2.2.1 Review of Electromagnetic Signal Transmission

A radio-frequency signal may be generated and amplified to the power P by means of a suitable transmitter (Figure 2.1).

Figure 2.1: An isotropic radiator radiates electromagnetic energy equally in all directions.

Suppose that there is at A an isotropic radiator: an antenna capable of radiating uniformly in all directions a signal of power P. At a distance R from A, the power transmitted will be distributed over a sphere whose surface area is 4 ? R ². Suppose the distance R is large enough to be in the Fraunhofer region (far field) of the antenna, such that

(2.1)

where D is the maximum dimension of the antenna. Then the power density p will be (it is assumed that the radiant efficiency is 1, that is, all the power reaching the radiator is radiated into space)

(2.2)

Defining the radiant intensity I( ?, ?) to be the power radiated per unit solid angle (watts per steradian) in the ( ?, ?) direction, and recalling that the solid angle is 4 ? steradians, one may write, for an isotropic radiator,

(2.3)

A nonisotropic radiator, or nonisotropic antenna, will radiate more in some directions than in others (Figure 2.2), so that the radiant intensity I( ?, ?) will not be constant, but will vary with ? and ?.

Figure 2.2: A directive radiator...