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Helix design question

Hi folks,

Since I'm STILL not having any joy with my S-band setup, and the Drake
plus Conifer preamp check out ok on the bench, I'm looking at the antenna
as the source of the problem.

The current antenna is an MMDS/ITFS grill-type from Lance.  I will try
making a 16 turn Helix (K5OE) this week to see if things improve, but
longer term I expect I'd have better reception if I keep the grill and
replace its feed with a 3 turn helix.

Two questions:

1) Am I correct that a small helix + Grill will outperform a 16-20 turn
helix?  I've heard that you shouldn't put a circular feed on a wire
"dish", since the wires run primarily in one direction.  Makes a mess
of the circularity of the antenna..

2) If I replace the grill's current feed, can I mount the helix on the
metal tube that sticks out from the center of the grill?  In other words,
can a helix antenna have a metal shaft running up through the middle of
the helix coil, and attached to the ground plane?  Or, does it need to
be non-metalic (wood/plastic)? 

Also, I assume that the helix should be mounted such that the base of
the helix (at the ground plane) is at the focal point of the dish.
How critical is that positioning?

Thanks for your help,

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