ewnes Guide to Radio and Communications Technology is required to travel parallel to the earth. In this way, it will follow the earth's surface and reach the maximum number of stations. Power directed upwards is often wasted, especially at VHF and above where this power is not usually reflected back to earth. The angle of radiation is also very important for ionospheric propaga- tion. A low angle of radiation will enable greater distances to be reached as a result of the geometry. However, many broadcast stations adjust the angle of radiation from their antennas to enable the signal to reach the required area for the transmission. Antenna system It is not possible to consider the antenna itself in isolation. It is necessary to look at the complete antenna system and optimize the operation of the whole system. The antenna system can be split down into a number of different sections. There are the antenna elements themselves. There is also a feeder that is required to transport the signal between the antenna and the transmitter or receiver. This is required because the antenna is rarely in the same place as the transmitter or receiver and a means is required to transport this power with the minimum of loss. Sometimes some form of matching arrangement is needed to ensure that the impedance match between the antenna and the feeder is optimized SO that the maximum power transfer can be effected. However, in many instances the antenna is designed to match the impedance of the feeder it is to be used with. Feeder The purpose of a feeder is to carry radio frequency signals from one point to another with the minimum amount of signal loss. In view of the fact that radio frequency signals are being carried, ordinary wire like that used for carrying mains power is unsuitable. Feeders with suitable radio Antenna system comprises the feeder, any matching units and the antenna elements Antenna Transmitter Feeder Matching receiver Figure 4.7 An antenna system Antenna systems 85 frequency characteristics are required. A poor feed