Re: NEC-LIST: Example helix antennas for NEC2?

Hi John,

The model you included in your last email
contains some geometry violations.

Segment 17 is within the volume of Segment 1
Segments 13 and 21 cross at the midpoint
Segment 17 is within the volume of Segment 16
Segments 16 and 19 cross at the midpoint

Although these are considered warnings they may be
affecting your results. I didn't have time to
validate the accuracy of the above but I believe
they should be looked into.

Regards,

Joe
NSI

At 04:32 PM 7/7/02 -0700, you wrote:
>A number of folks were kind enough to respond, but alas, not much proved
>to be very helpful. Not one person supplied a NEC2 deck. Three said i
>suggested using their software (two commercial and one not), none of which
>is very useful, especially given i'm neither running Windows nor using an
>Intel-compatible processor. The non-commercial person also supplied a
>source file, one of only two folks who did. A couple of people gave some
>general guidelines, much of which i already knew.
>
>The supplied file(s) were all in formats other than NEC2, and a third
>person supplied a link to one (also in another format) which was the best
>one i had found prior to my earlier posting. Alas, the one which did have
>a plausible impedence didn't use a reflector (used a ground card instead??)
>came out as 10dBi (where 18dBi was predicted by a formula found elsewhere),
>both forward/reverse. In the other case (a pair of helix antenna, with
>different reflector mesh intervals), was for NEC4. My attempts to convert
>the version whose compute time seemed feasible for my computer yielded an
>impedence of 154 +j281, whereas the author reported the pair as giving
>195 -j138 or 185 +j30 (not completely clear which was which).
>
>Attached is one of my own attempts, starting with ideas from AF9Y. It
>also only gets about 10dBi (where 17dBi is predicted), but at least has
>good impedence (approx. 50 ohms, j<8 for 1240-1300 MHz) and the pattern
>looks plausible. But it does use a ring reflector, which may or may not
>be as good as a simulated plate. It is constructed for 300 MHz (that is,
>1 meter = 1 wavelength) and scaled to the target frequency. I'm not sure
>i'm exciting it properly. Feedback/suggestions are welcome.
>
>=> Again, can anyone supply a NEC2 deck which models a helix well for both
>=> impedence and pattern?
>
>Also, do you get similar results with the attached file? Or is perhaps my
>copy of NEC2 (or NEC2 in general) broken in some way? Or does NEC2 not do
>well at modeling this type of antenna?
>
> -- KD6PAG
>
>-------------------------------------------------------------------------------
>CM Experimental Helix
>CM
>CM Reflector radius: 0.183
>CM Reflector element: 0.02
>CM Feed element: 0.02
>CM Feed length: -0.016
>CM Helix element: 0.02
>CM Segment 1 turns: 0.25
>CM Segment 1 radius1: 0.159
>CM Segment 1 radius2: 0.159
>CM Segment 1 spacing: 0.125
>CM Segment 2 turns: 5
>CM Segment 2 radius1: 0.159
>CM Segment 2 radius2: 0.159
>CM Segment 2 spacing: 0.25
>CM
>CE
>GA 1 16 0.183 0 360 0.02 0 0
0
>GM 0 0 90 0 0 0 0 0
1
>GW 2 2 0.183 0 0 0.159 0 -0.016
0.02
>GH 3 3 0.125 0.03125 0.159 0.159 0.159 0.159
0.02
>GM 0 0 0 0 0 0 0 -0.016
3
>GH 4 60 0.25 1.25 0.159 0.159 0.159 0.159
0.02
>GM 0 0 0 0 90 0 0 0.01525
4
>GS 0 0 0.2364
>GE
>EX 0 2 2 0 1
>FR 0 11 0 0 1200. 10
>XQ
>FR 0 6 0 0 1260. 2
>XQ
>FR 0 1 0 0 1269 0
>RP 0 1 73 111 90 0 90 5
>RP 0 73 1 111 90 0 -5 90
>EN
>===============================================================================
>--
>The NEC-List mailing list <nec-list_at_gweep.ca>
>http://www.gweep.ca/mailman/listinfo.cgi/nec-list
>
>

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