duplexers, antennas, repeaters
 

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.

duplexers, antennas, repeaters
 

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

duplexers, antennas, repeaters
 

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

duplexers, antennas, repeaters
 

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!

duplexers, antennas, repeaters
 

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.

duplexers, antennas, repeaters
 

Geoffrey S. Mendelson wrote:

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.


For you, perhaps. I have a mild learning disability and I can't use the
phone if there's too much hesi............tation. I have a twisted pair
that connects directly to an EOC at the push of a button. SMS works
great for me.

What's really funny is "music on hold" via mobile voice circuit.

duplexers, antennas, repeaters
 

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

duplexers, antennas, repeaters
 

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.

duplexers, antennas, repeaters
 

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

duplexers, antennas, repeaters
 

Jeff Liebermann wrote:

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.


An intermod study compares transmit freqs as well as input freqs.
Circulators are accepted practice. The intermod study will tell you if
you need more traps, BPF, etc. There is a lot of pseudoscience in
technology (and a comparable amount of "overkill"). By far, the worst
problem I encounter is XM radio on 2.5 gHz and ground radar from
airplanes (they like to use tower sites as benchmarks).

I have worked some of the premiere sites (Cedar Hill, Mt. Wilson, South
Mountain in Phoenix, Mt. Harvard, Senior Road in Houston, the John
Hancock building, the router room at Channel 4, etc.) and I have never
seen a blanket ban on LMR because it leaks.