INTRODUCTION

One of the more  important  parameters of EME operation is aiming the 
antenna at the  moon. with  the  tight  beam width of stacked antenna
arrays,  pointing  accuracy  becomes  very  important.  This  article 
describes a system which  can  obtain 1  degree  positional accuracy.

THEORY OF OPERATION

The circuit shown in Figure 1  was designed and  constructed in a few 
hours and put into operation just prior to the  first  weekend of the 
EME contest. It consists of a  precision  double  regulated 10.8 volt 
power supply  to  drive  the  precision  potentiometers and a  5 volt 
supply  to  power  the  Digital  voltmeters.  Two  3  turn  precision 
potentiometers  (0.1%  linearity)  are mounted  on the tower.  One is 
used  to  indicate  elevation  angle and the other is used to provide 
azimuth  readout.   The elevation pot is mounted and turned much like 
the conventional syncro.  Two  fishing  sinkers  were attached to a 1 
inch aluminum  bar  which  acts  like a pendulum.  As  the antenna is 
elevated the weight seeks gravity and turns the pot.  The azimuth pot 
is attached to the tower with an aluminum bracket.  A  plastic pulley 
was made by a  local  machinist the  some exact  diameter as the main 
vertical support most.  This  turned  out  to  be 2.016 inches. A 1/8 
inch  diameter "O" ring is used  as a  drive  belt. This  combination 
provides a  1 to I turning  ratio. 10.80 volts is fed into the top of 
each 3 turn precision pot.  By the use of high powered moth it can be 
shown that 10.8 divided by 3 is 3.6.  Therefore, 0-360  degrees  will 
equal 0-3.6 volts and 0-90 degrees will equal O-.9 volts.

CONSTRUCTION

Assembly of this indicator is  straightforward and easy to do.   Most 
of the parts can be obtained from Jameco (http://www.jameco.com). The
digital panel meters, M1 and M2 are available from NON-LINEAR SYSTEMS 
The basic model number of this meter is  X-30 with a -10 added for l0 
volt range and a /PR added for 0.1% accuracy. These meters are avail-
able directly from the factory for $35.00 each, plus shipping.   Tel. 
(619) 535-2161. Layout  of the  components is  not  critical and  any 
convenient layout should be acceptable. You might  consider including
the  AZ/EL controls and have the complete antenna control in one box.

ALIGNMENT AND TEST

To  calibrate  the  readout,  it  is only necessary to set the output 
voltage to  10.80v.  This  voltage  is critical  and  will  partially 
determine the  accuracy of the readout system.  Connect the precision 
pots prior to installation on the tower.   Adjust the pots and verify 
the  meters  are  reading properly.  note:  Do not apply more than 1O 
volts to the meter input.  No damage will occur, but the display will 
blank.) Standard weather proofing  techniques  should  be  used  when 
mounting the pots on the tower. (See Figure 2.) To  calibrate  the AZ 
pot, position the antenna to the O degree point  (North) and  set the 
AZ pot wiper to  the  ground  end.   Connect the  drive belt.  As the 
antenna turns  toward the  east,  the  voltage on  the  wiper  should 
increase. To calibrate the  EL pot, position the antenna at O degrees 
and set the EL pot  wiper to the ground end.  Elevate the antenna and 
verify the wiper voltage increases.  If either wiper voltage does not 
increase, you are probably not set to the ground end.   Reversing the 
outside wires on the ot should cure the problem. The digital  readout 
was installed prior to the EME contest  and  the only  problem I have 
encountered with the digital readout was a defective  LSI chip in the 
meter. NLS  supplied me  with a  replacement free  of charge and  the 
meter has performed  completely satisfactorily since. I am completely 
happy with the results of this project.

73,
Jerry KY4Z

Article reprinted from EME Newsletter
Editing and redraw by W5UN