Message-ID: <1A11EFA89B294E93B5158316DA289E0C@HughLaptop>
From: "Hugh M0WYE" <m0wye@radiouk.com>
To: <rsgb_lf_group@blacksheep.org>
References: <8E521D9BC6FA41FF801372D1EF8145C9@JimPC>
In-Reply-To: <8E521D9BC6FA41FF801372D1EF8145C9@JimPC>
Date: Wed, 7 Sep 2011 08:56:53 +0100
MIME-Version: 1.0
Subject: LF: Re: Curious local QRM
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Great story, Jim,
glad you were able to track it down.
73
Hugh M0WYE
----- Original Message ----- 
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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
>