Re: NEC-LIST: Ansoft HFSS vs NEC etc.

At 6:46 AM -0700 10/11/03, John Orr wrote:
>For precise global positioning, knowledge of the phase center of an
>antenna or antenna array (in the presence of adaptive beamforming)
>is critical. To begin to understand this problem I need to know the
>magnitudes and phases at the feed points for each element in an
>array (obviously including mutual coupling effects) for an incident
>plane wave from a given angle. This is the reverse of the typical
>antenna pattern calculation. I am a novice at computational E&M and
>the software that I have been using (Ansoft HFSS) does not seem to
>make it easy to do this calculation (but I may just be ignorant of
>some aspect of HFSS).
>Comments both on the problem and on the best choice of software
>would be welcome.

I don't know about HFSS, but NEC-2 or (better) NEC-4 can do what you
want quite well. I used NEC-2 to design a GPS dual-band (three L1
and four L2 elements interleaved, with L1 and L2 phase-centers
coincident) phased-array antenna for precise relative positioning via
carrier-phase measurements.

The measured input impedances of a couple of proof-of-concept models
of this antenna, constructed in close accordance with the NEC-2
model, were very close to the design value (50 ohms resistive) that
NEC-2 had calculated; the actual gain patterns were also close to
those calculated by NEC-2; and determinations of a short
relative-position-vector from carrier phase measurements of GPS
signals received by the proof-of-concept model antennas were accurate
within one millimeter.

For a description of this antenna see

Counselman III, Charles C., Multipath-Rejecting GPS Antennas, Proc.
IEEE, Special Issue on GPS, the Global Positioning System, Vol. 87,
No. 1, pp. 86 - 91, January 1999.

I probably still have this paper as a set of Microsoft Word 98 .doc,
and .jpg and .gif image files on an archive disk, and could email
them to you if you wished. I may also some relevant NEC-2 and/or
NEC-4 input files that you could play with.

I also had good success using NEC-2 or NEC-4 to model another
dual-band antenna for submillimeter-accuracy relative-positioning
with GPS: the Macrometer(r) antenna.

-Chuck.

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