At 02:02 PM 4/30/01 -0400, John Pelham wrote:
>
>I've got a noise problem, and although I perhaps haven't done enough legwork
>yet to be writing responsibly to this reflector, I thought I'd give a shot
>with a description to see if anyone's run across this type of thing.
>(Anyway, this reflector sure has been quiet lately.)
>
>Actually there are (at least) two noise sources. But they have similar
>characteristics. They sound, on AM, just like an electrical arc. Buzzing,
>with an ac-hum component plus higher harmonics. With a product detector,
>they sound more like higher pitched buzzing or hissing.
>
>One of the noises affects primarily 6M, but is audible on 10 and 15. The
>other affects primarily 10M and 15M. The noises come and go independently,
>but are most often present on dry days. They were a real problem during the
>dry winter, and are present less often now that the summer humidity has
>kicked in here in Georgia. They are never present when it's raining,
>although they can be present (very seldom) when it's drizzling. They are
>never present late at night, dry or wet, but they can be present in early
>evening when it hasn't been dark for too long.
>
>Also, the noises seem to be more prevalent when the band conditions are
>good, but that's gotta be my imagination!
>
>I would think that the above paragraph would indicate that the source is
>outside, and I would think probably it's power line noise, but my reading
>indicates that power line noise gets stronger at lower frequencies. These
>noises are not audible below 20M.
>
>Does anyone have any ideas? I've done some limited investigating, but I
>thought I'd keep this first e-mail short. I'd be glad to go into more
>detail about my noise problem; just ask me!
>From what I understand, this sounds like a classic power-line arc,
particularly because it never happens when it's raining. We found one such
on a pole on the edge of my property that was arcing between a bracket
(mounting a lightning arrestor) and a lag screw that attached the bracket
to the wood pole. A woodpecker had attacked the side of the pole and
undermined the wood that the screw's threads were trying to hold to,
causing the bolt to come loose. The part that surprised me is that none of
these parts was connected to any power-line connector, but because they
were in the field of the 7 KV power distribution circuit there was a
voltage gradient between them sufficient to cause the arcing.
Stephanie's problem may also be arcing -- and the time correlation may not
be to power demand but to precipitation -- at least it is a hypothesis
worth exploring.
The ARRL RFI Book has a chapter on powerline noise that is helpful, and
refers to a number of other references that are useful for the ham reader.
73, Pete N4ZR
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