Re: NEC-LIST: Image Plane Log Periodic Array

At 8:24 AM +0200 3/29/03, David de Haaij wrote:
>We are building an image plane log P (Log Periodic Monopole Array or
>LPMA in short) on a finite ground plane. I am trying to construct a
>model in NEC2, but I am currently unable to implement a finite size
>ground plane. Has anybody done something similar in the past? If so,
>what did you do to make it work?

I modeled a turnstile antenna on a finite, regular-hexagonal,
ground-plane whose "diameter" measured between opposite corners was
equal to 4.5 wavelengths (so the length of a side would have been
2.25 wl). I represented the ground-plane by an
equilateral-triangular mesh of one-segment wires. IIRC, the wire
length was 0.1 wl, possibly less; and the wire radius was set in
accordance with the "equal-area" rule. I exploited the six-fold
symmetry to reduce the number of unknowns in the solution by a factor
of six. (This required me to construct a six-arm turnstile.)

The gain calculated by NEC-2 matched the gain of a real antenna
measured in anechoic chamber at 1.57542 and 1.22760 MHz very well
down to at least 20 dB below the peak. I probably still have some
graphs that I could send you.

(I recall also that I tried densifying the innermost portion of the
ground-plane mesh (nearest the turnstile) by halving the wire length,
to 0.05 wl. I don't recall whether this made a significant
difference in the results.)

AFAIK, there's nothing magic about a triangular mesh or an overall
hexagonal shape. You could use a square mesh and a rectangular or
square overall shape.

-Chuck.

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