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Re: [RFI] Computer monitor RFI

To: rfi@contesting.com
Subject: Re: [RFI] Computer monitor RFI
From: "Ian White, G3SEK" <G3SEK@ifwtech.co.uk>
Reply-to: "Ian White, G3SEK" <g3sek@ifwtech.co.uk>
Date: Sat, 8 May 2004 09:29:09 +0100
List-post: <mailto:rfi@contesting.com>
Alan Beagley wrote:
The Sony 15" monitor connected to the computer I am using with my rig generates an enormous amount of RFI: S9 signals at intervals on 20m and up, the intervals depending on the screen resolution I set. The interference stops if I turn off the monitor or when it blanks after the set timeout interval.

The video cable has its own integral RFI filter, and I have tried adding a choke to the power cord and an additional one to the video cable, but to no avail.

Two thoughts...

I had some problems with a CRT monitor sitting right next to the transceiver, but they cleared up completely when I installed an isolated interface for the sound and computer-control lines. By "isolated" I mean a complete DC break in *every* signal and ground line between the computer/monitor and the rig - they are still connected via the mains ground, but there's no ground *loop* any more.

The second thought is that, when you say:
I have tried adding a choke to the power cord and an additional one to the video cable

- did you try adding *enough* of a choke? In a severe case like yours, one or two clip-on beads would be unlikely to make much difference because they don't create enough impedance at HF.

Most hams hugely underestimate how many "typical" clip-on beads it takes to replace one "typical" toroid with 5-10 turns. YMMV, but as a rough guide, think 20-30!

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

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