The design of this homemade variable capacitor was described in the USSR newspaper "News of the Radio", May, 1927. This variable capacitor is very suitable especially for VHF radio bands.
The variable capacitor (see Fig. 1) is housed in a small wooden box, the design includes two copper or brass plates. One of this plates - "F" is a stator, the stationary plate, and it mounted on the wooden bottom of the box. The plate is connected to the right terminal, placed on the side of the box. The plate is covered with a patch "D" of a mica or celluloid, or any suitable dielectric material.
Fig. 1. The homemade simplest variable capacitor, design of 1927
The another plate - "N" is placed above the dielectric patch. This plate is moving - this is the "rotor", it is attached to the left terminal. The plate is made of brass, its thickness is about 0.8-0.9 mm. The dimension of this moving plate is just a little less than the dimension of the stationary plate.
To make moving plate flexible, use a mallet - blow the plate for 10 minutes, changing its sides, the plate will become springy. The plate is fixed at some angle to the stationary plate. Using the screw with the knob, located on the top of the box, the moving plate can be moved closer to the stationary plate. When the distance between plates is maximum, the capacity of the variable capacitor is minimum, and vice versa.
The dimension of plates depends on the maximum capacity of the variable capacitor, it can be calculated using the known formula:
C = ε0*K*L*W/d,
Where ε0 = 8.8541x10-12 is the constant of vacuum permittivity;
K is the dielectric constant of the material;
L is the length of a plate;
W is the width of a plate;
d is the distance between the plates;
C is capacitance.
To calculate dimension of plates, can be used the online calculator. Both plates have the same size. All dimensions - length, width and distance are in millimeters.
Another design of a homemade variable capacitor was published in "Radio" magazine, April 1967.
Fig. 2. The homemade simplest variable capacitor, design of 1967
The design of this variable capacitor is shown in Figure 2. It has a wooden base 1. A cooper or tin plate 2 with dimension of 45x85 mm is attached to the base, the thickness of the plate doesn't matter. The plate is covered with a sheet of paper 3, it's the dielectric. A moving plate 4 is placed above the dielectric sheet. The design is assembled with two wood screws 5 with length of 5..6 mm. The moving plate is made of brass or bronze with thickness of 0.5...0.6 mm.
To make the moving plate more springy, it has to be blown from both sides. While this process is ongoing, the dimension of the plate will be changed, so it have to be consider to make the plate with the dimension of 45x100 mm. There is three small holes of 3 mm in diameter, two of them are for the screws 5, and the third on the other end is used to attach a line 6. Winding up the line on the rod 7 by using the knob 8, we approaching the moving plate 4 to the stationary plate 2, therefore we increasing capacity of the variable capacitor. The maximum capacitance of this variable capacitor is about 500 pF, it can be calculated using online parallel plate capacitance calculator shown above.