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Re: Helix feed for dish



Thanks Ed,

I found a very good online reference after I posted my question.  It is the
W1GHZ microwave antenna book at
http://www.qsl.net/n1bwt/contents.htm

In chapter 2 he gives a formula, BW3db=A*SQRT(3/taper), where BW3db is
the -3db beam width, A is the required beam width and taper is the field in
dB at the edge of the required beam.  Plugging in my numbers:
49=90*SQRT(3/10)

So a feed with a 49 degree -3db beam width should give a 90 degree -10 dB
beam width.

I'm hoping to have the antenna ready for tonight's pass even though it is
just after perigee.  Otherwise I'll try tomorrow afternoon.

Another thought that has occurred after reading W1GHZ's chapter (5) on
offset fed dishes is that -10db may not be the optimum taper since the feed
is looking up away from the Earth.  Your feed might not have the degradation
that you're expecting due to spill over.  He also discusses having measured
higher efficiencies on offset dishes since the feed doesn't block the
highest gain portion of the pattern.

Thanks again,
Lee-KU4OS

----- Original Message -----
From: "Edward R. Cole" <al7eb@ptialaska.net>
To: <amsat-bb@AMSAT.Org>
Sent: Saturday, April 07, 2001 14:00 PM
Subject: Re: [amsat-bb] Helix feed for dish


> Lee,
>
> I don't think I have ever encountered formula for antenna beamwidth other
> than -3 dB [i.e. HPBW].  This is the standard by which antennas are
> normally compared, and is sufficient for most terrestrial application.
> Unfortunately for space comms we need to concern ourselves with antenna
> noise temperature which is greatly affected by feed spillover at the dish
> edges.  You are obviously cognizant of this.
>
> I am not familiar with the Sony antenna so must rely on your measurements.
> The only way I have found for choosing feed antenna designs is by
> conducting pattern analysis [either by real measurements, or computer
> simulation].  I chose a 5-turn helix for my RCA DSS offset dish using the
> later [NEC4WIN95VM].
>
> I have reported its parameters in earlier posts, but have discovered that
I
> made an error measuring beamwidth from the polar pattern plot.  My 5-turn
> helix is only -9 dB at 90 degrees bw, and first [-20 dB] null occurs at
120
> degrees.  Not nearly as good as I first reported.
>
> Since you have determined your requirement to be a -10 dB Illumination
> angle of 90 degrees, the 5-turn helix would appear to satisfy.  For my 71
> degree requirement, my 5-turn helix is only -6 dB, so I will see
> significant degradation of system noise temp [but my first attempts to
hear
> AO-40 S2 will be using this since it is built].  Later I may play with a
> cavity helix design or pyramidal horns.
>
> I hold little hope for hearing the satellite in its current attitude from
> my latitude [60.675] since my AOS-LOS occurs only near apogee [>30,000
km].
>  The only station obtaining tlm in this part of the orbit is the 12m dish
> of the US Naval Academy [Bob Bruninga].  Well I'm only 29 dB down from him
> in gain :>0.
>
> Its my expectation that I will need to think about using my 2.5m dish
[then
> I will only be -15 dB from Bob's dish!].
>
> Anyway, Lee, it looks like a 5-turn helix would work for you.  BTW the
> effective aperture for your dish is probably that of a circular 18-5/8
inch
> dish [approx. 19.5 dB].  Good luck!
>
> Ed
>
> >From: "Lee" <lee-fl@cfl.rr.com>
> >How many turns should a helix be to give a -10 dB beam width of 90?  So
far
> >all I've been able to find is formulas for -3 dB beam widths.
> >
> >I've been given a Sony SAN-18S2 DSS dish which I'm going to try to use
for
> >AO40 on S-band.  I'm building a helix feed for it and am stumped trying
to
> >figure out how many turns I need to properly illuminate the dish.  Since
it
> >is an offset fed dish, I measured the angle from the top to the bottom as
90
> >degrees when viewed from the old feed horn location.  Measuring from side
to
> >side was also 90 degrees.  Based on what I've read so far I would want a
> >feed with a -10 dB beam width of 90 degrees.
> >
> >The dish dimensions are 473mm (18 5/8 inch) wide and 538mm (21 3/16 inch)
> >high.
> >
> >Thanks,
> >Lee-KU4OS
>
>
> ----
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