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
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