**From:** Ingraham, Andrew (*Andrew.Ingraham@compaq.com*)

**Date:** Tue May 29 2001 - 13:35:51 PDT

**Next message:**Doug McKean: "Re: [SI-LIST] : Antenna Problem on the Board"**Previous message:**Larry Miller: "RE: [SI-LIST] : RMS Definition"**Maybe in reply to:**subramanya C K: "[SI-LIST] : Antenna Problem on the Board"**Next in thread:**RMELLISON@aol.com: "Re: [SI-LIST] : Antenna Problem on the Board"

*> Anyone see any mistakes or disagree with these assumptions?
*

The first assumption was that you are in the far-field. This might be true

in some cases but not in others. (Not to criticize your analysis, just

bringing up yet another variable.)

For boards enclosed in boxes, I wonder how valid a far-field assumption is,

even if you go looking several wavelengths away from the outside of the box.

You might be far-field with respect to the box and the cables coming out of

the box; but the box and cables are in the near field with respect to the

board(s); so perhaps the first step is predominantly near-field radiation

from board traces to boxes and cables.

In the far-field, the electric and magnetic fields are related by the

impedance of free space (377 ohms). In the near-field they aren't. A

radiating antenna with lots of voltage but little current, imparts a greater

electric field than magnetic field, compared to an element with lots of

current but little voltage, when in the near field.

I am not sure what exactly happens to those fields as you transition from

near-field to far-field; but I would imagine that a ratio of electric to

magnetic fields that is close to 377 ohms in the near-field, may result in

greater energy transfer into the far-field. In other words, there may be

some trace impedance range, somewhere in the middle, (which may also depend

on geometry or other factors), that is more effective at establishing

radiation into the far-field ... all other things being equal ... and with

no other radiating or re-radiating elements present (a tricky proposition).

Too low a trace impedance, and the near-field is almost all magnetic; too

high, and it is almost all electric; neither of which is efficient for

establishing the far-field radiation. Maybe.

If you are in the near-field, and you notice "radiation" by detecting

voltage, I suspect you might notice more radiation from your hi-Z traces

than from low-Z traces, simply because the ratio of voltage to current is

greater ... even if you somehow made the geometries look the same.

Still, I defer to all you experts out there.

Regards,

Andy

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**Next message:**Doug McKean: "Re: [SI-LIST] : Antenna Problem on the Board"**Previous message:**Larry Miller: "RE: [SI-LIST] : RMS Definition"**Maybe in reply to:**subramanya C K: "[SI-LIST] : Antenna Problem on the Board"**Next in thread:**RMELLISON@aol.com: "Re: [SI-LIST] : Antenna Problem on the Board"

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