Editing Capture effect (section)

== FM phenomenon ==
The capture effect is defined as the complete suppression of the weaker signal at the receiver's [[limiter]] (if present) where the weaker signal is not [[Amplifier|amplified]], but [[attenuate]]d.  When both signals are nearly equal in strength or are [[fading]] independently, the receiver may rapidly switch from one to another and exhibit [[Flutter (electronics and communication)|flutter]].

The capture effect can occur at the signal limiter, or in the [[demodulation]] stage for circuits that do not require a signal limiter.<ref>{{cite journal |last1=Leentvaar |first1=K. |last2=Flint |first2=J. |title=The Capture Effect in FM Receivers |journal=IEEE Transactions on Communications |date=May 1976 |volume=24 |issue=5 |pages=531–539 |doi=10.1109/TCOM.1976.1093327 |access-date=1 June 2024|url=https://ieeexplore.ieee.org/document/1093327|url-access=subscription }}</ref> Some types of [[Detector (radio)|radio receiver circuits]] have a stronger capture effect than others.  The measurement of how well a receiver rejects a second signal on the same frequency is called its capture ratio. It is measured as the lowest ratio of the power of two signals that will result in the suppression of the weaker signal. 

The capture effect phenomenon was first documented in 1938 by [[General Electric]] engineers conducting test transmissions. Two experimental FM stations, located 15 miles (24 km) apart in Albany and Schenectady, New York, were configured to transmit on the same frequency, in order to study how this would affect reception. It was determined that, for most of the path between the two stations, only one of the signals could be heard, with the complete elimination of the other. It was concluded that this effect occurred whenever the stronger signal was about twice as strong as the weaker one.<ref>[https://archive.org/stream/broadcasting16unse#page/n186/mode/1up "Armstrong Soon to Start Staticless Radio"], ''Broadcasting'', February 1, 1939, page 19.</ref> This was significantly different than the case with amplitude modulation signals, where the general standard for broadcasting stations was that to avoid objectionable interference the stronger signal had to be about twenty times that of the weaker one. The capture effect thus allowed co-channel FM broadcasting stations to be located somewhat closer to each other than AM ones, without causing mutual interference.