aperture-to-medium coupling loss
The difference between the theoretical gain of a very large antenna, such as the antennas in beyond-the-horizon microwave links, and the gain that can be realized in operation.
Note 1: Aperture-to-medium coupling loss is related to the ratio of the scatter angle to the antenna beamwidth.
Note 2: The “very large antennas” are referred to in wavelengths; thus, this loss can apply to line-of-sight systems also.