tenna system in a narrow beam. As a result considerable levels of gain can be achieved. The gain is dependent on a variety of factors, but is mainly a function of the diameter of the parabolic reflector. The actual gain can be estimated from the formula: G = 1010g10 ("D) where G is the gain over an isotropic source k is the efficiency factor which is generally about 50 per cent D is the diameter of the parabolic reflector in metres 1 is the wavelength of the signal in metres From this it can be seen that very large gains can be achieved if sufficiently large reflectors are used. However, when the antenna has a very large gain, the beamwidth is also very small and the antenna 112 Newnes Guide to Radio and Communications Technology requires very careful control over its position. In professional systems electrical servo systems are used to provide very precise positioning. The reflecting surface may not be as critical as may be thought at first. Often a wire mesh may be used. Provided that the pitch of the mesh is small compared to a wavelength it will be seen as a continuous surface by the radio signals. If a mesh is used then the wind resistance will be reduced, and this may provide significant advantages. Ferrite rod antennas The ferrite rod antenna is almost universally used in portable transistor broadcast receivers as well as many hi-fi tuners where reception on the long, medium and possibly the short wave bands is required. As the name suggests the antenna consists of a rod made of ferrite, an iron-based magnetic material. Coils are wound around this as shown in Figure 4.31. Coils - normally one each for long and medium wave former for ferrite rod the coil Figure 4.31 A ferrite rod antenna This coil is brought to resonance using a variable tuning capacitor contained within the radio circuitry itself and in this way the antenna can be tuned to resonance. As the antenna is tuned it usually forms the RF tuning circuit for the receiver, enabling both functions to be combined within the same components, thereby r