Re: LF: Curious local QRM

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Fascinating Jim. Memories of rusty bolts!

73s
Roger G3XBM

On 6 September 2011 20:48, James Moritz <james.moritz@btopenworld.com> 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

-- 
http://g3xbm-qrp.blogspot.com/
http://www.g3xbm.co.uk
http://www.youtube.com/user/g3xbm
https://sites.google.com/site/sub9khz/