 known as the near field terms, length is small i.e. less than 0.1 wavelength. In this figure the current representing energy which ebbs and flows between the field and the current source. At d = 16A, a is equal to 0.01 and can effectively be set to zero. This leaves us with the radiation field terms. The total power radiated (Prod) by the current element may be Current element obtained by integration of the radial Poynting vector over a spheri- cal surface with the element at the centre, to give Equation 6.31, Current distribution where I is the peak current and is the r.m.s. current. (6.31) From Equation 6.31 it can be seen that the part in square brackets has the dimensions of resistance. This is known as the radiation resistance of the current element. 6.2.2 Radiation from an oscillating current in a small loop The magnetic equivalent to the infinitesimal linear element is the small loop, of radius a. Consider such a current loop at the centre of a spherical co-ordinate system. The electric and magnetic field components generated by such a loop may be derived, as for the infinitesimal linear current element, and are given by Equations 6.32 to 6.34, where X is given by Equation 6.35. Figure 6.9 The short dipole E = nx(1 - a) (6.32) distribution is given by Equation 6.38. (6.33) Current distribution -1-(1-2) (6.38) H = 2 x cotta(1 - a (6.34) The mean value of the current along the dipole is half the current x - Ba's i n o (6.35) at the centre. Thus, the magnitudes of the far field terms are half what would be given by Equations 6.26 to 6.30 for a current element Integration the Poynting vector over the surface of a surround- of the same length. Therefore, the total power radiated and the ing sphere gives the total radiated power as in Equation 6.36. radiation resistance, expressed in terms of the r.m.s. current 10 at the centre of the dipole, are a quarter of that given by Equation 6.31. 6.2.4 Radiation from a half wavelength dipole (6.36) The radiation characteristics of the half wavelength dipole may be deriv