NEC-LIST: Dipole reactance

I realize that over the years there have been a large number of
different calculated and measured values for cylindrical dipole input
impedance. However, in one region at least the results from both NEC-2
and NEC-4 seem to lie outside commonly-accepted values.

Unless I'm mistaken, a 0.5 wavelength dipole with wire radius of
.00674 wavelength will have a thickness parameter OMEGA of 10. For
this value, I would expect the reactance to be in the range of 38 to
43 ohms or so, based on theoretical calculations of various authors.
But Brown and Woodward (1945) show a somewhat higher measured
reactance of around 48 ohms for this case. I do notice that the
theoretically calculated reactance tends to increase from first to
second order analyses.

Here are what I calculate for the reactance of a 0.5 wavelength dipole
of .00675 wavelength radius. The NEC-2 runs included an EK card so
that the extended thin-wire kernel was used.

                                Segments
                          31 21 11

Program

NEC-2S 46.10 47.58 49.12
NEC81(1) 46.00 47.63 49.12
NEC-4.1(2) 44.78 47.53 49.81

(1) Version of NEC-2 distributed by ACES
(2) Results from single and double precision versions were identical

The range of segments were chosen to fit within the conservative
recommendations for maximum segment length and minimum segment
length/radius ratio.

Another interesting phenomenon was that NEC-2 and NEC-4 showed a
decrease in reactance as the antenna became thinner. This trend is
opposite the direction shown in the theoretical works I'm familiar
with, and with Brown & Woodward's measured data.

I know that many have been over this ground before, and that NEC-2 and
NEC-4 have been exhaustively compared to theoretical and experimental
results for simple antennas like a dipole, as well as more complex
antennas. Would someone be so kind as to briefly summarize the results
of these comparisons for 0.5 wavelength dipoles and/or where I can
obtain a copy of them? Were the theoretical reactances shown to be
low, or is there a small but known error in the NEC results? And what,
briefly, is the explanation for the fact that NEC-reported reactance
decreases as the antenna gets thinner but increases in at least some
of the theoretical and experimental measurements?

Thanks!

Roy Lewallen
Received on Fri Mar 12 1999 - 04:55:37 EST