In article ,
Jeff Liebermann wrote:

Beware of the Aluminum foil and shields though. Once it gets wet there is
no stopping the internal corrosion and will generate broadband noise under
power. This includes both RG6 and 9913 and LMR types. They have been
banned from all commercial sites around here.

Amazing. I can see it's possible to create a diode mixer junction
with electrolytic action between the aluminum foil and copper braid.
I've never seen broadband noise from coax cable. Usually the foil is
mylar or polyester coated and the copper is tinned or silver plated to
eliminate any points of contact. Most of the radio sites I've seen
are stuffed LMR-400 and LMR-600. Heliax is better, but also far more
expensive. Incidentally, if you want to generate RF noise and mixing,
there's nothing better than an abraided Heliax outer jacket, with the
copper shield touching the zinc galvanized tower.

If intermod were really a problem with foil shielded coax, then the
common residential CATV distribution system would have a big problem
with intermod and mixes. While the buried coax is semi-rigid with a
solid aluminum shield, the pole to house drops are usually quad
shielded RG-6/u (sometimes with a messenger wire) with foil and braid
shields.

I don't think those cases (commercial/repeater and CATV) are
comparable.

As I understand it (anecdotal report and discussions with other
repeater owners/builders), the problem with LMR-400 and similar cables
occurs when the cable is used in a duplex application, with RF
transmit power and the incoming receive signal being carried on the
same cable simultaneously.

Under these conditions, it doesn't take very much broadband noise
generation at all (from the cable, connectors, or nearby
metal-on-metal contacts) to cause a problem. The transmitter might be
pumping 50 volts RMS of power into the cable, while the receiver is
trying to pick off a signal of less than a microvolt at a frequency
only 600 kHz away. That's a rather critical environment which really
can't tolerate more than a tiny percentage of noise generation in the
cables, connectors, or duplexer.

I don't think there's any problem with using these sorts of cables in
commercial or amateur service in *simplex* applications... they can
handle transmitting, or receiving, just fine. It's only when you try
to do both, simultaneously, through the same cable. that the noise
generation can become a problem.

The same can be true of some classes of antenna problems. The
repeater system I help maintain developed a serious desensitization
problem, due to internal corrosion/oxidation which occurred in the
antenna after several years up in the weather. I doubt that the
corrosion/oxidation effect would ever have been noticed if the antenna
were in service as a normal (simplex) base antenna - it didn't affect
the transmit SWR or the receive sensitivity at all,

I believe that CATV distribution is a rather easier situation for the
cable to handle, since the signals being carried are all of a fairly
similar and moderate power level. The downstream carrier levels
delivered to the home seem to be around 1 millivolt... that's more
than 60 dB higher than what a repeater's receiver has to be able to
pick out when it's receiving a just-usable signal from the outer edge
of its service area.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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