Don Moman VE6JY wrote:
> 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
>
Chain link fences can be a really bad problem for this and particularly
long ones that pick up a LOT of RF coupled with all those contact points.
Just as a side note, and it was VHF, not MF, or HF. When we had the
repeater out here in my garage, it worked well for a number of years and
then started having desense problems The base of the antenna was at 90'
and the guys were broken at uneven intervals to prevent resonance on the
HF bands. One evening I took an 8" screwdriver and rubbed it against
the bottom section on one of the guys. This section was securely
grounded and less than 20' to the first insulator. Rubbing that
screwdriver against the still nice looking galvanizing produced enough
noise to completely drown out a 50 watt mobile only a couple miles away.
We do have a couple of chain link fences in this neighborhood, but all
are independent although a couple are nearly 200 X 100 X 200. So far
there is no noise that I've ever noticed from them. Baby monitors and
part 15 transmitters in the ham bands have been more of a problem. OTOH
as soon as I've let people know I can hear every word clearly said
monitors have disappeared as have the old 47 MHz cordless phones.
However I'm holding my breath, hoping no one near goes with the plug in
wireless systems that also run in the ham bands be they 50, 440 MHz or
HF. 4 Gig can be bad enough. Well that and the hope of no near by
audiophiles who put in a big wired speaker system when I run a KW on 160.
I'm thinking of running a 2.4 gig link to another ham a few miles away.
I wonder how many wireless networks are between us.
73
Roger (K8RI)
> 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@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
>>
>>
>>
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