Definition
The condition between two resonant electrical circuits, such as the primary and secondary windings of a radio-frequency transformer, in which the coupling is adjusted so that the maximum amount of energy is transferred from one circuit to the other at the resonant frequency.
Plain English
The exact amount of magnetic linkage between two tuned circuits that lets the most signal pass through at the frequency they are tuned to. Less coupling than this loses signal strength; more coupling than this spreads the signal across nearby frequencies instead of peaking at one.
Context Anchor
Seen in aircraft radio, navigation, and avionics maintenance discussions involving signal circuits and transformers.
Derivation
From Latin 'criticus' (decisive, turning point) and 'copula' (a link or tie). The term marks the decisive point at which the link between two circuits is tuned just right — neither too loose nor too tight.
Why Pilots Care
Pilots don't adjust this in the cockpit, but understanding it helps when troubleshooting radio reception problems with avionics technicians. A receiver that is over-coupled or under-coupled will not perform at its rated sensitivity or selectivity.
Analogy
It is like tuning the distance between two people passing a ball. Too far apart, the pass is weak or missed; too close, they get in each other’s way. At the right distance, the transfer works smoothly.
Grounding Statement
Think of two tuning forks placed near each other: at just the right distance, one makes the other ring loudest. Move them too far apart or too close together, and the response drops or smears.
Intuition Check
Do not read critical as “dangerous” here. In critical coupling, critical means “the exact best point” for transferring a signal between circuits.
Example Sentence 1
The IF transformer in the receiver was adjusted to critical coupling to maximize signal transfer at the tuned frequency.
Example Sentence 2
At critical coupling the receiver showed the strongest output without the double-hump response that appears with over-coupling.