jhs@Mitre-Bedford (07/16/85)
Subject: Re: "PL" and tweedles
In-reply-to: Your message of Thursday, 11 Jul 1985 20:25-EDT.
<204@redwood.UUCP>
Mr. Warnock's solution to the squelch tail problem would probably work if
the modulation method were AM rather than FM.
However, a well-designed FM receiver pays little attention to the amplitude
of the signal being processed, insofar as the audio output level is concerned.
Specifically, the discriminator output is ideally the time derivative of the
instantaneous phase of the IF signal output, and changing the gain of the IF
chain has no theoretical effect on this parameter.
As a matter of fact, it IS possible to make squelch circuits which turn off
considerably faster than the typical ones you hear. It is also possible
to insert a time delay network, e.g. using "bucket brigade" chips, in the
audio path, so that the squelch circuit can in effect turn off before the
signal does! Quite a few "ham" repeaters do precisely this to avoid the
"double squelch tail" effect of a repeater. (They haven't figured out how
to get rid of the one generated by YOUR receiver, though!)
Back when I was in the Applied Research Labs at Motorola in ought sixty five
or so, they had worked out a squelch that gave only the barest little "tick"
when the signal went off, and they thought it was the greatest thing since
sliced bread, or whatever. Then the marketing guys went out and found out
that the customers, mostly police departments and trucking fleets and such,
didn't WANT the squelch tail removed. Maybe they thought it was sort of
"macho-radio" to hear that deafening burst of noise at the end of each
transmission, or maybe they just thought it was a nice, handy, functional
feature to have a distinctive end-of-transmission indicator, but in any case,
they wanted their old squelch tail back. So the idea died right then and
there. The guy who invented it threw in the towel and went into patent law
instead of trying to invent things anymore. He also applied what he had
learned about signal detection theory, so he told me, to his dealings in the
commodities market, and got rich with his little squelch detector turned into
a "buy! buy! buy!" detector! S'truth, s'help me!
-John S., W3IKGlarry@extel.UUCP (07/26/85)
Actually the story I know is that at least one police chief used to listen to the dispatch channel for the squelch tails. The radios used in the cars used a reverse burst so there was no squelch tail while the HT-220's had no reverse burst and a squelch tail. This way he could tell who was possibly taking a break at an eatery and who was in the car. He was rather mad when he got his brand new Micor radio for his base station which did not have a discernable squelch tail on strong signals. Larry, WA9QOO
jhs@mitre-bedford.ARPA (11/05/85)
Subject: scanner antennas Austin Custom Antennas sells a fairly effective scanner antenna. If it is not available locally at your favorite ham radio store (if there are any left) you can order or obtain information from Dick Austin at: Austin Custom Antennas P.O. Box 357 Sandown NH 03873 Telephone: (603) 887-2926. I haven't tried his scanner antenna, but all of his products which I have tried have worked extremely well. The log-periodic is another quite valid approach, but it is of course directional. You might want one of Austin's units or something designed along the same lines even if you also buy or build a log-periodic. Incidentally, Dick will quote on custom antennas for specialized purposes, as the name of his company suggests. Hint: his current hot item is antennas for cellular mobile telephone service. 73 to the net, John S., W3IKG