Re: NEC-LIST: Trouble with dipoles

> Hello,
>
> I'm doing some simple dipole impedance calculations using NEC4. I'm
> modeling a 23.5cm half-wavelength dipole with a radius of .000425 in
> free space. I'm comparing these results to an finite-difference
> time-domain calculation of impedance for the same dipole. A frequency
> of 638 MHz corresponds to a half-wavelength of 23.5 cm. My NEC dipole
> contains 7 segments, and I'm using a frequency step of 22.5MHz in the
> NEC simulation to match the step-size in the FDTD simulation. I get a
> match frequency of 607 MHz from the NEC code, and a match frequency of
> 630 MHz from the FDTD simulation. (I have investigated running the
> NEC code with a much smaller frequency step of 1 MHz over the region
> of interest and again found a match at approximately 600 MHz) It would
> seem that the FDTD is giving more accurate results.
>
> I am looking for an explanation for the discrepancy. If anyone can
> give me any insight, suggestions, or corrections, it would be greatly
> appreciated. A listing of my NEC input file is below.
>
> Thanks,
> Tiffany
>
> CM LAMBDA/2 DIPOLE
> CE
> GW 0 7 0. 0. -.1175 0. 0. .1175 .000425
> GE
> EX 0 0 4 0 1.
> FR 0 255 0 0 22.5 22.5
> XQ
> EN

I would trust NEC. I cannot check your FDTD simulation.

Using NEC 4D (double precision), and a segment taper option, tapering
as one approaches the source to 0.0025 wavelengths --- in effect the
source is on the center of a wire 2.5 mm long --- 20 segments for my
model --- this is an overkill --- I calculate a resonant frequency of
607.1 MHz, impedance 65 ohms.

Jack
29 October 1998

_____________________________________________
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>
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Received on Wed Nov 04 1998 - 09:58:46 EST