[RFI] Computer monitor RFI

[Ian White, G3SEK]

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)

http://www.ifwtech.co.uk/g3sek