tical reception (5.61) purposes. More gain is required, together with the ability to reject unwanted signals coming from directions other than that of the wanted signal. This is often achieved with a yagi antenna consisting of a driven element with a passive reflector and directors (Yagi, 1928; Balanis, pp. 393-409). The reflector is more efficient if it 6.3.2 The hypothetical isotropic antenna consists of a mesh of rods effectively forming a back screen. Gain may also be achieved by having several dipoles spaced and com- The concept of an isotropic receiving antenna creates conceptual bined to give directivity. It may also be a requirement that the problems, since such an antenna has to be infinitesimally small. antenna has a wide band response as well as gain relative to a dipole; Nevertheless, an effective receiving aperture may be derived by this is neatly achieved with a log-periodic antenna (Balanis, 198, pp. taking that of a physically realisable antenna divided by its power 423-439). gain relative to an isotropic antenna. The terminated voltage (V expressed in decibels relative to 1uV, Equation 6.61 gives the effective aperture of a half wavelength for a practical receiving system with antenna gain, GdB, relative to dipole which has a maximum power gain of 1.64, relative to an a half wave dipole and attenuation of the feeder system LdB, is isotropic antenna. The effective aperture of an isotropic antenna is, given by Equation 6.64, assuming perfect impedance matching to therefore, given by Equation 6.62. the load resistance, R. (6.62) Yu20lo-LG-120 (6.64) 6.3.3 The loop antenna An application of Lenz's Law gives the open circuited voltage of a The space wave loop antenna as in Equation 6.63 where N is the number of turns in the loop and A is its area. 6.4.1 Free space field strength WHHNA (6.63) Earlier expressions were derived for field strength in free space. It is convenient to convert these to field strength expressed in decibels 6/14 Electromagnetic waves relative to 1uV/m, as in Equation 6.65, where