This idea about only grounding the shield ("Screen" in the King's English
hi hi) is in my opinion the best way to shield data cabling. We call it
the drain wire.
I have done a lot of industrial installations where the data cables ran
parallel to high voltage lines. An open wire in these runs of 3000 feet
or more would induce enough VA to light a small light bulb! Grounding the
shield at both ends had little if any effect on noise in the system while
just grounding one common end did. We could see the effect of it by
looking at the AC voltage in any open wires left in the shielded cable.
Some cables have shielded pairs with a shield and drain wire for each
pair of wires which had to be grounded at one common point as well.
This was around many primary high kilovolt power lines in the mining
industry and hopefully nothing anyone encounter in their shack. However
it was all at 60 Hz too so, the principle all scales down at HF freqs.
and you may have a scale model of this right in front of you!
The ferrite chokes that go around data cables are called 'common mode
chokes' and if you don't know what that means go look it up. They have a
true purpose and do a good job however it is much more effective to be
sure all the "Shielding " is done properly to start with. Remember the
idea of shielding in data cables is to drain off any unwanted AC current
not to act as a feed line.
Don't use the shield as a ground return path as in a DC system. Use a
separate insulated wire for that and let the shield cover that as well.
In a lot of pre made computer cables it's not possible to do this so a
common mode choke is the only answer. This is why I prefer to make my own
cables when possible. Off the rack store bought cables are not designed
to handle RF noise, they are made to be competitive in price.
John is right about the "ground Loops" as long as the shield is only
grounded at one end it can't be in a ground loop. There is no current
path so any other grounding conductors or methods outside of the cable
will have no effect on the shielded wires.
Aw heck I know this all sounds simple and a lot of you know more about
this than I do. However I do hear a lot of misunderstanding about
grounding and shielding and just thought I'd sent this up the pole too!
Yes you still should go to Church tomorrow!
73 Jeff aa8ve
On Sat, 18 Dec 1999 15:38:21 -0000 (GMT) John - G3JAG
<patents@dx0man.prestel.co.uk> writes:
>
> Seems I'm not the only one to have PC noise problems.
>
> Sounds like you do not have a screened cable. Or if you do, the noise
> is apparently on the screening braid. Have you tried disconnecting
> the
> screening braid at one end only of the cable, so you do not have an
> earth loop ?? I have never had much luck with ferrites, even though I
> have them on most of my PC leads. I also have a small stock of
> ferrite
> sleeves that were used on commercial data cables. Unfortunately, the
> makers code numbers give no clue as to the RF performance, and I
> cannot
> detect any useful effect from using them. They look good ...
>
> Most of my noise problems came from the monitor, which is necessarily
> close to the rig (very small shack). The only solution (not 100%
> effective on 30m) has been to acquire a fully metal jacketed
> surplus monitor designed for use in very close proximity (stacked
> sideways and vertically) with other monitors. Well worth the $75 I
> paid
> for it.
>
> Another prolific noise source may be the PC AC power supply outlet
> which
> supplies the monitor. Some (usually older) rice box PSUs come with
> minimal to zero filtering. Properly filtered outlets nake a big
> difference..
>
> John G3JAG
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