Good news!
I looked at the scope waveforms of the demodulated audio at 14MHz and at 150kHz
carrier frequencies. It turns out that there is a very pronounced
negative-going spike on the waveform when the carrier frequency is below about
1MHz. Above the AM broadcast band, that spike is gone. Aside from that spike,
the waveforms are identical. Until I looked closely at the waveform on the
scope, I was fooled by the sound because the spike at low frequencies caused
the noise to take on a completely different sound than what I hear at 14MHz.
The spike [and the whole demod waveform] repeats at 120Hz.
At this point, I am confident that the noise I hear with a handheld radio is
the noise we need to chase down. My TH-F6A isn't very sensitive at low
frequencies. I am going to look for a better radio to do the listening with.
Or, a better portable DF antenna for the HT might do the trick. I will try the
external loop idea.
Thanks again to everyone who has contributed with tips and suggestions - I
think they've helped with this progress.
I think the noise is coming from one of two neighbors' houses. One has given me
the ok to do the whole-house power-down test later today. The other has said we
can't do it for quite some time. Hopefully, the problem is in the house we can
gain access to.
73,
Jim
> Hmmm ...
> also very loud at 78.5kHz and at all multiples of 78.5kHz right
> up into the AM broadcast band.
We can work with this.
Any ham who is tying to track down 'noise' should have
at his disposal a battery-operated AM radio for the utility
of its highly directional ferrite 'loopstick' antenna.
While not a panacea, and you might have to 'watch out'
for coupling to fields as opposed to a 'propagated' signal
at AM broadcast frequencies, but stronger in these cases
still means 'closer to the source'.
I have hiked around in the past checking 'pole' noise by
holding the AM receiver in vicinity of the ground wire
running down the pole, but I digress ... if this signal is
receivable on the AM broadcast band I suggest a little
recon using the AM receiver in the car!
I have also had success using a consumer-class
transistorized portable battery-op AM/SW receiver
that utilized a ferrite rod antenna up through 6 MHz; this
has worked well DFing 80 Meter noise sources, one of
which was 3/4 mile or so distant (a power pole power-
factor correction cap bank complete with timer-controlled
switch on a 3-phase 14 KV line)!. Another ham had gone
out using a 2M rig and a beam and never found it; I
noticed that the amount of energy from this source was
considerably less in the 100 MHz and up bands
compared to that in the SW spectrum.
And FWIW - my hall light, w/an incandescent wired up
to a remote control BSR X10 light dimmer module is
only noisy with the light on and is quiet with the light off.
Jim P // WB5WPA //
----- Original Message -----
From: "Jim Idelson" <k1ir@designet.com>
To: "RFI Reflector" <rfi@contesting.com>
Cc: "CQ-Contest Post" <cq-contest@contesting.com>
Sent: Thursday, February 14, 2008 5:13 PM
Subject: [RFI] RFI Frustration
> Well, it's the night before the contest and the noise is still there.
>
> It's a 24/7 noise and I can DF it in one direction with yagis on 40-10
meters. It's pretty broadband and it has a sputtering sound, somewhat like
this light dimmer noise -
http://www.arrl.org/tis/info/HTML/rfi-noise/noise/noise-files/lightdimmer.mp
3
>
> found at
> http://www.arrl.org/tis/info/HTML/rfi-noise/appliances.html
>
> Is a noisy light dimmer going to be a constant source or does it change
with load and when turned off?
> I had been listening for this at high frequencies, but when I saw that
this is something that has a baseband frequency in the 150kHZ range, I tuned
my big rig in the station down there on a 160m dipole, and there is a very
loud noise at 157kHz. It is also very loud at 78.5kHz and at all multiples
of 78.5kHz right up into the AM broadcast band.
>
>
> Suggestions for better DF'ing techniques? I've been outside around the
neighborhood with my HT, sniffing power meters, wires coming down he power
poles, etc. There are some hot spots, but nothing like a smoking gun.
>
> I have a little time tomorrow to sniff around the neighborhood some more
to see what I can find. I've been in the two closest house with the HT, but
found nothing.
>
> Wouldn't it be cool to find and eliminate this just in tme for the
contest?
>
> 73,
>
> Jim Idelson K1IR
> email k1ir at designet.com
> web http://www.k1ir.com
>
<
Jim Idelson K1IR
email k1ir at designet.com
web http://www.k1ir.com
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