TABLE OF CONTENTS Much of the material presented in this chapter is quoted or adapted from [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11].
The same antenna can be used as a transmit and as a receive antenna. A transmit antenna is a transducer that transforms electrical power delivered to the antenna into RF radiation. If the radiation from an antenna is uniform in all directions, it is called an isotropic antenna. Such an antenna is not possible in practice; however, it serves as a useful reference.
The theoretical isotropic antenna is taken as the reference, and the gain of the antenna in a given direction is a measure of how the power level in that direction compares with the power level that would exist if the isotropic antenna had been present. The gain can be either less than 1 or greater than 1. Expressed in decibels relative to isotropic, it can be either positive or negative. For example, a typical tracking radar might use a parabolic dish antenna that produces a 1 1 degree main beam. There are 41,300 square degrees in a spherical solid angle. The directional gain in the main beam would then be about 41,300, or 46.2 dBi. At angles other than the main beam, there will be sidelobes. A typical power gain in the sidelobe directions might be ?10 dBi or less, depending on the design of the antenna.
The power gain of an antenna is less than the directional gain...
