ower accepted from the source, as in Equation 17.1. the practical implementation of antennas in communication sys- items. G( 0,0) Power radiated per solid angle in direction 0,4 (17.1) Basic properties Total power accepted source This is an inherent property of an antenna and includes dissipative The principle of reciprocity is one of the most important properties losses in the antenna. The dissipative losses cannot easily be pre- of an antenna. It means that the properties of an antenna when acting dicted SO a related parameter, the directivity, is used in calculations. as a transmitter are identical to the properties of the same antenna The definition of the directivity is similar to that of the gain except when acting as a receiver. For this to apply, the medium in between that the denominator is replaced by the total power radiated. The the two antennas must be linear, passive and isotropic, which is terms gain and directivity are often used interchangeably in the always the case for communication systems. literature. Normally only the peak gain along the boresight direction The directional selectivity of an antenna is represented by the is specified. If the direction of the gain is not specified, the peak radiation pattern. It is a plot of the relative strength of the radiated value is assumed. The value is normally quoted in dB's. The defini- field as a function of the angle. A pattern taken along the principal tions given above are in effect specifying the gain relative to a direction of the electric field is called an E- plane cut, the orthogonal loss-less isotropic source. This is sometimes stated explicitly by plane is called an H-plane cut. The most common plot is the using the symbol dBi. rectangular decibel plot, Figure 17.3 which can have scales of The efficiency of an aperture antenna is given by the ratio of the relative power and angle chosen to suit the antenna being charac- effective area of an aperture divided by the physical area. Normal terised. Other types of plots such as polar plots (used