Hi Jim
I had a similar experience on my 136 RX system after installing the VLF
antenna and preamp... The solution for me was installing an audio
unbalanced to balanced isolation transformer on the feed line of the VLF
system. My soundcard has the provision for balanced input and I didn't
have a pure unbalanced/unbalanced isolation xfmr to try...
This solved my 'mushy' noise on 136!
73,
Scott
VE7TIL CN89dk
On 9/6/2011 12:48 PM, James Moritz wrote:
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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