Aperture Efficiency
In general, the aperture of an antenna is not directly related to its physical size. However some types of antennas, for example parabolic dishes and horns, have a physical aperture (opening) which collects the radio waves. In these aperture antennas, the effective aperture Aeff must always be less than the area of the antenna's physical aperture Aphys, as can be seen from the definition above. An antenna's aperture efficiency, ea is defined as the ratio of these two areas:
The aperture efficiency is a dimensionless parameter between 0 and 1.0 that measures how far the antenna falls short of using all the radio power entering its physical aperture. If the antenna were perfectly efficient, all the radio power falling within its physical aperture would be converted to electrical power delivered to the load attached to its output terminals, so these two areas would be equal Aeff = Aphys and the aperture efficiency would be 1.0. But all antennas have losses, such as power dissipated as heat in the resistance of its elements, nonuniform illumination by its feed, and radio waves scattered by structural supports and diffraction at the aperture edge, which reduce the power output. Aperture efficiencies of typical antennas vary from 0.35 to 0.70 but can range up to 0.90.
Read more about this topic: Antenna Aperture
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