[RFI] Filtered connectors for video cable

Eric Gustafson guest account n7cl@mmsi.com
Thu, 25 Oct 2001 13:12:02 -0700

Hi Pete,

I'd go ahead and make the sniffer...

Are the monitor and computer powered from the same wall socket?
The physical separation of the monitor from the computer is not a
good idea from an RFI standpoint.  But I suspect this
circumstance is merely making the "real" problem worse.  Ideally,
the units should be near one another and powered from the same
power strip to avoid efficiently coupling circulating RF currents
into the house power wiring.

The worst possible scenario is a large physical separation with a
video cable whose shield integrity is not maintained and a power
distribution system that has power arriving at the monitor via a
separate circuit than the one by which it arrives at the computer
(separate wall outlet is bad enough but separate circuit -

By shield integrity, I mean that the shield fully encloses all
the wiring all the way from (metal) panel to panel.  Connectors
that terminate the shield to the shell by a single wire
connection rather than having the shield bonded all the way
around the shell (and fully enclosing all the signal wires) do
not count.  These devices are more properly described as "antenna
feedpoint" than as shielded connector.

Two cables in series are a bit dangerous but not necessarily a
fatal flaw _IF_ both cables are properly constructed as per above
_AND_ the shield integrity is maintained where they plug together
(and where they connect to the equipment - don't forget to
tighten up the jack screws).

Unforunately, many computer store aftermarket cables are not well
constructed.  So if you do determine that you have a video cable
shield integrity problem (using your sniffer), you will need to
do a bit of shopping for a well made cable.  Companies like "Black
Box" and others have the capability to make good cables and
identify them as such.  If you have to go to that trouble, you
may as well get a single cable that is long enough to do the job.

Most (but not all) of the cables I've gotten with monitors or
computer systems from the big players have been correctly made.

Good luck!

73, Eric  N7CL

>From: Pete Smith <n4zr@contesting.com>
>Date: Thu, 25 Oct 2001 14:34:14 -0400
>Hi Dale [WA9ENA] -- thank you for all the very good info.  I
>just discovered that my new portable shortwave RX has an
>external antenna jack, so I will use that and make myself a
>sniffer as you suggest.
>I suspect the video cable chiefly because when I disconnect the
>video cable from the back of the computer the noise goes away
>totally.  When I power down the monitor, there is little to no
>change.  Someone suggested, though, that the video card might
>automagically stop sending video in the disconnected state, so
>you couldn't know for sure.  Does that make sense?
>Because of the way my wife has her desk arranged, the PC is 2x6'
>video cables from her monitor.  I suppose that poor video cable
>shielding could account for a lot of this noise radiation.
>There isn't much slack in the cable -- would it make any sense
>to get yet another 6' cable and plan on winding most of its
>length on a series of toroids to try to get some common mode
>attenuation -- or would a leaky shield thereafter defeat the
>purpose?  Do you know of a maker of better-shielded video
>cables, short of TEMPEST or MILSPEC stuff?
>I have a new video card on the way - she had wanted one anyway -
>and hopefully that will be a quick and dirty solution.  Think
>I'll build up the sniffer anyhow...
>Thanks again, Dale.
>73, Pete N4ZR