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RE: 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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