Re: [SI-LIST] : Trace Impedance Selection

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From: S. Weir (weirsp@atdial.net)
Date: Sat May 06 2000 - 15:42:09 PDT


Actually, wouldn't a 377 ohm T-line by definition be ONLY the connection
formed by free space? Any T-line concentrates the flux density, and
therefore reduces the impedance from the free-space limit. How does one
get a 400 ohm impedance? Has someone invented "anti-waves"?

Maybe Mary's point is that if we construct a piece of coax with a REALLY
big diameter and very low dielectric coefficient core, then we could get a
pretty high impedance. However, I think that from a practical
standpoint, this just reinforces Vinu's point, the higher the impedance of
a T-line, the less concentrated the fields, and therefore for any fixed and
small local volume the more energy that exists outside that volume. 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,
( the box ), as would the shield in our really big coax. In a PCB
environment that fixed and small local volume is the volume around one
T-line but not surrounding any adjacent T-lines. So, we should expect that
higher impedance lines require more physical separation to maintain
a given percentage crosstalk than lower impedance lines, assuming that in
each case we do not have driving, or terminating problems.

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.

Regards,

Steve.

At 05:55 PM 5/5/2000 -0700, you wrote:
>If you were able to connect a transmitter to a receiver using a 377 ohm
>transmission line, this line would be in parallel to the "transmission
>line" between the two formed by free space. Therefore, one half the
>transmitted power would go through free space and the other half through
>the line. As the line impedance is lowered, more power would be
>transmitted through the line and less through space.
>
>What's wrong with this scenario?
>
>Thanks,
>Vinu
>
>Mary wrote:
>
> > Somone recently claimed that higher impedance transmission lines
> > radiate more because their impedance is closer to the 377-ohm
> > impedance of free space. This is not true. It is not possible
> > to judge anything about the radiation from a transmission line
> > based on the value of its characteristic impedance.
> >
> > Characteristic impedance is the ratio of voltage to current in a
> > forward traveling wave. The ratio of electric to magnetic field
> > strength in a free-space transmission line is approximately
> > 377 ohms regardless of what the characteristic impedance is.
> > Even if you were to build a transmission line with a 377-ohm
> > characteristic impedance, there is no reason to believe it would
> > radiate any better or worse than a 300-ohm or a 400-ohm line.
> >
> > Mary
> >
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