Hi Jim,
Really a good story! I can imagine you walking arround with the
headphones and throwing all the things out of the shed ;-)
So we can see, not only SMPSs and plasma TVs can be sources of QRM, even
well running broadcast transmitters with a clear spectrum can cause
trouble...
Ah BTW, yesterday i bought a 2nd handheld UHF TRX (FT-470) so now i
should be able to be completely /p without a car on LF. Will check that
soon, maybe this weekend :-) Have to build a 3 or 4 ele yagi for the
UHF-LF link...
73, Sefan/DK7FC
Am 06.09.2011 21:48, schrieb James Moritz:
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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