[RFI] DFing power line interference

Fri, 5 Jan 2001 08:37:49 -0600

Pete,

Yes, I have done some work (professionally related) with inexpensive AM
sets for noise or signal source detection for purposes other than tracking
power line noise.  I have read the QST article that was mentioned in your
other posting, and have the following comments:

1.  With most any AM receiver, all that you need to do is find the AGC bus
and take your metering from there.  The trick is finding that bus.  Some
sets use a "one chip wonder" for the guts, and others use a mess of
discretes.  Try to use one with discretes, as the AGC bus will be easier to
access.

2.  Once you find the AGC, take a small perf board (which could mount over
the rear meter terminals) and install a dual op-amp (general purpose type,
but best if you use one designed for single supply operation;  i.e. NOT a
747-type) and associated parts to provide some gain and possibly, an offset
adjustment.  You can power this from the same 9V battery (or pen cells)
that run the radio.  There are numerous "cookbooks" that give basic op-amp
circuits to achieve this.

3.  Now, for the bad news:  the "cheapo" sets that I worked with ALL
suffered from poor dynamic range and a total lack of shielding.  So, I
wound up building an attenuator and placing the radio guts into an aluminum
box, just like in the QST article.

4.  You might be better off taking one of the many currently available 2m
rigs that offer AM band receive and use that.  If the particular rig
happens to be a mobile, you could power it with one of those belt-clip gel
cell packs (since you are using RX-only, anyway), and you will still be
portable.  At least the ham rigs offer (usually) a metal case and an
S-meter.  However, my experience with most S-meters these days is that they
also suffer from incredibly poor dynamic range.  I have run tests on all of
my HTs and rigs for S-meter range, and it is unusual to find one with more
than 50 dB of usable meter dynamic range.  (I have rigs from Yaesu, Alinco,
and Icom - none conform to the "old" S-meter standard of S9 = 50 uV, or
where "10 over 9" really is 10 dB greater than S-9;  most have a swing of
only 3 to 5 dB between S9 and S9+10!  One rig goes from S1 to S9+40 with a
total input change of only about 30 dB!  My best receiver for S-meter
tracking and relative accuracy is the Hammarlund HQ-110, dating from 1960.)

5.  So, assuming you have (or borrow) a 2m rig with AM air receive and an
S-meter, you will still need an attenuator, for all the reasons stated in
the QST article.  Rather than building one, you might be able to find one
at a hamfest.  Stuff like old attenuators from H-P, Weinschel, etc.,
usually go real cheap.  One caveat:  "sniff" it in an attempt to confirm if
some previous owner cooked the guts by accidentally transmitting thru a
poor ol' receiving attenuator that was rated for 1 watt, max.
Alternatively, ask around any of your ham pals who are VHF/UHF fans;  most
serious folks on the higher bands have high quality variable attenuators
laying around for tweaking their pre-amps and converters.

Good luck.

73, Dale
WA9ENA

Pete Smith <n4zr@contesting.com>@contesting.com on 01/04/2001 05:07:18 PM

Please respond to Pete Smith <n4zr@contesting.com>

Sent by:  owner-rfi@contesting.com

To:   rfi@contesting.com
cc:

Subject:  [RFI] DFing power line interference

I'm seriously considering investing in a VHF AM receiver for DFing power
line noise.  Has anyone tried either the Radio Shack 12-615 or the 12-456
for this purpose?  The prices are sure right.  I've searched the QST/QEX
index looking for articles on modifying these or other VHF AM radios to add
a signal strength meter, without success.  Also no specific how-tos, that I
have found, in the ARRL RFI Book.  Anyone already been there/done that?

73, Pete N4ZR
Contesting is!

The World Contest Station Database
is back up and running at
http://www.qsl.net/n4zr

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