At 6/11/2005 10:06 AM, you wrote:
>Ray,
>
>I've seen the same symptoms occur when an inexpensive multi-band ham
>antenna is
>used, especially when the antenna is fairly close to the repeater and
>single-shielded coax feedline connects the duplexer to the antenna.
Multi-band antennas & single-shielded coax have nothing to do with
desense. Most amateur grade multi-band antennas duplex just fine out of
the box. What happens afterwards depends upon the environment in which
it's installed & how well it's weatherproofed. There are problems with one
or two specific models (Cushcraft and/or Hustler) which have been
previously discussed here. The problem with braided coax is the eventual
oxidation of the copper braid, which will cause desense; silver-plated
shielding does not have this problem. For antenna feeds it makes no
difference if the shield is single or double, only if it's silver plated or
not. As it turns out, you will probably have a hard time finding any
single-shielded silver-plated braided coax, but if you did it would work as
well as RG-214 for an antenna feed. The important point here is the
opposite case: double-shielded copper-braided feed will NOT work for
antenna feeds!
>Try using a good, broadband antenna that is known to perform well in duplex
>operation, and use Heliax or RG-214/U coax as feedline.
Good points, just make sure the RG-214/U is silver plated. As others have
previously pointed out here, there is some "RG-214 TYPE" coax out there
that is double-shielded but NOT silver-plated. Using that to feed your
duplex antenna will also eventually result in desense.
Bob NO6B
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