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Re: Helix Feed for a Dish



The beam width (B) of the feed should be 4*arctan(1/(4*F/D)) which is 116
degress for an F/D of 0.45. The length of the helix for -10 dB at the dish
edges should be 8400/b^2 which is 0.624 wavelengths or 3.07 inches at 2400
MHz.

Some people use a turns spacing of 0.23 wavelengths and reduce the number of
turns to acheive this length.  However a helix will work down to a turns
spacing of 0.13 wavelengths and the axial ratio (circularity) improves with
the number of turns. Either a 3 or 4 turn helix should work.

73,

John
KD6OZH

----- Original Message -----
From: "Woody" <kj4so@bellsouth.net>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, 06 June 2001 16:36 UTC
Subject: Fw: [amsat-bb] Helix Feed for a Dish


> Hi All:
>
> RE: "Is a 2 1/4 turn helix sufficient for use with a .9 meter,
center-focus
> dish
> with a focal length of .45?"
>
> I screwed up.  I should have said that the F/D is .45, NOT the focal
length.
>
> 73,
>
> Woody
> KJ4SO
>
>
> ----- Original Message -----
> From: "Woody" <kj4so@bellsouth.net>
> To: <amsat-bb@AMSAT.Org>
> Sent: Tuesday, June 05, 2001 22:36 PM
> Subject: Re: [amsat-bb] Helix Feed for a Dish
>
>
> > Hi All:
> >
> > There have been many very interesting messages on the BBS lately on the
> use
> > of a "cup" style reflector on a helix dish feed antenna to reduce the
> > sidelobes and thereby reduce noise pickup on S-band dishes.
> >
> > However, I have not seen any discussion on the size and depth of the cup
> > reflector.  Has anyone experimented with various diameter cups as well
as
> > the depth of the cup?  The diameter and depth of the cups would be very
> > helpful to many people, myself included.
> >
> > Does the diameter and depth of the cup change between feeds used for an
> > off-set feed dishe, versus a center-focus parabolic dishe?
> >
> > Is a 2 1/4 turn helix sufficient for use with a .9 meter, center-focuse
> dish
> > with a focal length of .45?
> >
> > Thanks & 73,
> >
> > Woody
> > KJ4SO
> >
> >
> >
> > ----
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> > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
> >
>
>
> ----
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