NEC-LIST: Re: Conical spiral: impedance convergency - current distribution

In another effort to investigate the quoted matter,
I can inform that I have tested the model against
data from an article [1,p. 638, Fig. 8] which deals
with the current distribution along an arm of
the conical spiral. I have simulated different
segmentation ratios, but there is still
no convergence in the pattern. However,
at ratios around 0.05, consistency with the
data from the article is apparent.
I would be glad to get comments on this issue.

The calculated graph is available on my personal page
http://www.e.kth.se/~awitte/segm.png

as well as the input file for the highest segmentation ratio
http://www.e.kth.se/~awitte/segm.nec

The current on one arm of the conical spiral
is plotted against the distance from the feed in wavelengths.
The different ratios are the maximum segment length in ratio
to the wavelength.

[1] Y.S. Yeh and K. K. Mei, "Theory of conical equiangular-spiral antennas.
Part I -- Numerical technique" IEEE Trans. Antennas and Propagation,
vol. AP-15 no.5, pp 634-639, Septermber 1967.

Best regards,
Alexander Witte

Electromagnetic Engineering
Royal Institute of Technology
Stockholm, Sweden

> Message: 1
> Date: Thu, 20 Dec 2007 13:26:04 -0500
> From: Alexander Witte <awitte_at_e.kth.se>
> Subject: NEC-LIST: conical spiral: impedance convergency
> To: nec-list_at_robomod.net
> Message-ID: <476A5624.3070607_at_e.kth.se>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Hi,
>
> I am modelling a conical rectangular spiral antenna for
> my thesis project. The nec-input file is generated
> by a perl script I have written. I have problems
> to see any impedance convergency, although the
> models went from just about 100 segments to over 3000.
>
> I sense that this might have to do with the
> unequal segmentation on parallel wires, even though
> the wires are at least hundreds of wire radii apart.
>
> Another factor might be the feed arrangement.
> I have currently two options:
>
> 1. The feed wire is diagonal and connected
> in the corners. This leaves a 45 degree angle.
>
> 2. The feed wire is parallel with the following
> spiral arms - 90 degree angle.
>
> Which feed arrangement would
> be more accurate for NEC?
>
> Does anyone have an idea how I could
> guarantee accuracy for the model,
> or how I change the model to get impedance convergency?
>
> Best regards,
> Alexander Witte
>
> Electromagnetic Engineering
> Royal Institute of Technology
> Stockholm, Sweden
>
>
>
> End of NEC-List Digest, Vol 5, Issue 58
> ***************************************
>

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