In telecommunication

Telecommunication

Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

, aperture-to-medium coupling loss is the difference between the theoretical gain

Gain

In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...

 of a very large antenna, such as the antennas in beyond-the-horizon microwave

Microwave

Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 links, and the gain that can be realized in practice.

Note 1: Aperture-to-medium

Medium (optics)

An optical medium is material through which electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability of the medium define how electromagnetic waves propagate in it...

 coupling loss

Coupling loss

Coupling loss also known as connection loss is the loss that occurs when energy is transferred from one circuit, circuit element, or medium to another...

 is related to the ratio of the scatter angle to the antenna beamwidth

Beamwidth

In telecommunication, the term beamwidth has the following meanings:1. In the radio regime, of an antenna pattern, the angle between the half-power points of the main lobe, when referenced to the peak effective radiated power of the main lobe....

.

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

Federal Standard 1037C

Federal Standard 1037C, titled Telecommunications: Glossary of Telecommunication Terms is a United States Federal Standard, issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, as amended....

 and from MIL-STD-188

MIL-STD-188

MIL-STD-188 is a series of U.S. military standards relating to telecommunications.-Purpose:Faced with “past technical deficiencies in telecommunications systems and equipment and software…that were traced to basic inadequacies in the application of telecommunication standards and to the lack of a...