RE: [SI-LIST] : Trace Impedance Selection

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From: Mary (
Date: Wed May 10 2000 - 05:50:06 PDT


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
>term (one of many in the equation) contributes nothing.

This is the way I interpreted your message at first, but if this is true,
magnitude of this term decreases as the characteristic impedance increases.
Since this was contrary to the point you were making, I assumed that this
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.
>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
lines will have a higher current if the voltage is the same. Did you keep
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
>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.


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