Wednesday, November 25, 2015

Choices on Adapting Prop Pitch Motors to Rotator Service

My plans for a larger station continue. I have the tower, the transmission line, a few antennas and assorted components for building long-boom yagis. I now also have a couple of rotators, or at least one rotator and one rotator-to-be. This is what I want to discuss today.

Big antennas require powerful rotators. The key metrics are turning torque and braking torque. Since purpose-designed large rotators are expensive many hams have adapted surplus prop (propellor) pitch motors from aircraft of an earlier era. They are ideal in that they have high turning and braking torque, and a shaft rotation of close to 1 rpm.

However a motor is just that: a motor. While important, a motor is only one component of a rotator. The two motors I now own came from the same ham that sold me the tower. One comes with a complete platform and drive system for the tower, and a homebrew controller, all of which is ready to go . The other prop pitch motor was in storage as a spare. Both appear to be working well.

On the left is the motor that had been up the tower. The hardware is almost all original equipment. When refurbished many choose to rebuild with new hardware, as seen on the right motor. The shell contains the motor and the body contains the reduction drive. The crown gear for the shaft is underneath (hidden in this view). I have the matching shafts for both motors. Wires on the motors have been rerouted to exit at the joint between shell and body. On the far right of the photo is a tower plate and thrust bearing to support the mast and chain drive. I am in the process of overhauling the bearings.

My plans for some large yagis require rotators that are up to the task, and prop pitch rotators fit the requirement well. I have some choices ahead of me and some work to prepare these motors for use in my next station.  In this article I'll walk through those points.


Rather than repeat here the history of prop pitch motors I will link to the excellent material posted by K7NV on his web site. My motors are the small size, perfect for the antennas I have in mind. The bigger motors are often used to power rotating towers.

Prop pitch motors have not been manufactured since around 1960, when (primarily military) aircraft technology advanced. It's humbling to think that the prop pitch motors sitting on my work bench are quite likely older than I am. That they continue to perform well in countless antenna farms is impressive.

Power and cabling

The clip leads get warm to the touch in this test
The motor within the small prop pitch motor runs at ~9,500 rpm. That's fast! The motors in the larger units are only a little slower. The reduction drive brings this down to ~1 rpm (I measured 0.75 rpm with 24 VDC), just about perfect for a rotator of this size. The motor is not only fast it is powerful. On a bench test one drew 8 A and the other drew 7 A at 24 VDC, or about 180 watts. This has implications for the power supply and cables.

The power supply is easy enough since a motor does not require low-ripple DC. A typical motor power supply consists of just 3 components: transformer, bridge rectifier and filter capacitor. The capacitor is sometimes omitted. Components should be rated for continuous duty.

In the simplest configuration 3 wires go to the motor: common, clockwise and counterclockwise. Power is applied between common (not necessarily tied to station ground) and one of the other wires. Alternatively a relay at the rotator can switch directions, with a lower gauge wire to power the relay. That can save some money at the risk of a relay failure at the top of the tower right when it'll hurt you most: a contest or DXpedition.

From Ohm's law we expect the motor to present a 3 Ω DC resistance (R = E/I), and that's just what I measured. This is low enough that the gauge of the motor wires must be carefully selected. According to K7NV 10 AWG wire can be used for up to 300 foot (90 meter) runs. That isn't much since the large antennas that are likely being turned on high up and far away. For example, 150' out to the tower and another 150' to the top.

Since 1,000' (300 m) of 10 AWG copper wire has ~1 Ω resistance his suggestion implies that a 20% voltage drop (from 24 to 20 volts) is acceptable. In a complete return circuit of 600' of wire we have 0.6 Ω in series with the 3 Ω motor. Smaller gauge wire can be used if the power supply voltage is raised. For example, the same run done with 12 AWG wire would require 28 VDC to achieve the same result. Clearly it is best to match the power supply voltage to the length and gauge of the motor wires.

Mast coupling

The drive shaft exits from the bottom of the motor. If it is to be used as a conventional rotator the motor must be mounted upside down (shaft pointing up) and fitted with a clamp for the mast. A typical installation would have the motor flush against a steel plate affixed to the tower, with a hole for the shaft. Optionally a thrust bearing can be used to direct the vertical force of the mast and antennas directly to the tower.

There are reports that this approach should only be taken if the oil in the reduction drive is replaced with grease. Otherwise the oil will eventually foul the motor. This procedure requires a complete disassembly of the reduction drive.

The other approach is to mount the motor upright (shaft pointing down) on a platform attached to the side of the tower, and use a chain to turn the mast. Rotating towers are similar but with the motor anchored to the ground. More mechanical work is involved for a chain drive system which, unless you have the skills and tools, will require the services of a machine shop. It is important that both the mast and motor shaft use thrust bearings since the drive torque will impart a large lateral force on both mast and motor shaft. Two bearing are needed for the motor to ensure no lateral force is transmitted to the motor.

There is just such a drive system in the tower package I purchased, all of it custom built many years ago. Since it is beautifully built and works well I intend to use it. However it is extremely heavy and had to be disassembled before I could carry it by myself. For the second motor I will have to decide how to proceed when (if) I have another tall tower and large array. I can defer this decision for a year or two.

Direction indicator

Apart from situating the shack window where you can see the tower and antennas, there are two common methods for indicating direction with a prop pitch rotator: pulse and pot.

In the former case, a pulse (momentary contact closure) circuit is made for every rotation of the motor or a selected point in the reduction drive. With a prop pitch motor this can be challenging since it's rotating from 7,000 to 9,500 rpm (115 to 160 times per second). K7NV sells a magnet-driven reed switch add-on to the motor axle.

The controller counts the pulses to calculate degrees of rotation. The controller requires a calibration circuit to assign a reference point and degrees per pulse. If pulses are missed the error will grow over time and require recalibration. This reminds me of my first rotator, the CDE AR-22R, which used a contact pulse to drive a solenoid to ratchet the direction indicator in the controller. It was flimsy and unreliable, requiring constant recalibration. Pulse technology has gotten much better over the decades!

In the case of a pot (potentiometer) the mast or shaft directly drives the wiper of a pot, the resistance of which indicates direction. The popular Ham rotator series uses this method. In this case a linear pot is attached to the motor & drive assembly and the pot wiper is turned by the bell housing as it rotates. You must carefully align the ring gear, pot, stops and bell housing during assembly. Trim pots in the controller set zero and full scale.

For the prop pitch motor a mechanism must be built to drive a linear pot. A calibration circuit in the controller is desirable to allow over-rotation (more than 360°) and not require climbing the tower to make any adjustments. A universal calibration circuit sets the resistance for the selected rotation stops in clockwise and counterclockwise directions.

The prop pitch motor and controller I acquired uses this style of direction indicator. An op-amp configured as a voltage comparator is calibrated with a trim pot for zero scale, and another in a buffer amp set full scale. Modern controllers typically use microprocessors and software calibration. Not all commercial controllers support both means of direction indication so this must be decided before purchasing a unit. For example the EA4TX controller does not support pulse direction indicators. The Green Heron built RT-21pp by K7NV supports pulse.

Regardless of the method used it is advisable to have rotation limits set by hardware or software to prevent destruction of the coax to the antennas.


In its original application the motor is protected from the weather by the propeller conical hub cover. In its designed horizontal orientation the motor is designed to keep the lubricant inside. To keep the weather out there are places in the body of the motor that must be sealed.

Both of the my motors have been weatherproofed with sealant in the all the right places. That was long ago so some maintenance is required. The two critical areas are the motor cover and the motor drive.

The motor wires in the original design often exit via a hole in the side of the motor cover. Most often the wires are rerouted to exit closer to the body of the motor where the hole is less exposed to the weather. The original holes and the new one must be sealed against moisture intrusion.

Unless the original adapter plate is used the shaft seal is exposed to the weather. This is usually not a concern when it is mounted upright (chain drive). But when mounted upside down to couple directly to the mast a cover over the motor ought to be used, even if the adapter plate is present. The oil seal around the shaft is old and is insufficient to repel wind-driven rain or melting snow and ice.

Controller choice

The ham I bought the motor from built his own controller. It is quite old so it is entirely analogue. The motor power supply is quite simple, as described above. Switches choose direction of rotation and another is for the transformer primary. Thus the motor power supply is off when the motor is idle. The direction indicator uses a pot with a planetary drive on the mast, as described earlier. The indicator itself is a meter from a junked Ham-M rotator.

My first inclination is to keep the retro controller but to move it to a surplus Ham IV controller. The brake and rotation switches can be rigged to operate the motor. However there is no limit mechanism so it is necessary to avoid going beyond the standard 360° rotation range. Well, you may occasionally want to do so to catch an elusive multiplier or new country, but only if you prepared by installing enough coax slack in the connection to the yagis.

The commercial controllers I am looking at are the K7NV/Green Heron RT-21pp and the EA4TX. Both are well regarded. I described their different approaches to direction indication up above (with links to their product pages).

The RT-21pp is the more sophisticated and expensive of the pair, including electronic start and stop power ramps, manual and computer control, power supply and motor pulse unit. The EA4TX requires an external motor power supply, which it switches with relays (no power ramp), computer control (manual control appears minimally adequate) and potentiometer-based direction indicator. It is also significantly less expensive.

Should I choose a commercial unit, especially to have computer control, I currently lean towards the EA4TX product. I have the pot already so I won't have to purchase and install pulse units for the motors. From my observations of the motor start and stop behaviour (see below) I should be able to get by without power ramping.

Stop, Start and Braking

It takes about 2 seconds for these two motors to spin up to full speed and about the same to spin down. Under load the time will increase a small amount. The reduction drive of ~10,000:1 cannot react instantaneously. This is good since it is not desirable that large antennas accelerate too quickly.

The K7NV/Green Heron controller chops the power with a solid state switch to electronically ramp power at the start and end of rotation. This is particularly helpful for the subset of motors that have an integrated brake, which could, if left as is, risk antenna damage. Since my motors are not of this type I do not absolutely require this controller feature.

The braking torque in a prop pitch motor mostly comes from the reduction drive; it is very difficult to turn the motor by applying torque to the output shaft. For example, momentum of a turning yagi or asymmetric wind load. That is usually sufficient with a prop pitch motor to make a brake redundant. On the commonly used Ham series rotators it is possible, with the brake released, to turn or stop a turning rotator with your bare hands. I've done this for laughs when I was younger, though I don't recommend it since you could injure your hands or wrists if you don't grab the bell housing in just the right manner.

Some of the large commercial rotators with worm drives (Alfa Spid, Prosistel, etc.) don't need a brake for much the same reason. However those rotators should use a controller with an electronic ramp (manual or automatic) to gradually accelerate and decelerate the rotator.

Preparing for use

The first motor will not go into service until at least late 2016. The other will require a tower mount and chain drive system to be designed and built. However that is not possible until I know what model of tower it will be used with. That takes me to perhaps 2017. Until then the second motor is a spare. They are easily interchanged on the existing motor platform and drive system.

Other maintenance includes the thrust bearings, weatherproofing and, possibly, internal inspection and lubrication. I also want to refurbish the controller. This is a project for next year since the motors will not be required until my first large tower is installed.

For me this will be an excellent learning experience. I've never had the chance to directly play with prop pitch motors, only to use them a couple of times at other stations. Learning new things is good for the soul. But right now I need to go and fish a thrust bearing out of the oil and solvent it's been soak in for the past 2 weeks. I am attempting to salvage it rather than buy a new one.


  1. Just working on a rebuild of a small prop pitch myself to turn some antennas with coming up on my next tower build in late 2017 or early 2018. Just finished a medium duty rebuild for a friend. If you have any questions on how to remove anything or how it goes back together drop me an email at my QRZ address. Also a word to the wise on these the original gaskets that are between the case halves AND the mounting plate/output housing are asbestos on the original units. Another gotcha is be sure to assemble the entire unit loosely before applying grease to the gears/surfaces. I did that and suggest taking a small engraver to mark the housings, gears, etc to ensure you get it "timed" exactly right because if you do not you will hate life trying to get it all mated back up... With grease on everything it makes a mess!

    73 de
    Shane KI4M

  2. Oh and if you need help modifying a M2 rotor controller for these drop me a note I have modified mine and it appears to be working good on the test bench. I have not tested it under much load for long periods of time but in 15 minutes of testing it has performed nicely with these prop pitch units. The price on the used M2 RC2800PXAZ are much cheaper than the new Green Heron boxes. Sure they aren't as nice but they work just as well!

    Shane KI4M

  3. Thanks, Shane. As mentioned in the article I am not planning a rebuild near future. Both motors are running nicely at present. I appreciate your pointers on how to go about it. I'll keep you in mind when/if I get there!

    Ron VE3VN

  4. What grease do you use in your PP rotator? mine has 3 grease nipples on it (previous owner) and before putting into service really feel that I should give it a squirt. Thanks Gavin ZL1BBW

  5. Gavin, sorry but I don't know since I have never opened up either one. However one looks to have the original seals and thus probably has the original oil. Since it mounts upright (motor on top) on its platform the oil doesn't foul the motor. The other has been redone by a previous owner but I have not opened it so I don't know what was done. Since both work fine I haven't meddled with them. I have no current plan to open them unless service is necessary.

    73 Ron VE3VN


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