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Re: Helix Feed for a Dish



John, Woddy, et al.,
Kraus' dimensions are a logical starting point, and are about what I used (ditto K&W in their work).  Neither, however, address the use of the helix w/cup as a parabolic dish feed in general, nor specifically for small (less than 10 lambda) dishes.  

My throw-down cups are 4" in diameter and 2" deep, just like the references above.  In further analysis of the application, however, these dimensions appear to be less than optimum for application as a dish feed.  As it turns out, a simple geometric analysis reveals the dimensions of the cup are dependant on the dish geometry (ignoring effects on helix) and applying a little trigonometry to the spacing reveals a potential design criterion for the helix size (number of turns).  Here's my reasoning (no field trials yet):

1.  The f/D ratio of the common MMDS BBQ antennas appears to be (commonly) 0.55, and is the basis for this discussion.
2.  The often-recommended 3-turn helix has a -10 dB beamwidth of about 120 degrees.  With a nominal focal axial distance of 12", that turns out to be ( 2*f*tan(120) ) a nominal 1 m dish for the conventional illumination "standard" of -10 dB.  Different f/D's will give different results, but the implication is clear.
3.  If you have a smaller dish (like mine, barely 0.5 m dia. equiv.), the spillover from a short helix is very, very large.  This is where adding a cup, acting as a feed horn, plays into improving the G/T of the dish.  By reducing illumination pattern, the spillover is reduced and the G/T is improved.
4.  While the location of the phase center of a helix is subject to much debate, my "tribal knowledge" leads me to assume it is the 180 degree point of the first turn (current maxima closest to the feed).  Nominally, this will locate it about 1/4" to 3/8" from the feed/reflector.  So... with geometry of a .55 f/D dish limiting you to a nominal 90 degree beamwidth, I come up with the follwing two design parameters:
a) the helix should be -13 dB (for best G/T, see W1GHZ chapter 4) with a 90 degree beamwidth, corresponding to about 5 turns, assuming 1 dB gain from cup.
b) the cup axial dimension should be [(0.5 * D) + PCO].  PCO is Phase Center Offset.  This corresponds to an optimum axial length of 2.25" based on my practice of close-spacing the helices' first turn.  

These parameters are for this dish only!

Comments, suggestions for improvements, kindly offers of other pertinent references, gentle explanations of where my logic is flawed, are all welcome.  Now, where can I find an al can with a 4" OD...
73,
Jerry, K5OE



In a message dated Wed, 6 Jun 2001  3:35:31 AM Eastern Daylight Time, "John Stephensen, KD6OZH" <kd6ozh@gte.net> writes:

<< 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

----- Original Message -----
From: "Woody" <kj4so@bellsouth.net>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, 06 June 2001 02:36 UTC
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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