Aperture-to-medium coupling loss
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In telecommunication, aperture-to-medium coupling loss is the difference between the theoretical antenna gain of a very large antenna, such as the antennas in beyond-the-horizon microwave links, and the gain that can be realized in practice.
Note 2: The "very large antennas" are referred to in wavelengths; thus, this loss can apply to line-of-sight systems also.