[SI-LIST] Re: Dispersion
- From: C Deibele <deibele@xxxxxxx>
- To: steven.corey@xxxxxxxxxxxxxx
- Date: Mon, 16 Dec 2002 08:55:16 -0500
Even circuitry adds dispersion -- I did an example, privately for someone,
where I had a simple transmission line (lossless in this example) and
put a short circuited
stub in the middle of it. The resonance causes dispersion.
This brings up some wonderful questions about matching circuits....but I'll
leave that for another day.
I don't know if I agree, that physical materials are always dispersive.
Do you think
that superconductors are dispersive? In all honesty, I've measured so
many properties
of them, but I don't know if I've ever taken the time to measure its
dispersiveness. I am
guessing it is pretty damn small, given the fact that I can make
superconducting cavities
with Q's on the order of 10^10, when copper is on the order of 40 x 10^3.
The notion that dispersion implies loss is nonsensical.
Even the notion that loss implies dispersion is nonsensical. A simple
attenuator
is, for example, dispersionless and lossy.
I think I am going beyond the initial question though. The answer to
the original
question is most likely, "It depends."
Craig
Steve Corey wrote:
>List members -- I am resending for the second time, since the list
>server seems to be eating my posts. If at some point it decides to
>regurgitate the originals, my apologies in advance...
>
>***
>
>Craig -- I think we both agree. As Xin Wu pointed out earlier in this
>thread, if we step away from physical materials and go to pure
>mathematics, we can come up with a system that is causal, lossless, and
>dispersive. Your perfectly conducting waveguide built with a lossless
>dielectric and lossless conductors is a good example. For the same
>reason, we can also come up with a non-physical "material" that is
>causal, lossless, and dispersive.
>
>Simplified in the interest of brevity, the Kramers-Kronig relationships
>are derived under the assumption that free space (with its purely real,
>constant permittivity) is the only lossless "material", and it is
>therefore used as the limiting case. This is not overly restrictive if
>we are discussing physical materials, since it follows directly from the
>second law of thermodynamics -- interaction with matter always increases
>entropy. It has also stood the test of time to some extent, since it
>still stands today as postulated independently by the two originators in
>1926 and 1927.
>
>Based on this assumption (and the assumption of linearity), if we
>analyze the permittivity of any physical material, it will be lossy and
>it will be dispersive. I'm sure we both agree on this point. The more
>interesting part is that via the Hilbert transform, the real part of the
>permittivity completely determines the imaginary part, and vice versa.
>
> -- Steve
>
>-------------------------------------------
>Steven D. Corey, Ph.D.
>Time Domain Analysis Systems, Inc.
>"The Interconnect Modeling Company."
>http://www.tdasystems.com
>
>email: steven.corey@xxxxxxxxxxxxxx
>phone: (503) 246-2272
>fax: (503) 246-2282
>-------------------------------------------
>
>
--
Craig Deibele, PhD PE MBA
Spallation Neutron Source
Oak Ridge National Laboratory
PO Box 2008 MS 6473
701 Scarboro Road Room 301
Oak Ridge, TN 37830
deibele@xxxxxxx
office: +1 865.574.1969 cell: +1 865.719.4381 fax: +1 865.241.6739
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