What is the capture effect in an FM receiver and how does it affect performance with multiple signals?

The capture effect in an FM receiver is the phenomenon where, when two or more FM signals are received simultaneously on the same frequency, the receiver 'captures' (demodulates) only the strongest signal and suppresses the weaker signal(s). This is a fundamental property of the FM limiter-discriminator demodulator: the FM limiter removes amplitude variations (clipping the signal to a constant envelope). When two FM signals are present: the limiter tracks the resultant signal, which is dominated by the stronger signal. If the stronger signal is at least 3-6 dB stronger than the weaker signal: the limiter-discriminator effectively produces only the audio of the stronger signal, and the weaker signal is suppressed (its audio contribution is greatly reduced). The capture ratio is the minimum power difference (in dB) required for the FM receiver to capture the stronger signal with acceptable quality. Typical capture ratios: 1-3 dB for high-quality receivers, 3-6 dB for standard receivers. How the capture effect affects performance with multiple signals: beneficial effect (the capture effect enables FM broadcasting and FM two-way radio to reject co-channel interference: if the desired signal is stronger than the interferer by more than the capture ratio, the interferer is effectively eliminated; this is why FM broadcasting works in environments with co-channel reuse: stations on the same frequency in different cities do not interfere as long as the local station's signal is sufficiently stronger). Detrimental effect (if the desired signal is slightly weaker than the interferer: the receiver will capture the wrong signal, completely losing the desired signal; there is no gradual degradation as in AM or digital systems; the transition from capturing the desired signal to capturing the interferer is abrupt; this makes FM more susceptible to sudden signal loss in mobile environments where the signal-to-interferer ratio fluctuates rapidly).