[RFI] Spray Shielding?

Eric Gustafson Eric Gustafson <n7cl@mmsi.com>
Mon, 12 Jun 2000 14:04:11 -0700

Hi Jim,

Yes, this stuff is still around.  But...

1.  It is quite expensive

2.  You will probably not want to go to the trouble required to
    apply it effectively

3.  Even if all the other mods to the cabinet that would be
    required are done, it is unlikely that this will cure the
    radiated EMI problem since the radiation is most likely
    coming from one or more of the cables that exit the case.

Unfortunately, it is very difficult for the end user to
compensate for lack of consideration or poor design in the EMI
control aspects of a product.  So it is imperative that the EMI
characteristics of a product be considered by the end user before
purchase.  Only this will cause the producer to have any interest
at all in the matter.  Since the user has only the FCC and
European certification information to go by, and the FCC
requirements are entirely inadequate, maybe we should simply not
buy anything that doesn't meet the European standards.

Anyone interested in (or already) maintaining a database of "ham
suitable" low radiated EMI computer system components?

A well designed system (monitor in this instance) can operate in
an unshielded plastic case without causing excessive radiated
EMI.  A poorly designed one cannot benefit from making the same
case from solid silver.  I have seen instances where converting
the cabinet from plastic to metal in order to solve an EMI
problem actually made things worse.

The noise you describe doesn't appear to be coming from the high
power sweep circuits but rather from the digital signalling
between the computer and the monitor (or possibly internal to the
monitor).  If it is strong enough to interfere with a TV which
has a watchable (relatively noise free) picture, it is most
likely being radiated by one or more of the cables connected to
the monitor.  Direct radiation from the PCB in the monitor
(unless it is _horribly_badly_ designed) is unlikely to be strong
enough to interfere with a watchable signal.  (The TV antenna
_is_ outside the house is it not??)

I don't have the name or source of supply for the silver loaded
paint you asked about handy.  If you really do want to chase this
ghost, I can track it down for you.  But it may take some time.
We're pretty busy right now and we haven't bought any for a
number of years.  My suspicion is that the most effective (both
time and cost) way to use it is to spray it directly on the main
PCB in the monitor (both sides of the board).  Then, when the
monitor no longer works, you can discard it and get one that
doesn't have such a bad EMI problem.

Good Luck.

73,  Eric  N7CL

>From: "J. Bradshaw" <ac6tk@cybertime.net>
>Date: Mon, 12 Jun 2000 06:32:54 -0500
>I once heard of a product that came in a spray can that could be
>used to coat the inside of a plastic cabinet for shielding.  Is
>this still around?  Where can I find some?
>I have an IBM 9525 monitor (svga)that is so noisy, that it
>interferes with ch 2, sometimes 4 on my TV down the hall.  It
>appears to be some kind of digital hash that looks like numerous
>tiny dots that line up into diagonal lines.
>Jim, ac6tk

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