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Author Topic: Signal Source ever 77kHz on HF  (Read 16084 times)
N4MB
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Posts: 15




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« on: September 24, 2013, 10:43:45 PM »

Here's a weird noise that is on almost all the time - peaks 77kHz apart at -80dBm from the broadcast band to 30MHz.

Two spectrum displays, the upper is zoomed in, lower zoomed out.

https://www.dropbox.com/s/0zqbgtvl80noe02/noise%20every%2077%20KHz.JPG

Frustrating... goes away sometimes on no apparent schedule, not it my house, but I'm in an urban environment.

Ideas before I call the power company?

Thanks and 73,

Mickey N4MB
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WX7G
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Posts: 6321




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« Reply #1 on: September 25, 2013, 06:12:21 AM »

If the frequency drifts the noise source is a switching power supply and if the frequency does not drift it is a PC video monitor.

The noise source is not due to any equipment owned by the power company but if they have an RFI engineer they can locate it.
« Last Edit: September 25, 2013, 06:15:17 AM by WX7G » Logged
K0OD
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Posts: 2591




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« Reply #2 on: September 25, 2013, 08:11:36 AM »

Did that graphic come from one of the new Flex radios?  Nice use of a graphic to show a problem.

I had just about the same thing a few years ago. Drove around with a portable SW radio in the car. The signal peaked S-9+ about 1/2 mile away in a home. Some charger most likely. Soon afterward it stopped.

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N4MB
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Posts: 15




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« Reply #3 on: September 25, 2013, 10:42:41 AM »

>> Did that graphic come from one of the new Flex radios? 
Yes, Flex 6500

>> Nice use of a graphic to show a problem.
Thanks! At this point in the development cycle, it is a better spectrum analyzer than an HF transceiver though...

73,

Mickey N4MB
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K0OD
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Posts: 2591




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« Reply #4 on: September 25, 2013, 01:45:53 PM »

Here's a shot from my Flex-5000 panadaptor showing a remarkably similar rattling noise

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N4MB
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Posts: 15




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« Reply #5 on: September 25, 2013, 02:26:56 PM »

Indeed.

I can't make heads or tails out of that one, either. Wait. I can't make tails out of it. And I think I see the outline of a rat at that first peak...
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W8NF
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Posts: 53




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« Reply #6 on: October 11, 2013, 12:39:28 PM »

Here's a weird noise that is on almost all the time - peaks 77kHz apart at -80dBm from the broadcast band to 30MHz.

Two spectrum displays, the upper is zoomed in, lower zoomed out.

https://www.dropbox.com/s/0zqbgtvl80noe02/noise%20every%2077%20KHz.JPG

Frustrating... goes away sometimes on no apparent schedule, not it my house, but I'm in an urban environment.

Ideas before I call the power company?

Thanks and 73,

Mickey N4MB

It is VERY local to you.  The only signal sources that would have such uniform strength all the way to 20MHz have to be within a wavelength of your antenna.  And, those frequencies are way too high to be propagated in through the power line.

Cell phone chargers can do that
iPod/iPhone chargers
Cable/FiOS modems and routers, and their power supplies
The SMPS in any electronic appliance in your home (not all SMPSes vary in frequency, some actually use crystal controlled clocks)
The SMPSes inside the devices themselves (modern consumer devices all have them)
Don't forget your ham gear: any radio that can handle a negative-going ALC signal from an amplifier has an SMPS inside to generate that negative voltage.  I can sometimes hear those SMPSes on the receiver they're part of.

Diagnostics:
Turn off ALL battery operated devices in your home by removing the battery.  You cannot assume they're off until the battery is removed.
Operate your receiver from a battery.
Start turning off breakers in the house, looking for a change.

I once had a coffee pot generate hash like that - and it was only "barely" electronic, in that it had a clock so you could set a morning brew time.

I haven't scoped it out yet, but at my present QTH, I do suspect something outside the house.  The power pole out front has a cable TV box on it and it emits a very audible acoustic buzz...no doubt there are SMPSes in that thing.

GL ES 73,

Dave W8NF
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WX7G
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« Reply #7 on: October 13, 2013, 07:14:41 AM »

Someone said "...those frequencies are way too high to be propagated in through the power line."

BPL sends HF data up to 1500 meters on AC power lines.

        http://en.wikipedia.org/wiki/IEEE_1901



« Last Edit: October 13, 2013, 07:16:45 AM by WX7G » Logged
W8NF
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Posts: 53




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« Reply #8 on: October 14, 2013, 12:47:20 PM »

Someone said "...those frequencies are way too high to be propagated in through the power line."

BPL sends HF data up to 1500 meters on AC power lines.

        http://en.wikipedia.org/wiki/IEEE_1901





I was involved in the testing of early BPL chips....late 1990s.  At that time, nothing above a few hundred kHz could make it through a power line transformer.  Bypass networks had been designed and had to be manually (and expensively) installed in order for those frequencies to make it past the transformer.

I suppose, if a powerline is BPL-equipped then such signals could indeed, propagate down the line.  But, to my knowledge, BPL has not been broadly distributed in the United States.  Those bypass elements were the costliest part of BPL deployment, according to my customers.

Dave W8NF
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W7VO
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« Reply #9 on: October 15, 2013, 04:54:23 PM »

Indeed.

I can't make heads or tails out of that one, either. Wait. I can't make tails out of it. And I think I see the outline of a rat at that first peak...

Looks like the signal received from one of those fancy "snake on ground" Beverage antennas......

Mike, W7VO
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KB7IUV
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Posts: 5




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« Reply #10 on: November 11, 2013, 02:40:21 PM »

I have a Flex3000 and I used to get a similar "signature" of noise on my radio about 66KHz apart.  Although the Flex3000 has no ability to plot a frequency spectrum that wide.  But I did have that same "suspension bridge cable" effect.

You really need to verify it is not your house by turning off all breakers except your shack.  No need to try to operate on a battery if the signal goes away from some other breaker.  If still there then you can try to operate your rig on battery (or run a long extension from neighbor if possible to power your radio).

I found that the source of my noise was my Cox HD Cable box.  I opened it up and put as many ferrite beads as I could inside and was able to greatly reduce the noise but not eliminate it.  I even tried building a Faraday cage for my cable box with no effect even though I was able to suppress all my cell phone bars with the phone inside the cage.   I no longer have the suspension bridge cable "look" but simply a small bump where each noise peak used to be.  But the signal must be very strong because all other RFI I have found was able to completely suppress using ferrites.  If I recall, the noise peaks were at least 20db above the noise floor for me.  Now I have about a 4db bump that I am able to copy signals even when they occur precisely on the noise frequencies.  When TV is off and cable box is not recording a show the noise goes away.
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