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Author Topic: Source of loud RFI type noise on 3815 KHz ?  (Read 345 times)
K7PEH
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« on: December 09, 2006, 08:00:23 AM »

I am trying to figure out the source of a strong noisy raspy signal that appears on 3815 KHz in the evenings.  First let me describe this noise.  On my Icom Pro III I can view its spectrum, it is about 25 KHz wide and symmetric like an AM signal but with sharp spike like signals about 3 to 4 KHz wide that are spread through the roughly 25 KHz bandwidth.  These signals have a peak strength that follows a rough AM signal envelope with its peak on or near 3815 KHz.  At its peak, the signal is 5 to 10 over 9.  And, about 5 KHz out from peak, on either side, the signal is still about S9.

Right now, at 7:45 AM PST, the signal is not there.  In the evenings, it is usually there but not always.  And, last night, I saw it switch off and then about an hour later it was back.  So, I am obviously it is a device that is turned on or off.  Given the operating hours, I have ruled out things like a fish tank device but I am leaning towards something like maybe a plasma TV. (And, I don't own a plasma TV).

So, last night I got in my truck, tuned the Icom 706 to 3815 KHz and without bothering to retune my screwdriver to the 80 meter band I still had the signal at about S8 or so.  I drove around the neighborhood up to a mile distant from my home and could still copy the signal to various levels though at times it was weaker.  But, a few times I had spikes in the signal strength when I was maybe a mile away so I am thinking maybe there is more than one source -- more than one plasma TV?

So, some questions.

Since I have never heard a plasma TV RFI signal, what does it sound like?  Is there sufficient power to have it heard over the range I described?

Also, I have not found this signal to appear anywhere else on any of the ham bands.  I have not searched outside the ham bands for any harmonics.  However, I have not ruled out the possibility it is a very, very, bad transmitter located somewhere.
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N3OX
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« Reply #1 on: December 09, 2006, 12:00:06 PM »

http://www.arrl.org/tis/info/HTML/rfi-noise/

has some example sound files of various things; unfortunately there's no plasma TV there AFAIK, but maybe you can find your noise.

You might record it and post a sound file somewhere... maybe someone will recognize it.  It's surprising you get it so far away... that suggests something powerline or other utility related to me, though I suppose multiple sources can't be ruled out completely.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
A9KW
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« Reply #2 on: December 09, 2006, 07:19:01 PM »

I have the same noise on 3812 up here in IL
TOM
N9ZV
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KI4NX
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« Reply #3 on: December 10, 2006, 03:53:27 AM »

I hear something like that around 3936 t0 3946 in the  morning about 5 am. It sounds just as you describe your signal.  I have not been able to track it down - mobile station being rebuilt and all.  I'm in Leesburg VA.  Most annoying.

73
Ken
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K7PEH
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« Reply #4 on: December 10, 2006, 09:00:38 AM »

The RFI never made a show last night.  I am suspicious that this may be due to a plasma TV.  I have heard that they can produce strong RFI but I myself have never heard one.  I am making my guess because my next door neighbor, who I think has a plasma, had a dinner party at his house and I suspect that during that time he was not watching TV.  OK, it is a little bit of a wild stab but this RFI does have all the makings of a switched appliance of sorts that follows a kind of pattern suggesting evening "entertainment".  It is almost never there in the morning.

My plan is this.  The next time I hear it, I will photograph the spectrum display and I will record the sound and I will make those available here (maybe via a link to my web site which would be easiest).

The other night when I was driving around the neighborhood, I thought that I was picking up a different source of the same kind of noise.  The fact that others here on this forum report a similar RFI experience seems to say that this source is not unusual.  Again, I am leaning towards a plasma TV.

Strong signal though.  

phil,K7PEH
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K7PEH
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« Reply #5 on: December 10, 2006, 09:16:59 AM »

I am now fairly certain this is a plasma TV.  There is an article that I found on the net that seems to describe the same kind of symptoms.  Here is the link at dx engineering:

http://www.dxengineering.com/TechArticles.asp?ID=%7B1D9AAC83-E591-46D4-86AE-B84F1CBF1D62%7D

phil, K7PEH
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N3OX
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« Reply #6 on: December 10, 2006, 09:44:51 AM »

"I am making my guess because my next door neighbor, who I think has a plasma, had a dinner party at his house"

That's a good thought... hope you can get some resolution if it's the case.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K7PEH
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« Reply #7 on: December 10, 2006, 01:14:51 PM »

A little more looking around and I find this same RFI all over the place.  The RFI is back now as I think my next door neighbor is watching a football game.  The noise came on around 11 AM.

I looked up Panasonic 42 inch TV guide to see what kind of FCC statement that they had.  I repeat it here:

FCC STATEMENT:
This equipment has been tested and found to comply with the limits for Class A digital device, pursuant to part 15 of the FCC Rules.
These limits are designed to provide reasonable protection against harmful interference when the equipment is operated in a commercial environment.
This equipment generates, uses, and can radiate radio frequency energy and, if not installed and used in accordance with the instruction manual, may cause harmful interference to radio communications.  Operation of this equipment in a residential area is likely to cause harmful interference in which case the user will be required to correct the interference at his own expense.


Now, I scanned through the Panasonic document and I did not find anything mentioned about correct usage in order to prevent harmful interference as hinted by the FCC notice above.  Also, how is it that a manufacturer such as Panasonic can get part 15 approval for a device that so blatantly broadcasts RFI over the HF spectrum.  This is worse than BPL.

Also, what is the poor unsuspecting consumer supposed to do with a device that radiates RFI such as this Panasonic plasma TV?  Do they put a full Faraday cage around it or maybe around their media room (OK, whereever they keep their TV set: family room, basement, living room, etc.).
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N3OX
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« Reply #8 on: December 10, 2006, 06:30:15 PM »

 "Also, how is it that a manufacturer such as Panasonic can get part 15 approval for a device that so blatantly broadcasts RFI over the HF spectrum."

They can get away with it because it generates complaints that are very few and far between.  Part 15 emissions limits are really useless as far as protecting ham radio; the protection to licensed services comes in when a licensed user brings a complaint.  The licensed service "wins" in a legal sense, but of course, that's not so useful at friendly neighbor relations.  

I think there should be emissions limits and shielding requirements that are sufficent to protect ham communications under all but the most pathological circumstances (satellite dish coax is an electrical quarter wavelength on 40m, that kind of thing), but part 15 limits aren't really sufficient.  We don't have any financial clout in this regard... why would the manufacturers of these things care?

"Also, what is the poor unsuspecting consumer supposed to do with a device that radiates RFI such as this Panasonic plasma TV?"

It might still be conducted to the house wiring and cable on 80m, so good common-mode chokes on the coax and power cord might help some.  Hard to say for sure, but even a giant-screen TV is going to be a pretty inefficent antenna in and of itself at 3.8MHz.

I think the particularly distressing thing about plasma TV RFI problems is the price of the darn things.  You drop $4999 on a television and your ham neighbor comes over and starts saying that something's wrong with it... still, if you've got a good relationship with your neighbor, just talk to him about it and have a fistful of ferrites on hand.  He might understand, one gadget-lover to another, but it would be a much easier fix if it was the power supply for his $35 wireles router, just swap it out.

The ARRL's site has a suggested plan of action that makes sense... they have some volunteers that specialize in RFI resolution issues who you can call for advice and who are willing to act as intermediaries in situations like this, where it could be messy to tell someone their expensive TV is ruining your weird hobby.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KB4QAA
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« Reply #9 on: December 11, 2006, 03:46:14 PM »

One trick to help in locating a signal while mobile, is to 'detune' your radio when you start.

Don't stay centered on the offending signal, but tune off frequency till you can just barely hear it.  Then start moving around.  If the signal gets stronger, tune off frequency some more.  When you lose the signal, DON'T TOUCH THE DIAL, but note your location and turn around until you gain the signal again and keep going till you lose it again.  

You will quickly locate the source.  This is the same technique used for locating emergency beacons on aircraft and boats when you don't have a direction finder.

Good luck,  bill
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W5ONV
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« Reply #10 on: December 19, 2006, 06:32:15 AM »

 I hear it 24/7 here in Dallas on 3885.It seems to be in the AM mode.It is a pulsating sound. It makes that area of the band useless..
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