At 09:36 AM 08/19/08, you wrote:
How could it be more sensitive with CTCSS? I'm not sure I
understand.
de N5ZTW
A CTCSS decoder looks for a precise tone frequency in a narrow band,
so it's fairly sensitive. A squelch circuit looks for noise (no
signal present) and loss of noise (signal present) in a much broader
band above the voice passband, so it's not as sensitive.
73,
Bob, WA9FBO
Another trick - if you run full time tone mode you can run
open squelch (i.e. and blow raw carrier into the tone decoder)
and maximize the system sensitivity.
This is because a weak carrier that otherwise wouldn't open
the squelch will decode.
If your users are using radios with reverse burst on the encoder
you will not have ANY squelch noise burst.
When the user unkeys the reverse burst slams the tone
decoder shut, muting the receiver, and there is no squelch
tail at all (note that the proper definition of a "squelch tail" is
the noise burst at the end of the carrier, NOT the carrier
delay timer that keeps the transmitter up for several seconds).
The first MSY repeater (Motrac vintage) I saw had a dead
squelch pot and only after a couple of evenings of tracing
connections and poring over the pull-out schematics
did I figure out why it didn't work. It was built that way.
I then sat down with WA6KLA at a Dennys coffee shop and
walked him through what I found, and he graciously explained
that running open squelch was normal on a community repeater
with a multiple tone panel (6 receive reeds and 6 transmit reeds
with jumpers to select what incoming tone selected what
outgoing tone).
Several of the factory stock Micor repeaters that I have seen
have come this way - the squelch pot was not hooked up,
and the soldered-in jumpers on the interconnect board were
set up for PL-only. This was factory stock and a PITA to
change to what hams would consider "normal" operation.
Moto actually made a few low-end mobile models that didn't
have a carrier squelch circuit at all, and depended on the PL
decoder and the reverse burst on the encoder.
Many years ago I saw a machine that made use of the
maximize-the-sensitivity-by-blowing-squelch feature.
Carrier squelch mode was just what you'd expect, but
selecting PL mode caused an additional SPDT reed
relay to close.
The contacts were wired into the squelch pot to cause
the squelch to open (it effectively switched the wiper of
the squelch pot to the full open position).
All of the users were running Progs, Mastr-Pros, TPLs,
Motracs and Motrans (I did say it was many years ago)
and tone mode was actually more sensitive (and used
more) than carrier mode. And there was no squelch tail.
If you didn't know it was toned you'd think it was a well-designed
carrier squelch system (at least until the first YaeComWood
showed up with it's no-reverse-burst and the PL reed in the
system receiver had to freewheel to a stop to mute the audio).
Mike WA6ILQ
