**From:** Mary (*mary@advocate.net*)

**Date:** Wed May 10 2000 - 05:50:06 PDT

**Next message:**Daniel, Erik S.: "RE: [SI-LIST] : Match of differential pairs"**Previous message:**Christoph Hillen: "[SI-LIST] : VeriBest / Mentor Tools"**In reply to:**MikonCons@aol.com: "Re: [SI-LIST] : Trace Impedance Selection"**Next in thread:**Aubrey_Sparkman@Dell.com: "RE: [SI-LIST] : Trace Impedance Selection"

Mike,

In your last message, you said:

*>"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.
*

This is the way I interpreted your message at first, but if this is true,

the

magnitude of this term decreases as the characteristic impedance increases.

Since this was contrary to the point you were making, I assumed that this

term

appeared in the denominator of an expression for the radiated field.

If this term is added to other terms, I guess we'd have to know what the

other terms were before drawing conclusions about the effect of the

characteristic impedance. However, since characteristic impedance is not

inherently related to radiation, I believe that that equation can only be

applied to a fairly specific trace configuration.

*>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?
*

This is an interesting experiment. As others have pointed out, lower

impedance

lines will have a higher current if the voltage is the same. Did you keep

the

voltage or the current constant when you switched impedances?

*>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.
*

I'll bet we can all agree on that. I appreciate the response.

Mary

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**Next message:**Daniel, Erik S.: "RE: [SI-LIST] : Match of differential pairs"**Previous message:**Christoph Hillen: "[SI-LIST] : VeriBest / Mentor Tools"**In reply to:**MikonCons@aol.com: "Re: [SI-LIST] : Trace Impedance Selection"**Next in thread:**Aubrey_Sparkman@Dell.com: "RE: [SI-LIST] : Trace Impedance Selection"

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