"Bob Bob" wrote in message
...
Kind of off topic but maybe interesting.

In Sydney in I guess the late 70's two broadcast FM stations came online
on 105 and 106MHz (Approx). Their antennas were both mounted on one of
the big TV towers in the northern suburbs.

Individual tests proved positive, good range etc etc. When they were
both turned on however they caused severe interference to the AM BC band
(1MHz or so) and also the CH10 TV (212MHz or so). As I remember hearing
the fault was a rusty rivet holding some tower sections together.

This information is anecdotal. I havent verified it!

Oh, yes! The "rusty bolt effect" is well known to those of us who have
worked US Navy EMI problems. I did it off and on for almost thirty years.
A non-linear junction mixes two or more signals and the structure involved
reradiates the sums and differences of the fundamentals and their harmonics.
The structure can be any topside metallic item or collection of items.

The result is a comb of frequencies which can cause widespread interference
literally for miles. In some cases broadband noise is also present. In
really bad cases, HF receive is effectively non-functional.

The solution is to bond or insulate everything topside in areas of high RF
energy -- eliminate as many non-linear junctions as possible.