A  PRACTICAL  ROTATOR  SYSTEM  (APRS)®

 

APRS  SUMMARY:   This project is s simple 2 wire interface between the ubiquitous  Radio Shack antenna rotator and 2 bits on a parallel port.  APRStk.EXE (version k20) supports it.  I hope other APRS programs might consider it as well.   Any rotator which operates on about 30 volts AC and rotates at 1.0 RPM should work as well.   Read these instructions ENTIRELY before you start working on your rotator to be sure you understand which rotator you have and which wire is which…

 

WHY THIS WORKS:   For at least 12 of the amateur satellites a modest beam antenna is more than adequate to bring in signals from the digital packet and voice FM downlinks to your dual band FM radio. This Practical Rotator System can add full automatic antenna control to your PC for under $90.  This is a bargain compared to the $750 required for a Kansas City Tracker and Yeasu AZ/EL Rotator system.

 

The APRS rotator takes advantage of the geometry of low earth orbiting satellites which are within view of a 10 degree fixed elevation beam over  96% of all pass times.  In this region, using a short 4 to 6 element beam, your station will be able to maintain over 10 dB of relative gain on the bird whenever it is in view.  Low Earth orbits are unique because:

               

                 * 4-6 elements are enough gain to be above digital  and  FM voice thresholds

                 * Most LEO birds are mostly vertical most of the time.

                 * LEO satellites are below 45 deg 95% of the time and 6 to 10 dB closer when above it

 

The following table  is a plot of overall gain for the elevation angles shown.  Notice the cumulative percent column showing 96% of all pass times are below 50 degrees yet still give you at least 10 dB gain.

 

EL  PCT CUM-% RANGE RNG-GAIN ANT-GAIN OVERALL-GAIN

--- --- ----- ----- -------- -------- ------------

10   32   32  3030     0       10         10

20   35   67  2440     2       10         12

30   17   84  1827     5        8         13

40    8   92  1460     6        6         12

50    4   96  1190     8        2         10

60    2   98  1020    10        0         10

 

* Data for an 800 km orbit.  For the ISS at 370 km, the times below 30 degrees are 6% higher.

* If your horizon is blocked below 5 degrees anyway, elevate the beam to 20 deg to improve gain (+2 dB)

 

CONSTRUCTION:   The interface is built on a small perf board and placed inside the rotator and accomplishes the following objectives:

 

1)       Adding an ON/OFF switch to select manual or PC controlled operation. 

 

2)       Adding ventilation holes to keep the transformer cool.  After this mod, the primary of the

transformer is always on, so the transformer runs somewhat warmer than normal.

 

3)       Adding the TRIAC's and optoisolators to interface the PC to the ARCHEROTOR wiring.  This circuit uses a special balanced optoisolator design so that there is no chance that the bits on the parallel port can cause both TRIAC's to switch on at the same time.  The rotator goes left or right depending on the polarity of the logic level applied to the two wires.

 

 

 

 

MODIFICATION PROCEDURES:

 

The very day I finished this complete set of procedures for the ARCHEROTOR model 15-1225 I went to buy another and now it is a new 15-1245 model.  Fortunately electrically they are the same.   This one does have better ventilation, but the cover is harder to get off.  Differences  are noted where they apply.

 

1) OPEN:   Remove the ARCHEROTOR top plastic cover by inserting a thin screwdriver in the four slots around the edge and pressing the lower halfshell outward. (New model, remove the recessed screw in the bottom and then insert a knife in the front split between the top and bottom half.  When it pops open slightly, work the knife around to the side and it should come off easily).

 

2) ADD VENTILLATION, BOTTOM:  Remove the two screws holding the transformer in place and lift it out of the way.  Using a 1/4" drill bit or larger, drill 14 holes in obvious places.  There would be  4 rows of 4 holes but you do not drill out the mounting posts for the 2 screws you just removed.  Also drill a 1/4" hole in extreme lower right front of the base for our new ON/OFF switch. (This is still needed for the new model, because there is still no ventilation for the transformer).

 

3) ADD VENTILLATION, TOP:  In the back and left sides of the top, drill 5 or 6 evenly spaced holes near the transformer.  Do NOT louse up the appearance of the top by drilling through the top top.

 

4)   JUMPER AC, ALWAYS ON:  There are two black wires to the controller’s switch.  One goes to a transformer lug and the other to a wire nut.  Jumper between the transformer lug and the wire nut so the AC is always on to the transformer primary.  (New model, the wires are black and white.  Clip off the crimp-on wire nuts on these TWO wire junctions and join all FOUR wires together under one new screw-on wire nut.  The original one with the surge protector grounded to the Transformer remains untouched.)

 

5) MOUNT SWITCH: 

                *  Slide back heatshrink covering the connection of the two yellow motor wires and the      transformer wire and separate them.   Thread the two motor wires over to the top of the

                left side of our switch.  Solder a yellow jumper to extend the yellow transformer wire over

to the middle post of the same switch side. (New model.  Cut lose the tie-wrap holding all the wires in place.  You will find a sleeve over a junction between a RED from the transformer to the two wires  (now black and clear) going to the indicator motor.)

 

* Unsolder the brown wire (which goes to the controller’s manual switch knob) from the

Transformer and feed it under and around to the top post of the right side of our new ON/OFF  switch.    Take a new 10" brown wire from the middle post of  our new switch back to this same post on the transformer.  (New model.  Its still a brown wire but spliced to a black transformer wire instead of a transformer post )

 

*  Add a new 8" brown wire to jumper from the bottom post of our new switch and run it to the "common" connection on the TRIAC board.

               

5)  BUILD TRIAC BOARD according to the sketch and mount under the upper right screw that holds the transformer in place.  Solder two short jumpers from the Triac outputs  down to the screw terminals #1 and #2 going to the external rotator. (New model, just stuff it in the back right corner not touching the cap.)

 

* Insert the Triacs from the bottom and let pins protrude about 3/16ths inch on the top.  Bolt the

two Triacs together to let them heatsink each other.  Bend the two center pins over each other.  This is the TRIAC COMMON point.   Insert the two optoisolators taking care to bend out pins#4 so that they can plug into the same perf-board hole as the pin's #2 on the Triacs.  Solder.

 

                * Cut one lead on each resistor to about 3/4" and bend it over in the shape of a horseshoe.  Insert   the other end wire through the board such that the horseshoe can be soldered to the gate of the

                Triac.  Route the other lead under the board to the optoisolator's pin 6 and solder.

 

                *  Install the diodes so that the cathodes go off  the edge of the board to form a connection loop

                and then route to the optoisolator pin #1.  The anode goes under the board to the pin#2 on the

                opposite opto-isolator.  On the bottom of the board be sure to jumper these two Pin#2's to each

                other with a spare piece of tinned wire.

 

               

6) PC CABLE: Solder the 2 pin connector onto the end of the zip cord and add strength and protection with the two pre-cut pieces of heat shrink tubing.  Sorry the first batch are right angle.  You may want to flatten a little with a pair of pliers.  At least they are gold plated. Thread it through the back of the rotator housing, put the two leads through the ferrite bead and solder to the TRIAC board as bit inputs for right and left.  Obviously, you may want to replace my two pins with a real DB-25, but I find the two pins far easier to take in and out of my laptop every morning and evening than fighting all 25 pins of a DB25 when only 2 are use.

 

7) TESTING:  Before replacing the cover, take a 9 volt battery in series with a 560 ohm resistor to test your circuit.  Momentarilly connect it one way to the PC leads and then the other.  The rotator should rotate one way and then the other.

 

8) FINISH:  Replace top cover.  Insert the two-pin plug into the pins 2 and 3 of your parallel printer port LPT-1.  Sending a 1 to the port 888 will cause the rotator to go LEFT.  Sending a 2 to the port will cause the rotator to go RIGHT.  Pins 2 ans 3 are the 2nd and 3rd ones in from the upper right on the top row (the longer row) . If it goes the wrong way, just plug the two pins in the other way.

 

 

WARNINGS AND CAUTIONS:

 

1)  Any antenna and rotator system is a natural lightening rod which will attract lightening.  Connecting any such device to your computer is risky in the case of a near or direct lightening hit.  This circuit has been designed with optoisolators for over 5000 volts of protection of the PC circuit from the AC and antenna circuits; but this will not protect your station or computer from a direct hit. USE AT YOUR OWN RISK.

2)  The APRS modifications to this ARCHEROTATOR will keep the transformer primary circuit always connected to 115 volt power no matter what the position of the ON/OFF - AUTO/MANUAL switch.  There will be some heat build up in the plastic housing and this must be allowed to escape through the ventilation holes provided in the instructions above.  Whenever the rotator is not to be used for an extended period of time, it should be unplugged.

 

 

If your rotator is different than 1 RPM, change the constant in the APRStk config file to match your speed.   See CFIGK20.APR in your root directory.  The value “6” near the end is for 6.0 degrees per sec.