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In a radio receiver, the capture effect is a phenomenon associated with reception in which only the stronger of two or more signals received within the bandwidth of the receiver passband will be demodulated. The Capture effect therefore enables frequency reuse of the same frequency by imposing a sufficient distance separation, e.g. used in AM communication in the AM(R)S (Aeronautical Aeronautical mobile (R) service), or between FM-BC transmitter for the capture take effect. Alternatively the capture effect enables two frequency ILS-Localizer (ILS-LOC) and ILS-Glide-Path (ILS-GP) to operate at airports in presence of strong reflections, e.g. due to terrain and buildings.
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 amplified, but attenuated. When both signals are nearly equal in strength or are fading independently, the receiver may rapidly switch from one to another and exhibit flutter.
The capture effect can occur at the signal limiter, or in the demodulation stage for circuits that do not require a signal limiter.[1] Some types of 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.[2] 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.
When AM (Amplitude Modulation) transmitter share the same center frequency, the weaker signal will introduce distortions in form of beat frequencies or both signals interfere completely with each other. If only a carrier devoid of modulation is received or added in a receiver, e.g. using a BFO (Beat Frequency Oscillatior), a tone with the frequency offset between the two carrier frequencies will be heard.
By introducing a frequency offset between the carrier frequencies in the magnitude of at least the sum of the highest modulation frequencies employed by both transmitter, will eliminate generation of beat frequencies, e.g. highest modulation frequency is 2.4 kHz therefore the min. frequency offset between both carrier is 4.8 kHz (=2 x 2.4 kHz).
The capture effect is actively employed in Europe for aeronautical VHF-communication in the band 118 MHz to 137 MHz to provide coverage for aircraft flying under ATC (Air Traffic Control) in large ATC sectors that cannot be covered by a single transmitter site. For ATC large sectors up to five transmitter sites are required to provide continuous coverage within the area of a large ATC sector. Transmitter are strategically placed therefore ensuring that not all transmitter will be received at any point in space simultaneously.
Operation of ILS-Localizer and ILS-Glide-Path at airports is often impossible due to strong reflections, e.g. on terrain and buildings. The solution lead to the development of two frequency ILS-LOC and ILS-GP.[3] Two frequency ILS systems use an additional carrier frequencies with a frequency offset of at least 6 kHz (2 x 3kHz as the highest voice modulation frequency), two separate antenna pattern that overlapp only in part and a higher EIRP for the ILS-localizer course signal for a range of up to 25 NM and a lower EIRP for the ILS-Localizer clearance signal for a range between10 NM to 17 NM. For difficult terrain and weather impact 2f ILS-GP are employed.[4] Two frequency ILS-LOC and ILS-GP are an ICAO standard precision approach system and are standardized for worldwide use in ICAO Annex 10.[5]
AM receiver will demodulate any carriers and their side bands that are strong enough for demodulation within the receiver passband resulting in an audio mix, if their carrier frequencies are not separated by the sum of the highest modulation frequencies of employed in both transmitter. If only a carrier devoid of modulation is received or added in a receiver, e.g. using a BFO (Beat Frequency Oscillator) the resulting tone is the frequency offset between the two carrier frequencies. [a]
For digital modulation schemes it has been shown that for properly implemented on-off keying/amplitude-shift keying systems, co-channel rejection can be better than for frequency-shift keying systems.
This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).
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