Dish Controllers

Most hams are familiar with aiming beam antennas. HF beams typically are rather wide beamwidth, perhaps a high gain 5 or 6 element HF beam might have a 30 degree beamwidth. The radiation direction is also usually quite obvious, it runs in line with the boom; the relector is bigger than the directors, so the direction is obvious as well. And most importantly, the HF beam just goes around in azimuth, never elevation.

But a dish is quite different. It is high gain, and gain is achieved by by concetrating all the radiation in one narrow path. And if it is sending or receiving from a satellite, it not only goes around in azimuth, but also moves in elevation. And then there are the offset fed dishes that have nothing pointed in the direction of maximum radiation.

All of the medium cost rotators on the ham market use precision wirewound potentiometers coupled to the azimuth axis and the elevation axis. Potentiometers have 2 problems; they are not that accurate, rarely achieving one degree of accuracy, and they get noisy from outdoor corrosion.

If you are using a high gain dish, you need something better than potentiometers, like some kind of a digital shaft encoder, but then the problem comes up of how to readout the shaft encoder. Shaft encoders come in 2 flavors. An incremental encoder, which is relatively simple and much cheaper, and the complex absolute encoder. The incremental encoder tells you haow many steps to took in what direction, but if you ever turn off power without storing the past movement history, you have to return to a known calibration point to start over.

Another problem a dish controller has to contend with is motor protection. Many dishes have microswitch limit switches that automatically stop the motor when the dish comes to an end of travel point. But what happens if something jams before the end is reached? You may wind up with a burned out motor or damaged gears.

The tracking software is generally quite accurate, but if tracking a LEO satellite, you better have a very accurate clock in you PC, which is seldom the case. In this case the dish controller may require an overriding control that allows you to apply a supplemental tracking offset to maximize the signal, sort of like tuning the rig to compensate for doppler shift.

Satellite Tracker Jr

Several simple interfaces have been designed for VHF/UHF antennas used to track amateur stellites. Those designed some time ago used many componenets, resulting in a rather complex design, difficult to build and costly to repair. Microchip in Phoenix has a line of microcontrollers that had almost everything in a single IC, great reducing the cost to manufacture. Now it is more the cost (and imagination) to program. Updating or repair now consists of exchanging a single part, or at most, a simple PCB.

The Satellite Tracker Jr. is designed for low cost manufacture and repair. The software is very complex. A general rule of software is, "the easiest and simplest to use takes the longest to develop". I have been using PIC controllers in a number of other projects, but if I had to develop it for just this project, I would claim about 2 years as the software development time.

In auto racing they have a saying, "if its got tits or wheels, it's trouble." In electronics I have a similar saying, "if its got fiberglass or metal, it's trouble." PCB makers have long been a trouble source. Sometimes the etching is too much or too little, resulting in shorts or opens. Usually it's just late. Metal cases are even worse. It doesn't quite fit, or they paint the screw holes shut. And always it's late. Hams are used to making one unit. One is easy. A bunch that works exactly like the first one, that's something else!

Satellite Tracker Mini

Everybody wants something for nothing, well, maybe a little money. The Satellite Tracker Mini was designed as a unit that could sell for under $100, yet have all the functionality of its big brother above, except for the display. It actually has more features than all the competive units cost twice as much! The most obvious difference between the Mini and the Jr. is the Mini uses 2 LEDs for user feedback and the Jr uses a 128 x 64 dot Graphic LCD. The software inside the two units is identical.

If you have a Ken Pro, G-5400, or G-5600 rotator with their N centered scales and want to work the S center AO-40, I suggest getting the Jr, as you can make your older rotator function like the later G-5500 and use the Graphic LCD for bearings instead of the dinky little meters.

Satellite Tracker Sr

I am presently using several automatic digital controllers for running a number of large TVRO dishes. In fact I connect a multiple connector line from my computer's com port to my 10' dish's Satellite Tracker Senior, a 7 1/2' portable dish, and my PrimeStar dish controlled by a a Satellite Tracker Jr. As you can see from my main homepage, I have been manufacturing battery chargers for R/C airplanes for several years. My latest charger uses a 128 x 64 dot graphic LCD for its display. The microcontroller is a Microchip (PIC) 17C756.

The Satellite Tracker Senior's dual displays give operational and diagnostic messages, as well as displaying azimuth coordinates on the right display and elevation coordinates on the left display. The twin displays are expensive backlit 128 x 64 dot graphic LCDs. Four pushbutton switches provide the operator with the capability to move the TVRO dishes around, and to select options and offsets. This controller is designed to be reasonably priced, but provide exceptional capability to the serious amateur with a large TVRO dish to control.

The controller inputs from the dish can be analog precision potentiometers, incremental encoders, or a pair of 14 bit absolute encoders. . Another input is provided by a socket (below the switches) for plugging in an IBM PC keyboard for direct entry of constants and coordinates.

Output motor control of either DC or AC gear motors, or relays is provided by the controller. Two asyncronous ports provide communications with a tracking program, and an altitude/azimuth position encoding controller remotely located at the dish site. Communications can be via RS-232 (50'), RS-422 (1 mile), or Fiber Optics for full electrical isolation.

The controller will operate automatically, manually, and park the dish. It will also turn the dish to selected locations by entering a name or call sign whose coordinates have been preloaded by the user, and perform a preprogrammed search pattern for a desired signal source.

Field Day 2002 provided a nice test of its abilities. It kept a 7 1/2' TVRO dish centered on AO-40 for 10 hours straight.

This powerful dish controller costs $495.