RE: [SI-LIST] : Trace Impedance Selection

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From: S. Weir (weirsp@atdial.net)
Date: Sun May 07 2000 - 23:20:11 PDT


Mary,

At 01:42 PM 5/7/2000 -0500, you wrote:
>S. Weir wrote:
>
> >Actually, wouldn't a 377 ohm T-line by definition be ONLY the connection
> >formed by free space?

The assumption was that the impedance seen by a point source. T-line isn't
really the right term here, because we would be talking about
antennas.. Antenna theory is not my specialty.

>No, a free-space connection does not have a characteristic impedance.
>Characteristic impedance is defined only for transmission lines.
>Characteristic impedance is not the same thing as intrinsic impedance.
>
> >How does one get a 400 ohm impedance? Has someone invented "anti-waves"?
>
>Twin-wire pair with 2-mm wire diameter and 3-cm wire separation or a
>coaxial cable with 1-inch outer diameter and 1.3-mil inner diameter.
>The coax is not a very practical cable in this case due to resistive
>losses on the inner conductor, but it does not radiate and does not
>employ "anti-waves".

Stupid me, wrong coefficients for the upper limit. Telco voice T-lines are
commonly 600 ohms.

> >If we use higher impedance T-lines, then from an external EMI standpoint
> >we are more dependent on making sure that secondary shielding contains
> >the fields ...
>
>This is the presumption that brought me into this discussion. The
>characteristic impedance of a transmission line does not convey any
>information about it's field containment or radiation.

Mary, I think the cross purpose here is the geometry assumptions and where
the physical boundaries are drawn. I believe we agree that there are
constructions( coax ) where all of flux is contained. I believe your point
( and Neven's as well ), that the 50/50 voltage divider model proposed by
Vinu is not supportable.

As much as I agree with that aspect, the notion I believe Vinu was
attempting to address, ( and I believe of practical value ) is that the
flux density surrounding T-line on a PWB as typically a microstrip or
stripline goes up as the impedance goes down and vice-versa.. For those
constructions a significant amount of the energy transmitted down the line
does so in the dielectric surrounding the signal and return etch. That
flux is free to intercept any antenna we place in the field, whether it is
a chassis slot, or a susceptor trace.

> >The funny thing about Doug's first query is that only asked whether there
> >is a specific EMI side effect to one impedance or another. There have been
> >a lot of good responses pointing-out that practical choice of an
> >appropriate impedance entails several other factors.
>
>Yes, I agree.
>
>Mary
>
>
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Regards,

Steve.

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