Computer Interference / Driving Me Crazy!

[azl]

Hi Everybody. I realize there is a lot of discussion on this topic already but I am having a difficult time coming up with a test method to resolve my RFI problem. I'm just going to lay everything I know out on the table and hope that somebody out there can assist.

Certain frequencies (for instance, 42.5MHz FM) pick up a hiss/static background noise when my PC is turned on. My scanner is mounted to a shelf about 4' above the PC itself. I am using a discone-style antenna with about 6' of RG-58A coax and it is hanging from the ceiling about 7' above my desk. I am using a Uniden BCT-15. The only thing connected to the scanner is the 12VDC input and the antenna BNC.

If I am tuned to 42.5MHz with the squelch at zero, I hear the typical radio static. Turning my computer ON and OFF has no effect on that (I can't hear any difference). However, when there is a transmission on 42.5MHz, I can easily hear the static generated by my PC in the background. In fact, if I have the squelch turned up and am listening to a distant transmission, sometimes turning on my computer will create such a RF racket that my radio will lose the signal and resume scanning.

* I have tried connecting the radio to a 12V battery to isolate the power source - no effect.
* I tried "shielding" the radio in aluminum foil - useless.

When I get off of work I am going to try moving the radio to another part of the room to see if it is a proximity thing (I assume it is).

The worst part is that I can only seem to test this when there is an active transmission (I guess that is a "carrier" - I don't know the radio lingo). When there is nothing going over the airwaves, I can't observe the issue.

I really cannot relocate my radio permanently (it would be useless if it weren't at my desk). I was wondering if my RG-58 coax could be leaky or something else? Is there a better sort of coax that would work in my situation? If the problem is the cable/antenna, I can move the antenna to another part of the room (can't put it outside, I'm living in a rental).

Again, I don't even know if it is the RADIO or the antenna/coax that is picking up the RFI, because without the antenna connected I cannot receive any transmissions with which to test the problem!

I also wonder if things need to be grounded in some way. Obviously the PC is grounded via the AC line, but the radio and antenna are not.

Thank you all for any help you can offer!

 

[iMONITOR]

When you are getting the interference, try turning off your monitor and/or printer, and see if that makes any difference.

 

[azl]

Thanks for your help SIG!

I should clarify: I have many different devices near the radio (wireless router, three LCD panels, printer, bluetooth keyboard/mouse) but none of them are the source. The issue is clear as day and can easily be controlled by the power switch on the front of my computer. Everything else can be off or unplugged, but the problem occurs as soon as I hit the ON button on the computer itself.

 

[ka3jjz]

If you have your PC and BCT15 connected to the same outlet, there's a chance that interference might be radiating from the AC line and being picked up by the 15 either inductively (through the power cord to the 15) or from the common connection. You might try moving the 15 to a completely seperate circuit - if the interference goes away you have identified your issue. You may need to get one of those RF Kits that has the transformer that seperates (name escapes me at the moment) and wrap the AC cord from the PC around it.

Another way to test this would be to run the 15 on a completely seperate power supply - say a car battery, or perhaps even a filtered power supply.

I'd also look at choking off all leads running to/from the PC. They too can act as RFI antennas in some cases. I've had some USB devices that were issues, too, such as an old Zip drive I still use from time to time. That's a real bad case, that one.

73 Mike

 

[fineshot1]

It sounds like its airborn wideband noise from the pc switching power supply which are famous for generating lots of noise on many freqs. You have a gain antenna in the same room as your pc so
your going to need to experiment around with moving the antenna outside or in another room.

 

[torontokris]

Make sure other items arent plugged in as well on the same fuse/breaker.
I had a problem with an ipod adapter (non Apple - cheap brand)

Caused interfearence on everything from radio to scanners even when NO ipod was plugged into the adapter.. long as it was plugged into the AC outlet it caused interference.

Sounds like your switch or something connected to it is causing the problem (Since u say it goes away when you click off)

 

[azl]

Thanks everybody for your advice. As mentioned, I have already tried running the radio off a 12V battery - no change.

Fineshot: I think you're right. I tried relocating the radio during my lunch hour and found that the best reception is when everything is in the middle of the room (10' away from the PC).

I think I am going to either:

a) Suck it up and deal with it
b) Find a way to get the antenna outdoors

 

[altec]

Computer cases

If you are computer building kinda guy maybe a better insulated case might help on the interference? I know store bought stuff the cases are made of really cheap thin metal that like to cut you.

 

[azl]

Altec,

This case is an Antec and I'd think it would be one of the better metal cases.

If there were a way I could shield the case better, I would. Maybe a metal panel or something.

 

[zz0468]

I've probed around my computers with a spectrum analyzer to see where the signals radiate from. I see it radiating from any and all openings in the case (drive bays, etc) but RF radiates strongest from cables leading from the computer. Snap on ferrites help some, but the easiest effective fix is to get antennas outside and as far away as possible.

I agree that the switching supplies are the worst offenders in generating rf noise, and I've considered replacing them with simple analog iron core transformer supplies, but haven't done that... yet. I want to tackle cleaning up the switchers first, to see if it's even possible.

In some other projects I have built that use switching supplies, the only way I've been able to fully suppress noise is full shielding, and all conductors in and out going through either feed through capacitors or, in the case of AC power, a brute force filter.

 

[k9rzz]

In some other projects I have built that use switching supplies, the only way I've been able to fully suppress noise is full shielding, and all conductors in and out going through either feed through capacitors or, in the case of AC power, a brute force filter.

I have an older computer that was giving me grief on HF ... soldering .01uf ceramic caps to ground from every wire coming out of the power supply completely cured my problem. It wasn't too big of a project either, I guess I got lucky !

 

[hoser147]

Give this a try, get some alum. window screen and wrap it around the cables and possibly a piece on the computer tower were the source is. We had issues between the radio'sand computers in our watchroom, similar to what you are having, after wrapping the wires with the screening it stopped. Worth a try and not to expensive to do. Good Luck Hoser

 

[chrismol1]

If I turn my Bose speakers up just about all the way, I will get a local radio station. i know that the tower is about 2 miles from my locations but its fun to hear the radio over ur speakers even though they are not hooked up to any radios
If my move the speakers around I get better reception LOL

 

[azl]

I have solved my RFI problem. Thanks for your advice everyone.

I tried moving my antenna out of the house; I hung it out my 2nd story bedroom window. I was still receiving the same horrible noise.

So I got down into the computer and started disconnecting things one by one - hard disk drive, optical drive, motherboard. There was a 12V line coming from the power supply connecting to a rear exhaust fan. This line apparently was arranged in such a way that it was emitting noise out of the vent on the side of the PC case.

Second, I disconnected all cables from the rear of the PC. I had been using a Prolific USB-Serial adapter to control the BCT15 from Windows. I was actually using a 6' serial extension cable in between the USB adapter and the radio. This was a cheapo cable and I suspect it is not shielded very well. Even though I disconnected this line while I was doing my testing, the cable's proximity to the scanner was apparently a problem.

Once I rerouted the internal power supply wiring and removed the 6' serial extension, my reception improved dramatically. I was able to return the antenna indoors with no effect. I can now receive signals from 35 miles away with very minimal static.

Thanks again!

 

[iMONITOR]

Super, good trouble shooting!

 

[roadranger]

Keybaord

I have solved my RFI problem. Thanks for your advice everyone.

I tried moving my antenna out of the house; I hung it out my 2nd story bedroom window. I was still receiving the same horrible noise.

So I got down into the computer and started disconnecting things one by one - hard disk drive, optical drive, motherboard. There was a 12V line coming from the power supply connecting to a rear exhaust fan. This line apparently was arranged in such a way that it was emitting noise out of the vent on the side of the PC case.

Second, I disconnected all cables from the rear of the PC. I had been using a Prolific USB-Serial adapter to control the BCT15 from Windows. I was actually using a 6' serial extension cable in between the USB adapter and the radio. This was a cheapo cable and I suspect it is not shielded very well. Even though I disconnected this line while I was doing my testing, the cable's proximity to the scanner was apparently a problem.

Once I rerouted the internal power supply wiring and removed the 6' serial extension, my reception improved dramatically. I was able to return the antenna indoors with no effect. I can now receive signals from 35 miles away with very minimal static.

Thanks again!

Try this while you are at it. Unplug your keyboard, if all that great noise goes away, consider a wireless board.