These capacitors are used as tuning capacitors in radio receivers. The two requirements of these capacitors are:
(1) Frequency stability
(2) Preventing micro phony
For frequency stability, the spacing between vanes should not change. For this the vanes are relived of all the internal stresses before assembly. To prevent micro phony, there must not be mechanical resonance over audio range of frequencies. At very high frequency (V.H.F.); vanes are either thick or widely spaced.
CONSTRUCTION: For tuning receiver and low power transmitters air dielectric capacitors are usual. A two-gang variable capacitor suitable for radio receivers contains two variable capacitors operated to a single tuning control. The various parts of a capacitor are: Frame, rotor, stator spindle and bearing. In this construction, fixed metal plates or vanes connected together form a stator and movable vanes connected together from a rotor. Capacitance is varied by rotating the shaft to make the rotor plates mesh with the stator plates. The fixed vanes are insulated from the frame work of capacitor by mounting which are made of ceramic materials. The movable vanes are spaced on the spindle by spacing washers to that they do not touch the fixed vanes at the point. Full mesh of the two sets of vanes means maximum capacitance. Moving the rotor completely out of mesh provide minimum capacitance. Losses in such air dielectric capacitors are small. The power factor at 1.5 MHz may be as low as 10-4 corresponding to a Q of 10000. Variable capacitors may be having their capacitance intestinally and rapidly change over the life of the device. They include capacitors that use a mechanical construction to change the distance between the plates, or the amount of plate surface area which overlaps, and variable capacitance diode that change their capacitance as a function of the applied reverse bias voltage. Variable capacitance is also used in sensors for physical quantities, including microphones, pressure and hygrosensors.