in mV/m, P in kW and D in km. If the field strength in a particular direction is E the gain relative to an isotropic antenna is given by Equation 6.54. E = 313 VP (6.50) (6.54) 6.2.7 The isotropic radiator The hypothetical isotropic radiator, which radiates power equally in all directions, is widely used as a reference with which to compare If a choice is made such that E is in the direction of maximum the performance of practical antennas. radiation, the maximum intrinsic gain of the antenna is obtained. Such a radiator located at the centre of a large sphere of radius, d Gi is the gain solely attributable to the directional characteristics metres, and radiating a total power, P Watts, produces a uniform power flux density EAH over the surface of the sphere, as in of the radiation pattern of the antenna and takes no account of Equation 6.51. dissipative losses in the antenna itself or in the transmission line between the antenna and the transmitter. These losses may be (6.51) combined and expressed as a power ratio, K SO that the antenna gain referred to the transmitter output becomes K G; for the gain in any direction. This gives the effective gain relative to an isotropic antenna. The product of transmitter power and effective antenna Therefore, in free space, the electric field strength in Volts/metre, gain relative to an isotropic antenna is called the Effective Isotropi- for P in Watts and d in metres, is given by Equation 6.52. cally Radiated Power (EIRP). Generation of the electromagnetic wave 6/11 When the half wavelength dipole is used as the reference antenna, instead of the isotropic antenna, the gain figures are reduced by the maximum intrinsic gain of the half wavelength dipole. The product of transmitter power and effective antenna gain relative to a half wavelength dipole is called the Effective Radiated Power (ERP). The numerical power gain of a half wave dipole relative to an isotropic source is 1.641 (2.15dB). The computation of the intrinsic or directive gain of an antenna requires a complete 