NEC-LIST: Wave speed on wires and TL modeling

Hello,

I've followed with interest the recent discussions concerning
transmission-line modeling and the speed of wave propagation on a thin
wire. Some colleagues and I have sproadically examined the latter
problem in the time domain over a number of years (using TWTD) and I
offer these observations:

1) A space-time contour plot [the x-axis is space and y-axis is c x t
   with c the speed of light] of the current on a wire excited at its
   center by a Gaussian voltage pulse that varies as V = exp[-(at)^2],
   where t is time and a is a parameter, exhibits contour lines at
   approximately 45-deg slopes. This shows that the current/charge
   pulses are propagating at essentially light speed, although we have
   not really attempted to quantify this to a resolution of a few per
   cent.

2) A comparison of the spatial I and Q pulses reveals that I ‰ Qc,
   again indicating that the speed of light is applicable.

3) It's known that a plane wave propagates at light speed in a medium.
   Considering that the fields around a wire on which such I/Q pulses
   propagate reasonably approximate a TEM wave [i.e., it acts as a
   single-wire transmission line], and furthermore taking into account
   the applicable boundary conditions at the wire, especially that the
   normal E-field is terminated by charge there, this seems to imply
   that the I/Q pulse also must propagate at the speed of the field,
   or at light speed, in the medium.

Also, concerning modeling transmission lines using NEC or MININEC,
this problem is discussed in the Febuary 1997 "PCs for AP" that
appears in the IEEE AP Magazine, pages 96-97.

Ed Miller

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Edmund K. Miller
3225 Calle Celestial
Santa Fe, NM 87501-9631
505-820-7371 (voice and FAX)
emiller_at_popmail.esa.lanl.gov
emiller_at_nm-us.campus.mci.net
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Received on Thu Jun 18 1998 - 10:03:19 EDT