NEC-LIST: Re: Conical Spiral: Impedance Convergence

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> Today's Topics:
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> 1. Re: Conical spiral: impedance convergency - current
> distribution (Alexander Witte)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Thu, 03 Jan 2008 15:53:19 -0500
> From: Alexander Witte <awitte_at_e.kth.se>
> Subject: NEC-LIST: Re: Conical spiral: impedance convergency - current
> distribution
> To: nec-list_at_robomod.net
> Message-ID: <477D4B3F.5090900_at_e.kth.se>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> 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

I can see a number of errors in your code:

1. All tags show - Seg. Len/Rad < 2. "In NEC 2 this is especially
   important to keep segment lengths about 2 times the wire diameter"
   (where EK is invoked).
   (Cebik - Intermediate Antenna Modeling, p1-9).
2. Warning 978 is within volume of 1969. (Tag 102)
3. Warning 1969 is within volume of 979. (Tag 103)
4. Warning 1957 is within volume of 1959. (Tag 101)
5. Warning 1959 is within volume of 1958. (Tag 103)
   These warning refer Specifically to the sharp junctions
   at each end of tag 103.
6. The segment lengths of tag 103 is significantly different
   than all other tags in the model. They should be equal.
7. Probably not important, but I would have placed the source
   in the center of tag 103.

I am using GNEC V 1.6 running the NEC 2 core.

Regards,

Frank Meredith
Calgary AB

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