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

Chuck Counselman wrote:
>
> 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.
>

I have done a lot of work with parallel wire lines using NEC2 and
agree with the above. The smallest centre-to-centre spacing to
diameter ratio (s/d) which I have found to give an acceptable model is
about 1.5, corresponding to a 115 Ohm line. Even this requires very
careful segmentation of the load and feed points and probably use of
GC cards to give varying segmentation along the line. I think the best
VSWR I achieved for a matched line at 115 ohms was about 1.13 (~6%
reflection) and for a 220 ohm line (s/d = 3) about 1.08 (~4%
reflection).

Regards

Ian

-- 
Dr Ian David Flintoft           Email:      idf1_at_ohm.york.ac.uk
Applied Electromagnetic Group   Tel:        +44 1904 432391    
Department of Electronics       Fax:        +44 1904 433224    
University of York  
Heslington          
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