# Re: [SI-LIST] : What's your favorite Screwy SI Concept?

From: Michael Vrbanac ([email protected])
Date: Wed Jan 12 2000 - 08:42:41 PST

Good point. (See RB's reply below.) However, that option is not
allowed us in the examination of this rule. The rule assumes that both
planes are not both backed away by 20H so we probably should stick
with that. There may have been something in the original situation which
precluded that but I have no information on that or any idea why.

Perhaps your point would be best taken with some idea of going beyond
"the 20H rule" or something. Perhaps something like "intentionally spacing
all conductors, even planes" in the attempt to electromagnetically isolate
circuits???

Michael E. Vrbanac

[email protected] wrote:

> Michael E. Vrbanac wrote:
>
> >Starting with two adjacent power/ground distribution planes, the entire
> power
> >plane perimeter is designed to be smaller than the ground plane by a
> factor of
> >20 times the distance between the two planes when measured at any point
> >along the ground plane perimeter edge.
>
> >To the forum,
>
> >It looks as if we have found what the 20H rule is and who originated it
> >and who wrote something about it. That's good progress but we need
> >more. It is obvious that its effectiveness is disputed, so let's continue
> >to explore the possible reasons why the 20H rule might exist for as long
> >as it has. Without making any claims one way or other, I will offer some
> >"thought experiments" to hopefully help us along.
>
> >We first have to assume that there was a beneficial intent by the
> application
> >of the rule and it was thought to have been effective in its original use.
> >Otherwise, the whole thing would be an intentional sham and we should
> >forget about it altogether. Since there would be no benefit of throwing
> it
> >out without adequately researching any possible ways it could be of
> benefit,
> >let's assume that it had some perceived well intended benefit from the
> >application of it.
>
> >Then by logical deduction, we could conclude that a few things might
> happen
> >as a result of this 20H rule (from a general point of view) just from a
> >physical description of it and applying some basic knowledge of physics to
> >see if there are any plausible avenues of understanding the 20H rule's
> usefulness.
>
> And while we consider that, we shoiuld also consider the effect
> of backing both the power and ground planes back by 20H.
>
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