 for small aperture antennas have efficiencies in the range 50-80%. antennas and two dimensional), contour plots (or three dimen- As far as circuit designers are concerned the antenna is an imped- sional), and isometric plots are also used. A radiation pattern is ance. Maximum power transfer will occur when the antenna is characterised by the main beam and sidelobes. The quality is speci- Generic antenna types 17/5 matched to the transmission line. The impedance consists of the self impedance and the mutual impedance. The mutual impedance ac- counts for the influence of nearby objects and of mutual coupling to other antennas. The self impedance consists of the radiation resist- ance, the loss resistance and the self reactance. Loss resistance is the ohmic losses in the antenna structure. Radiation resistance measures the power absorbed by the antenna from the incoming plane waves. It is one of the most significant parameters for small antennas where the problem is often to match very dissimilar impedances. A receiving antenna is both a spatially selective filter (measured by the radiation pattern) and a frequency selective filter. The band- width measures the frequency range over which the antenna oper- ates. The upper and lower frequencies can be specified in terms of a number of possible parameters: gain, polarisation, beamwidth and impedance. A communication link consists of a transmitting antenna and a receiving antenna. If the transmitter radiates P. watts, then the received power, P at a distance r is given by Equation 17.2 where and G are the transmitter and receiver antenna gains respec- tively. (17.2) This formula is known as the Friis transmission equation. It assumes that the antennas are impedance and polarisation matched. If this is not the case then extra factors must be multiplied to the equation to account for the mismatches. Figure 17.4 Radiation aperture in a ground plane 17.3 Generic antenna types aperture antennas. If the field in the aperture is constant, the radiated field is given from Equa