On Thu, 8 Sep 2011 22:43:26 -0700, (Dave Platt)
wrote:

You could build a very nice full duplex repeater on a single
frequency that way.

Yes, except that the ARRL has decided to only petition for a waver for
single time slot TDMA, which can't be used for a repeater.

You're scheme would certainly work, and I too am wondering why nobody
has bothered to do it. Possibly because nobody really wants full
duplex (with echo, reverb, feedback, etc).

I wonder whether you may not also have to be really careful with your
transceiver/receiver switching design. You'll really need to be able
to trust (and drive) those PIN diodes properly... goof up on even a
single time-slice and you could put enough TX power into your receiver
to turn its front end into a pile of smouldering char in a millisecond.

Been there, blown up enough diodes to make me want to go back to
relays. However, that was about 1975, when AMTOR and SITOR were the
next big thing. High speed reliable RTTY at umm.... 3 characters per
second.
http://en.wikipedia.org/wiki/SITOR
At the time, everyone was trying to do SITOR-FEC and ARQ mode using
existing HF radios, including my employer, Intech. Few of our radios
were really very good at fast TX/RX switching. The basic spec was
that it had to be less than the flight time from West to East coast.
Speed-o-light is 186,000 miles per second, or 186 miles per
millisecond, or 16msec from coast to coast. That actual turn-around
spec was something like 35msec as I vaguely recall. I was thrilled
that the synthesizer PLL would settle down in maybe 25msec, and the
T/R relay in maybe 20msec. Suffice to say that some major reword was
required to get the timing correct, during which I sacrificed my share
of fried PIN diodes. Then, we dragged it out for a field test
(parking lot test), and found that the high Q antenna coupler stored
just enough energy to vaporize an ocassional PIN diode. I got fed up
with expensive Unitrode PIN diodes and substituted cheap 1N4007 power
diodes, which were far more rugged, and lived with the slight increase
in loss.

This isn't a problem with normal split-frequency repeaters, thanks to
the isolation in the duplexer cans.

True. However, expensive duplexers are a problem with the very narrow
TX/RX frequency offset found on 2m, 6m, and 10m repeaters. It's
fairly easy on 440 and 1215Mhz repeaters, which have wider offsets.

Do any of the commercial TDMA systems use the same frequencies for
base-mobile and mobile-base? My recollection is that TDMA cellphone
systems operate with split uplink/downlink frequencies.

TDMA is IS-54/136, which is always on split frequencies. There was an
HF system in Africa that I worked on in about 1979, that used single
slot audio compression, and a single channel. It's idea of time
slicing was trivial with a 50% duty cycle, and one cycle every 128
stolen for control. Absolutely nothing was standardized, but was very
suitable for use on HF. I suppose it would be easy enough to take one
of the HF digital encoders, add a synchronous t/r switch, and you have
a workable start. I think this is close, but without full duplex:
http://www.aorusa.com/others/ard9800.html

But "no filtering" comes with its own set of concerns.

Yep. If the repeater is going in any of the buildings where I have
equipement, it will need a cavity and ferrite isolator to prevent
intermod problems. I know of one building that demands Heliax, no
braided coax including LMR-xxx coax, all silver plated connectors, and
other draconian anti-intermod measures. Tempest like packaging on all
computahs. All I can say is that it works (if you can afford it).


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558