RE: [SI-LIST] : Antenna Problem on the Board

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Tue May 29 2001 - 13:35:51 PDT


> 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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