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More on Stearing Antennas

I am yet another neophyte Satellite wanabee wrestling with the same antenna Az/El
questions.  I have been agonizing on this subject for a couple of months now.
There is a design concept for elevation in some versions of “The Radio Amateur's
Satellite Handbook” published by the ARRL.  It is a hinged design and has
simplicity and ruggedness going for it.  This hinged design, coupled with a 100
oz-inch or so stepper motor and a 12in or longer lead screw, can provide
tremendous torque to a horizontal boom.

The price paid for the mechanics of this approach is some “non-linearity” during
the lead screw travel and a rather stately speed of 120 seconds for a full
traverse.  The lead screw is essentially a secant for much of its travel but the
mathematical details of its traverse can be compensated for by software.  The
speed of the traverse is dependent on how fast you can push the stepper motor.
The 120 second traverse figure is conservative.  It presumes a maximum of 2 rps
from the stepper motor.  Since stepper motors come in all manner of different
torque’s, steps per revolution, voltage, current… etc. it is hard to predict the
rotational speed of a stepper motor shaft.  I expect to cut that figure in half
and achieve 60 seconds or better for 90 to 100 degrees of elevation.  In any case
I am not aware of any useful satellite that traverses horizon to horizon in 4

Control of the stepper will be via PIC chip and MOSFETs.  Control of the system
will ultimately be an RS232 conversion or straight USB port of a MAC G3 running
“MacDoppler Pro”.  Interim control will be by rotary encoder and LCD display.

Azimuth, at this point, will be crude.  I intend to use a Radio Shack TV antenna
rotator since my first set of beams will be modest.

I hope to have this all running in time for P3D.

Bronson Crothers
5764 SERC
University of Maine
Orono,  ME.  04469

Ph: 207 581 2252
Fx: 207 581 2255

Email: bronson@eece.maine.edu

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