Butterfly
A butterfly capacitor is a form of rotary variable capacitor with two independent sets of stator
plates opposing each other, and a butterfly-shaped rotor arranged so that turning the rotor will
vary the capacitances between the rotor and either stator equally.
Butterfly capacitors are used in symmetrical tuned circuits, e.g. RF power amplifier stages in
push-pull configuration or symmetrical antenna tuners where the rotor needs to be “cold”, i.e.
connected to RF (but not necessarily DC) ground potential. Since the peak RF current normally
flows from one stator to the other without going through wiper contacts, butterfly capacitors
can handle large resonance RF currents, e.g. in magnetic loop antennas.
In a butterfly capacitor, the stators and each half of the rotor can only cover a maximum angle
of 90° since there must be a position without rotor/stator overlap corresponding to minimum
capacity, therefore a turn of only 90° covers the entire capacitance range.
Split stator
The closely related split stator variable capacitor does not have the limitation of 90° angle
since it uses two separate packs of rotor electrodes arranged axially behind one another. Unlike
in a capacitor with several sections, the rotor plates in a split stator capacitor are mounted on
opposite sides of the rotor axis. While the split stator capacitor benefits from larger electrodes
compared to the butterfly capacitor, as well as a rotation angle of up to 180°, the separation of
rotor plates incurs some losses since RF current has to pass the rotor axis instead of flowing
straight through each rotor vane
