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Re: Helix Feed question



Another note:  The "bar-b-que" dishes need to be covered with window
screening or a similar surface to be operated in CP mode.  The grid in one 
axis and the lack of grid in the other axis means that the dish will only 
refect signals in one polarization plane, and will otherwise act like a 
diffraction grating.  The net result is that your helix would act like a 
dipole feed if you didn't resurface the dish with screening!!  (-3db)

I picked up a couple of Conifer dishes w/o feeds and I intend on building 
helix feeds for them time permitting (so many projects... so VERY little 
time...)  I'll post info on how to do this when I'm done.

My experiments with the 18" RCA DSS dish listening to AO-40 show that from 
the end of the helix to one turn in at the focal point (12.75" for Conifer 
dishes off the top of my head) is correct, but I'd advise (with any amateur 
dish setup) to make a few inches of adjustment room as our homebrew feeds 
are not perfect, and having the ability to adjust this while listening to 
the bird (peaking) is very nice indeed...

Fred Spinner, W0FMS


>From: "Bill Magnusson" <williamb@netc.net.au>
>To: "Bill Hurlock" <bill@hurlock.net>, <amsat-bb@AMSAT.Org>
>Subject: Re: [amsat-bb] Helix Feed question
>Date: Thu, 1 Feb 2001 09:12:06 +1100
>
>
>
> >If I have a 3 turn helix and I want to use it with my Bar-b-Q dish.
> >Where is the helix placed in regard to the focal point of the dish.
> >Example, Is the helix reflector placed a focus?
> >
>Hi Bill,
>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.
>Bill...vk3jt
>
>
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