Re: NEC-LIST: MININEC Shows Standing Waves at Resonance on Twinline

Jerry Ehman <jehman_at_postbox.acs.ohio-state.edu> wrote:
> ...We are using MININEC Broadcast Professional by EM Scientific, Inc.
> to model several antennas. We are getting strange results from the
> twinline and dipole model. Can someone help us determine what is
> wrong? The bottom line is that at resonance (as defined by zero
> reactance), there are standing waves of current on the twinline; this
> is not supposed to occur....
   ---[snipped]---
>...Due to the fact
>that the propagation of information (i.e., group velocity) in a
>conductor is a few percent slower than the free-space speed of light....

This is not what you asked about, but I believe I detect a
misconception or two here. I'll stick out my neck and hope that I'm
_helping_ by commenting:
(1) If the conductor is perfect, then wave propagation along it is at
the speed of light in vacuuo.
(2) Group velocity is irrelevant here. It's phase velocity that
matters. The dipole resonance you're dealing with is a
single-frequency, continuous-wave, phenomenon. Group velocity would
matter if, e.g., we were discussing the time taken for a _pulse_ to
propagate some distance.
(3) Perhaps you're thinking of the well-known fact that a "half-wave"
wire dipole resonates at a frequency slightly lower (by a few percent,
depending on wire diameter) than the frequency for which the wire
length equals one-half wavelength in vacuuo. This effect is due to
the lumps of excess capacitance at the open ends of the wire, where
the E-field "fringes", i.e., spreads out from the end of the wire
radially in three dimensions, rather than in two dimensions in the
plane perpendicular to the wire. BTW, if the resonance-frequency
reduction _were_ due to a modification of phase velocity along the
wire, the necessary modification would be an increase, not a decrease.

> ... Wires 1 and 2 make up the twinline.... The diameter of each wire
> (d) is 0.3 cm and the center-to-center spacing between the wires (D)
> is 0.36 cm....

Here's your problem, I believe. MININEC can not accurately model a
parallel-wire transmission line, or "twinline," with such close
spacing between the wires. It's been five years since I used MININEC,
but I think I recall that the practical lower limit for the
center-to-center spacing is about three times the wire diameter.
(Perhaps someone can refine this estimate.)

I suggest doing the following little experiment with MININEC, which I
remember finding very instructive. Define a parallel-wire
transmission line of length somewhat greater than a half wavelength,
so that you'll be able easily to see the standing-wave pattern. Put a
source at one end and a resistive load (not an antenna) at the
opposite end. (You can halve the number of segments required and cut
execution time by using a perfectly conducting ground plane and
modeling one wire of the twinline rather than both wires.)

Observe the standing wave pattern with different values of load
resistance, to find the characteristic impedance of the line. Compare
this value with what the usual formula gives.

Hope this helps.

Best regards -Chuck.
Received on Thu Jun 11 1998 - 17:31:10 EDT