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Old 03-12-2018, 5:11 PM
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Default SDR VHF Interference

Alright well I guess it's my turn to ask as i've exhausted all of my ideas. Recently in the last 24 - 48 hours some sort of VHF interference has shown up on my sdr. It causes the noise floor to bounce up and down every couple of seconds by about 2-3 db. Nothing has changed in my office and i've shut down multiple devices trying to pinpoint the source and nothing has worked.

I'm wondering if anyone else has experienced such a thing. I've had solid interference before from LED lighting that i've since corrected and spikes but nothing in the form of an on - off wave like this. The darker blue at the top is after I shut the bias-t LNA off. The three brighter columns aren't related to this, I think they're FM interference even though i'm using an fm stop filter.

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Old 03-12-2018, 6:08 PM
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A few questions:

1. When you disconnect the antenna cable from the receiver does the interference cease?
2. Is your antenna outside or indoors?
3. What type of antenna are you using?

I have been using SDR dongles for many years and yes occasionally the noise floor jumps up erratically but eventually it ceases.
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Old 03-12-2018, 6:57 PM
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The interference goes away when the antenna is not hooked up. It's a discone inside in the attic on the other end of the building about 40 feet away. I found it's even worse on the ham bands.



The signature almost reminds me of a battery charger I have that would mess up vhf big time. I've unplugged all of that though and nothing else has been added or removed from the building.
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Old 03-12-2018, 7:10 PM
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Looks like wideband noise from a poorly filtered/shielded digital device. I would get a rubber ducky antenna, a USB extension cable, and a laptop, and walk around the building (and neighborhood if need be) to locate the source of the noise.
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Old 03-13-2018, 5:48 AM
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Thanks for the additional info. The duty cycle of the interference (every 2 -3 seconds) is interesting. Devices like wireless weather stations and other remote wireless monitors have short duration transmissions but most are low power and are located in specific areas of the spectrum (ISM bands) so that can be ruled out.

Are you the only occupant in the building, is it possible another business is the cause?
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Old 03-13-2018, 12:44 PM
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I'm the only occupant in the building. A thought came to me today when I got back in here, i've only had this airspy for a couple of days now and before that I was using an rtl receiver which had a noise floor of no greater than -60 db. This interference never peaks higher than that so it's possible that whatever this is has been around all along here and I just never knew it because it was below the noise floor.

When I turn on my fluorescent lights it makes the problem worse in the same signature which makes me think someone else is using fluorescent lights in the neighborhood and i'm picking up a weak interference from them. I'll do some more investigating later with more equipment.
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Old 03-13-2018, 12:52 PM
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Assuming you don't have people on life support in your building ( ), and are running from a laptop, you could kill power to your building for a couple of minutes to ensure it isn't anything in your building (that isn't battery powered, anyway). If that kills the noise source, then do each breaker one by one until you find which circuit the offender is on. That could at least help narrow down the source if it is in your building.
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Old 03-13-2018, 2:31 PM
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Alright well i'm pretty sure I found the culprit. The only other building close to me is a park and recreation maintenance building across the street and I know from being in there that it's full of fluorescent lights. The interference was faint but present all day today but right when they were getting ready to leave for the day at 3:30 I made it a point to watch the sdr. Almost three minutes before they left the interference spiked in the same pattern on and off, sometimes really bad and then not so bad but still there. Then right before they all got in their vehicles to leave it went away.

I can only guess they were clearing the building or checking doors before they left and that meant they were turning the lights on and then off in different rooms before they left. As for last night they probably accidentally left one on which contributed to the faint signal.

Just for fun I then turned on my own fluorescent lights in my office and the same exact signal came back in the same pattern.

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Old 03-14-2018, 5:44 AM
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Good detective work. Glad you found the source.
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Old 03-17-2018, 11:54 AM
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The real disappointing thing is about a year ago I went through my office and replaced the old magnetic ballasts with electronic ones very happy that I was able to get rid of the hum. Little did I know I created another problem! I've put some ferrites on the ballast output leads which helped a little but it only knocked down about 1/2 the problem.

Oddly enough it doesn't affect HF much at all.
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