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Re: short helix feeds



What turns spacing are you modelling? The spacing can be decreased to
squeeze more turns into the same length helix and improve the axial ratio
without increasing gain.

73,

John
KD6OZH

----- Original Message -----
From: "Howard Long" <howard@howardlong.com>
To: <nx7u@arrl.net>; <amsat-bb@AMSAT.Org>
Sent: Tuesday, 24 July 2001 19:22 UTC
Subject: RE: [amsat-bb] short helix feeds


> Hi Scott
>
> > My question to that gentleman
>
> ... you obviously don't know me ;-) ....
>
> > (or anyone else for that matter)...I have
> > had pretty good success, in general,
> > modelling helices with NEC.  My
> > modelling shows some pretty poor axial
> > ratios/circularity for these
> > short (< 5 turn) helices.  Does anyone
> > else's modelling/field experience
> > show the same thing?
>
> That's a good point, so I just re-did the models including axial ratios.
> Note that this is based on a pretty faithful model (I hope?) of a G3RUH
> helix (except matching shim - I just don't seem to be able to model that
> right), using a 125x125mm square reflector. Otherwise all wire dimensions
> and all distances quoted in that spec. are used in the model. No original
> thought on my part.
>
> For the sake of completeness, here's the on-axis axial ratios at 2401MHz I
> just calculated:
>
>  Turns  G (dBi)  F/B (dB)  3dB BW  10dB BW  Axial Ratio (dB)
>   2.25    9.6      13.3     61.2     116      2.24
>   2.75   10.0      14.8     57.2     107      2.50
>   3.25   10.6      14.6     52.8      95      2.46
>   3.75   11.0      13.2     50.8      88      2.35
>   4.25   11.2      13.3     49.7      85      1.91
>   4.75   11.4      15.0     48.1      81      1.36
>   5.25   11.8      15.7     46.0      78      1.37
>   5.75   12.0      14.2     44.3      74      1.72
>   6.25   12.2      13.8     43.3      71      1.72
>   6.75   12.5      15.4     42.1      69      1.27
>   7.25   12.7      16.8     40.6      66      1.06
>   7.75   12.9      15.5     39.4      64      1.28
>
> I'm not qualified to say if these axial ratios are 'good' or 'bad', but it
> suggests that you are right that the helix has to learn its circularity.
> Between 1 & 2 dBAR seems to be within most commercial antenna's
> specifications I've seen. It's likely that using a coaxial match behind
the
> reflector with the helix presenting a ~ 140 ohm impedance may improve
things
> somewhat, but at the expense of simplicity.
>
> 73 Howard G6LVB
>
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