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RE: AO-40 Helix Feed ...How many turns ???



Hi Jeff

> I had a few people claiming that 2 1/2 to 3 turns is optimum. Is this
> pretty much a rule of thumb ??
> I am presently using 5 turns, should I scale it back to 2 to 3 turns ??

It depends primarily on the illumination angle at the feed point to
illuminate the dish. There was also some investigation done by Ed Krome K9EK
documented in his "Mode S" book which suggests that one should aim for an
odd number of quarter turns for matching purposes (this may assume that
you're using a quarter turn matching section). I can confirm that
empirically odd numbers of quarter turns did improve matching on the dozen
plus helices I've made.

I have some NEC modelled optimum turns at
http://www.g6lvb.com/el/addendum.htm#HelixCalcs based on a 10dB edge taper
(the beamwidth of the helix was 10dB down at the dish edges). For space
communications it might be better to use say 13dB or so as you are pointing
into cold sky. This last point in itself may be moot because of the
sidelobes on a helix. This is perhaps why a well-made patch seems to come
out better than a helix due to its reduced sidelobes. (In my experience a
patch is probably slightly more difficult to get working well as it is far
narrower band. Any small inaccuracies which may have little effect on a
helix will be more likely to upset things on a patch.)

73 Howard G6LVB

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