Strange interference, 27/7 for the past few months

[k4kyv]

Interference is spaced at intervals of approximately 30.475 kHz, throughout the entire HF spectrum - from 80m to 30 mHz.  On an AM detector it sounds like a tone modulated carrier, but careful observation shows two distinct carriers heterodyning against each other; one is unmodulated and the other has a spastic but barely perceptible frequency shift of a few Hz, and each shift is accompanied by a click. The interval between frequency shifts varies, but averages about twice a second.

The higher in frequency, the greater the spacing between the two adjacent carriers thus the higher the pitch of the heterodyne.  At 30 mHz, they are a little over 800~ apart, on 20m. they are a little over 400~ apart, on 40 they are 200+ hz apart, and on 80m about 100~ apart.  Although the signals appear approximately every 30.475 kHz, some frequencies are much stronger than adjacent ones.  There are a few holes in the pattern, where nothing is audible on the expected frequency.

The signal appears to be local, since it comes in on 10m all the time, and the S-meter remains stationary, thus excluding  the liklihood that it is skywave.  The higher in frequency, the stronger the signal.  It is louder than my xtal calibrator on 10m, but on 80m it is almost buried in the noise.

Here are a few of the frequencies I measured that  fall inside the ham bands:

29.998826/29.999660 mHz

14.884912/14.885340 mHz

7.373734/7.373935 mHz

3.618225 kHz

The signal is extremely stable; I notice no drift over a 24-hour period.

It is not something in the house, since I cut the a.c. power at the main utility entrance and the signal is still there.

Anyone ever heard anything like this?

Don K4KYV

[W1GFH]

Don,

VE3HLS has done a great job of cataloguing known and unknown RF noise. See if it matches anything on his page.

http://www.ve3hls.com/noise/rfihome.html

Also see if you can manage to record the noise and post it here.

Joe, W1GFH/6

[K1JJ]

Don,

I listened on all of your listed freqs in various directions and hear no sign of it hear. Not even a faint carrier.

Do the lower freqs have fading and is there a relationship to your sunset/grayline with peaking signals?   You are sure it is NOT an internal birdie I assume... :-) [disappears when ant disconnected]

The 3mhz signal will show you distance by it's afternoon weakness. Also if you have a directional antenna on the higher freqs that will give clues.  If you decide it's local, a drive around with a portable HF RX may uncover it.

Be thankful it's not in the prime ham spectrums where you operate.

Let us know what you discover, OM.

BTW, how do you get the freq measurement with so many significant digits to the right of the decimal point?  Are you zero beating with a hi-fi system to hear a few cycles/sec or some other method?

T

[w3jn]

Don, a more appropriate question might be "who HASN'T seen interference of this sort."  With the proliferation of switchmode power supplies, oscillating high-end TVs, etc., it's just gonna get worse.

It appears to me that you're receiving 30 KHz sidebands of a VHF or high HF oscillator, because the sig is stronger the higher you go.  Then again you could be receiving harmonics that are coupling to a poor (at HF) radiator.  However harmonics, as you know, will decrease in strength as you go up in freq.

Remember Master Vu's DFing rig?  Suggest you find the fundamental with a VHF receiver, build up a small yagi, and go-a-huntin'!

73 Johnny Novice

[k4kyv]

Turned out to be my cable modem.  I just unplug the thing if I want to listen above about 10 mHz (which is very rare).  May have to look into it more seriously if and when 10m opens back up.

[W1AEX]

What kind of Cable modem is it Don? Also, does it have an internal power supply or an external wall outlet supply. I found that the little wall outlet switching power supplies that come with modems, routers, and hubs are quite nasty little HF noise generators that spew out warbling little oscillations that travel up and down the bands. I got rid of them all and built linear supplies to replace them and that resolved it once and for all.

Rob W1AEX