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Re: Dish aiming



 > From:          "John Boudreau" <john@newnorth.ca>
 > To:            "Roy Welch" <rdwelch@swbell.net>
 > Cc:            <amsat-bb@AMSAT.Org>
 > Subject:       Re: [amsat-bb] Dish aiming
 > Date:          Fri, 3 Mar 2000 09:33:24 -0700

 > I am currently writing software and designing a position feedback system for automatic pointing.  
The solution I am using to get around the AZ accuracy problem is to 
use two inputs for azimuth, one f> John - VE8EV
 > 

Dear John,

Some comments and ideas for thought.

Your going to need a better solution for the voltage then simple diodes.  Zenors do not have
all that a sharp "knee" where they start and stop conducting.  Ordinary silicon diodes make poor
zenors.  Plus both change their break down point as their temperature changes.
You can use a diode as a temperature sensor by making use of this effect.
I would think you would be better off with staying with 0-5 volts for 
the entire 360 degree range and going to a higher resolution A/D converter.

Another problem with high resolution partly analog systems is noise.  Mechanical
sensors like pots are noisy.  At least mine are.  How will the noise be filtered out?
How will it affect the response and accuracy of the system.  Maybe you should 
consider a optical encoding system.

I can see some mechanical problems with the potentiometers as well.  You have
to couple (any sensor) to the rotating antenna.  Slop in the coupling/gear train/levers
can exceed the tollerance of your design goal.

A better solution is to be more realistic about the beam width.  What's wrong with 10-15 degree
beam widths?  Has anyone seriously considered what the trade offs are for beam width
vs. transmitter power output on the different bands?

If for example, if I have 15 watts of SSB at the antenna at 1.2Ghz 
for uplinking, is that enough RF power with a 15 degree beam width 
antenna? 

The reverse could be asked.  If I have a 15 degree beam width antenna 
on say the 6cm band, how much RF power will I need at the antenna?  A better
RF designer then me needs to answer these questions.

Keep in mind that as the antenna gain increases (ERP increases as well),
the beam width decreases and the pointing problems increase.   It might be
better to design your system for wider beam widths to make antenna pointing
and tracking simpler.  

--73-- David WA0AUQ


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