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Re: [RFI] Quiet Shack PC Systems?

To: rfi@contesting.com
Subject: Re: [RFI] Quiet Shack PC Systems?
From: "Ian White, G3SEK" <G3SEK@ifwtech.co.uk>
Reply-to: "Ian White, G3SEK" <g3sek@ifwtech.co.uk>
Date: Thu, 18 Mar 2004 21:40:58 +0000
List-post: <mailto:rfi@contesting.com>
Deni wrote:
I'm always intrigued by the photos of other peoples shacks with the PC and
monitor mounted right in among the radio gear.

I have a conventional tower and CRT monitor which creates nasty QRM in my HF
receivers, I also have a laptop with TFT screen which is not too bad but
it's switched mode PSU/Charger is chronic anywhere near my radio.

I'm interested to know how others solve the PC / Radio interface  and
maintain low interference levels.

There's been plenty of good advice already, and the main question to ask is *why* your receiver is so sensitive to something like a small switchmode PSU held near to it?

What happens if you disconnect the antenna? If the problem goes away, it means that you're picking up the noise on your feedline in common mode, and then it is coming back down into the RX in differential mode. Alternatively, very similar effects may be taking place in your ground system.

You probably have both a mains ground connection and some kind of "RF" ground connection. In that case all ground currents will divide between those two paths, so you will have transmitted RF flowing through the mains ground, and also computer noise currents flowing along your RF ground.

The solutions are to clean up your antenna feedlines as others have advised, and be as disciplined as Tom is about the grounding.

At the very least, separate your mains feeds into the "radio side" and the "computer side". Then feed each side separately through its own *three-wire* mains filter that includes a ground choke. This strategy will keep RF out of the mains, and it also breaks up any local ground loops. A prime suspect for a ground loop is the triangle between the radio, the PC, and the RS232 interface cable between the PC and the radio.

The good news is that the same package of precautions will reduce noise pickup, reduce risks of RF feedback, reduce risks of causing RFI, and at least help reduce the risks of damage by lightning-induced mains surges... because all of those problems actually have very similar origins, in uncontrolled ground currents and/or common-mode currents on the feedline.

-- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)

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