Variable Capacitors
A variable capacitor allows capacitance to be adjusted. In a traditional mechanical variable capacitor, one set of conductive plates moves relative to another. Changing the overlapping plate area changes capacitance. Variable capacitors were once among the most recognizable parts inside radios because turning the tuning knob physically changed the resonant frequency.
Air-variable capacitors
A classic air-variable capacitor has two groups of interleaved metal plates. The stationary plates are the stator; the rotating plates are the rotor. Air is the dielectric between them. As the rotor turns into the stator, overlapping area increases and capacitance rises.
The plate shape can be designed so that rotation gives a useful relationship between knob position and tuned frequency. Multiple capacitor sections may be mounted on one shaft so several resonant circuits track together in a radio.
Trimmer capacitors
A trimmer is a small adjustable capacitor intended mainly for calibration rather than constant user adjustment. A screwdriver, ceramic tool, or small tuning tool changes plate overlap or dielectric spacing. Trimmers are used to align oscillators, filters, antenna networks, and RF stages.
Vacuum variable capacitors
High-power transmitters and RF equipment may use vacuum variable capacitors. Their electrodes operate in a vacuum, allowing high voltage and high RF current with low loss. They are larger and more expensive than ordinary trimmers but are well suited to antenna tuners, matching networks, plasma equipment, and high-power transmitters.
Varactor diodes: electronic variable capacitance
Not every variable capacitor is mechanical. A reverse-biased semiconductor junction has a depletion region that behaves like a capacitor. A varactor or varicap diode is designed to use this effect. Changing the reverse voltage changes the junction capacitance.
Varactors permit electronic tuning without a motor or moving plates. They are used in voltage-controlled oscillators, phase-locked loops, radio tuners, frequency synthesizers, and RF filters.
Resonant tuning
A capacitor and inductor form a resonant circuit. For the ideal case:
Increasing capacitance lowers the resonant frequency; decreasing capacitance raises it. This simple relationship is the foundation of traditional radio tuning.
Stray capacitance
Variable-capacitor circuits often work with only a few picofarads. Wiring, switch contacts, transistor terminals, circuit-board pads, a metal enclosure, and even the operator's hand may contribute capacitance comparable to the component being adjusted. RF construction therefore depends heavily on short connections and consistent physical layout.
Mechanical considerations
Air-variable capacitors must keep their rotor and stator plates from touching. Bent plates, dirty bearings, or shaft movement can cause intermittent shorts or noisy tuning. Dust and corrosion can also affect high-voltage spacing.
Typical applications
- AM and shortwave radio tuning.
- Antenna tuners and matching networks.
- Oscillator calibration.
- RF filter alignment.
- High-power transmitter tuning with vacuum variables.
- Electronic tuning with varactor diodes.
Selection checklist
- Minimum and maximum capacitance.
- Adjustment range and mechanical rotation.
- Voltage and RF-current capability.
- Q and loss at the operating frequency.
- Number of ganged sections.
- Shaft, mounting, and physical clearance.
- For varactors: capacitance-versus-voltage curve, Q, series resistance, and bias range.
Further reading
- NXP — Varactor diode applications and tuning concepts
- For mechanical variable capacitors, RF voltage, current, plate spacing, and Q should be taken from the exact component specification.