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Re: "Precision" Antenna Pointing



Hi Kevin,


>I just got around to reading this thread now since I finally have S-band
>capability using a 60-cm dish ( G3RUH special) and 2.5 turn helix.  I just
>put together my S-band receive this weekend and am double checking things
>like focal length.  By calculation the focal length of my dish is 18.7-cm.
>What part of the feed section should be at that distance, the reflector
>plate, front, middle, or back of the helix?  I currently have the reflector
>plate set at the calculated focal length.


This question came up on the list just a week or so ago. I'll post the
original reply again.

It is generally recommended that placing the focal point at the end of turn
one is about right. That's the position I've always used and it seems to be
fine. I made an adjustable setup years ago using a 2.25 turn helix as
recommended by James Miller, and tried this very point. My experience seemed
to indicate the "turn-1" position was correct although the adjustment was
fairly broad and not all that critical.

The "first turn" is of course the first turn that the RF encounters when it
leaves the co-ax. (someone asked for clarification of this point).


Bill...vk3jt
Milawa
Australia



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