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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|