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I'm very much a beginner with radio. I have a simple set of Baofeng BF-888S radios, and in my basic use with those, I got very curious about something: when you push the PTT button on one radio to speak, what is happening in the other radios on the same frequency that lights their LEDs green and they are seemingly unable to transmit until the one radio stops transmitting?

PDF manual, 7 pages

I'm imagining that the single radio starts transmitting at a certain threshold, even before the user starts speaking, and the other radios are configured to go into this receive/prevent-transmit mode when they pick up signals beyond a threshold? But I have no idea, and whatever keywords I'm using to search aren't revealing the answer.

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    $\begingroup$ Hello and welcome to ham.stackexchange.com! $\endgroup$
    – rclocher3
    Aug 6, 2020 at 0:38
  • $\begingroup$ Hello Gregir, does this involve a repeater or is it simplex? And what make and model of radios? $\endgroup$
    – Mike Waters
    Aug 6, 2020 at 2:38
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    $\begingroup$ Hi, Mike. Nope, no repeaters or anything. Just simple 2-way radio use got me curious. I edited my question a bit. $\endgroup$
    – Gregir
    Aug 6, 2020 at 3:45

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Well, it's a combination of two features: The squelch setting, and the busy channel lockout setting.

Namely, when a sufficiently strong signal has been received, the squelch system will activate the audio part of the receiver, and allow you to hear transmission.

Many radios also have more or less complicated busy channel lockout feature, which prevents the radio from transmitting. If the squelch is open, and the BCL is active, then the radio won't transmit. The general idea is, since you can't interrupt the person transmitting, you should get notified and prevented from transmitting yourself. This way, you can't "step" onto other users using the channel.

On the BF-888, you should be able to disable this feature with programming software.

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    $\begingroup$ That's one of my favorite Hollywood chuckles. The villain starts talking on the radio and then the hero angrily slams down the PTT button to interrupt. And of course the villain wouldn't care because either the hero can't transmit, or they're both transmitting. $\endgroup$
    – Duston
    Aug 6, 2020 at 13:08
  • $\begingroup$ Haha, that happened in Die Hard a lot ;) $\endgroup$
    – rclocher3
    Aug 6, 2020 at 14:22
  • $\begingroup$ also in Stranger Things :( $\endgroup$
    – Phil Frost - W8II
    Aug 8, 2020 at 1:25

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