Larry, I , too, have thought about the same thing. I got a kit from Parallax, Inc (www.parallax.com
) to explore the usage of step motors for just such an application. My approach is to use a step motor for each individual incuctor and include a routine for calibrating and zeroing the stepper motors on the inductors so that they are synchronized and a separate stepper motor for the capacitor. Using two separate stepper motors for the inductors does increase cost and complexity but a Basic Stamp controller can easily drive both motors. At the same time, such an arrangement reduces the torque requirement for each stepper motor and eliminates the mechanical compexity in the pulleys, belts and/or gear drives required if you use only one stepper motor to turn the indutcors. It also allow you to mount the inductors at right angles to each other so that you get negligible mutual inductance/coupling between them.
A simple software counter is all that is needed to keep track of position and can run off a register that counts (accumulates) the stepper motor pulses given by the Basic stamp controller. Parallax sells LCD output displays for such a purpose. The memory (RAM) and (ROM) allow one to bandswitch easily to a preset inductor and capacitor setting and would even allow one to automatically switch in a fixed capacitance in parallel or series with the variable capacitor if the range of capacitance is not enoungh. The stepper motors would be driven to mid band on my set-up. Another approach would be to just write down the desired settings and have your program go to them. This would allow antenna set up changes. Another approach would be to allow the controller to store settings that can be input to non-volatile storage and changed as desired. The settings could have alphanumeric labels that would show up on the display by scrolling through them and when the correct setting shows, then push a button to go to them. As long as you keep the state of the present setting save in non-volatile memory of some sort, this is easy. In the internal porgram, the presets would be encoded as pulses.
I have thought a lot about this and I have decided to go with a Basic Stamp controller, as you can (possibly) guess. Right now I am learning how to use the controller. In addition such a controller offers all sorts of possibilities of allowing one to remotely control a narrow bandwidth antenna from the shack. Steppir antennas by Fluid Motion did it first, but I don't think they will be the last to use this sort of device. A simple variable iductor or capacitor at the antenna would allow remote tuning of a highly efficient radiator such as a vertical antenna. You could even use homemade inductors or capacitors.
Brad A. Steffler