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.

On 9/19/2011 12:48 PM, dave wrote:
[ dribble snipped ]

I see you're over here trying to sound impressive too.

Jeff


--
"Everything from Crackers to Coffins"

On Mon, 19 Sep 2011 14:59:53 -0500, Jeffrey Angus wrote:

On 9/19/2011 12:48 PM, dave wrote:
[ dribble snipped ]

I see you're over here trying to sound impressive too.

Jeff

I don't need to sound "impressive". I have been paid to conduct many
intermod studies using proprietary software.

On Mon, 19 Sep 2011 12:48:09 -0500, dave wrote:

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.

Yep. And after I've done the math, I still have to get rid of the
intermod. The problem is not the math. That's well known and easy to
do. The problems a
1. Finding which of the hundreds of signals found on a typical
mountain top is causing the problem.
2. Finding where the likely culprits are located (i.e. which
building).
3. Finding any and all sources of non-linearity that are producing
the mixes. That could be anything from a gold on nickel connector to
insufficient reverse power protection on a broadband power amp.
4. Site management and politics.

It's no longer single "known frequencies" causing the intermod. In
these days of broadband everything, it's fairly wide swaths of digital
noise that's causing the intermod. For example, CDMA phone is 1.25Mhz
wide, WCDMA is 5Mhz, and CDMA2000 is up to 25Mhz wide. The worst part
is that most of the culprits can't be decoded on my service monitor,
so I can't tell for sure if they're causing the intermod.


--
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 Tue, 20 Sep 2011 00:16:29 -0700, Jeff Liebermann wrote:


Yep. And after I've done the math, I still have to get rid of the
intermod. The problem is not the math. That's well known and easy to
do. The problems a
1. Finding which of the hundreds of signals found on a typical mountain
top is causing the problem.
2. Finding where the likely culprits are located (i.e. which building).
3. Finding any and all sources of non-linearity that are producing the
mixes. That could be anything from a gold on nickel connector to
insufficient reverse power protection on a broadband power amp. 4. Site
management and politics.

It's no longer single "known frequencies" causing the intermod. In
these days of broadband everything, it's fairly wide swaths of digital
noise that's causing the intermod. For example, CDMA phone is 1.25Mhz
wide, WCDMA is 5Mhz, and CDMA2000 is up to 25Mhz wide. The worst part
is that most of the culprits can't be decoded on my service monitor, so
I can't tell for sure if they're causing the intermod.

Cellular phones are a different animal. I worked on fixed and mobile,
mostly analog, mostly FM radios. Theory and practice are quite different.

The tower owner should have an inventory of every transmit and every
receive frequency, plus all the standard I.F., plus nearby external high
powered sources. The owner should have cleared each frequency before it
went on the air, and should not add a tenant if doing so would create a
harmful spur to existing users. This is site management 101.

I don't care how the WL people run their data streams. Cellular folks
don't like high mountains (except for backhaul). I know they use very
advanced techniques to hear signals below the noise floor; keeping that
noise floor as low as possible is of paramount importance when you are
looking at 100 mW devices in people's pockets 5 miles away.

FWIW, Tek has a real nice analyzer that will reverse engineer TDMA spurs.
make time-lapse spectrum analysis, and can even write on a map for you.

On Tue, 20 Sep 2011 07:16:40 -0500, dave wrote:

Theory and practice are quite different.

One day, you're going to eat those words, when you have to decide
whether to follow theory or practice. When I find that they're
different, it's usually because I'm doing something wrong. Also, if
you understand the theory, you can probably figure out the practice
(what to do). However, if you know the practice (i.e. seat of the
pants engineering), you're highly likely to fumble somewhere.

The tower owner should have an inventory of every transmit and every
receive frequency, plus all the standard I.F., plus nearby external high
powered sources. The owner should have cleared each frequency before it
went on the air, and should not add a tenant if doing so would create a
harmful spur to existing users. This is site management 101.

You almost made me spill my hot chocolate. You're correct. Site
managers should do all that. The problem is that all but one of the
site managers that I know of are business types, not engineers. They
hire engineers, tower jockeys, construction crews, and generally run
the business. It's not unusual for me to get a call or email with "I
just signed on to have [insert name] company put their radios in the
building. I'll let you know if anyone complains". This translates to
"Don't burn any billable hours doing calculations until AFTER someone
experiences interference. In short, I get paid to clean up the mess,
not to do the planning. If I want to enforce any engineering
standards, it's also done post mortem. At best, I would get an email
asking where in the building and tower I would guess the new radios
should be installed, usually without telling me the frequencies or
equipment. Interrogating the prospective new customer is something I
try to do, but often they contract out the repeater service to a comm
shop, which claims that they don't know anything because they're
afraid I might steal the customer. I don't wanna talk about
licensing, HAAT calcs, and coordination. Hopefully, your operation is
a bit closer to theory than practice.

I don't care how the WL people run their data streams. Cellular folks
don't like high mountains (except for backhaul).

Generally true. The CDMA crowd doesn't like high mountains for the
same reason they don't like CDMA operation in airplanes. The noise
floor is much higher up high and there are not enough channels
available to handle all the potential users if in a metro area.
However, they do like medium high mountain tops with fairly well
controlled coverage areas. They also like to share site ownership and
management with public agencies to reduce costs.

I know they use very
advanced techniques to hear signals below the noise floor; keeping that
noise floor as low as possible is of paramount importance when you are
looking at 100 mW devices in people's pockets 5 miles away.

100mw is about the maximum that a cell phone can belch. Power control
will usually keep that down to about 30-50mw.

FWIW, Tek has a real nice analyzer that will reverse engineer TDMA spurs.
make time-lapse spectrum analysis, and can even write on a map for you.

Well, the 20+ year old P25 radios are finally being forced into
service by FCC edict, along with various incompatible TDMA
implementations. Meanwhile, cellular is heading towards various CDMA
spread spectrum technologies (CDMA200, WCDMA, LTE, etc), which makes
TDMA look kinda dated. Anyway, I can't afford much in the way of
expensive test equipment and usually borrow or rent what I need. I
haven't actually seen a spur, mix, intermod, or noise on a spectrum
analyzer for many years as the receiver sensitivities are well below
the analyzer noise floor. Same problem with PIM (passive intermod).
It takes quite a bit of power to produce PIM making it almost
impossible to measure PIM while the xmitters are in operation. Trying
to see PIM on a spectrum analyzer is futile.



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

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!