LF: Curious local QRM

["James Moritz" <james.moritz@btopenworld.com>]

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