Dish Positioners

Dish Positioners

You can always look out the window at the dish and guess where it is pointed. Even with a center fed antenna, I rarely guess within 10 degrees. I don't even try with an offset fed PrimeStar dish. When a big dish only has 3 degrees beamwidth, my chances of hearing the satellite are not good.

There are several ways to aim the dish towards a desired point. The mechanical way utilizes protractor-like scales to indicate where the dish is pointed in elevation and azimuth degrees. Another way is to connect potentiometers to the elevation and azimuth shafts and read out the voltage on either an analog or digital meter. Most ham rotators use this method. Another method is a pair of WWII selsyns for each axis. Modern techniques use some device to create pulses as the shaft turns and use a small computer to keep track of the pulses and direction.

Mechanical

I can look out the window and try to set the dish to around 5 degrees of the desired heading. But the dish beamwidth is less than that, so I have to do a bit of juggling around to find the AO-40 2.4 GHz beacon. My 10' Janeil dish included a plastic "protractor" and rod "plumb bob" that help set the polar angle. After rotating the polar axis to become my elevation axis, I reattached this unit to serve as an elevation indicator. The Azimuth axis uses an oversized protractor, which I made by hand, calibrated in 5 degree increments between 90 degrees through 180 degrees to 270 degrees. I can easily achieve about 1 - 2 degree accuracy with these protractor systems. This level of accuracy exceeds the beamwidth of the dish, so after setting the dish where the satellite is supposed to be with the "protractors" I can generally at least hear the satellite when I get back in the house. The main problem with this system is that I have to walk outside to read the "protractors." I have heard of people aiming solid state TV cameras on the protractors for remote operation. Seems like a waste of video equiptment.

Potentiometers

My Ham-M type rotators and my Yaesu 5500 Az-El rotator use potentiometers which send back a zero to fullscale voltage that deflects analog meters. It works ok for my beams that have about 20-40 degrees beamwidth, but I find the 2.4 GHz barbecue grill dish with about 5-10 degrees beamwidth is difficult to set.

I built up a small (7 1/2" W x 6" H x 4" D) control unit with a mirror finish front panel to manually run my converted 10' Janeil dish. The control unit has two 3 1/2 DPMs (Digital Panel Meter) on the front, and three power supplies - 30 VDC @ 3 amps for motors, 12 VDC @ 100 ma for the DPMs and another 12 VDC @ 100 ma for the potentiometers, and two center off toggle switches for the Azimuth and Elevation control.

Outside on the 10' dish, I put a 1 turn 10K precision wirewound pot on the elevation axis, and a 5 turn 10K precision wirewound pot on the 10 tooth / 8" pulley shaft (runs 4 times faster than the Azimuth top cap - it was more convenient there.) I got my precision potentiometers surplus many, many years ago. I believe that I got some of them at C&H Surplus in Pasedena 13 years ago. I recently got 3 one turn 10K precision potentiometers from MECI (Mendelson) in Dayton, Ohio for $3.75 for all 3. Took a lot of digging around, though. The actual resistance is not critical, but linearity is very critical. To get 1 degree of accuracy, .25% linearity is needed! Make sure the potentiometer is a wirewound model too. My telescope uses Bourns 3543S-1-103 3 turn pots 10K @ .25% linearity. These are listed in the Mouser catalog (800-346-6873) as p/n 652-3543S-1-103 for $22.62 each. A 5 turn Bourns 3545S-1-103 is p/n 652-3545S-1-103 for $23.30 each Spectrol makes a 1 turn precision 10K .5% linearity pot listed in the Allied Catalog (800-433-5700) as a Spectrol 132-0-0-103 p/n 970-1680 for $21.27 each. A 3 turn version with 10K .25% linearity is Spectrol 533-1-1-103 p/n 970-1830 for $12.00 each. .

Small self-contained 3 1/2 digit LCD panel voltmeters from Jameco, selling for $13 each, are attached to the elevation and azimuth potentiometers' output. The basic "meter movement" is factory set to 199.9 mv full scale.

The control unit places 12 VDC across outside terminals of each potentiometer. Mount the potentiometers so that at Elevation and Azimuth 0 the output voltage is 0, and the ouput voltage increases as the Azimuth or Elevation axis is increased..

Then make an Azimuth voltage divider that converts the voltage coming from the middle terminal of the Azimuth potentiometer so that at 360 degrees the indicated voltage at the Azimuth LCD panel voltmeter is 36.0 mv.

Make an Elevation voltage divider that converts the voltage coming from the middle terminal of the Elevation potentiometers so that at 90 degrees the indicated volatge at the Elevation LCD panel voltmeter 9.0 mv for degree accuracy or 90.0 mv for 1/10 degree accuracy if your mount is up to it.

The LCD display will then directly readout Azimuth in degrees and Elevation in degrees or 1/10ths of degrees. If you want a 1/10 of a degree Azimuth readout, use a 4 1/2 digit LCD panel voltmeter. My dish mechanics are not 1/10 degree accurate so a degree readout is fine. The .6" high digits DPMs are available from Jameco @ 1-800-831-4242. The 3 1/2 version is p/n 175951 for $12.95 and the 4 1/2 Digit version is p/n 123140 for $27.95. A backlit 3 1/2 digit version p/n 146835 is available for $17.95.

Click here for a schematic of my AO-40 digital position indicator. Values and voltages are not critical; the concepts are.

I move the toggle switch on the control unit N, S, E, or W until the digital position on the DPMs matches the satellite's position given by my NOVA tracking program. AO-40 coordinates move slowly so non-automation is not a real problem, just a nuisance.

Pulses

The TVRO actuators send out pulses to the attached control box as dish moves. The control box has a small computer in it which keeps track of the # of pulses and the direction the dish turned. The TVRO tracker control box has batteries which keep the pulse count in memory when the unit is turned off or there is a power outage. My Janeil Drive sent out a linear number of pulses/degree because it was driving a curved gear. The Houston tracker arms are straight so the number of pulses/degree changes as the arm pushes/pulls the dish around. But the tracking of the TVRO dishes was not done by degrees but by how many pulses were required to reach each satellite. The TVRO pulses were usually generated when a magnet passed a reed relay or Hall effect transistor.

The pulse system has the most potential for precise pointing, but at a price. It will take a small computer of some type to convert the linear pulses to meaningful spherical degrees and display them digitally. The same small computer is needed to receive coordinates from a PC running tracking programs, compare the "should be" with the "is" for the 2 axis, and move the dish accordingly until each set of coordinates match.

A shaft encoder for azimuth and a shaft encoder for elevation will be required. Either an absolute encoder or a incremental encoder can be used. An absolute encoder directly digitizes the position of its rotary shaft and has an output position word available at any time. This is a great advantage, but very costly (~$270 to $400 each) due to the number of bits required to achieve sub degree accuracy. The incremental encoder is much simpler internally, so less costly. Incremental encoders can be either mechanical or photo-optical, and can be either single phase or 2 phase in quadrature for telling what direction the shaft is turning. Prices vary between $6 and $85. OE5JFL shows how to build a 12 bit absolute encoder ( good for 1/2 degree accuracy)on his website. USDIGITAL produces a 14 bit absolute encoder (good for 1/10th degree accuracy), called the A2 model for $270 and a 2048 steps/rev incremental encoder,called the model E3, for $49. For those desiring very high accuracy, JMI of Colorado (1-800-247-0304) supplies incremental encoders for optical telescopes with 4000 steps/rev - E4000 - for $75 and 8192 steps/rev - E8192 - for $85. JMI also has a 2048 steps/rev version - E2160 - for $75 that would work well on the shaft of the small sprocket that drives the larger azimuth axis sprocket.

The picture at the left shows a mechanical incremental shaft encoder with detents (top left) and an optical shaft encoder with detents (bottom left). The red encoder is a 256 step 2 phase optical encoders. The large encoder is a 8192 step/rev incremental optical encoder, ideal for obtaining better than 1/10th degree of accuracy. At the right are two optical disks, the larger one being the pattern for a 10 bit (1024 step) absolute encoder, and the smaller disk being for a 60 step incremental encoder. The large disk requires 10 photo-optical readers, and the small disk only needs 2 photo-optical readers. Each disk would be attached internally to the encoder's shaft. My Satellite Tracker Senior is able to utilize all types of encoders: potentiometers, incremental encoders and absolute encoders.

Various interfaces, such as the "Kansas City Tracker" or the SASI interface, currently unsupported by its manufacturer, only support analog potentiometers and are not highly accurate, but if your beamwidth is over 10 degrees they work great. A much better tracking code is EASYCOM I or NOVACOM I. These are Serial Com Port Azimuth and Elevation systems with readout accuracy to 1/10 of a degree. An interface for the more accurate digital encoders which uses EASYCOM I or NOVACOM I needs to be built for use with the larger dishes. My Satellite Tracker Senior, Satellite Tracker Junior, and Satellite Tracker Mini all use an EASYCOM I or NOVACOM I interface. Because the interface is a one way interface, PC to Tracker, I usually build a special cable with multiple DB9F connectors and connect several trackers, each running a dish, to the computer running NOVA.