If you and I enjoy talking using simplex, but Billy always butts in, you and I could use the same PL tone on transmit and receive and never hear Billy, unless he figures out the PL tone and uses it when transmitting. If he does, we let the air out of his bicycle tires.
Reading your post again, well…we could use a different PL when transmitting. You could use PL 67 when transmitting and I would use PL 67 on receive. I could use 141.3 on TX and you would use 141.3 on RX. That may perplex Billy.
Well, that may come close to being true, but if he's using no PL or a different PL, Billy's transmission could still interfere with, if not block your transmissions to each other. If 2 fm signals are of a close signal strength, there isn't a pronounced capture effect and you get a buzzing bees like interference and if Billy's signal were strong enough, it would be captured and you wouldn't hear a transmission from your friend occurring at the same time. I used to deal with a community repeater that had multiple PLs. The mobiles were generally configured to disable Rx PL when the mic was off hook, as you were supposed to monitor to make sure there wasn't any activity on the channel before transmitting. Handhelds had no such automatic PL disable(usually they had a button to push to go into monitor or carrier squelch, which usually wasn't done.), so handheld users often interfered with other users.
What PL does do fairly well is allow multiple users on a single frequency to not "have" to hear everyone else on the channel as long as you use both TX & Rx PL and to reduce the annoyance caused by various types of interference that would break squelch and be heard with a radio with normally adjusted carrier squelch. Of course in a repeater system, PL can be used as an access control device and can also reduce or eliminate having the repeater keying up for interference or a high rf noise level, but PL(subaudible tones, outside the receiver audio passband) is, despite its trademarked name(Private Line), not at all any type of privacy device, but a listeners' comfort / filter system.
that allows a repeater/reciever to discriminate what it wants to repeat, and allows anyone with any reciever, that can recieve the frequency, to listen to it. Used to be used frequently in the public saftey market to allow people to switch to a local 'scene' channel that everyone could still here, but would'nt tie up a repeater.
J-Roc steals two of your radios. He turns off the PL on TX of the radios he's using. It now won't go thru your repeater (which is set up to require a PL on TX), but your radios will hear it if they are in the immediate vicinity. Julian sneaks around right near you and talks on the radio which is now only talking to you, but you think it's going thru the repeater and that everyone can hear it...and he sends you, and only you on a wild goose chase. Randy, who is much farther away, hears nothing as it didn't go thru the repeater.
The repeater was listening for PL on TX, didn't hear it...didn't send it. Radios were in what's called carrier-squelch (CSQ) mode (RX no PL) will listen to anything on that chanel w/o discrimination.
That's not quite right, if Tx PL were disabled on a channel configured for a repeater, you'd be transmitting on the repeater input and listening on the repeater output. You could interfere with the repeater under some circumstances, but no one would hear you other than the interference because you aren't keying up the repeater and other users aren't listening on that frequency, they're listening on the repeater output, no matter whether they're progrogrammed for PL or carrier squelch(CSQ). The "scene" channel, if not a separate frequency, would generally be simplex on the repeater output frequency, which is known as "talk around" and is especially useful if you're using a handheld or are out of range of the repeater system or in a dead spot. If the stolen radio were programmed or reprogrammed with talk around, then what you described could happen, but others would hear what if anything was said by the person being sent on the wild goose chase because it would still be going out over the repeater and would likely, especially in public safety, be questioned as to who he was talking to or what was going on. The person being sent on the wild goose chase might also notice, because most repeaters have at least a short hang time to facilitate communications, and it's absence is noticeable.
