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Author Topic: curious QRM  (Read 4017 times)
KC0GDN
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« on: February 11, 2010, 11:50:18 AM »

About a week ago I began to notice what appears to be an alternating tone showing up on the twenty meters band.  It sounds similar to *very* slow CW and is space roughly every 30 KHz throughout the band and a bit higher.  This tone shows up about 9 or 10 in the morning MST and then typically shuts off abrubtly at around 5:30 pm.  

I haven't done extensive searching yet, but my first impression is that the interference is local since it doesn't vary in strength throughout the day (always about S5-6).

I've experience a varitey of types of interference in the past such as power supplies, power lines, appliances, but this one is damn perplexing.  Has anyone heard of anything quite like this?

Ian
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KH6AQ
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« Reply #1 on: February 11, 2010, 11:58:36 AM »

Can you measure the frequency spacing more exactly? That can tell us alot. Also, is it stable like a crystal or does it drift like a switching power supply will?
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KC0GDN
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« Reply #2 on: February 11, 2010, 01:13:54 PM »

Wouldnt you know it!  No sooner than I posted that question on my mysterious tones then it stopped--at least for now.

If it comes back I will get more exact frequency info, but it was definitely a narrow tone, almost exactly as if someone were transmitting CW simultaneously on a variety of frequencies--extremely slowly.
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K7LRB
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« Reply #3 on: February 11, 2010, 01:59:42 PM »

This may be a shot in the dark. IF.... the frequency is around 14.076, it MAY be jt65. I can see where someone would interpret this as slow speed CW. Also note if it starts at exactly the top of the minute (or within a couple of seconds after the top of the minute). Then see if it stops at around 45 to 48 seconds after the minute has begun.

If the "signal" is on continuously from 9 am to 5 pm, forget what I just said.

Good luck and 73,
de Larry
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KC0GDN
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« Reply #4 on: February 12, 2010, 08:44:22 AM »

It's back now and it's really starting to piss me off because it's exhibiting signs of being intentional QRM.  

Some of the frequencies it appears on are:  14.289, 14.283, 14.250, 14.244, 14.216, 14.205, 14.177.  There seems to be a pattern, but I can't determine if it is actually CW because the tones are so slow (and my CW copy is almost non-existant).  Whatever is tranmitting is obviously broadbanded and is creating incredible harmonics.

Seriously,  I've even set up runs of fibre optics in my house to replace standard ethernet runs to reduce that interference, so I've tackled RFI problems before.

There are no other hams that I know of in the neighborhood and this is so far from the sound of any other kind of RFI I've ever heard.  Is it possible someone is intentionally jamming with broadband CW???
(I know it's *possible*, but what the....!!)
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N3OX
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« Reply #5 on: February 12, 2010, 03:50:52 PM »

  Is it possible someone is intentionally jamming with broadband CW???

No.

You'd have to build a special transmitter to transmit simultaneous signals over more than 100kHz bandwidth.... either frequency multiplying up a narrow comb or wideband frequency modulating one.

You can't do that with any ham radio except maybe a Flex and there you'd have to write your own software.  Unless you've really made some enemies it's not intentional ;-)

Can you record some of it?

73
Dan

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

Monkey/silicon cyborg, beeping at rocks since 1995.
KC0GDN
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« Reply #6 on: February 17, 2010, 09:29:52 AM »

I've recorded about a minute from one of the frequencies being interfered with, but I'm not sure if I can attach the MP3 to this forum.  Does anyone know if this is possible and perhaps how it's done?

Upon further listening, it doesn't appear to be coherent CW, but the actual tones sound about the right type and width
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N3OX
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« Reply #7 on: February 17, 2010, 11:28:28 AM »

You can't upload files to eHam as far as I know, even with the new style, but you can put it somewhere and put a link to it.  If you don't have a place to put it, email it to me at n3ox.dan@gmail.com and I'll toss it up on my website and put a link here.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
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« Reply #8 on: February 17, 2010, 01:34:57 PM »

Here are the mp3 files Ian sent me, renamed so I remember what they are in a few weeks :

Near 14.289MHz

http://n3ox.net/files/KC0GDN_noise_14289.mp3

And near 14.205MHz

http://n3ox.net/files/KC0DGN_noise_14205.mp3



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

Monkey/silicon cyborg, beeping at rocks since 1995.
AE5JU
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Posts: 244




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« Reply #9 on: February 23, 2010, 09:25:42 AM »

In discussing this with a friend he looked at your QTH via Google and noticed the number of nearby businesses.

He commented:  "I suspect he'll eventually find it is some piece of gear with a fault - equipment that runs retail store hours."
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AB4D
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« Reply #10 on: February 23, 2010, 09:51:17 AM »

I agree, the time frame seems to correlate to the operating hours of a business or office. Do you have a portable radio with an S meter? Sounds as though it's time to do some mobile detective work.
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KC0GDN
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« Reply #11 on: February 23, 2010, 10:19:13 AM »

I was actually leaning that way myself.  After listening to the signal on FM I'm beginning to think it's a carrier emitted by some kind of a detection device perhaps.  My guess is that it's retail or maybe even local government, but it could also be a cheap consumer gadget splattering all over the place  

I'm going to break out a little shortwave radio I have and tour the neigborhood once the weather improves.

I appreciate all the feedback I've been getting
thanks for the help!

Ian
KC0GDN
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W6RMK
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« Reply #12 on: February 26, 2010, 07:49:27 PM »

what about the dot clock from a video source. If you'll recall that early IBM PCs used a 14.31 MHz clock oscillator. You could easily have point of sale terminals or some such that use a VGA screen or some sort of NTSC video.. Imagine a video switcher with surveillance cameras which puts some text at the bottom of each screen with the camera number and time/date.  They might use old circuitry derived from the CGA to do the titling. Is the interference coherent on all frequencies (that is, does the modulation appear at the same time on all frequencies) or does the frequency shift around.  You could have a whole bunch of these character generators (one on each camera feed).

Or some sort of instore display system that changes images every few seconds.

A bit of hunting with a loop antenna that has a decent null, and an AM receiver..
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W6RMK
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« Reply #13 on: February 26, 2010, 07:50:43 PM »

speaking of harmonics.. think about the horizontal retrace rate at 15.75 kHz (nominal..most monitors will accept a pretty wide variation)
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KH6AQ
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« Reply #14 on: February 26, 2010, 09:17:30 PM »

Being different and interesting we can refer to this QRM as curiously refreshing.
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