Coupling loss is a term used to describe any loss of energy potential across a physical joint in two conductors. The term is generally applied to the electrical, telecommunications, and data transfer fields, although it may also have relevance in the area of fluid dynamics. Any coupling loss is expressed in terms of the unit of measure of the original energy source. For example, an audio signal measured in watts will suffer a coupling loss of X watts across any couplings in the signal path. These losses are typically the result of issues such as mismatched component impedance or misaligned joints, and are essentially an inescapable reality in most coupling applications.
Any source of energy, including electric current, audio, or data signals, and even fluid flows, will lose little potential when passing through a continuous, unimpeded conductor. Continuous conductors that run from source to destination are not always possible though, and the need for conductor joints or couplings are a reality in most systems. Unfortunately, coupling points are always a source of potential energy depletion commonly known as coupling loss. These losses are generally only a concern in high-fidelity applications such as electronics and data transfer networks, but may, in the case of high-performance systems, be applied to fields such as fluid dynamics.
A coupling loss will generally be expressed in the same unit of measure as the original energy source, with a good example being a 1-decibel loss in an audio signal measured in decibels. It may also be expressed as a percentage of the original, as in a 2% coupling loss in a 100-watt audio signal. This would be translated as a 2-watt loss across the coupling.
The loss of energy potential across coupling points is generally caused by a number of physical conditions. Mismatched impedance of components is one of the most common sources of coupling loss. Impedance is the total resistance or opposition a circuit offers to the current flowing through it. Careful matching of component impedance helps to reduce these losses, but it is almost impossible to achieve exact matches, so some losses will be experienced.
Misaligned optical couplers are also a common cause of coupling loss. When optic fibers are not perfectly aligned, a certain volume of the signal will not enter the secondary fiber and be lost. Misalignment also causes reflective losses of signal potential. Again, perfectly-aligned joints are generally not feasible, so losses will occur. Although much can be done to minimize coupling loss, they are an integral part of any energy transfer system that includes any type of joint or coupling in the transfer network.