[SI-LIST] Re: characteristic impedance at DC
- From: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
- To: HUBING@xxxxxxxxxxx, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 10 Apr 2012 12:14:53 -0700 (GMT-07:00)
Nice to have a clear, concise statement now and then!
-----Original Message-----
>From: Todd Hubing <HUBING@xxxxxxxxxxx>
>Sent: Apr 10, 2012 8:00 AM
>To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
>Subject: [SI-LIST] Re: characteristic impedance at DC
>
>This has been an interesting discussion, but not everything presented has been
>correct. Some people may have come away with the wrong idea. In the interest
>of sorting the good information from the bad, here are some of the true
>statements that were made (with some paraphrasing):
>
>1. The concept of characteristic impedance assumes that there is TEM
>propagation on the line. If there are evanescent modes or higher-order modes,
>characteristic impedance is no longer a particularly useful (or well defined)
>concept.
>
>2. At high frequencies, where higher-order modes propagate, each mode has its
>own characteristic impedance and the RLCG transmission line equations are no
>longer useful.
>
>3. For lossless lines (R=0, G=0), the characteristic impedance is not a
>function of frequency and has the same value at f->0 as it does at any other
>frequency that supports TEM propagation.
>
>4. For low-loss lines (R<<wL and G<<wC), the characteristic impedance is
>nearly independent of frequency. This is why it is so useful to the members of
>this list.
>
>5. In a lossy line, as f->0, at the point where R and G are no longer small
>relative to wL and wC; TEM propagation is no longer supported and the
>characteristic impedance is no longer well-defined.
>
>Here are a few corrections to incorrect statements that were made:
>
>1. The characteristic impedance does not approach infinity at DC. Some of the
>equations for characteristic impedance approach infinity, but only under
>conditions where TEM propagation is no longer supported.
>
>2. The characteristic impedance is not the AC input impedance of a
>transmission line. (Unless the line is low-loss and long enough that you never
>see reflections from the far end. In that particular case, the characteristic
>impedance IS the AC input impedance of the line. This is one way to measure
>the characteristic impedance of very long cables.)
>
>2. The characteristic impedance is not the ratio of E to H in the line. The
>ratio of E to H is the intrinsic impedance of the dielectric in the
>transmission line and is not a function of the transmission line's geometry.
>
>Todd Hubing
>http://www.cvel.clemson.edu
>http://www.learnemc.com
>
>
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>
I just used the energy it took to get mad and wrote some blues. Count Basie
Lee W. Ritchey
Speeding Edge
P.O. Box 2194
Glen Ellen, CA
707-568-3983
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