One wonders why someone isn't pushing for a digital TDMA scheme for
amateurs. You could build a very nice full duplex repeater on a single
frequency that way. Sure it's totally incompatible with current FM
repeaters, but then, D-star isn't totally compatible either. D-star and
it's ilk are sort of half measures in that sense.

No filtering, much less intermod issues in multi station at onee site
systems... all kinds of good comes of it.

Digital schemes on HF to replace SSB I can see having real trouble (the
biggest is the lack of a "party line" capability, the other is the long
propagation delay on HF paths), but on VHF and up FM, you already have a
"one person talks at a time" by virtue of the standard FM demodulator.

On Wed, 31 Aug 2011 09:38:51 -0700, Jim Lux
wrote:

One wonders why someone isn't pushing for a digital TDMA scheme for
amateurs.

That's easy. Because there's no emission designator for FCC approved
TDMA mobile/HT for ham radio. The ARRL is working on the problem.
http://www.arrl.org/news/arrl-files-em-petition-em-em-request-for-temporary-waiver-em-with-fcc-regarding-vhf-voice-and-data-e
Multiple time slot systems, such as DStar are currently approved. This
is nothing new. Just approval for P25 radios.

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).

Sure it's totally incompatible with current FM
repeaters, but then, D-star isn't totally compatible either. D-star and
it's ilk are sort of half measures in that sense.

Dstar duz 5 independent simultaneous conversations through the
repeater. That's not what I would call a half measure.

No filtering, much less intermod issues in multi station at onee site
systems... all kinds of good comes of it.

Yep.

Digital schemes on HF to replace SSB I can see having real trouble (the
biggest is the lack of a "party line" capability, the other is the long
propagation delay on HF paths), but on VHF and up FM, you already have a
"one person talks at a time" by virtue of the standard FM demodulator.

Yep. We did that with Amtor (Sitor) for data. The big problem was
getting the switching time between T/R to less than the prop delay. If
you increase the time slicing to where it could handle voice (about
200Hz) but not be audible, the occupied bandwidth increases
unacceptably wide for HF. It's possible to decrease the switching
time, but then the latency (delay) increases to unacceptable levels. I
don't think anyone really wants a repeater with a 1 second audio delay
(even though they exist).

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

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.

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

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.

No filtering, much less intermod issues in multi station at onee site
systems... all kinds of good comes of it.

Yep.

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

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

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

Jeff Liebermann wrote:



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).


I've never seen that anywhere. Intermod is a math problem. No amount of
silver plating will fix bad coordination.

On 09 Sep 2011 19:50:43 GMT, dave wrote:

Jeff Liebermann wrote:
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).

I've never seen that anywhere. Intermod is a math problem. No amount of
silver plating will fix bad coordination.

Sounds like you've never had to deal with transmitted intermod.
Consider yourself lucky as getting rid of it is a PITA. Also, not all
intermod comes from mixing in the receiver. TX intermod is real and
preventable. The xmit cavity and ferrite isolator prevent any RF from
adjacent antennas from going down the antenna, into the power amp,
mixing there with the xmit signal, and having the power amp amplify
the intermod.

Google for "intermod suppression panel".
http://www.google.com/search?hl=en&q=intermod+suppression+panel&um=1&ie= UTF-8&tbm=isch
http://www.repeater-builder.com/antenna/ant-sys-index.html
http://antennasystems.com/product/sinclair-PC3/PC3113.html
http://www.telewave.com/pricelist/impanels.html
http://www.sinclairtechnologies.com/catalog/group.aspx?id=32
http://birdtechnologies.thomasnet.com/item/components/intermodulation-suppression-panels/81-series?
http://www.ferrocom.com/products.htm
http://www.taradios.com/IM_Supression_Panels.html
etc...

Incidentally, my Rotomola UHF MSF-5000 repeater has *THREE* isolators,
connected in series, in addition to a band pass cavity filter. Lossy,
ugly, but clean.

Next, silver plating is not a magic fix. Getting rid of bright nickel
plated cheap connectors *is* a magic fix. I've cleaned up several
systems by simply getting rid of cheap connectors and adapters and
replacing them with silver plated brass connectors (or just brass as
in Heliax connectors).

Google for "Passive Intermod Distortion":
http://www.amphenolrf.com/simple/PIM%20Paper.pdf
Materials: Ferromagnetic materials
such as nickel or steel must be
eliminated from the current path due to
their non-linear characteristics. Brass
and copper alloys are generally
accepted as linear materials. Tests
have shown that nickel plate under gold
on the center contact will typically result
in a 40 to 50 dB increase in PIM.
Stainless Steel in the body will usually
give a 10-20 dB increase in PIM.

Mo
https://engineering.purdue.edu/IDEAS/PIM.html (nice video)
In case you've seen magnets taped to coax connectors, this might
explain why.

More on PID.
http://aeroflex.com/ats/products/prodfiles/articles/8814/Intermod.pdf


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

On 9/8/2011 10:43 PM, 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).
Not so much full duplex, but single frequency half duplex, with
negligible time delay (implying 100ms frame time) between Rx and Tx.



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.

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

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.


Sure.. 802.11 is half duplex on a single channel, for instance.

Lots and lots of radars have fast and reliable T/R switching at pretty
much any frequency you care to name from DC to light.

TDMA cellphone uses split bands probably because it was on top of
existing AMPS systems. There is also a frequency allocation issue (e.g.
no need for new licensing). Having separate forward and reverse bands
also helps with frequency reuse and near-far issues. I hardly think
that hams are going to carpet the country with repeaters to the extent
that cell sites do.

On Mon, 12 Sep 2011 09:19:29 -0700, Jim Lux
wrote:

Not so much full duplex, but single frequency half duplex, with
negligible time delay (implying 100ms frame time) between Rx and Tx.

Most of the delay will not come from the flight time or mux switching.
It will come from the necessary audio compression. It won't work
without audio compression, which means that some types of
uncompressible pre-randomized data is not going to work (no big deal).
Also, the more compression, the longer the latency.

TDMA cellphone uses split bands probably because it was on top of
existing AMPS systems.

Yep. The problem was that TDMA (IS-54/IS-136) had to be compatible
with the then existing analog cell systems. Therefore, all early TDMA
phones had to offer analog compatibility. Digital only phones weren't
available until about 2003.

There's also little justification for making the change. It will not
double the number of available channels as some pundits have
suggested. Since the return audio now has to be sqeezed into the
previously transmit only channel, the number of users per channel is
cut in half. The result is no capacity change.

There is also a frequency allocation issue (e.g.
no need for new licensing). Having separate forward and reverse bands
also helps with frequency reuse and near-far issues. I hardly think
that hams are going to carpet the country with repeaters to the extent
that cell sites do.

I've looked into butchering cellular handsets into something usable on
ham radio. I have internals on some of the old Motorola flip phones
and bag phones and could probably modify the firmware sufficiently to
turn it into a conventional radios. The fatal flaw was the fixed
45MHz T/R offset. There were simply too many components that would
need to be replaced in order to operate on the smaller offset
available to hams, or on simplex. In addition, it's usually fairly
easy to go down in frequency, but the phones would require going up
from the 850MHz cellular bands to the 915MHz ham band. I gave up on
the idea.

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

On Aug 31, 11:38*am, Jim Lux wrote:
One wonders why someone isn't pushing for a digital TDMA scheme for
amateurs. You could build a very nice full duplex repeater on a single
frequency that way. *...

No filtering, much less intermod issues in multi station at onee site
systems... all kinds of good comes of it.

Digital schemes on HF to replace SSB I can see having real trouble (the
biggest is the lack of a "party line" capability, the other is the long
propagation delay on HF paths), but on VHF and up FM, you already have a
"one person talks at a time" by virtue of the standard FM demodulator.

Uhhh. It's been a long time since I worked on such a system (1975 I
think),
but IIR the prop delay through space for moderate distances kills the
idea.
Rough calculations gives a round trip delay, at 10 miles from the
repeater,
of about 0.1 ms. For two stations at that distance that's 0.1 ms not
available for sampling, bit width and processing. Keeping the BW down
also needs
rise and fall time as well as guard times. It added up quiclky back
then.
The vocoder becomes very important to reduce the data rate.

Hope I got that right...
73, Steve, K9DCI

Noskosteve wrote:

Uhhh. It's been a long time since I worked on such a system (1975 I
think),
but IIR the prop delay through space for moderate distances kills the
idea.

That was 1975. The only people who ever were exposed to any propigation delay
where those rich enough to make a long distance call that was routed over
satellite. If I remember correctly it was about $5 a minute to call New York
from L.A.

This is 2011, everyone is used the the propigation delay of digital telephones,
and VoIP. 150ms is tollerable.

Geoff.

--
Geoffrey S. Mendelson N3OWJ/4X1GM
Making your enemy reliant on software you support is the best revenge.