Can two FM radios transmit two separate PL tones, and be received by two radios with matching PL tones? Or will the receivers hear a mix of the two, or nothing at all?


Assuming that the two transmitters operate at the same carrier frequency, and that the receivers receive similar power from each transmitter, then the FM signals will suffer destructive interference and the demodulated signal will be heavily distorted. PL tone, which is part of the transmitted signal, will also be lost.

If one of the RF signals gets to the receiver with much more power than the other transmission, then there is FM masking, where the stronger signal prevails and the weaker signal is undetected.

You cannot expect to receive a nice mix of the two transmitted signals in FM.

P.S. this is why aviation VHF uses AM and not FM. In AM, all received signals add up in the demodulated audio, so you can hear all signals at once.

  • $\begingroup$ Another reason aircraft use AM is because propellers can produce doppler modulation on FM, causing strong noise in both transmitted and received signals. $\endgroup$ – HarveyB Jun 10 '14 at 21:28

The subaudible "PL" tone is only there to control the receiver's squelch circuit. If the tones don't match, the receiver stays muted. Once the squelch is opened, the received FM signals continue to behave as they normally would.

  • $\begingroup$ This post doesn't answer the question. The question was whether two simultaneous transmissions would prevent these PL tones from being recognized by the receivers (and of course whether the human receiver could recognize the content of any transmission). The question is not about squelch control as much as what happens when two FM transmissions overlap. Especially in average consumer radios, marketers often package a radios as having "582 channel combinations!" meaning 14 frequencies and 38 PL tones which confuses end users as you could not carry out 582 simultaneous conversations. $\endgroup$ – BenSwayne May 20 '14 at 15:49
  • $\begingroup$ Further to my first comment, lets say the first transmission begins a few seconds earlier than the next, while both continue for some 15 second message. The first transmission may initially open the squelch on its paired receiver, while the second transmission to begin will cause destructive interference to the already started transmission. So your statement above "Once the squelch is opened, the received FM signals continue as they normally would" isn't really constructive (although you could interpret what I'm saying as how simultaneous FM transmissions "normally would" work). $\endgroup$ – BenSwayne May 20 '14 at 15:55
  • $\begingroup$ I got the impression the OP thought the tones allowed you to magically put two stations on the same channel (much as your 582 channel example implies). I was attempting to show that two stations "doubling" would still sound the same in the receiver. It's true, if you look at it closely, that there really are 3 cases. If station A is stronger, you hear it with B largely suppressed; if B is stronger, B captures the Rx (with the wrong tone) and it mutes. If they're equal, you get mess, with them trading places back and forth (if they're mobile) and the Rx probably cuts in and out. $\endgroup$ – gbarry May 21 '14 at 5:11

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