Dave Platt wrote:
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.

Yeah, I've seen some of that. I spent several days finding the
culprit on a UHF repeater where the over the air rx sensitivity varied
substantially and erratically. Watching the IF noise level, showed it
going up and down with the sensitivity changes. In frustration, I
grabbed a broomstick and beat on the accessible coax cables. I
eventually found a length of 9913 coax that was apparently involved. I
replaced it, and the noise problem disappeared. Inspecting the coax
carefully, the outer jacket was slightly corroded and white dust was
visible.

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.

Yep, but the mechanism isn't obvious. All transmitters belch some
level of synthesizer or oscillator noise. The notch type duplexer
does a great job of getting rid of the noise in the receiver bandpass
produced by the transmitter. However, when there's a diode present,
the very low level tx synthesizer spurs, or other signals picked up at
the antenna, mix with the tx synthesizer noise, and land on the
receiver frequency. It's intermod, but instead of dealing with a
collection of individual frequencies, it deals with broadband noise.
The same mechanism is a problem in broadband mux, broadcast, and
cellular systems.

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.

Requiring Heliax is a good but expensive solution. Requiring Heliax
on initial installation makes is somewhat less expensive.

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

Sorta. I have problems securely attaching connectors to RG-213/u.
Unlike the rigid and semi-rigid cables, crimp type connectors are
problematic. In addition, much of the RG-213/u floating around is NOT
silver plated, but bare copper. That will corrode, and form diodes.

I also don't like the attenuation of RG-213/u. 5.1dB/100ft at 450MHz,
while LMR-400 is 2.7dB/100ft at 450MHz.

On 11 Sep 2011 13:32:15 GMT, dave wrote:
Interesting. I always hear people bragging about LMR. Are we using the
term "Heliax" generically? Is semi-flex no good too, (It's all aluminum
and brass metallically isn't it?)

A picture is worth 1000 words:
Heliax:
http://www.google.com/search?tbm=isch&q=heliax
LMR type coax:
http://www.google.com/search?tbm=isch&q=LMR+coax

I've never heard of semi-flex. Perhaps you mean semi-rigid coax,
which includes aluminum outer jacket coax as used in the CATV
industry? The coax is fine, but where it transitions to a brass or
silver plated connector, there's a problem. In general, it's a bad
idea for reducing PIM (although I use CATV coax because I'm cheap).

Did your guru make a profit on the replacement cables?

Guru's do not stay in business very long unless they're profitable.
Even the glorified poverty style of guru has to eat.

How do you know a
$15 can of Cramolin wouldn't have helped just as much?

Cramolin is now DeOxit and has been reformulated.
http://store.caig.com/s.nl/sc.2/.f
It's a total disaster on RF connectors because it contains oleic acid,
which is great for removing oxides from electrical contacts, but
equally good at rotting off the plating from connectors over long time
periods. The reformulated DeOxit allegedly contains a different
anti-oxidant, which allegedly has the same effect.

Tinned copper braid is OK, no?

Dunno. I never use tinned braid except for some semi-rigid microwave
coax, which is quite stiff. Most often I see tin plated braid. Since
tin is not magnetic, there's no PIM problem.

http://www.corrosionist.com/Corros1.gif

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