[TowerTalk] Shields

[Tom Rauch]

> At any rate, running a tad over 100 watts into the Heliax
with the base
> of the antenna at roughly 90 feet, you could walk up to
any of the guy
> lines near the anchors and completely wipe out a 50 watt
mobile 3 to 5
> miles away by just sliding a 10" screwdriver up and down
the steel guy line.

That's why you never mount antenna near metallic guylines,
and make sure the antenna you use is properly designed to
prevent exciting the tower with common mode current. You
either bond guylines passing the antenna level, or better
yet insulate them.

Noise on repeaters caused by poor connections in antennas
and hardware around antennas is common. My two-way business
maintained and installed dozens of repeaters and we found
all sorts of things causing problems. Despite that, I never
once noticed a problem from using single shield cable. Quite
frankly, I think it is IMPOSSIBLE for single shield to
behave any differently than double shield on VHF or higher,
assuming the inner shield is of similar construction in all
cables or the cable simply isn't defective.

You be amazed at the current on the outside of cables (and
on supporting structures) caused by easy to correct antenna
design flaws, equipment port design flaws, and poor
installation procedures. Problems like that could easily be
blamed on cable leakage.

The repeater out back, for example, has 50 ft of RG-213 in a
lightning retarding and RF decoupling choke. It has RG213
jumpers at both ends, and not a lick of noise problems. I
just tried grounding and ungrounding the connector at the
jumper, and scratching my car keys on the floating cable
connector and there isn't the faintest trace of noise on a
10dB quieting test signal injected from an antenna 500 feet
away.

73 Tom