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Re: Helix Feed for a Dish



>From: "Woody" <kj4so@bellsouth.net>
>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

Woody,

I would say that a 2-1/4 turn helix would have too wide a beamwidth for a
f/d=.45.  The Helix I modeled and built for my 8-foot prime-feed radio
telescope, which has an f/d=.35 is 3-1/4 turns. So a guess would be to use
a 4-turn helix for your dish.  You can adjust the beamwidth of the helix
wider slightly by squeezing the turns closer together.

Another approach is to use the "cup" cavity to narrow beamwidth.  Then
maybe a 3-turn helix would be right.  Helical feeds are pretty simple to
build and are cheap.  Why not try different length helix's to see which
works best?  Ideally you would ude the same mounting method for rapid
exchange of feeds.

Ed

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