**From:** *MikonCons@aol.com*

**Date:** Mon May 08 2000 - 16:36:32 PDT

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Hi, Mary:

In a message dated 5/8/00 10:50:15 AM Pacific Daylight Time,

mary@advocate.net writes:

<< So apparently this software predicts infinite radiation from a 377-ohm

line?

* >>
*

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.

Mike

Michael L. Conn

Owner/Principal Consultant

Mikon Consulting

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**Next message:**Neven Pischl: "Re: [SI-LIST] : Trace Impedance Selection"**Previous message:**John Spohnheimer: "RE: [SI-LIST] : Trace Impedance Selection"**Maybe in reply to:**Doug Brooks: "[SI-LIST] : Trace Impedance Selection"**Next in thread:**Mary: "RE: [SI-LIST] : Trace Impedance Selection"**Reply:**Mary: "RE: [SI-LIST] : Trace Impedance Selection"

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