g, 1973), which assumes that the reference polarization is that due to a Huygen's source. It most closely corresponds to what is measured with a conventional antenna test range. Chapter 9 Antennas The power gain in a specified direction is defined by the ratio of the power radiated per unit solid angle in direction 0, Q to the total power accepted from the source, as in equation 9.1. Power radiated per unit solid angle in direction 0, Q G(A,0)=4 (9.1) Total power accepted source This is an inherent antenna property and includes dissipation losses in the antenna. The dissipative losses cannot easily be predicted, SO a related parameter, directivity, is used in calculations. The definition of directivity is similar to that of gain except that the denomina- tor is replaced by the total power radiated. The terms gain and directivity are often used interchangeably in the literature. Normally only the peak gain along the boresight direction is specified. If the direction of the gain is not specified, peak value is assumed. The value is normally quoted in decibels. The definitions previously given are, in effect, specifying the gain relative to a lossless isotropic source. This is sometimes stated explicitly using the symbol dBi. The efficiency of an aperture antenna is given by the ratio of the effective area of an aperture divided by the physical area. Normal aperture antennas have efficiencies in the range 50 to 80 percent. As far as circuit designers are concerned, the antenna is an impedance. Maximum power transfer will occur when the antenna is matched to the transmission line. The impedance consists of the self-impedance and the mutual impedance. The mutual impedance accounts for the influence of nearby objects and of mutual coupling to other antennas. The self- impedance consists of the radiation resistance, the loss resistance, and the self-reactance. Loss resistance consists of the ohmic losses in the antenna structure. Radiation resistance measures the power absorbed by the antenna from the incoming plane wav