Antenna aperture
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As a receiver, antenna aperture can be visualised as the area of a circle constructed broadside to incoming radiation where all radiation passing within the circle is delivered by the antenna to a matched load. (Note that transmitting and receiving are reciprocal, so the aperture is the same for both.) Thus incoming power density (watts per square metre) x aperture (square metres)= available power from antenna (watts). Antenna gain is directly proportional to aperture. An isotropic antenna has an aperture of (wavelength)²/4π, and one with gain G has an aperture of G.λ²/4π
Generally, antenna gain is increased by directing radiation in a single direction, while necessarily reducing it in all other directions since power cannot be created by the antenna. Thus the higher the gain, the larger the aperture and the narrower the beamwidth.
Large dish antennas, many wavelengths across, have an aperture nearly equal to their physical area.
See "Antenna (radio)" for more details.
