Message-ID: <4E6695C4.8090906@telus.net>
Date: Tue, 06 Sep 2011 14:51:00 -0700
From: Scott Tilley <sthed475@telus.net>
User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:6.0.1) Gecko/20110830 Thunderbird/6.0.1
MIME-Version: 1.0
To: rsgb_lf_group@blacksheep.org
References: <8E521D9BC6FA41FF801372D1EF8145C9@JimPC>
In-Reply-To: <8E521D9BC6FA41FF801372D1EF8145C9@JimPC>
Subject: Re: LF: Curious local QRM
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
X-AOL-SCOLL-URL_COUNT: 0  
x-aol-sid: 3039ac1d604e4e66960367f2
X-AOL-IP: 195.171.43.25
X-AOL-SPF: domain : blacksheep.org SPF : none


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
>
>