Re: Unintentional VHF transmissions
Unknown if it's a FCC rule for marine band radios.
I can tell you this. A TX Timeout is very common, and a standard feature on most mid to higher end walkie talkies. It's normally a option of feature that you have to activate.
On our radio system at work (Motorola EDACS trunking) we have all our radios set to timeout out and alert (audible beep) after only 30 seconds. You will then have to release the PTT button and hit a shift key on the radio to clear the timeout message, and allow the radio to TX again. We work in a steel mill and everything around here is big hot and heavy. A open mike for a longer period than that has the potential to cause a safety hazard should someone need to call for help, or to stop a lift etc........ Our FCC license states that the radio system, is for "general communication" thus transmissions should be kept short and to the point.
Most open MIC's are caused from hand held radios. Normally from guys putting them in their back pockets and sitting on them. It's easier than you think to manipulate the radio to hit the PTT button.
On a funny side note, when there is a open mike after the transmitting stops someone will always come on the air and state "open mic" or something like that. It's funny watch every one grab their radios out of their pockets and say "oh S..., hope it wasn't me, the boss doesn't need to hear that coming from me"
Since were diving into it. Ours along with most police departments have the ability to "listen in" on a specific radio. If a dispatcher or supervisor suspects something is wrong they can dial up that radio, and key it up to determine if the officer is in trouble. Most newer police radios have a button on top, Normally red that acts as a panic button. More on that later.
Also our county fire dispatch uses a semi-duplex repeater system. This system uses "PL" codes. With out getting to technical PL=private line. When you key up the radio the radio sends out a IN-audible tone with the transmission. The repeater listens for this tone, when it hears it the repeater will then break squelch and TX the signal to everybody else, but at a higher strength. The point here, only radios with this correct PL code(fire and rescue personnel) can use the repeater, even if others are transmitting on the same frequency.
Why did I bring this up?????? O yea that's right open mic's..... Duh
OK. In the event of a open mic, the dispatcher can do one thing to stop it. They can basically tell the repeater to stop transmitting for a split second. This causes the repeater to listen for that PL code again before it starts transmitting. Now even though there's a open carrier, (open mic) the radio only sends that PL code on for a split second on TX. Since the radio never stopped transmitting, and started again, the repeater dosen't hear the PL code and won't key up. It's a pretty good way to stop a open mic and free up the channel.
Ahhhhhhhh.... But there's a catch. IF someone Keys up their radio while the previous open mic is still going on, the repeater will hear the PL code and begin to transmit, BUT even after the second radio ceases to stop transmitting, the first radio with the open mic will continue the carrier, thus continuing the repeater to continue TXing. Thus starting the process over of having to momentarily stop the repeater from listening. This is why we still have and use time-out timers in the Fire and rescue field.
I'm not really sure why I dove in that far, but there's the basics on Open mics, and ways to stop them. Yes I work with this stuff all day so bring on the questions.
Bill
