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
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|