RE: [SI-LIST] : Trace Impedance Selection

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From: Mary (mary@advocate.net)
Date: Sun May 07 2000 - 11:42:55 PDT


S. Weir wrote:

>Actually, wouldn't a 377 ohm T-line by definition be ONLY the connection
>formed by free space?

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".

>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.

>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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