[TowerTalk] BCB Intermodulation

[Don Moman VE6JY]

In lieu of a spectrum analyzer. a portable SW rcvr tuned to one of the IM
frequencies is quite adequate to show a change if you manage to physically
upset the source of the IM.  The chain link fence is absolutely FULL of
galvanized contact points, none of which are solidly connected,  and is
certainly one area I'd look at first.  There is also a long continuous
 heavy wire usually at the top (and perhaps at the bottom of the fence) that
would make a pretty effective gatherer of the MW energy as well.   When the
ground wasn't frozen, each of the fence pipes would have provided some sort
of ground, but now that they are now suspended in solid frozen earth it is
less so, perhaps allowing the MW RF to build up enough to cause the
problem.

Also as Dan N3OX pointed out, ball bearing in rotors can be a source - and
extend that to your mast bearings as well.   But that would usually change
the noise as you rotate something, easy enough to check.  That isn't a
problem I've seen much of, but then I'm not that close to the MW
transmitters.

Check out that fence first.... no climbing required.

73 Don




On Fri, Jan 9, 2009 at 5:23 AM, Ed Richardson <ed_richardson at shaw.ca> wrote:

> Thanks for the replies gents.
>
> It looks like the problem started several weeks ago. I should mention
> that the tower is a 50' freestanding is brand new, just installed this
> past summer. No corrosion or loose joints (at least I hope not). All
> feedline and connectors are also brand new and well weatherproofed.
>
> Since the temperature has not risen above 0F for the last month, I have
> not done any tower climbing to look for problems. The problem may have
> started with the sub 0 degree temps but I can not be sure.
>
> High power transmissions do not reduce/eliminate the intermod.
>
> Living in a suburban area with 55' wide lots.
>
> As I type this, I am wondering if the 8' tall, 1/4 mile long chain link
> fence that borders all the house on my street could be a suspect.
>
> 10kw AM Transmitters are 8 and 12 kms away.
>
> Hopefully it warms up soon to see if the problem fades.
>
> Ed
>
>