[SI-LIST] Re: Traces don't cause EMI - really?

  • From: "Ingraham, Andrew" <a.ingraham@xxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Tue, 14 Oct 2003 10:43:43 -0400

Brent wrote:

> ...  If, in your statement, the words
> "matched transmission lines" had been exchanged for "traces", I doubt
anyone
> would disagree.

> ...  As an intermediate case, I expect you would not
> disagree with the concept that mis-matched transmission lines, even over a
> continuous ground plane, may radiate significantly.

My understanding is that it was a common but false belief that only
mismatched lines radiate.  Either matched or mismatched lines can
radiate equally well.

This popular misconception probably came about because of
misunderstandings about the normal mode vs. common mode
signals on the transmission lines; plus the fact that you can use a
transmission line that usually doesn't radiate, as an efficient
antenna element if you drive it differently, like the folded dipole
antenna built from twinlead.

I suppose that mismatched lines might have higher near field
emissions because of the greater voltage maxima on the line; but
otherwise they can both radiate.  What determines how well a PCB
structure radiates has more to do with its geometry than anything
else.  You can design geometries that radiate very well (and are
designed to do so), or plain traces over continuous grounds that
radiate poorly, and might be neglected because of other sources
of emissions in your system.

Regards,
Andy







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