Talk:Capture effect

How does it work?

The article doesn't explain why the weaker station is rejected or what a "receiver limiter" is. 08:17, 23 February 2007 (UTC)

Similarly it would be great to know why AM radio transmissions are not subject to this effect, as the article states. -- TripleF (talk) 14:50, 23 May 2009 (UTC)

As far as I remember "Capture Effect" is basically inherent in ALL FM modulation sytems. I think it can be proven mathematically, I can't think of any other way. Apart from experimentally.

Each FM signals frequency is changing constantly. An AM signals center frequency remains the same. An AM receiver sees the sum total of any sigals presented to it's antenna. If that signal consists of multiple transmissions on the same frequency, they will all be heard. There is no way of telling them apart. Not a good explanation, but the best I can give for now.
--220.101.28.25 (talk) 14:42, 5 December 2009 (UTC)

Compared to (for example) FM Limiter & Capture Ratio, by Dietmar Rudolph, this Wikipedia article does nothing to explain FM capture effect. -- AHMartin (talk) 20:51, 26 January 2012 (UTC)

AM immunity

I think the section explaining AM's immunity to the effect needs to be clearer. —Preceding unsigned comment added by 96.230.232.9 (talk) 04:51, 7 March 2011 (UTC)

FM

This article is more than half on AM not having a capture effect. I somewhat understand the FM capture effect, but came here to remember the effect on S/N ratio for FM signals. Specifically, the S/N ratio after demodulation is much higher than before demodulation, for similar reasons. Or, in other words, the capture effect removes lower amplitude signals as noise. But I forgot the details in terms of S/N. Gah4 (talk) 23:29, 25 May 2018 (UTC)

I added some info from the 1939 Armstrong paper. The whole paper is available through the IEEE paywall, or a decent scan at world radio history WhaleFarm (talk) 17:38, 19 January 2026 (UTC)

Everyone seems to associate the capture effect always with certain modulations. The principle however is based on nonlinear devices used for demodulation in receiver in order to provide sufficient suppression of a weaker signal without creating to much distortion on the desired signal contents. In addition to certain modulation types, like e.g. AM, FM and PM, in order for the effect to take effect some conditions apply.

The first requirement is that a a certain ratio in signal strength between the Desired and the Undesired (D/U) stations involved are required. It depends on the definition of the acceptable level of distortion that can be generated during demodulation of both signals.

The second requirement applies not to Frequency Modulated (FM) or Phase-Modulated (PM which are similar to FM and therefore should be the same) stations, which is that for AM (Amplitude Modulated) stations using the same channel within Radio Line Of Sight (RLOS) the staions have to have an offset in all their center frequency of at least the sum of the highest audio frequencies used. If the center frequencies are separated by less than the sum of the highest audio frequencies used, you will hear additional beat frequencies where the signal contents overlaps in the channel. If they are separated by at least the sum of the highest audio frequencies used the beat frequencies are higher than the highest frequency in the defined signal bandwidth and can therefor be suppressed by a low pass filter.

The required D/U therefore depends mainly on the acceptable distortion of the desired signal information you want to receive. The D/U can therefore differs with the definition what is an acceptable distortion, e.g. for AM-VHF ILS-LLZ receiver a different D/U is required compared to Aeronautical AM communications, where a human listener can suppress distortion with some training more efficiently than any circuit. Think of it to you in a room full of voices and you still you can focus on one voice and ignore/sort it out of all the other voices.

In the 1990s I measured the required D/U on the ILS-LLZ receiver we had available in our lab in Langen Germany and measured the required D/U for all receiver to be around 8 dB. For a lower D/U the distortions increased above a value not tolerated by ICAO for ILS-LLZ receiver. The 8 dB D/U figure was also used in part for aeronautical VHF-Com separation calculations.

The capture effect works also for digital modulations. I measured for the pulse-pair reception of DME/N- and TACAN-Interrogator that in order to achieve a lockon to a desired DME/N or TACAN Transponder the required D/U to be also around 8 dB, with only minor variations for different receiver designs.

I hope that helps you to understand that the process of suppressing distortion of undesired signals and their information depend on the definitions what is an acceptable distortion and what is not. If you need more information ask. Snn47 (talk) 17:36, 20 January 2026 (UTC)

The Armstrong paper describes thie effect well for the FM case, maybe somebody could add a short generalization and find references? I've got a 1947 copy of Terman, I think it describes the AM case with offset frequencies. WhaleFarm (talk) 19:44, 20 January 2026 (UTC)

US ATC

As I understand it, the US ATC uses AM specifically to avoid a capture effect. Capture effect could result in an airplane completely missing requests from ATC, and not even know that they missed them. With AM, they will hear either two voices, or garbled noise, in both cases knowing that they missed something. Gah4 (talk) 20:41, 20 January 2026 (UTC)

The capture effect principle is based that nonlinear devices used for demodulation in receiver in order to provide sufficient suppression of a weaker signal without creating to much distortion on the desired signal contents, therefore is independent of the modulation used. This capture effect principle works mainly for AM, FM PM, and some digital modulations to suppress a weaker signal if the Desired and the Undesired (D/U) ratio between stations is about 8 dB without producing to high distortions during demodulation.

In Europe two or more transmitter are used to provide coverage for large VHF-COM ATC-sectors, e.g. up to 5 transmitter. As I described in my reply to the FM discussion FM above, Stations using the same channel within Radio Line Of Sight (RLOS) have to have an offset in all their center frequencies of at least the sum of the highest audio frequencies used, e.g. for a max. modulation frequncy of 3 kHz that means in a 25kHz channel both have to be seperated by 6 kHz to avoid interference.

If the center frequencies are not separated by less than the sum of the highest audio frequencies used, you will hear additional beat frequencies where the signal contents overlaps in the channel. If they are separated by at least the sum of the highest audio frequencies used the beat frequencies will be above 3kHz and suppressed by a low pass filter that allows less than 3kHz to pass to the audio amplifyier. If you need more information ask. Snn47 (talk) 20:57, 20 January 2026 (UTC)

PS.: In my reply AM refers to is carrier with two identical sidebands, with a max. modulation frequency of 3kHz each sideband is 3 kHz wide, therfore two adjacent AM stations require a min. separation of 6 kHz between the center frequencies of the carriers, e.g. for a center frequency of 133.550 MHz the carrier center frequencies would be 133.547 MHz and 133,553 MHz. Snn47 (talk) 21:03, 20 January 2026 (UTC)

OK, so if two are transmitting on adjacent frequencies, there will be a beat signal, and they will know. But if they are on the same frequency, within tolerance, there might not be. They won't know.

This came up in discussion related to an airplane accident with two planes on the ground, talking to ATC. For AM, and within frequency tolerance, receivers will get some combination of both, assuming they are close to the same distance away. Someone should ask for retransmission. As well as I know, US ATC uses VHF AM frequencies. Gah4 (talk) 22:05, 20 January 2026 (UTC)

OK you mix several effects.

Climax operation is only used to provide service to aircraft en route for large sectors that cannot be provided by a single ground transmitter.

Only the VHF-COM ground stations use the frequency offset to the nominal center frequency of a channel for climax operation, while aircraft are always on the centerfrequency ±frequency tolerance and ±their Doppler shift due to their speed.

AM climax operation for VHF-COM is defined by ICAO for worldwide use for ATC in ICAO Annex 10 Vol 3. The only difference is that in addition to 25 kHz channel spacing, we use in Europe also 8.33 kHz channel spacing due to frequency congestion in the European core area. This limits however climax operation to only two stations, unless the ground transmitter would use carrier single side band instead of double sideband.

No, the receiver bandwidth ensures ther is interference between transmitter on adjacent channel unless they are very very close to a transmitter.. What do you mean by crosstalk, intermodulation?

However if more than one ground transmitter utelize the same antenna transmitter intermodulation may occur if this in not avoided during technical planning of the ground infrastructure and frequency planning.

If you refer to the airplane accident accident on the ground in Tenerifaa (?), that was double talk of the pilot s, since as I wrote climax is not used for e.g. ground, APP, DEP services Snn47 (talk) 04:35, 21 January 2026 (UTC)