"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.

Sal M. Onella wrote:
A non-linear junction mixes two or more signals ...

I once had a cracked/cold solder joint in a 40m downspout
vertical that seemed to partially rectify the source
signal generating undesirable harmonics on 15m detected
by another ham 1/2 mile away. A jumper across the bad
junction cured the problem.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

"Cecil Moore" wrote in message
...
Sal M. Onella wrote:
A non-linear junction mixes two or more signals ...

I once had a cracked/cold solder joint in a 40m downspout
vertical that seemed to partially rectify the source
signal generating undesirable harmonics on 15m detected
by another ham 1/2 mile away. A jumper across the bad
junction cured the problem.

Yup. Navy ships have from a few dozen to several hundred such jumpers,
depending on ship type. Many are welded but some are bolted, especially if
the object must be moved for use, etc.

I have heard that hams who use a tin roof as an HF ground plane are apt to
generate harmonics.

Sal

"Sal M. Onella" wrote in message
...

"Cecil Moore" wrote in message
...
Sal M. Onella wrote:
A non-linear junction mixes two or more signals ...

I once had a cracked/cold solder joint in a 40m downspout
vertical that seemed to partially rectify the source
signal generating undesirable harmonics on 15m detected
by another ham 1/2 mile away. A jumper across the bad
junction cured the problem.

Yup. Navy ships have from a few dozen to several hundred such jumpers,
depending on ship type. Many are welded but some are bolted, especially
if
the object must be moved for use, etc.

I have heard that hams who use a tin roof as an HF ground plane are apt to
generate harmonics.

Sal


Some of the towers and masts used for broadcasting in the UK have welded-on
tabs that are welded together after the sections are bolted together. A
particular potential problem has existed since Band II was extended up to
108 MHz: that of generation and radiation of IPs in the band above, used for
VOR and ILS.

Chris

Sal M. Onella wrote:

I have heard that hams who use a tin roof as an HF ground plane are apt to
generate harmonics.


As well as those who spend too much time around the wimmin!


- 73 de Mike N3LI -