Re: NEC-LIST: NEC2 versus NEC4

>If you have compared modelling between the two versions and
>compared NEC4 to actual measurements of real ground based
>antennas I would appreciate your evaluation as I am sure
>others also would.
>//
> Forrest k2bt

Forrest, a comparison of NEC-2 and NEC-4 depends on the model. The
principle limitations of NEC-2 follow:

1) Everyone knows for that with NEC-2 wires making up the antenna
model cannot touch lossy (real) ground --- but wires can come very
close to real ground (several wire diameters). With NEC-4 wires can
touch ground, or penetrate ground, hence buried radial wires and
ground stakes can be modelled. I have no detailed comparison
experiment vs. simulation, but simulation looks reasonable --- and
compares well (at least for buried radial wire systems) with other
methods (e.g. methods developed by Jim Wait).

2) NEC-2 does not like joining wires having very different diameters
--- the guideline suggests a maximum difference of 2:1. That is why
some programs employ a diameter taper option when using NEC-2. There
is no problem with NEC-4 (also see below).

3) NEC-2 is not good for modelling multi-junctions, particularly a fan
antenna fed by a single wire. The treatment of charge distribution at
a junction is a problem with NEC-2. But NEC-4 and MININEC should do
this right. The condition used to determine the distribution of
charge at the junction with NEC-2 considers only the radius of each
wire, and not the proximity of neighboring wires. NEC-4 uses a
quasistatic approximation taking account of neighboring wires, so it
should do better.

For the simple wire antenna under discussion (a quad beam) there is no
difference. My TC yesterday assumed free space. For this antenna
over average ground (height as specified) 236.22 inches (I have
assumed that this height is to the bottom wire for the reflector):

        Gain = 13.52 dBi (13.52 dBi NEC-2)
        Take Off Angle 11.8 degrees (12 degrees NEC-2)
        Impedance 47.4 ohms* (47.4 ohms NEC-2)

73, Jack, VE2CV

* Copper wire, my note yesterday assumed aluminum wire.

_____________________________________________
John S. (Jack) Belrose, PhD Cantab, VE2CV
Senior Radioscientist
Radio Sciences Branch
Communications Research Centre
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2779
FAX 613-998-4077
e-mail <john.belrose_at_crc.ca>
_____________________________________________
Received on Thu Mar 02 2000 - 05:36:40 EST