Prop Pitch Direction Pot Updates

When I raised the first of my big towers in 2017 there was no direction indicator for the chain-driven prop pitch motor rotator. I made one using a Bourns 10 kΩ 10-turn potentiometer, a device used by other hams of my acquaintance. 

These components are not hardened for outdoor use and require protection. I built a plastic enclosure on a bracket dangling below the motor platform and hoped for the best. It lasted 5 years, far longer than I expected.

It was time to replace the pot on that prop pitch rotator. The pot had water damage and there might also be internal mechanical wear. The resistance jumped around a lot during rotation and there is evidence of bald spots on the resistance wire.

I went through at least two plastic "hoods" for the pot. The container seen above replaced the original, which was too small to adequately protect against wind driven rain. I repurposed food containers to save cost for what I expected to be a temporary measure . The work great when they're new. Of course they are not resistant to UV and thermal cycling also takes its toll.

As the plastic cracked the silicone sealant around the gaps couldn't block water incursion. Notice the rust at the base of the pot shaft. It was always a danger of this design because the pot shaft points upward, and open to rain and snow without excellent water protection.

An underappreciated problem with ordinary pots in rotator service is wear due to rocking of the mast and antenna system. This is directly coupled to the direction pot, with the pot wiper moving back and forth over the resistance wire where it is resting. 

The wire in a typical pot is not designed for this amount of abuse. They have a limited service life in this application. In contrast, the pot in a Hy-Gain rotator (photo credit: MFJ) is much larger with thicker wire and a wide wiper. It may help to vary the direction of the yagis when they are not in use.

The multi-turn linear pot has its advantages and it is not too much of a burden to replace them every 5 years or so. To better protect the Bourns pot from the elements I undertook a redesign. I had planned this for some time but put it off until the pot failed.

The chain-driven drive shaft is 2" OD with a wall thickness of about ¼". I projects less than 1" below the bottom of the housing that sits on the side of the tower. There is just enough room to fit the Bourns pot. So that's what I did. As you can see it's completely enclosed and well isolated from the elements. There is no opening above the drive shaft where water can enter.

The difficulty with this design, and the reason I didn't do it this to begin with, is that with the pot fixed in place it is the shaft that turns. The wires must turn with the shaft. Alignment of the shaft is critical to avoid bending force that can bind the pot during rotation. As for the first version, the pot is mounted on thin aluminum flashing that flexes to protect against a minor misalignment.

I tied down the wires to prevent tugging on the pot terminals. The 3 wires are coiled around the coupling. The wires are AWG 24 scavenged from stranded Cat5 cable. The plastic insulation is very pliable but it is stiffer than the wire when used in this application. Disciplining the wire coil is therefore not easy. It looks ugly but it works. Now that winter has arrived we'll see how it does when coated with snow and ice. A snow shield might be necessary.

On the other prop pitch motor turning the 15 and 20 meter yagis I encountered a different problem with the direction pot. A few months ago I replaced the temporary rope belt with higher quality ⅛" nylon weave rope. Since little friction with the mast and pulley is needed to drive the pot I kept the tension low. The rope loosened only slightly but that was enough for it to slip. Worse, it preferentially slipped in one direction. 

At first this could be dealt with by adjusting the zeroing pot on the op-amp direction indicator. With the continual back and forth rocking of the mast the pot soon hit the end of its 10-turn rotation and could no longer be adjusted. I climbed the tower and confirmed the nature of the problem. I centred the pot and pulled out the adjustment bolt to increase tension on the rope.

With the higher tension the mechanism has been working well for the past few weeks. I hope that it survives the winter. Another design may be necessary. One example I've considered is a chain drive. It would provide positive engagement without fine adjustment of chain tension.

I have discarded bicycle chains and cogs (½" pitch) to work with. The challenge is that there is no way to slip a cog over the mast. The cog would have to be cut in half and machined and attached to a collar that wraps around and is affixed to the mast. The pulley on the pot shaft would be replaced by a small chain cog. A master link solves the chain wrap problem. It's straight-forward work that I hope to avoid. Time will tell.

The direction indicator circuit described in an earlier article works perfectly well but it is more complex than currently needed. The goal was a circuit to drive either a physical meter or an analogue Arduino GPIO. The needs are different because a meter responds to current and the GPIO ADC responds to voltage. A second gain stage provides the required voltage range without risking the linearity of the differential amplifier.

Since my immediate plan is to use physical meters the second stage has been eliminated in the schematic shown above. I used the freed space on the breadboard to build two of the circuits. A few days before CQ WW CW I discovered that the meter movement of the old direction indicator (for the 40 and 10 meter yagis) had malfunctioned. The single op-amp circuit is a temporary fix since I needed it for the contest. As these things go, I expect it'll be in service for a while yet.

The circuit provides a minimum 10 kΩ load for the op-amp to keep it stable. The pot is tapped with a current limiting resistor to protect the meter movement. The resistance range was selected to support a large range of current requirements and drive ratio at the direction pot. The circuit is simple and works well. Calibration is performed in the same way as for the original circuit. Substitution of the current limiting resistor may be needed for meters other than the ones I have tried.

Because there's just one meter, one of the meter wires must be moved between circuits to monitor the direction of each prop pitch rotator. An SPDT switch would make this job easier. The arrangement is far from ideal but acceptable in the short term while I decide on a final design. 

I may do what some others have done, by repurposing an orphaned Hy-Gain rotator controller. The meter indicates direction and the levers operate the motor. Unused circuitry in the controller can be removed or left in place in case it is later returned to service as a Hy-Gain rotator controller. It also looks good on the operating desk. A home brew professional looking controller is a lot of work.

There is RFI, which shouldn't be a surprise considering the layout. You can see one RF choke at left on one of the pot wiper lines. This was proof of concept to see how it would help. It suppressed most of the RFI but not all. When properly built the lines will be choked and bypassed to suppress RF getting into the circuitry. Lightning protection must also be included, but probably at a connection point far from the shack for maximum safety.

My next step is to find a used orphan rotator controller and build permanent direction indicator circuits. The ±12 VDC for the op amps is not easily built with the stock transformers in Hy-Gain rotators so I may use the existing power supplies and wire them up to the controllers. The motor controls can be implemented at the same time or later.