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Date: Tue, 6 Sep 2011 21:07:09 +0100
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From: Roger Lapthorn <rogerlapthorn@gmail.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Curious local QRM
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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/

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Fascinating Jim. Memories of rusty bolts!<br><br>73s<br>Roger G3XBM<br><br>=
<div class=3D"gmail_quote">On 6 September 2011 20:48, James Moritz <span di=
r=3D"ltr">&lt;<a href=3D"mailto:james.moritz@btopenworld.com">james.moritz@=
btopenworld.com</a>&gt;</span> wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex;">Dear LF Group,<br>
<br>
While doing some tests with 136kHz ferrite rod antennas recently, I had not=
iced a problem with some local wideband &quot;mush&quot; QRM. =A0In some lo=
cations in my garden this was 30dB or more over the band noise level. Movin=
g the RX antenna a few metres made a big difference to the QRM level, so th=
e source was obviously very local, and so this evening I decided to track i=
t down.<br>

<br>
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. Pre=
tty quickly, I was able to localise the source to a shed in my garden that =
I use to store larger &quot;junk box&quot; items, but there are no power or=
 other electrical connections to the shed, so what could be making the QRM =
was a mystery.<br>

<br>
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 E=
MF 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 loo=
p. Turning the broadband loop at right angles to the broadcast stations res=
tored peace and quiet!<br>

<br>
Cheers, Jim Moritz<br>
73 de M0BMU <br>
<br>
</blockquote></div><br><br clear=3D"all"><br>-- <br><a href=3D"http://g3xbm=
-qrp.blogspot.com/" target=3D"_blank">http://g3xbm-qrp.blogspot.com/</a><br=
><a href=3D"http://www.g3xbm.co.uk" target=3D"_blank">http://www.g3xbm.co.u=
k</a><br>
<a href=3D"http://www.youtube.com/user/g3xbm" target=3D"_blank">http://www.=
youtube.com/user/g3xbm</a><br><a href=3D"https://sites.google.com/site/sub9=
khz/" target=3D"_blank">https://sites.google.com/site/sub9khz/</a><br><br>

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