So, given PT and G it is possible to calculate the power density at any point in the far ®eld that lies in the direction of maximum radiation. A knowledge of the radiation pattern is necessary to determine the power density at other points. The power gain is unity for an isotropic antenna, . one which radiates uniformly in all directions, and an alternative de®nition of power gain is therefore the ratio of power density, from the speci®ed antenna, at a given distance in the direction of maximum radiation, to the power density at the same point, from an isotropic. | 16 The Mobile Radio Propagation Channel So given PT and G it is possible to calculate the power density at any point in the far field that lies in the direction of maximum radiation. A knowledge of the radiation pattern is necessary to determine the power density at other points. The power gain is unity for an isotropic antenna . one which radiates uniformly in all directions and an alternative definition of power gain is therefore the ratio of power density from the specified antenna at a given distance in the direction of maximum radiation to the power density at the same point from an isotropic antenna which radiates the same power. As an example the power gain of a halfwave dipole is dB in a direction normal to the dipole and is the same whether the antenna is used for transmission or reception. There is a concept known as effective area which is useful when dealing with antennas in the receiving mode. If an antenna is irradiated by an elecinmiagnetic-wave. the received power available at its terminals is the power per unit area carried by the wave x the effective area . P WA. It can be shown 1 Ch. 11 that the effective area of an antenna and its power gain are related by A PROPAGATION IN FREE SPACE Radio propagation is a subject where deterministic analysis can only be applied in a few rather simple cases fk extent to which these cases represent practical conditions is a matter for individual interpretation but they do give an insight into the basic propagation mechanisms and establish bounds. If a transmitting antenna is located in free space . remote from the Earth or any obstructions then if it has a gain GT in the direction to a receiving antenna the power density . power per unit area at a distance range d in the chosen direction is P Gt jy -A k 4W 2 The available power at the receiving antenna which has an effective area A is therefore Rv G-v Pr 4 A Pt Ct 4nd 2 22GR 4 y where GR is the gain of the receiving antenna. Thus
