[SI-LIST] : Regarding Slots ...

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From: Doug McKean (dmckean@corp.auspex.com)
Date: Mon Apr 24 2000 - 10:44:26 PDT


I've seen Doug Smith do an excellent demonstration
of the "trace crossing a slot" phenomena.

Analytically, if you borrow Babinet's method from
optics, a slot is the complement of a wire. In
principle, a slot is the dual of wire and can
even be drawn as such. Whereas the wire is current
driven, a slot or more generally an aperture, is
voltage driven. As long as an electric field is
developed across the slot, it will radiate. Return
currents from the trace across a slot accomplish
this quite nicely. Good to keep traces away from
slots and only cut up ground planes if absolutely
necessary.

Slots in product housings can be another matter
if return currents develop a voltage across them.
Where the directivity of a simple dipole is about
1.5 or so, the directivity of a slot can run upwards
much higher by roughly a factor of 10 or so. So,
given roughly the same amount of power, slots become
more of an issue than wires during emissions testing.

For a more formal mathematical analysis, might I
suggest _Elements of Electromagnetics_ by Rao from
Prentice Hall copyright 2000, ISBN 0-13-013201-2.
Excellent discussion.

Regards, Doug McKean

Dan Swanson wrote:
>
> I remember one article in the past year or two
> in IEEE Circuits and Devices Magazine.
> Showed coupling of energy from a trace that crosses
> a split ground plane. Coupling into slot mode was
> clearly evident.
>
> Dan
>
> Dan Swanson EMAIL: d.swanson@ieee.org
> Bartley R.F. Systems TEL: (978) 241-1091
> 38 Water Street FAX: (978) 388-7077
> Amesbury, MA 01913

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