My understanding of the 20H rule, was with regard to planes.

If the planes have noise fields between them (I don't know how this can be
avoided, regardless of component/trace/via placement), when the planes end
equally at the edge of the board, this makes a nice dipole antenna, and the
noise radiates off the edge of the board in a nice wide pattern like you
would want from a dipole.

By applying the 20H rule, you distort the field between the two 'poles', and
the effective radiation is greatly reduced as this is a very poor antenna.

How well this applies to traces over a plane near the edge of the board, I'm
not sure, but I would imagine the effect would be local, and would only be
significant if there were a strong field between the two conductors (poles).

-----Original Message-----
From: Ian Wilson [mailto:[EMAIL PROTECTED]]
Sent: Thursday, July 12, 2001 3:56 PM
To: Protel EDA Forum
Subject: [PEDA] 20H rule, Planes etc (Ex: perimeter stitched ground vias
question)


G'Day all.

I have been wondering about this (the 20H rule for planes) for some 
time.  I have been trying to think of practical situations (rather than 
theoretical) why there may be some advantage.

First off - I agree that if both planes are equally noisy then keeping them 
the same extents is likely to be the best.  However this may not be the 
case where both planes are not equally noisy.

(clip)

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