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

$\frac{\lambda^2}{4\pi}$

where $λ$ is the wavelength. An antenna with a gain of G has an aperture of

$\frac{\textrm{G}\lambda^2}{4\pi}$ .

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 a larger aperture produces a higher gain and narrower beamwidth.

Large dish antennas, many wavelengths across, have an aperture nearly equal to their physical area.