t (Jordan and 1. Space wave, below 10 metres i.e. frequencies above 30MHz. Balmain, 1968, pp. 317-321; Balanis, 1982, pp. 164-176). For such 2. Sky wave, 10 to 200 metres i.e. from 1.5 to 30MHz. an element, the current may be assumed to be constant throughout 3. Surface wave, above 200 metres i.e. below 1.5MHz. its length. Consider a current element of length 81 carrying a current This division into three categories should not be regarded as I cos W 1, located at the origin of a spherical co-ordinate system in a rigidly defining the propagation mechanisms for the frequency homogeneous, isotropic, non-conducting medium as shown in Fig- ranges specified. Firstly, it should be appreciated that the stated ure 6.8. The formulation of the electric and magnetic field magni- ranges are only approximate. Secondly, there can be a considerable tudes generated by such a current element is made more succinct by Figure 6.8 Radiation from an oscillating current in an infinitessimal linear element (8L) at the centre of a spherical co-ordinate system 6/8 Electromagnetic waves the introduction of two variables, as in Equations 6.26 and 6.27, Again the part in square brackets is the radiation resistance, which from which Equations 6.28 to 6.30 are obtained. may be increased by constructing the loop from N turns, wound SO that the magnetic field passes through all the turns. The radiation (6.26) resistance is then given by Equation 6.37. (6.37) (6.27) Ho = 4 ( a ) (6.28) 6.2.3 Radiation from a short dipole (6.29) The derivation of the radiation characteristics of practical antennas, from the equations for the infinitesimal current element, requires a E,=2nya(1-a)cot 0 (6.30) knowledge of the current distribution in the practical antenna. In the case of the centre fed short dipole (Figure 6.9) the current distribution may be regarded as linearly tapering from a maximum Terms in a and a2 are known as the induction and electrostatic value at the centre to zero at the tips, provided that the electrical fields respectively. They are also