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
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