Variable Tuning Capacitor

With many variable capacitors becoming hard to get I wanted to see if it was possible to build a homemade version suitable for a medium wave radio. Looking on the web there are a few examples. Unfortunately these tend to be mechanically challenging, needing workshop tools and precise assembly.  On the other hand, there are some designs that are really just too experimental and flimsy for practical purposes. My design described here can be made with just a few basic tools. As for the materials and parts, these are readily available from high street stores / ebay.  The variable capacitor made has operated reliably in my radio circuit and covers a capacitance range from 120pF to 400pF.

Tuning Vane (rotor)- made from aluminium sheet 1.0 mm thick. Hole drilled 5.5mm dia.

The tuning shaft is made from a M5 screw 30mm long. The electrical connection to the rotor is via a flexible stranded wire soldered to the solder tag. The bushes are made from the flat ⅛ x ⅜ brass with a suitable sized hole drilled through. They are recessed into the plywood base and plywood support.

Base board plywood 4 mm thick- hole 5.5 mm dia

Wood spacer block (2 required). 12 mm thick, 20 long, 12 wide

Hardboard stator case – hardboard is 3mm (2 off  required). The two sides of the stator case come together in a sandwich with an opening at the top for the rotor. The parts are held in place by 2x M3 screws.

Side view of Stator Case

The tin foil should be glued down absolutely flat to the hardboard stator case sides and fully covered with parcel tape. Wrap the tin foil around the edges-  overlap by 15mm . Ensure the M3 solder tag makes contact with the foil (see photo below).