 Therefore, in free space the electric field strength in volts/meter, for P in watts and d in meters, is given by equation 7.52. E=V30P (7.52) Antenna Gain and Effective Radiated Power The term antenna gain defines the degree to which an antenna concentrates radiated power in a given direction, or absorbs incident power from that direction, compared to a reference antenna. The logical reference antenna is the isotropic radiator, in view of its lack of direc- tional properties, though the half-wavelength dipole is also often used. Antenna gain is most easily derived by considering all antennas as transmitting antennas. If the r.m.s. field strength in free space produced at a distance, d, is given by E, the total power radiated by the antenna is given by the surface integral, as in equation 7.53. (7.53) If the field strength in a particular direction is En the gain relative to an isotropic antenna is given by equation 7.54. (7.54) Chapter 7 Electromagnetic Waves If a choice is made such that En is in the direction of maximum radiation, the maximum intrinsic gain of the antenna is obtained. Gi is the gain solely attributable to the directional characteristics of the radiation pattern of the antenna and takes no account of dissipative losses in the antenna itself or in the transmission line between the antenna and the transmitter. These losses can be combined and expressed as a power ratio, K, SO that the antenna gain referred to the transmitter output becomes KG for the gain in any direction. This gives the effective gain relative to an isotropic antenna. The product of transmitter power and effective antenna gain relative to an isotropic antenna is the Effective Isotropically Radiated Power (EIRP). When the half-wavelength dipole is used as the reference antenna, instead of the iso- tropic 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 the Effective Radiated Power 