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

Gary Schafer garyschafer at comcast.net
Fri Sep 7 19:17:42 EDT 2007


I don't quite see where you get "tens or hundreds of thousands of pounds of
pressure" on joint bolts on a typical ham tower? The towers are not that
heavy. Joints can easily move especially on smaller towers as the wind
blows. Watching joints on a self supporting tower is interesting as it moves
in the wind.

Direct copper connections should never be made to galvanized material as it
will destroy the galvanized surface.

Often tower joints are bonded to make sure that there is a low resistance
joint for lightning conduction. Even a few milliohms of joint resistance can
allow the voltage to rise high across a tower section with very high
lightning current thru the joint. Tower bolts don't always provide good low
resistance connections as the joint can still move.

73
Gary  K4FMX


> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Tom Rauch
> Sent: Friday, September 07, 2007 6:12 AM
> To: k7fm; Nils Petter Pedersen
> Cc: amps at contesting.com
> Subject: Re: [Amps]crossmodulation in PA ? Thanks for all good  advice
> 
> 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
> 
> 
> 
> 
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