I think you may have put your finger on the source of the confusion here.
There was a lot of talk of reducing emissions by shrinking the power planes
slightly. That may have created the impression that emissions directly from
the power planes were reduced rather than reducing the power plane coupling
to something else that was radiating. In the products where I have had
problems that caused me to cut back the power planes it was always coupling
to a trace near the edge of the power plane, or the power plane at the
board edge coupling to part of the chassis or an air vent. These cases
always involved small distances (fractions of inches).
John Lockwood
Juniper Networks
At 09:09 AM 5/28/99 -0700, you wrote:
>Wow! What an interesting discussion! Since we have recently been
>investigating this issue, I can't resist adding my own 2 cents worth.
>
>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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