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

"Jeff Liebermann" wrote in message
...


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.

Such is the case on USN ships of my acquaintance. If the ship buys a
commercial transceiver and throws the antenna any old place, the front
end becomes a mixer. There's a reason why (most) military gear is
pricey. It's been engineered not to do that.

Aside:
It's not always an active device that causes problems. I had one ship
that was getting massive interference on UHF comm circuits between
about 325 MHz to 399 MHz. from a radar operating around 430 MHz.
Normally not a problem. The cause was a tangled hunk of wire I
found in the field of the radar. It had been used to secure scaffolding
during the ship's previous inport period.

Every time the radar lit up that bailing wire, the resulting arcing
and sparkling generated broadband RF pulses at the radar's rep rate.

I was climbing around on the mast, looking for just something of the
sort. When I saw that wire, I actually spoke out loud to it.

I said, "Well, hello there!" True story.

It's one of several reasons we preached "Topside Housekeeping"
to our Sailors during ship visits. Leave nothing on the mast that
doesn't have to be there.

"Sal"

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.

On 10 Sep 2011 18:27:58 GMT, dave wrote:

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

Perhaps you didn't notice but several of the examples of PIM and
rotten coax induced intermod were for non-cellular systems. The
problems are much the same with any service type. If you have
moderate TX power, magnetic materials in the connectors, and sensitive
receivers, PIM might be a concern.

Also, it may not be obvious unless you've worked on a cell site, but
the state-o-de-art in radio is currently defined by cellular. The
current generation of cellular radios, pre-distorting power amps,
tower mounted amps, cryogenic front ends, steerable antennas, channel
loading, codecs, and environmental protections, are far superior to
what I've seen in commercial radio, and light years ahead of the 3rd
hand garbage commonly found in ham repeaters. Yes, cellular is
different, but it's also quite superior. If the cellular people think
they have a PIM problem, it's probably quite real.

Unfortunately, cellular can't always get everything right:
http://802.11junk.com/jeffl/SCCARC-talk-2010-06-18/Burning-Towers.htm
http://www.cellsiteanalysis.net/cell_site_analysis_images/Cell_Site_Mast_Loaded.jpg

YouTube is pretty intense.

Duz that mean that you still haven't watched the video clip?

--
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 Sun, 11 Sep 2011 16:05:44 -0700, Jeff Liebermann wrote:

On 10 Sep 2011 18:27:58 GMT, dave wrote:

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

Perhaps you didn't notice but several of the examples of PIM and rotten
coax induced intermod were for non-cellular systems. The problems are
much the same with any service type. If you have moderate TX power,
magnetic materials in the connectors, and sensitive receivers, PIM might
be a concern.

It's still a math problem. You can predict intermod products from known
frequencies whether the non-linear device is active or passive.