Convert gain to effective aperture
A = gain2aperture(G,lambda)
returns
the effective aperture in square meters corresponding to a gain of A
= gain2aperture(G
,lambda
)G
decibels
for an incident electromagnetic wave with wavelength lambda
meters. G
can
be a scalar or vector. If G
is a vector, A
is
a vector of the same size as G
. The elements
of A
represent the effective apertures for the
corresponding elements of G
. lambda
must
be a scalar.

Antenna gain in decibels. 

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