What are Air Capacitors?

Air Capacitors are capacitors which use air as the dielectric medium.  Air capacitors are used in tuning circuits.  

An Air capacitor is made of two banks of semicircular plates which are mounted on a single shaft.  The rotation of the shaft controls the amount of overlap between the two plates.  This determines the capacitance value of the capacitor. 

Since air has a low dielectric constant, it cannot be used for high voltages.  The capacitance of air capacitors ranges between 100pF to 1 nF. 

Air capacitors are not polar which means that they can be connected in any polarity.

There can be a shift in the capacitance value due to deposition of dust on the plate surfaces and the change in air parameters such as temperature and humidity.  Small insects can also get caught between the plates and create a short circuit.