Re: [TowerTalk] Shields

[VR2BrettGraham <vr2bg@harts.org.hk>]

WB2WIK/6 added:

> Actually, in my experience using hardline or 100% (or at least very good
> double) shielded cables in VHF-UHF repeater systems and cavity interconnects
> in duplexers does make a difference, not necessarily in notch depth but in
> generated noise that can bother receivers.  I also see this "noise" issue
> when using many antenna designs that are perfect for simplex operation and
> stink in duplex operation with any power applied.  It seems the oxidation
> between shield strands, which is always there even with brand new cable and
> worsens with age, creates minute noise-generating interfaces (or something).
> I've put up repeaters using brand new mil-spec RG213/U and running 100W
> output power with a very good duplexer had desense problems resulting in
> poor receiver performance.  Replacing absolutely nothing in the system
> except the transmission line, going from 213 to 9913 or hardline or some
> other 100% shielded cable, completely eliminates the noise -- it's gone.
> This has happened to me so many times it cannot be coincidence.

<snip bit on clamped vs welded antenna construction>

With all the energy floating around hilltops, any diode action anywhere is
bound to bring grief.  Certainly sounds like the mechanism here & going
to better shielded cable might have simply gotten rid of a bunch of diodes
& not really improved things because of the greater amount of shielding.

Ditto for the antennas.  I can just imagine what sort of nasty joints there
are on some of our repeater antennas here, as it has been hard to spend
the money on the better ones & everything lives in a very moist, salty &
dirty atmosphere that eats into just about anything in short order.

73, VR2BrettGraham

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