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Re: Primestar Dish Characterized?



>From: Jim Walls <k6ccc@earthlink.net>
>Bill Acito wrote:
>
>> I have seen references to DSS dishes (e.g. Paul Wade), but none for the
>> larger Primestar dishes.
>>
>> Has anyone characterized one for f/D, proper illumination, optimal turns
for
>> a helix feed? Do you optimize beamwidth on the long or short dimension of
>> the dish?
>
>What I did is build a 6 turn helix and put the antenna with the helix as a
feed
>on an antenna range and start cutting the helix until I got maximum signal.
>Turned out to be 2 3/4 turns in my case.  I actually trimmed the helix
down to
>only 1/4 turn (just to see what happened) and then rewound it to the optimum
>length.

Jim,

Optimizing dish performance is a difficult task for us amateurs considering we
normally don't have professional measurement equipment and antenna ranges.
But
we do the best we can.

One caveat in your procedure.  The process you used is to determine maximum
signal strength...i.e. max antenna gain.  But it has been previously discussed
on this list that for optimum receive performance at 2401 our objective should
be minimum system noise figure.  The antenna portion of this is a balance
between gain and clean pattern.  It is well known that the feed
illumination of
a dish that produces max gain does not produce lowest sidelobes necessary for
low antenna noise temperature.  In general some sacrifice of gain is needed to
achieve min silelobe pickup of ground thermal noise.

The way to determine this performance is by making G/T measurements, usually
using sun noise {or ground/cold sky} observations.  Observing the
difference in
noise seen from the sun vs cold sky is achievable with simple equipment.  The
improvement you will realize will be subtle usually showing around one dB
improvement in S/N or MDS.  Yet that is often the difference between making a
contact or not. 

My intuition on the proper size helix feed for a Primstar would be longer than
2-3/4 turn.  But I do not have one or any specific knowlege on them, so I do
commend you on making and sharing your results.

Ed 
from the paper given at Symposium: 
Noise Figure and G/T measurements:
1. http://www.setileague.org/software/spreadsh.htm (cascade.xls) (g-t.xls)
2. G/T article http://www.setileague.org/articles/g-t.htm
3. 2400 MHz solar flux
http://solar.sec.noaa.gov/getftp.cgi?get=lists/radio/rad.txt 

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