[amsat-bb] FODTRACK Interface

[Jesse Morris <w4mvb@xxxxxxxx>]

Good Morning to All,

Recently there was some discussion here about the value of the resistors
in the comparator circuits which create the "dead zone".  I have been
using the interface for three or four years and have always had some
hunting instability, especially in azimuth.  I had always attributed the
problem to my haywire antenna system which consists of a CDE TR-44 for
azimuth and an "Orbit 360" TV type rotator for elevation.  The TR-44 does
not have a brake and the use of a 22 element 2 meter antenna on an 18
foot boom and two 22 element 435 antennas on 14 foot booms with PVC for a
cross boom gives me a lot of twisting and torquing.  I had intended to
try changing the value of the 1 meg resistors in the comparator for some
time but that has always been one of those "round-to-it" jobs.  Instead,
I had just increased the step size to try and minimize the problem.  Last
week I finally completed the "helix project"  (I'll report on the results
of that separately soon) and 15 degree steps were just too large.  I
reduced the step size to 5 degrees and was unable to live with the
instability.  Yesterday I decided to try the resistor change so I found 4
1.5 meg resistors in the junk box and put them across the existing 1 meg
resistors.  To my surprise, the resultant 600k resistance solved the
problem completely!   Even down to the southeast where I have a bad place
on the pot I only got one overshoot out of six satellites I tracked
yesterday.  I don't know what the resultant "dead zone" is but my
accuracy seemed to always within 1 degree.  My pointing accuracy is not
that good because of the pots in the rotors but the antenna always went
to within 1 degree of the value the pots were outputting so I don't think
the reduced value is significant as far as point accuracy is concerned 
(unless you are trying to point a 10 foot dish!).

Results with the quad 30 turn helix array were somewhat encouraging and I
will report on that soon.  I also completed building a 4 foot dish last
week but the initial test results with it were not very encouraging. 
More on that later.


73's de Jess - W4MVB
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