Direction Pot for an Upside Down Prop Pitch Rotator

Prop pitch motors make excellent rotators for the very largest antennas and rotating towers. The DC motor is powerful and the gearbox has exceptional torque and turns at just the right speed. What they lack is a convenient means to attach a mast and a direction indicator.

There are two ways to drive a mast -- I have one of each: 

• Motor on top, with a chain drive to the mast
• Motor on bottom, direct drive of the mast

In both cases the weight of the mast and antennas and radial thrust are supported by bearings. The motor is only designed for torque.

There are several ways to add a direction indicator to a prop pitch motor:

• Magnetic sensor on the motor axle that pulses on every motor rotation. The motor spins at ~9500 RPM (0.1 ms per rotation) so the technical requirements are non-trivial. Perhaps the most common example is the control system sold by K7NV.
• Magnetic sensor (compass) mounted on the antenna, which communicates by wire or, in some cases, by radio. There are numerous products available of diverse capability and reliability. One successfully used by a few hams that I know is the magnetic sensor sold by 4O3A.
• Use a belt or chain drive around the mast to drive a potentiometer. I don't know of any commercial products, and most everyone builds it themselves. It isn't difficult but there are different methods. Making it mechanically and electrically reliable is the challenge.

I chose the last option since it's inexpensive and I enjoy home brew projects like this. The challenge was to design a belt drive system for the pot, which is unlike the method I used for the chain drive system on the other tower. 

Getting the unit built took longer than it ought to have taken because I fidgetted over the design and then winter arrived. Now it's done. It was a nice little project to start the tower season with. The system is simple, the details are critical to success.

We'll look at the belt and tower mounting shortly. Let's first review the details.

The C-shaped steel chassis was pulled from my junk box. It's the right size and it's rigid enough for the application. I didn't saw off the "lips" on the left since they aren't in the way. If you don't have a similar scrap of metal handy it isn't difficult to fabricate or purchase something similar. The hold down screws and nuts for the bearing are #6 stainless. The holes in the steel chassis are large enough to allow the inner ring of the bearings to spin freely.

The pulley was found in a local farm supply store. For the 2-⅞" mast, there is about a 2.5:1 drive ratio. The pot turns ~2.5 times for a 360° rotation. The 10 kΩ linear Bourns pot has a 10:1 reduction drive, so this equates to a 2.5 kΩ range, or a 3 VDC range with a 12 volt power supply. Doing it this way supports over-rotation and improves resolution when applied, with suitable voltage division, to the 10-bit ADC of an Arduino. Should I decide on a software controller of this type, the ultimate resolution is about 2°. That's more than adequate for HF yagis.

There are ¼" sealed bearings at the top and bottom of the ¼" aluminum drive shaft. There is no friction with normal or excess belt tension, and no radial force on the pot. The shaft is slightly oversize due to the mill finish. This came in handy since only the section that fits inside the bearings must be reduced. I mounted the shaft in a drill press and used a flat file to remove material. A metal lathe is the proper tool, but for this small job the drill press worked well. 

The shaft bottoms in the lower bearing at the transition from the turned down section. Those stops ensure the mast sits firmly within the bearings without slipping. The pulley has a ½" press fit bore. A reducer was made with 1" lengths of ½" and ⅜" tubes slit along their lengths. I have a box of tube scraps from my many yagi projects. The 5/16" set screw was fabricated from scrap screw stock by cutting a slot for a screwdriver. A few minutes of work saved a trip to the hardware store.

The bolt on the left connects to angle stock attached to a tower girt (see below). Moving the position of the inner nut adjusts belt tension. The hole in the chassis was already there and it was perfectly placed for the bolt.

The bracket for the pot must be carefully bent, drilled and attached so that the pot and shaft are aligned over the full range of rotation. I made the mounting holes larger than required for adjustment room. The bracket is made from ⅛" aluminum stock. Final alignment required careful bending in a vice to make the top and bottom faces parallel. The ¼" shaft coupler is a flea market special. In the final installation (see further below) I used stainless screws cut to size.

Here we see the completed unit mounted on a tower section that I keep as a jig in my workshop. The pipe is manually turned to mimic mast rotation.

As you can see, the belt doesn't need to be anything fancy! The friction due to the pulley, bearings and pot is so low that almost anything will do. The knot passes the pulley without any problem. I will eventually come up with a better belt. Initial testing suggests that the rope belt does not creep down the mast as it turns if the belt isn't measurably slack. I may add a thin flange around the mast to make sure that it stays in place.

Alternatives to a rope include a V-belt, an adjustable belt and a rubber band. Finding a V-belt of exactly the right size is difficult and there is no good way of getting it around the mast and pulley. The adjustable belts I've found online look promising but they are very expensive. I tried rubber strips from my junk box and found that the excellent grip caused them to easily derail by riding up the pulley rims. Rope is cheap, easy to fit and won't jump out of the pulley.

For weather protection, the unit is mounted on a tower girt to place it directly beneath the plate for the middle mast bearing. The pot is wrapped to resist corrosion from moisture, which is a common problem for pots located outdoors. The plate and girt should offer sufficient protection to the pulley and belt from wind driven, rain, snow and ice. I won't know for sure until it survives a cycle of the seasons. 

The pot slider is grounded, both to the tower and via a control cable wire. Several feet of Cat5 run from the pot to the termination of the main run of control cable. There I spliced the wires into the bundle. The other wires of the control cable are for the 15 and 20 meter stack switches.

The bit of scrap rope I used for the belt is temporary (ha!). It was about the right length so I carried it up the tower and wrapped it around the mast and pulley. That double granny knot is not going to last long! I just want it to last long enough for for the first phase of testing and experimentation. It is easy replaced.

Lucky for me, when I did the control cables for the 15 and 20 meter stack switches I made sure that all wires of the control cable were connected from the tower top to the shack desk. It was easy to turn over the "temporary" stack switch controls to access the 3 wires (pot slider is grounded) and measure the resistance as the motor is turned. So far so good.

Software controllers for the two prop pitch motors are an item in my 2022 station plan. I have the chips and other components, but not the software or hardware UI (user interface). I haven't yet decided whether to go fully with software, as with the ongoing antenna controller project, or a mix of hardware controls and software. I'll build a prototype or two before deciding on a final design.

Expect an article later this year once I have a prototype controller. It'll be nice to get the large and retro controller relegated to below the operating desk rather than adding to the clutter on top of the desk.