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[RFI] A Switcher it was!!

To: Dan Kovatch <w8car@buckeye-express.com>
Subject: [RFI] A Switcher it was!!
From: Pat Thurman <k7kr@earthlink.net>
Date: Mon, 22 Dec 2003 11:40:21 -0700
List-post: <mailto:rfi@contesting.com>
I have recently been plagued with a very similar problem and just discovered the source! Here's the story...

Several weeks ago I started noticing a peculiar almost time related broadband interference largely confined to 40m. It sounded very much like white noise and was S9 in strength. Everywhere. From below 5 mhz to over 9 mhz. Not sure but that the range is maybe just a reflection of the bandpass filter characteristics of my radio. After seeing Dan's post I started listening more carefully to the noise and sure enough I noticed a number of 'growlers' just like he posted.

In the course of hunting the noise, I had noticed that it seemed to come on at what seemed to be almost a predetermined time. Eventually, I found that my son's clock radio was going off at about the same time. A couple of times, I was able to get the noise to go away simply by unplugging the clock radio so I began to think this was the source. About this time, W6WRT posted that it sounded like a switching power supply. Armed with that, I continued looking. I noticed that I could get the pitch of the 'growlers' to change slightly when I would turn one particular ceiling fan on and off.

To make a long story short, it turned out to be my wife's computer power supply. It seems almost like the particular circuit in the house was acting somewhat like a tuned circuit. A disturbance anywhere on the circuit could trigger some sort of instability in the power supply and make it start spewing this noise.

Since it's time to upgrade her computer anyway, I won't bother with just swapping out the supply so she'll be happy! And I won't be plagued with any more noise for a while.

Life is good!!
73, Pat K7KR

Dan Kovatch wrote:

I have noticed a strange  'group' of signals mainly centered on the 40 meter
band. The signal sounds a bit like power line noise but may actually be at a
higher frequency-kind of a 'growl' more than a raspy 120 hz . The 'noise'
seems to peak every 140KHZ and is detectable from below 6 mhz to about 8
mhz-this range may be greater but I haven't tuned more spectrum yet  It does
appear to fade quickly as the freq increases from 8Mhz on. The strongest
peaks are about 20 over s9 in the 40 meter band (which may be due to a
resonant antenna,etc) The noise is relatively narrow band at about 5 KHZ max
before beginning to fade out. Using 500hz filters in the AM position the 40
meter peaks seem to be at (6880), 7020. 7160, 7300. The frequency of these
peaks varies over time by about 5 khz. This interference disappears once in
a awhile but I have found no apparent patterns

I have killed every circuit in the house and am pretty sure it does not come
from inside the QTH (it's always possible I overlooked something) . The next
step is to put the 706 in the truck with the 40 meter ant and 'cruise' the

Any wisdom or ideas will be greatly appreciated.

Merry Xmas and Happy New Year!

Dan Kovatch
zone 4 and 8/ohio/EN81oj

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