Antenna
From Freeradio
The Antenna, otherwise known as the Aerial, is what radiates the RF signal (produced by the Transmitter) into the atmosphere (or into space, as the case may be).
In some ways, the antenna is at least as important as the transmitter - a well engineered, well placed antenna transmitting a low power signal can often achieve greater coverage than a poorly engineered or poorly situated antenna transmitting a high power signal.
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Positioning of the antenna
Height is the most important factor, especially on VHF FM - which has "line of sight" coverage. This is why pirates (in fact, most broadcasters) tend to transmit from a high tower block, a house on high ground, or a high hill overlooking the intended coverage area.
On the other hand, a high antenna is more easily visible to the DTI, so a compromise must be struck between maximum coverage and mimimum visibility. Occasionally, it may be possible to camouflage an antenna, or otherwise place it so it is hard to see from ground level.
Antenna Gain
The Gain of an antenna is how much the antenna appears to amplify the signal from the transmitter. An antenna with high gain will transmit further, with the same transmitter power, than one with low gain.
A VHF (or UHF) antenna achieves gain by concentrating the signal in one plane, ie beaming the signal outwards to the horizon (horizontal plane) while radiating very little signal upwards or downwards (vertical plane).
The Gain of an antenna is measured in dB. This is a Logarithmic measurement
Gain − in − dB = 10logEffective − radiated − power / Transmitter − power
- eg 3dB=2x gain, 6dB=4x, 10dB=10x, 20dB=100x.
A simple antenna, which is considered as a 'reference' because it has an almost spherical radiation pattern, is the Dipole. Antennas with Gain include the "Slim Jim" (6dB gain).
Directional Antennas
If you are on the fringe of the area which you intend to cover, eg on a hill overlooking a town, it may be worthwhile to use a directional antenna. A directional antenna can achieve a high degree of gain by beaming the signal in one direction, while attenuating it in the opposite direction.
Most directional antennas are Beam Antennas, the simplest of which has 3 elements - a Reflector (longer than the Radiator), a Radiator (which is connected to the Transmitter), and a Director (shorter than the Radiator). The Director should be facing towards the centre of the area which you wish to cover.
Extra gain may be achieved by adding more Director elements.
Polarisation
Polarisation can be Horizontal, Vertical or Mixed (both Horizontal and Vertical).
The best coverage range is achieved in the transmitting and receiving antennas are of the same Polarisation. Most broadcasters, Pirates included, tend to use Vertical polarization because most FM radio listeners - car radios, radios with extendable antennas, walkmans - generally have receiving antennas with Vertical polarisation.
On the other hand, Horizontal polarisation has a few advantages. The main one is that it tends to penetrate into valleys slightly better than Vertical polarisation. Also, many roof-top antennas are Horizontally polarised. Finally, if you're sharing a frequency with a distant (or adjacent frequency) station which uses Vertical polarisation, you may end up getting better coverage if you transmit using Horizontal polarisation.
Link Antennas
For antennas used for links (see Main transmitter), it is highly advantageous to use directional antennas to make the signal less easy for the DTI to trace (they would need to be in the area which the signal is being beamed towards). On the other hand, a directional antenna at the Link Receiver(near the Main Transmitter) may give the DTI a clue in which direction the Link Transmitter( and therefore the Studio) is. So a highly directional Link TX antenna and a less directional Link RX antenna may be the optimum compromise.
