Gain for an antenna is expressed in decibels relative to an isotropic radiator
(point source that radiates equally in every direction). Physical attributes of
an antenna, in conjunction with the operating frequency, results in constructive
and destructive interference patterns being set up a points distant from the antenna.
Where the fields add constructively a positive gain is experienced, and where fields
add destructively a negative gain is experienced (relative to isotropic). These
drawings and parameters were derived from the
EW and Radar Systems