[TowerTalk] Shields

[Tower (K8RI)]

Tom Rauch wrote:

>>At any rate, running a tad over 100 watts into the Heliax
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>with the base
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>>of the antenna at roughly 90 feet, you could walk up to
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>any of the guy
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>>lines near the anchors and completely wipe out a 50 watt
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>mobile 3 to 5
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>>miles away by just sliding a 10" screwdriver up and down
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>the steel guy line.
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>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.
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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.
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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
>away.
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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
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-- 
Roger Halstead (K8RI ARRL Life Member)
Worlds Oldest Debonair (N833R S# CD-2)
www.rogerhalstead.com