Great story, Jim,
glad you were able to track it down.
73
Hugh M0WYE
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Tuesday, September 06, 2011 8:48 PM
Subject: LF: Curious local QRM
Dear LF Group,
While doing some tests with 136kHz ferrite rod antennas recently, I had
noticed a problem with some local wideband "mush" QRM. In some locations
in my garden this was 30dB or more over the band noise level. Moving the
RX antenna a few metres made a big difference to the QRM level, so the
source was obviously very local, and so this evening I decided to track it
down.
I used one of my small loop antennas with a long coax extension lead going
back to the shack, and a pair of wireless headphones so that I could
listen to the QRM level on the RX in the shack as I moved the antenna
around. Pretty quickly, I was able to localise the source to a shed in my
garden that I use to store larger "junk box" items, but there are no power
or other electrical connections to the shed, so what could be making the
QRM was a mystery.
After emptying out half the shed contents, the QRM source turned out to be
another, experimental, loop antenna that I made some years ago. This was a
1m^2 loop with a broadband preamp fed via a step-up transformer. The local
MF broadcast stations produce enough field strength to induce a few volts
EMF at the un-powered preamp input, where rectification and
intermodulation occurs. Since there are 4 modulated broadcast signals, the
result is a wide ,almost uniform spectrum of noise, some of which is
re-radiated by the loop. Turning the broadband loop at right angles to the
broadcast stations restored peace and quiet!
Cheers, Jim Moritz
73 de M0BMU
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