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Re: Helix Feed for a Dish



The original message asked about a helix as a feed for a dish and the cup
sould be as long as the helix (usually between 3/8 and 3/4-wavelength) in
order to reduce sidelobes. I haven't tried a cup on a long helix.

The Kraus book shows two solutions for a long helix. One is a short
3/4-wavelength diameter by 3/8-wavelength long cup which increases the
forward gain. No claim is made about decreasing sidelobes.

The other is a "helicone" which is touted as having low sidelobes. Here the
cup is changed to a conical horn with a diameter inceasing to sqrt(2.5*L)
wavelengths at the open end of the helix, where L is the length of the helix
in wavelengths.

73,

John
KD6ZOH

----- Original Message -----
From: "John Wright" <g4dmf@AMSAT.Org>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, 06 June 2001 17:35 UTC
Subject: Re: [amsat-bb] Helix Feed for a Dish


> At 07:30 06/06/01 +0000, you wrote:
> >The dimesions suggested by Kraus in "Antennas" for a cylindrical cup are
> >3/4-1 wavelength in diameter and the same length as the helix. Mine is 4"
> >square by 2-1/4" long.
> >
> >73,
> >
> >John
> >KD6OZH
>
> Do you mean the *length* of the helix, or just of 1 turn? My helix is 3.5
> turns and 3" long. Do you mean for the cup to be as deep as the helix is
> long?. If this is so, what sort of length cup would be suitable for say a
> 20 turn helix? Would it still be effective if it was say only 1" deep,
this
> would make mounting the feed easier in my case.
>
>
> ____________________________
>
>       G4DMF  QTHR   IO93HA
>       Amsat-UK Member 5372
>           g4dmf@amsat.org
>       http://www.g4dmf.co.uk
> ____________________________
>
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