In telecommunication, aperture-to-medium coupling loss is 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.

Source: from Federal Standard 1037C and from MIL-STD-188