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[RFI] DFing

To: RFI@contesting.com
Subject: [RFI] DFing
From: Tom Thompson <tlthompson@qwest.net>
Date: Wed, 04 May 2011 08:43:02 -0600
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
Here is a link to a Picasa site showing some pictures of some 
inexpensive noise finding tools.

http://picasaweb.google.com/990toedtli/NoiseFindingTools#

The loop using the plastic tubing labeled  Tuned Loop Lite is what I 
would recommend.  The big loop is 16" in diameter and uses #22 wire 
inside the 1/4" inch plastic tubing.  The smaller pickup loop is 
similarly constructed and is 4.5" in diameter.  The big loop is 
resonated with a 220 pF fixed capacitor in parallel with a 5-175 pF 
trimmer capacitor on 40 m.  The loop can also be used on 80 m and 20 m 
with different capacitance values.  The coax is connected across the 
small loop through a choke balun.  The choke balun has about 20 turns of 
coax on an FT114-43 core.

I modified a small Sony shortwave receiver by adding an input attenuator 
and an S- meter.  I did not have a schematic, but most of these 
receivers use a single receiver chip.  I just poked around with a meter 
until I found the AGC line.  I used a dual op amp and a reference diode 
to offset and amplify the AGC voltage.  The input attenuator uses four 
switchable, 5-dB pads for a total of 20-dB attenuation.  You can also 
de-tune the loop slightly  for attenuation.

I have found that most noise radiating from houses is vertically 
polarized.  My theory on this is that both horizontal and vertical wires 
are excited inside the house, but the horizontal polarized signals are 
much weaker than the vertical polarized signals because the horizontal 
wires are  low to the ground which causes most of the signal to go 
straight up.  That being said, the loop will null broadside and peak off 
the edge.  I usually start out by peaking the signal and walking while 
looking for the signal to get stronger.  When the signal is pretty 
strong, I use the null to get a more accurate direction since the null 
is much sharper than the peak.

I have found 2 halogen lights, 4 grow light ballasts, and a leaky 
surveillance video camera with this setup.

If you don't feel comfortable with the loop, excite your own house 
wiring by capacitive coupling a low level signal source into your own 
house wiring and practice outside finding it.

Finding the noise is the easy part.  Fixing it is more difficult.

Good luck.

Tom   W0IVJ
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