lier solution to propagation phenomena came via Huygen's plex function F, given by Equation 6.24, where V is given by principle, which states that every point on a wave front acts as a Equation 6.25. source of a secondary wavelet. This provides an explanation for some propagation phenomena. Most importantly, it helped to quan- F(v)= 1 dt (6.24) tify diffraction. Diffraction effects can be formulated via the wave equation and Kirchhoff's integral (Longhurst, 1967), but the original derivation by Fresnel was via the Huygen principle. A semi-infinite opaque Generation of the electromagnetic wave 6/7 overlap, where more than one mechanism is relevant. For example, m.f. broadcast stations normally provide coverage via the surface wave, but this coverage may be reduced, at night time, by sky wave interference. Opaque screen 6.2 Generation of the electromagnetic Electromagnetic waves, as distinct from electrostatic fields or steady magnetic fields, involve a continuous net transfer of energy, Figure 6.7 Fresnel diffraction parameters or radiation, through a medium and are produced by current and charge distributions which vary with time. The types of antenna employed to generate such waves are many, depending on the radio frequency to be transmitted, the radiation pattern required to fulfil the needs of the system and the environ- (6.25) ment in which the antenna is required to operate. The following sections deal with several fundamental antenna elements from which the performance of many practical antennas may be derived. 6.1.5 Propagation mechanisms 6.2.1 Radiation from an infinitesimal linear The principles of propagation enable the communications engineer element to produce convenient calculation techniques. This process is facili- tated by recognising three major propagation mechanisms, which The electromagnetic field equations for many complex antenna tend to dominate in different wave bands: systems may be derived from a knowledge of the fields produced by an oscillating current in an infinitesimal linear elemen