Convert effective aperture to gain
G = aperture2gain(A,lambda)
returns
the antenna gain in decibels corresponding to an effective aperture
of G
= aperture2gain(A
,lambda
)A
square meters for an incident electromagnetic
wave with wavelength lambda
meters. A
can
be a scalar or vector. If A
is a vector, G
is
a vector of the same size as A
. The elements
of G
represent the gains for the corresponding
elements of A
. lambda
must
be a scalar.

Antenna effective aperture in square meters. The effective aperture
describes how much energy is captured from an incident electromagnetic
plane wave. The argument describes the functional area of the antenna
and is not equivalent to the actual physical area. For a fixed wavelength,
the antenna gain is proportional to the effective aperture. 

Wavelength of the incident electromagnetic wave. The wavelength
of an electromagnetic wave is the ratio of the wave propagation speed
to the frequency. For a fixed effective aperture, the antenna gain
is inversely proportional to the square of the wavelength. 

Antenna gain in decibels. 
[1] Skolnik, M. Introduction to Radar Systems, 3rd Ed. New York: McGrawHill, 2001.