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RE: Helix Calculations



I've added prime feed focus dish f/D figures if you happen to be using one
(i.e., _not_ an offset fed dish) and know the f/D but not the illumination
angle. If you're using an offset fed dish, stick with matching the 10dB
beamwidth to the angle subtended by the dish at the focal point of the dish.

 Turns  G (dBi)  F/B (dB)  3dB BW  10dB BW    dBAR  Prime f/D
  2.25    9.6      13.3     61.2     116      2.24     0.45
  2.75   10.0      14.8     57.2     107      2.50     0.49
  3.25   10.6      14.6     52.8      95      2.46     0.57
  3.75   11.0      13.2     50.8      88      2.35     0.62
  4.25   11.2      13.3     49.7      85      1.91     0.64
  4.75   11.4      15.0     48.1      81      1.36     0.68
  5.25   11.8      15.7     46.0      78      1.37     0.70
  5.75   12.0      14.2     44.3      74      1.72     0.75
  6.25   12.2      13.8     43.3      71      1.72     0.78
  6.75   12.5      15.4     42.1      69      1.27     0.80
  7.25   12.7      16.8     40.6      66      1.06     0.84
  7.75   12.9      15.5     39.4      64      1.28     0.87

I've also had a question about why I used odd numbers of quarter turns. This
is for two reasons:

a) Ed Krome on page 118 of Mode S - The Book (2001 edn) describes how the
return losses minimize at odd numbers of quarter turns;

b) Especially at turns < 5, the far field patterns look more symetrical and
prettier with odd numbers of turns than when using even numbers of 1/4
turns.

FWIW, I've seen there's a school of thought which suggests that one should
use, say, 15dB beamwidths for weak signal extra-terrestrial operation
because of the additional earth noise you get on your antenna as the feed
overspill points to the ground.

By the way, I invite fellow modellers and experimenters to submit their
results to either support or dispute any of these results.

73 Howard G6LVB

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