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

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.

All it takes is ONE user who believes himself to be above
any technical standards in his quest to maximize profit.

These are the same clowns that strip a site of any and all
hardware that isn't nailed down or currently connected to
something.

Or throw together a "repeater" out of junk bought at the
swap meet and nailed to a piece of plywood.

Jeff-1.0
wa6fwi

--
"Everything from Crackers to Coffins"

On Sat, 10 Sep 2011 08:12:23 -0500, Jeffrey Angus
wrote:

All it takes is ONE user who believes himself to be above
any technical standards in his quest to maximize profit.

Yep. However, sometime they actually have a clue. One of my friends
recently orchestrated a site cleanup and purge, emphasizing coax
cables and isolators. After the complaining, yelling, and screaming
stopped, so did the intermod. On the other foot, the county decided
to do the same things on a crowded tower that we were sharing. All
the LRM-400 came down, and was replaced by Heliax. Much of the
intermod went away, but the mixes generated in the receiver front ends
remained.

These are the same clowns that strip a site of any and all
hardware that isn't nailed down or currently connected to
something.

They're probably the same clowns that steal my scope probes that I
leave plugged into the scopes at various sites.

I had a weird problem related to unused equipment. There was an
unused "smog alert" receiver at one site, connected to an external
ground plane antenna half way up the tower. It was turned off as the
system was obsolete. Someone noticed that if they unplugged the
antenna connector, some of the intermod would magically disappear. The
outside antenna was picking up RF from the tower, delivering into the
building, and the badly shielded receiver front end was re-radiating
it all over the rack. The building manager immediately instituted a
reign of terror, demanding that all unused equipment and antennas be
removed, resulting in most of the junk exiting the building and tower.
There was a slight but noticeable decrease in intermod. Oh well.

Or throw together a "repeater" out of junk bought at the
swap meet and nailed to a piece of plywood.

Ahem... You must have been looking at my photos. Please don't do
that. Here's our unfinished plywood APRS weather station, built on a
plywood (with ash veneer) bookshelf:
http://802.11junk.com/jeffl/WR6AOK-WX-Station/
The 2m bottle is not in the picture. I use screws, not nails. There
was a good (political) reason to use plywood. Also, the rack in my
living room has plywood shelves, as are the radios in my Subaru.


--
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 10 Sep 2011 12:57:07 GMT, dave wrote:

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.

It's not leakage. The problem is the plated steel wire used over the
foil wrap on the shield. The steel is non-linear and subject to PIM
(Passive Intermod) problems. The aluminum foil to steel junction can
easily become a diode if the mylar coating is penetrated. I've seen
it with LMR-400 on a lab test similar to the YouTube video that you
apparently didn't watch. The problem was bad enough that Times had to
conjure a special mutation of LMR-400 with low PIM:
http://timesmicrowave.com/products/lmr/downloads/126-129.pdf
I think (not sure) that the only difference is that the braid over the
foil is now aluminum.

The initial reaction of most techs is that the PIM is sufficiently low
level that it would not have an effect on receiver performance. Wrong.
In cell sites, where squeezing every dBm of sensitivity out of the
receiver is necessary to deal with perpetually marginal cell phone
handset signals, that install cryogenically cooled front ends and
tower mounted preamps to do this, can definitely see the effect. Look
at a cell site install and try to find anything other than Heliax.

"SITE MANAGEMENT EQUIPMENT INSTALLATION RULES" (sample)
http://www.repeater-builder.com/antenna/site-stuff/radiositerules.html
"C. All cabling from the building to tower including on the
tower to the antenna, shall consist of a minimum of 1/4 inch
jacketed corrugated copper "Heliax" type cable. Semi-rigid
"LMR-400", "LMR-600", etc. cable and non-rigid cable, such as
RG8, RG, 213, RG-214, RG8X, etc. will NOT be used as
transmission cable exiting the building."

In most cases, this has been extended to include internal coax cabling
that carry transmit RF.

--
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:

On 10 Sep 2011 12:57:07 GMT, dave wrote:

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.

It's not leakage. The problem is the plated steel wire used over the
foil wrap on the shield. The steel is non-linear and subject to PIM
(Passive Intermod) problems. The aluminum foil to steel junction can
easily become a diode if the mylar coating is penetrated. I've seen
it with LMR-400 on a lab test similar to the YouTube video that you
apparently didn't watch. The problem was bad enough that Times had to
conjure a special mutation of LMR-400 with low PIM:
http://timesmicrowave.com/products/lmr/downloads/126-129.pdf
I think (not sure) that the only difference is that the braid over the
foil is now aluminum.

The initial reaction of most techs is that the PIM is sufficiently low
level that it would not have an effect on receiver performance. Wrong.
In cell sites, where squeezing every dBm of sensitivity out of the
receiver is necessary to deal with perpetually marginal cell phone
handset signals, that install cryogenically cooled front ends and
tower mounted preamps to do this, can definitely see the effect. Look
at a cell site install and try to find anything other than Heliax.

"SITE MANAGEMENT EQUIPMENT INSTALLATION RULES" (sample)
http://www.repeater-builder.com/antenna/site-stuff/radiositerules.html
"C. All cabling from the building to tower including on the
tower to the antenna, shall consist of a minimum of 1/4 inch
jacketed corrugated copper "Heliax" type cable. Semi-rigid
"LMR-400", "LMR-600", etc. cable and non-rigid cable, such as
RG8, RG, 213, RG-214, RG8X, etc. will NOT be used as
transmission cable exiting the building."

In most cases, this has been extended to include internal coax cabling
that carry transmit RF.


Cell sites are a different animal . We were talking about 2-way,
point-to-point, VHF/UHF broadcast type sites. YouTube is pretty intense.

In article ,
Jeff Liebermann wrote:

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.

It's not leakage. The problem is the plated steel wire used over the
foil wrap on the shield. The steel is non-linear and subject to PIM
(Passive Intermod) problems. The aluminum foil to steel junction can
easily become a diode if the mylar coating is penetrated. I've seen
it with LMR-400 on a lab test similar to the YouTube video that you
apparently didn't watch. The problem was bad enough that Times had to
conjure a special mutation of LMR-400 with low PIM:
http://timesmicrowave.com/products/lmr/downloads/126-129.pdf
I think (not sure) that the only difference is that the braid over the
foil is now aluminum.

That same diode-like effect also seems to be capable of causing the
cable to generate a nontrivial amount of broadband noise, when
energized by a sufficiently strong transmitter signal. In simplex
applications this seems not to matter, but in repeater applications it
tends to cause enough of an increase in the noise floor at the
receiver to appreciably de-sensitize the receiver.

The system I work on, was originally build with LMR-type feedlines
within the cabinet, and didn't "hear" particularly well. When the
chief hardware guru threw out all of those (well-constructed)
pigtails, and replaced them with 1/4" heliax... the problem went away
and has not returned.

Heliax is good. Double-braid shielded cable (with silver-plated
copper braid, not aluminum) seems to be almost as good.

--
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!