Re: [SI-LIST] : Trace Impedance Selection

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From: [email protected]
Date: Mon May 08 2000 - 16:36:32 PDT

Hi, Mary:

In a message dated 5/8/00 10:50:15 AM Pacific Daylight Time,
[email protected] writes:

<< So apparently this software predicts infinite radiation from a 377-ohm
No, not quite. The actual term is as I stated in the last E-mail.

"The actual equation that computes the radiation for a given line uses the
log of the ratio of the trace characteristic impedance to the impedance of
free space (120*Pi=377 ohms)."

Therefore, when Zline = Zfreespace, the log(base 10) of one is zero and this
term (one of many in the equation) contributes nothing.

 << The radiation from a microstrip trace is not proportional to the log of
 the characteristic impedance. In fact, for a narrow trace it is
 approximately proportional to the height of the trace above the plane
 while the characteristic impedance is proportional to the log of the
 height. The radiation expressed in dB would be approximately proportional
 to the characteristic impedance for a fixed trace width. Perhaps this
 is what the software is assuming. If so, the expression used by the
 software is a convenient approximation that is only valid for a fairly
 specific geometry.

Although I do not know (nor does the EMCAD1 literature indicate) the
boundaries within which their software/equations apply, I agree with you. My
tests on a demonstration board with 50 ohm and 100 ohm microstrip traces
(verified by HP TDR equipment) indicated field increases of 8-9 dB for the
100 ohm line relative to the 50 ohm line. This result is between the 6 dB
and 12 dB boundaries of our discussion. Isn't that always the way it is?

Nevertheless, in case any of the above has misled anyone, the fact remains
that surface traces DO emit electromagnetic fields and therefore DO radiate.
The measured levels in the far field, even for simple topologies, will be
influenced by any and all near by materials (conductors, dielectrics,
permeabilities > 1, etc.) that can concentrate or otherwise distort the
radiated TEM field.


Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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