Sunday 22 September 2013

Directive Gain and Directivity of Antenna



The Directive Gain (DG) is defined as the ratio of radiation intensity due to the test antenna to isotropic antenna (hypothetical antenna that radiates uniformly in all direction).
                       
                                    DG =     U        =    4πU 
                                                 U0             Prad
Where,
U   = radiation intensity due to test antenna, in watts per unit solid angle
U0  = radiation intensity due to isotropic antenna, in watts per unit solid angle
Prad =total power radiated in watts

Since U is a directional dependent quantity, the directive gain of an antenna depends on the angle θ and Φ. If the radiation intensity assumes its maximum value then the directive gain is called the Directivity (D0).
i.e.                               DG =     Umax    =   4πUmax
                                                   U0              Prad

Gain of an Antenna

Power Gain of an antenna is defined as the ratio of its radiation intensity at a point to the radiation intensity that results from a uniform radiation of the same input power.
i.e.                               Gain = 4π x radiation intensity           = 4π (θ, Φ)
                                                    Total input power                     Pin

Whereas, relative gain is defined as the ratio of power gain of the test antenna in a given direction to a power gain of reference antenna (whose gain can calculated or known). In most of cases reference antenna is a lossless isotropic source.
i.e.                               Gain =                  4π (θ, Φ)
                                                 Pin (lossless isotropic antenna)
 When the direction is not stated, the power gain is usually taken in the direction of maximum radiation.

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