Antenna Directivity

Directivity is the ratio of radiation intensity to the radiation intensity averaged over a sphere.It is different from gain although both terms are used interchangeably. Directivity is also different from beamwidth and is calculated using the beamwidth angles at half-power as follows,

D = 40,000 / (theta * phi)

where

theta = E-plane half-power beamwidth angle in degrees
phi = H-plane half-power beamwidth angle in degrees

Directivity can also be calculated if the aperture size and wavelength are known as follows,

D = 4* PI * A / lamda^2

where A = aperture area, lambda = wave length and PI=3.14