. A piece of wire slung between two poles was hanging in the air, and so became known as an aerial wire. It really does not matter which term you use, people will know what you mean.