[Amps] crossmodulation in PA ? Thanks for all good??advice

[Robert Chudek]

This misplaced bonding effort applies to bonding around a rotator as well. We already dispelled the myth (on the TowerTalk reflector) of rotator bearnings being welded by a lightning strike. Common sense tells us the rotator is protected by a Faraday shield anyway (the tower).

73 de Bob - KØRC in MN


---------------Original thread---------------

Message: 6
Date: Fri, 7 Sep 2007 07:11:41 -0400
From: "Tom Rauch" <w8ji at contesting.com>
Subject: Re: [Amps] crossmodulation in PA ? Thanks for all good
??advice
To: "k7fm" <k7fm at teleport.com>, "Nils Petter Pedersen"
<la7sl at online.no>
Cc: amps at contesting.com
Message-ID: <002b01c7f13f$e37c3430$640fa8c0 at radioroom>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=original

Hi Colin,

>I mentioned this morning that I had some question about the 
>copper bonding
> across the tower joints, and raised the question that it 
> could create
> corrosion.  Copper has a .35 volt potential and 
> hot-dip-zinc has a 1.20 volt
> potential.  Even though the tower is bolted together with 
> bolts, there is a
> process called "fretting" that can  cause corrosion to 
> occur between the
> metals that are otherwise solidly joined.

My point was that with rare exception bonding a tower joint 
is a waste of time. There are tens or hundreds of thousands 
of sheer pressure on the bolts in a typical cross-bolted 
tower joint.

While I agree dissimilar metals should be avoided, placing 
them across a tower joint is meaningless. How good would the 
diode be if it is shorted end-to-end with what we could 
consider a zero ohm connection? The same is true for 
lightning. Lightning doesn't care a bit if the joints are 
bridged or not.

There are some rumors that bonding the joints helps things, 
but they probably came from looking at early broadcast 
towers with pad joints. In many cases those joints would 
have brazed connections jumpering the joint, but in later 
installations that was practice abandoned after it was found 
unnecessary. This probably spawned the idea Hams should 
jumper joints. Anyone who thinks a couple stainless steel 
clamps with a few dozen pounds per square inch clamping 
force will significantly change the connectivity in a joint 
bearing tens of thousands of pounds force probably hasn't 
thought about the system.

It really is meaningless. The possible exception is in 
systems ready to fall down anyway.

73 Tom