[GreenKeys] qrm

[Paul Kasley]

First you need to ascertain the source and nature of the noise. Are you sure 
it's the computer? Does it go away when you turn it off? How much of the RFI 
is coming from your monitor? Over what range of frequencies does it occur? 
Is it broadband noise (for example, as generated by an arc) or is it 
discrete frequencies? If discrete frequencies, the spacing of the spikes 
will give you clues as to the source. Is your network wiring radiating? 
Watch network activity on your DSL modem/cable modem/router/etc while you 
tune around on the radio and watch for any correlations. Any wire entering 
or leaving a box is a point of potential RF ingress and egress. Slots and 
holes in metal boxes can make great antennas at VHF and up. How does the 
noise get into the radio? Is it picked up by your antenna or is it being 
conducted in on the power or other lines?

Typical solutions involve:
* common mode chokes on cables
* feedthru capacitors
* powerline filters ("corcoms")
* replacing power supplies (cheap switching power supplies can really throw 
off RFI)
* "RFI proofing" enclosures by applying metal gasketing where cabinet parts 
meet
* Replacing unshielded cables with shielded ones
* Carefully and intelligently re-arranging shield ground connections

Beating down RFI is like peeling an onion. You fix the first layer, then 
peel it away and work on the next layer and so on. There was one instance I 
had where the last layer required gold flashing the display window of an 
instrument to supress the noise spewing out the metal cutout. And all that 
stuff made in China is made to be cheap, not good so they don't and won't 
spend the few extra cents to keep the junk they make for Walmart RF clean.

-w9ts, E36736
------------------------------------------------------------------------------------
Paul Kasley
Accelerator Division / Controls Hardware
Fermi National Accelerator Lab, PO Box 500, MS 360
Batavia, IL 60510


----- Original Message ----- 
From: "Mike E" <ka2sph at comcast.net>
To: "greenkeys send email" <GreenKeys at mailman.qth.net>
Sent: Thursday, June 25, 2009 12:43 AM
Subject: [GreenKeys] qrm


>I have a question and as a ham I am ashamed I don't have the answer for 
>myself.
> I am trying to use my multiband radio to copy RTTY using MMTTY to the 
> computer.
> The problem is that the computer is putting out so much RFI that I cannot 
> hear anything on the radio. I know toroids will take care of the problems 
> through the cables but what about RFI from the computer itself?
> It does not matter where the radio is, I get nailed by off the scale RF.
> Any thoughts gang?
> 73
> Mike KA2SPH
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