**From:** Mary (*[email protected]*)

**Date:** Mon May 08 2000 - 07:25:20 PDT

**Next message:**Mary: "RE: [SI-LIST] : Trace Impedance Selection"**Previous message:**Cusanelli, Tony: "RE: [SI-LIST] : or3t125a.ibs"**In reply to:**[email protected]: "Re: [SI-LIST] : Trace Impedance Selection"**Next in thread:**Ron Miller: "RE: [SI-LIST] : Trace Impedance Selection"

Mike,

Thanks for your very thorough response. Your comments did not seem harsh

at all. In fact, I do not disagree with most of what you said. You said:

*>If your last sentence quoted above were true, a lot of high-priced
*

*>software that purports to predict radiated emissions from circuit boards is
*

*>taking the industry on a scam ride.
*

I am not familiar with the inner workings of software that purports to

predict the radiated emissions from circuit boards, but my initial reaction

is that software that predicts the radiation from microstrip patches is

highly complex and requires a great deal of computer resources. Microstrip

patch antennas are much simpler than printed circuit boards, so I am

guessing that software that purports to predict radiation from printed

circuit boards is vastly oversimplifying things.

*>The far-field radiation from a one inch surface trace on a printed circuit
*

*>board is predicted by EMCAD1 software (produced by CKC Labs) to be 4.75 dB
*

*>higher for a 119 ohm line relative to a 69 ohm line; i.e., the higher
*

*>radiation is proportional to the log of the ratio of the two impedances.
*

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). The closer (higher) the line characteristic
*

*>impedance is to that of free space, the higher the radiation. The lines
*

*>noted were 5 mils wide and use an Er of 4.5. The 69-ohm line used a 5-mil
*

*>dielectric thickness, and the 119-ohm line used 20 mils.
*

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

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.

I do not want to suggest that anybody's software is incorrect. My point is

that the characteristic impedance of a transmission line does not convey

information about how much the line radiates.

Thanks again for responding to my post. I am enjoying the discussion (and

learning a lot).

Mary

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**Next message:**Mary: "RE: [SI-LIST] : Trace Impedance Selection"**Previous message:**Cusanelli, Tony: "RE: [SI-LIST] : or3t125a.ibs"**In reply to:**[email protected]: "Re: [SI-LIST] : Trace Impedance Selection"**Next in thread:**Ron Miller: "RE: [SI-LIST] : Trace Impedance Selection"

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