SDR Interference - Electronic Devices

KenH999

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New to SDR. Have some public safety comms background and have a decent grasp of basic RF concepts but definitely not to a high technical level. Having a problem with what I think is interference from other electronic devices on/around my desk (see images 1 and 2).

Image 1.
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Image 2

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Using SDR#, a 2 week old RTL-SDR Blog V3, Tram 1411 Discone on the roof, RG-6/U with a small F-female to SMA male patch cord into the dongle.

My primary interest is monitoring MilAir and occasionally P25 ~800 MHz trunking although will need better or more dongles to effectively do that. Down the road maybe some HF.

Dongle is in a powered USB 3.0 hub on desktop. I'm getting major interference at ~292-293 MHz. On a shelf above my desk is a WiFi Router, Internet Modem, Security Camera DVR w/WiFi. On desk is dual Dell E2013H monitors. CPU (Dell Optiplex 7040) is under the desk on floor.

I initially tried adding a 6' USB extension cable to separate the dongle some. Put it in a drawer. No help. Wrapped it in foil, no help.

I connected the kit's small dipole and pointed it near and against the various devices. Noise floor jumped up and signals spiked. See image 3. Note that this interference also occurs with exterior antenna. I also unplugged the power supplies from each device. Saw less signals but still there. Touched the antenna to the unplugged power jacks and signals jumped.

Image 3 - Dipole touching devices.
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I turned the gain down to 3.7dB, that's the first level that freq. searches on 225-380 don't stop on the interference.

I searched here and read a few other posts and saw mention of ferrite chokes and beads. Totally unfamiliar with those.

Suggestions are appreciated. I can't relocate the 3 devices. Assuming it's the devices' power supplies what can I do?

Thanks!

Ken
 

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KenH999

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Looks like RF leakage from a television or computer monitor. Are you using any HDMI switches?
No. I think I fairly well described the probable sources in my post. Hoping for some guidance on mitigating same.
 

air-scan

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If same interference is still introduced from outside antenna into your sdr program it might suggest leaky coax or the pc wireless radios have poor filtering. Could be internal inside the dongle. RFI can be tricky. RFI might have you think its certain devices but it may be a proxy of the real problem. You should also consider the fact RTL-SDR at your mentioned frequency band also is spikey on my RTL-SDR's. Thagt could be the real source. If you have an SDRPlay or Airspy to compare on the same frequency bands you mentioned. If those do not show spikes then RTL-SDR is the culprit. These are best in the 800mhz and up bands. 400-512mhz is okay too.
 

KenH999

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If same interference is still introduced from outside antenna into your sdr program it might suggest leaky coax or the pc wireless radios have poor filtering. Could be internal inside the dongle. RFI can be tricky. RFI might have you think its certain devices but it may be a proxy of the real problem. You should also consider the fact RTL-SDR at your mentioned frequency band also is spikey on my RTL-SDR's. Thagt could be the real source. If you have an SDRPlay or Airspy to compare on the same frequency bands you mentioned. If those do not show spikes then RTL-SDR is the culprit. These are best in the 800mhz and up bands. 400-512mhz is okay too.
1. The described interference is present whether the exterior antenna OR the RTL-SDR kit's little dipole is connected. I would think that kinda rules out leaky coax?
2. The PC has no WiFi radios.
3. With the little dipole when I touch it to the devices (camera DVR, router, modem) the signals and noise floor spike. Especially when I touch their power jacks.
4. When I move the dipole away from the desk and devices, turn around and 'shield' the antenna with my body in between the antenna and devices the signals drop significantly.
5. Do not have a second SDR.

Any advice on something (chokes?) that might filter this???
 

KE5MC

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Step 4 suggest it radiated (over the air) and not conducted (wiring) interference. I don't believe the specific offending device or most offending device has been identified as I read thru the thread. Individual turning off equipment is best if it doesn't impact SDR/computer operations to identify the offender. Then it's chokes on cables to and from that device and/or shielding of that device. It seems the interference is very strong and may never be eliminated without physical distancing (more than 6 feet :) ) or a device with better Part 15 compliance.
Good Luck!
Mike

P.S. Good chance that it is a combination of radiated and conducted interference and not in equal parts.
 

KenH999

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Step 4 suggest it radiated (over the air) and not conducted (wiring) interference. I don't believe the specific offending device or most offending device has been identified as I read thru the thread. Individual turning off equipment is best if it doesn't impact SDR/computer operations to identify the offender. Then it's chokes on cables to and from that device and/or shielding of that device. It seems the interference is very strong and may never be eliminated without physical distancing (more than 6 feet :) ) or a device with better Part 15 compliance.
Good Luck!
Mike

P.S. Good chance that it is a combination of radiated and conducted interference and not in equal parts.
Thanks. Distancing the devices is not something I want to do, not an easy task that would require a good bit of cable re-routing. Will keep it in mind though.

Please point me to some good guidance on selection of and using chokes?
 

cg

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Shut off AND unplug each device until the offending signal drops.
Once you have identified the offending device/power supply, you then can target it for mitigation. Wall warts are notorious sources of interference.
 

bagmouse7

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A couple of usual suspect to check:
Computer monitors, LED or CFC lighting, any USB connected devices, any wall wart type power supplies.
I had the exact same type of interference come from LED lightbulbs,
The manufacturers of these LED lights change the internal designs of these lights all the time and some of these are very noisy while others (even from the same manufactures) are quiet.
It’s an annoying process of elimination to find the real source.
Good luck!
 

KenH999

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Assuming you have cables, are your ethernet network cables shielded? Does your router have a plastic case or a metal case?
Just standard Cat 5e and 6 cables as far as I know. Will the cable labeling say 'shielded' if it is?

Router (Asus AC1900) is plastic. I need to replace it though as I'm having unrelated issues with it.
 

KenH999

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I've ordered a bunch of chokes. They'll be here later this week. Will report back after trying them.

Question: If the EMI is at ~ 292 MHz do I need to do loops in the various chokes. Some of the power cords are small, flat ~3mm while others are round 6-7mm. If the small ones don't fit tightly in the choke will it still work?

Thanks
 

KE5MC

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Loop the smaller cords as each pass will improve the choking impedance.

P.S. I wondered off into Google to look at shielded cat cable and RJ45 connectors. My take is that the cable will have metal shields on the RJ45 connectors and the sockets will have the same on the device. The typical cable most often used has 8 twisted pair for signaling. Shielding comes into play only if the cable has outer braid like coax and exposed metal on the RJ45 to make the shielding connection. Nothing in the wiring of the 8 pairs supports shielding that I can tell without the exposed metal bits on the cable.
 
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