First of all, the 20-H rule was developed years ago, before radiation
directly from the power planes was a common problem. As a couple of
people
have pointed out, pulling the power plane in away from the edge of the
board
reduces near-field coupling to other boards, cables, or the enclosure.
This
can be a very good thing, because it keeps energy from coupling to the
things that may be good antennas.
However, in a board with only 1 return plane, pulling the power plane in
away from the edge of the board does not reduce the power bus
structure's
ability to radiate. In fact, slightly more power can be radiated when
the
power and ground planes are not of equal size. (I liked Larry Smith's
intuitive remarks regarding the loss of balance.)
Placing a ring of return trace around the perimeter of a board and
stitching
it to the return plane also does not reduce radiation directly from the
power bus. This is something we have experimented with in our lab. The
gap
between the power plane and the return ring becomes the new "edge" and
radiates just as effectively.
I am not saying the 20-H rule is not a good idea. It can be very
effective
at eliminating EMI problems resulting from near field coupling off the
edge
of the board. However, it does not generally reduce EMI at power bus
resonant frequencies by making the power bus a less efficient radiator.
Todd Hubing
University of Missouri-Rolla
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