Demonstrating the FM Capture Effect – Why Aircraft use AM

Over on his YouTube channel Tall Paul Tech has uploaded a video that demonstrates the FM (frequency modulation) capture effect. Apart from the costs and difficult logistics to change from AM to FM worldwide, the FM capture effect may be one additional reason as to why aircraft still choose to use AM modulation for communications instead of FM.

The FM capture effect is a phenomenon that occurs when two FM transmitters transmit on the same frequency at the same time. What will happen with FM is that the stronger of the two transmissions will be the only one heard, with the weaker one totally muted. This is in contrast to AM where both signals can be heard, albeit garbled like two people talking at the same time.

With aircraft this is important as for example if some aircraft accidentally leaves a blank transmission open, another aircraft can still transmit on top of the blank transmission and still be heard. Or allowing air traffic control to hear if multiple aircraft are trying to transmit at once, and handle communications appropriately based on urgency. The disadvantage is that without the capture effect, AM is more prone to interference from interference and atmospheric noise like lightning.  

In his demonstration Paul uses two HackRF's with their clocks linked and an RTL-SDR to simulate two transmitters and a receiver.

Demonstrating FM Capture Effect

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I advise to correct the text to eliminate the false claims by the author which are BS, because either the author does not know or does not understand how ATC VHF-COM Climax operation works.

ATC VHF-COM Climax operation is only used to provide coverage from ground AM-Tx to aircraft flying in a large ATC sector that cannot be serviced by a single ground VHF-COM AM Tx.

The trick to avoid the interfernce of simultaniously decoding several AM signals, =overlapping of voices and generation of beat frequency tones, is that all transmitter that operate at the same channel, are shifted by at least by 2.5 kHz , from the fc (center frequency) of the other transmitter. The >2.5 kHz is necessary to make allowance for Doppler shift and frequency tolerance of ground transmitter.

Due to the capture effect the stronger sreceived signal supressed the weaker ones, thus eleminating beat frequency tone and/or interference from overlaping voices since they are not transmitting at the same fc! For 3 transmitter one tx would transmitt at fc (center frequency of a channel), one at fc+5 kHz, and one at fc – 5 kHz.

Any pilot or ATCO would get fast loose concentration and get a headache if the by the author claimed use ” allowing air traffic control to hear if multiple aircraft are trying to transmit at once, and handle communications appropriately based on urgency” would really be the standard operationl use.


This is only true for modern radios whichs carrier is less than 100Hz off, else this frequency offset will create a very loud audio tone, which would be less troubling in FM. Everybody who listens to Airband a lot is aware of the screaming interference.


If the screaming interference would be regularily the case, both pilots and ATCO would classify the frequency to be unuseable.

While you may receive at random several signals at once since you are not under the same restrictions as pilots and ATCO are. Pilots and ATC only use the published frequency/channel (8.33kHz spacing) which has been coordinated to provide interference free operation within the limits of a Designated Operational Coverage (DOC) or ATC sector.

The pilot is protected from interference in sectors which require more than one transmitter to cover a sector by ground climax operation (described in my previous post), while the ATCO is protected since more than one receiver are spread georgraphically over a large sector, and only the aircraft closest to the receiver is received by the receiver unless all would transmitt simultaniously.


PS.: Your properbly have screaming interference at your location, because you most likely ignore the limits defined by the Designated Operational Coverage (DOC) or ATC sector in order to ensure interference free reption.