W9KE Satellite Tracker - Rotor

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The goals for the rotor project were - completely scratch built (no tv rotor conversion), rugged robust construction, low cost, relatively easy to construct, strong enough to handle a pair of small beam antennas such as the Elk 2M/440L5 and 440L8 (Arrow antennas would probably work but I do not have any), run off DC, small, at least 450 degree rotation, easy to control and easy to set up and take down for portable or indoor operation. Things that are not included - the ability to handle large antenna arrays and weatherproofing. The rotor shown below meets all these requirements. The diameter of the antenna cross-boom in the picture is 1-1/4" for size reference.

This youtube video shows the rotor in action moving the two Elk antennas described above. http://www.youtube.com/watch?v=JPhavWyShMo

The rotor is about as simple as it can get. At the heart of the rotor is a unipolar 1.8 degree stepper motor. The motor used here has a rating of 3 Amps at 3.3 Volts. The holding torque is 181 oz-in. The stepper motor voltage is not critical. Motors with almost any voltage/current rating will work as long as they have enough torque. The 181 oz-in rating on this motor is more than enough.

The stepper motor used here is a dual shaft motor (this means there is a shaft sticking out of both ends of the motor). A dual shaft motor is not required or even desired. The dual shaft motor was used because I had one on hand. The output shaft of the motor has a 10 tooth sprocket for #25 chain attached to it. The #25 chain is connected to a 36 pin sprocket attached to a 3/8 inch main shaft (see Notes: at the bottom of the page). The ratio is not critical. The sprockets were selected based on availability and size. Putting the smaller sprocket on the motor multiplies the torque and reduces the effective step size. The sprocket/chain/main shaft system also isolates the stepper motor from the radial and axial loads of the antennas/elevation rotor. The 1.8 degree motor combined with the 10 -> 36 tooth sprockets results in a 0.5 degree per step. This value is used to configure the controller software and is not critical. A lower degree per step will be more accurate but will slow the motor speed. The picture below shows the sprocket/chain detail.

The main shaft is held in place with a self-aligning thrust bearing (see Notes) at the top and bottom of the rotor box. There is a 3/8 inch shaft lock (obtained from Ace Hardware) near each bearing. The bearings handle only radial force - the axial force is handled by another bearing at the top of the rotor box. The two shaft locks do not touch the bearings, they are spaced slightly away from them. They are there to prevent the shaft from slipping out while the rotor is being built or worked on. The picture below shows the detail of the shaft lock and self-aligning thrust bearing

The self-aligning thrust bearings handle the radial load on the main shaft. The axial load from the antennas and possibly an elevation rotor is handled by a thrust bearing. This picture shows the main shaft sticking out of the top of the rotor box. For initial testing a simple thrust bearing was used.

The thrust bearing is slipped over the shaft as shown in the picture below. The washer in the middle is needed because the thrust bearing did not have the correct inner diameter. The washer and bearing assembly is free floating.

The mount for the antennas / elevation rotor is then slipped over the shaft and rests on the thrust bearing. Lift the main shaft slightly and tighten the rotor mount to make sure all the axial load is taken by the bearing. The picture below shows the antenna mount resting on the thrust bearing. This bearing is well suited to handling the axial load but offers almost no radial load support. It was used only for initial testing. Think of it as a tower that needs support from guy wires. If the guy wires are located very close to the base they will do almost no good. The guy wires need to be moved out from the base. The ball bearings act like the guy wires so they must be moved as far as possible out from the main shaft.

McMaster-Carr carries a product they refer to as turntables. These are corrosion-resistant and very inexpensive. The 3 inch galvanized steel version costs less than two dollars. The 3 inch yellow chromate finish version with stainless steel ball bearings shown in the picture below was selected. It is attached to the top of the rotor using a small aluminum plate to allow clearance for the screw heads in the top of the rotor. The turntable as shipped is not lubricated - a little Superlube Synthetic Grease from Ace hardware works quite well as a lubricant on the turntable and the chain. The turntable has a load capacity of 200 pounds which is more than adequate.

A small aluminum plate is attached to the top of the turntable as shown in the picture below.

The only difficult part of the project is connecting the cross boom to the rotor. The method used is shown in the picture below. The mount is attached such that the axial load is taken by the turntable and passed through the rotor box to the mast. Most all of the radial load is also taken by the turntable. The only significant torque handled by the main shaft is the rotational torque required to turn the antennas.

The rotor box is attached to the tripod mast using a very simple base as shown in the picture below.

To Elevate or Not To Elevate:

Perhaps the most interesting question for a rotor system is if an elevation rotor should be included. An interesting article titled "A PRACTICAL ROTATOR SYSTEM" on the aprs.org web site states that 70% of all satellite passes are at an elevation below 22 degrees. This fact coupled with the fact that most short boom beams have a fairly wide bandwidth and high elevation passes result in a stronger signal indicate that an elevation rotor may not be required. The article suggests that a fixed elevation angle of 15% is ideal. Read the article and decide for yourself.


The self aligning 3/8 inch thrust bearing was made by Spyraflo and obtained from Amazon. The initial design used a 3/8 inch bronze bushing from Ace Hardware, this was a very bad choice as it made alignment of the main shaft almost impossible due to my very limited construction skills and tools. If the shaft is not aligned some of your motor torque will be wasted by the friction due to the shaft misalignment,

The sprockets and #25 chain were obtained from Amazon.

Amazon has a large selection of bearings. They are not expensive but most have metric dimensions. If you purchase a thrust bearing with the correct i.d. you will not need the washer.

McMaster-Carr is an excellent source for parts.

73 de W9KE

For more information on various parts of the project click on the links at the top of the page.