Four methods of determining the absolute power gain of microwave aerials are described, and some indication of their limitations and advantages is given. Two of them require no knowledge of the aerial geometry or polar diagram, and can therefore be used for finding the gain of any type of aerial. The other two, however, require a knowledge of the field across the aerial aperture, and can only readily be applied to horns of rectangular cross-section; even then several approximations have to be made. A method for comparing other aerials with the standard is also described.
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