[Towertalk] tower joint Conductivity ???

[Chuck Counselman]

At 7:22 AM -0500 11/20/02, Pete Smith wrote:
>I'd be surprised if 5 percent of the shunt-fed galvanized steel 
>towers out there had any special measures taken with joint 
>preparation.  Wouldn't we have heard about this by now if it was a 
>problem?

I've never heard of it, but I wonder about a so far unmentioned 
consequence of bad joint conduction in a tower: the so-called 
rusty-bolt effect.

Within a couple of miles of my house (which is near the top the 
highest hill around) are a 50-kW and a 10-kW AM broadcast station; 
and several other 50-kilowatters are not much farther away.  The 
vertical component of the electric field at ground level in my yard, 
from these broadcasters, is 0.25 V/m.  This is from actual 
measurement with a commercial instrument; and NEC-4 simulations 
confirm that it is to be expected.

Shortly after moving here I discovered that the 80-meter band (among 
others) was jammed full of very strong intermodulation products of 
these strong broadcast signals.  My ham antenna was a well-balanced, 
center-fed, horizontal doublet, well isolated from my house, with a 
balun, antenna tuner, and several common-mode chokes.  I quickly 
determined that the intermodulation was not occurring in my antenna, 
feed system, or receiver.  Then, using an electrostatically shielded 
B-field sensing loop I explored to find where it _was_ occurring.  I 
found several culprits, but the worst was an electric-power conduit 
(common plated-steel "EMT") that ran from the main circuit-breaker 
panel in my basement to the air-handling equipment in the attic of my 
2 1/2 story house.  Significantly IMO, this conduit had a substantial 
vertical extent.  This conduit had several joints, and these joints 
were nonlinear conductors.  Merely tightening all the screws in the 
joint bushings/clamps cleared up the problem.

A tower with an imperfectly conducting junction could also generate 
AM-broadcast intermodulation products, although I expect that all 
three legs would have to have bad joints.  Has anyone observed 
AM-broadcast intermod from a ham tower?

73 de Chuck, W1HIS