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Re: [TowerTalk] Shields

To: Tom Rauch <>
Subject: Re: [TowerTalk] Shields
From: "Tower (K8RI)" <>
Date: Wed, 13 Oct 2004 21:30:14 -0400
List-post: <>

Tom Rauch wrote:

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.

It gets even more interesting. The guy lines were insulated from the tower and bonded to the ground posts I was rubbing the screw driver on the lines only 3 feet from the anchors.

It was definitely rectifying the transmitter output on 147.00 as it only happened when the transmitter was up. None of the other bands bothered.

"I think" the noise we never located was probably an abandoned, or unused TV antenna preamp. The area had just gone to cable around that time. It may have still been connected to a TV set. OTOH I guess it could have even been a poor ground on a nearby electric fence.

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.

I don't think we had any problems there. "It appeared" to be external to the system, but appearances can be deceiving. Something external was rectifying the signal and reradiating it covering the input.

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

This would have been about 130 feet from the base of the antenna. The guy line was galvanized wire rope.
Of course this was close to 20 years ago so my memory could be a tad foggy on that. I think you may have still been in the Toledo area at that time.

73 Tom

-- Roger Halstead (K8RI ARRL Life Member) Worlds Oldest Debonair (N833R S# CD-2)


See: for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.

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