[SI-LIST] : 20H rule: a theory?

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From: Abd ul-Rahman Lomax (marjan@vom.com)
Date: Wed Feb 02 2000 - 13:27:35 PST


A little while back there was a discussion on this list of the so-called
20H rule, which suggests backing off a power plane from the edges of its
co-extensive ground plane by 20 times the distance between the planes, in
order to reduce EMI.

There seemed to be some scepticism that this was actually effective, but
one writer noted that this would change the radiation pattern, which brings
me to my suspicion as to why this rule might be effective in some situations.

I would expect that co-extensive planes would show peak radiation in the
plane of the planes; i.e., out the edges. Where one is concerned about
coupling between one section of a PCB and another, which is often the case,
setting one of the planes back might move the radiation peak out more
toward normal to the planes, and thus result in less coupling to a
sensitive on-board area.

Thus the rule might work to reduce interference between one section of the
board and another, even if overall emission was not reduced, or even if,
possibly, it even increased a little.

Comments? Thanks....

marjan@vom.com
Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433

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