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Re: Helix feed for dish


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!


>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)

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