Dual section air variable capacitor

Dual section air variable capacitor

was invented to provide tracking between two related LC-tuned circuits, as in a radio receiver. Such capacitors are basically two (in the case of Fig. 5) or more variable capacitors mechanically ganged on the same rotor shaft.

In Fig. 5, both sections of the variable capacitor have the same capacitance, so they are identical to each other. If this capacitor is used in a superheterodyne radio, the section used for the local oscillator (LO) tuning must be padded with a series capacitance in order to reduce the overall capacitance. This trick is done to permit the higher-frequency LO to track with the RF amplifiers on the dial.

In many superheterodyne radios, you will find variable tuning capacitors in which one section (usually the front section) has fewer plates than the other section. One section tunes the RF amplifier of the radio, and the other tunes the local oscillator. These capacitors are sometimes called cut-plate capacitors because the LO section plates are cut to permit tracking of the LO with the RF.