The return current of a trace that crosses a gap
in the ground plane will excite the gap as a slot
antenna. The area of most radiation will be out
at the ends of the gap. That's due to the current
being the most dense and taking the most complete
change in direction. This is of course assuming
that the gap does not completely dissect the
ground plane.
My own interpretation is that the return current
smears itself out from the trace to the perimeter
of the gap inversely according to frequency, i.e.
the higher freq, the more the return current tries
to follow in the *shadow* of the trace until the
gap completely interrupts a return path. The return
currents of all the frequencies will focus, though,
at the ends of gap.
Forgive the following ASCII art but ...
High Speed Trace



Area ++ Area
of  Gap  Gap  of
most ++ most
radiation  radiation


This has been demonstrated and empirically measured
for seminar purposes by my manager Doug Smith.
Regards, Doug McKean
At 02:09 PM 3/23/99 0800, you wrote:
>>>>
<excerpt><fontfamily><param>Arial</param><smaller>Hi all,
I am trying to find the effects of the gaps on the ground plane for the
highspeed circuits.
Even though I have a couple of references about the via, which has the
ground gap problem, I need more specific data about the ground gap
problem regarding the gap size, gap location.....
Either measured data or simulated data will be fine ( papaers or refernce
book...)
Looking forward to your answers.
</smaller></fontfamily>
<fontfamily><param>Arial</param><smaller>Thanks
Sangil Lee
</smaller></fontfamily>
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