Total capacitance can be adjusted by how wide you make the capacitor plate area. Tuning can be made straight-line capacitance or straight-line frequency by how you cut the slot in the lower plate. Number of turns is adjustable by how long you make the device. This is a differential capacitor so you can make it increase capacitance with either left-hand rotation of the knob, or right-hand rotation. If you want absolute minimal capacitance at one end or the other of the range, you could probably use plastic angle pieces in place of the aluminium angle stock, but you would then have to add a grounded washer of other metallic grounding means for the shaft to provide reliable grounding of the sliding plate. Most of the versions built thus far have used 1/4 inch diameter threaded rod, but a few have been made with 1/8 inch diameter rod to fit small-bore knobs.
A little tension on the Nylock nuts will eliminate any backlash from shaft movement. Make the PCB plates rub slightly to minimize any rotational backlash. Thinner PCB substrate (dielectric) will provide increased capacitance. Low voltage versions of these capacitors have been successdfully made using tinplate (galvanized roofing material) sections with stick-on adhesive labels as the insulation in place of using PCB material.
The driver nuts should be soldered to the sliding plate section. Gluing them in place did not work because it did not provide a reliable ground path. Soldering these nuts in place is easier if they are brass instead of iron, but with enough soldering paste and heat you can use either type. If you accumulate enough wear in the driver nuts to cause noticable backlash, just unsolder one of the nuts and tip it sideways enough to take up the slack and re-solder it.
There are a number of possibilities for modification and improvement of this design. Think it through and make the design fit your particilar needs and material availability.
NOTE: It is not necessary but on my own builds of these I usually lathe-turn half of the flatted area on the Nylock nuts so that they fit inside a 3/8 inch diameter hole in the end-plates. This provides a non-threaded bearing surface for rotation of the shaft. You don't need to do this if the thickness of your angle-stock at the ends is wide enough to bridge a couple of threads on the threaded rod, and thus prevent feeling the threads as you rotate the knob.
For low power levels you could use this same design as a motor-driven tuning capacitor for a magnetic loop antenna. Higher power levels would probably require something better than PCB material for the dielectric.
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