wjc@ll-vlsi (Bill Chiarchiaro) (07/09/85)
Any of the tone schemes you mention would be legal for ham use. A repeater supposedly filters out the mobile's PL tone (it's supposedly "subaudible" anyway). However, on my Motorola HT-220, I can hear the PL tone from another HT-220 after going through our Motorola MSY repeater (MIT 449.2). I can't hear it continuosly, but only as a quick, chiming tone at the beginning of the transmission. Maybe what I am hearing are higher- frequency transients as the electromechanical PL reed is starting to vibrate. There could be a few reasons for the apparently shorter tail heard when using PL squelch. Some radios (repeaters in particular) transmit a so-called "reverse burst". When the radio is about to unkey its transmitter, it quickly reverses the phase of its PL tone. This causes the PL squelch circuit to turn off faster. This is useful with the electromechanical reed type of PL circuit; I don't think it would be necessary with an all-electronic PL squelch. Hams tend to use PL only for keying up a closed repeater or preventing a repeater from being keyed by interference or grunge. However, it can be very nice to have an HT or mobile rig that can provide PL squelch as well as "PL transmit". If you are driving around an urban area where there is a lot of grunge, you will often find your rig unsquelching on all sorts of garbage. Enabling your PL squelch will keep your receiver quiet until the repeater you are listening for starts outputting the proper tone. In fact, this is how we run the 449.2 machine; we do not require a PL on the input to key the machine, but the machine does output PL for those who want to use it. Also, I often open up the noise squelch and enable the PL squelch on my HT when I am trying to work a weak simplex contact. I find that my PL squelch is more reliable on weak signals. Of course, the other guy's HT has to transmit the right tone. One other ham use of PL is known as "anti-PL". About a year ago, our repeater started talking to a new machine in Philadelphia that was "upside-down" from us (444.2). The two machines talk to each other about once a week, often strongly enough to carry on a QSO with the people in PA. In order to prevent this from becoming annoying, the operator of the Philly machine installed a circuit that prevented his machine from keying if it heard our PL tone. Of course, when a local Philly user keys his HT, his (hopefully) much stronger signal will capture the Philly machine. This makes our tone "disappear" and their machine will key up. Lastly, the only made-for-hams HTs that I can think of that support PL squelch are those made by Yaesu. The FT-208R and FT-708R and now the FT-209R and FT-709R allow for options that give both PL encode and decode. Hope this helped... Bill N1CPK MIT UHF Repeater Association