LC resonance and reactance calculator

Resonance

An inductor and a capacitor together resonate at the frequency where their reactances are equal: f = 1 ÷ (2π√(LC)). In series, the pair has very low impedance at resonance, and in parallel, very high impedance. That's the basis of every tuned circuit in a radio, from the front-end filter of a receiver to the traps in a trap dipole and the tuning of a magnetic loop.

Reactance

An inductor's reactance rises with frequency, X_L = 2πfL, and a capacitor's falls, X_C = 1 ÷ (2πfC). Reactance is measured in ohms like resistance, but stores energy rather than turning it into heat. The figures are useful for choosing coupling and bypass capacitors, RF chokes and matching networks, and they come up often in licence exams.

In practice

Real components have stray capacitance and inductance, and wiring adds more, so a tuned circuit built from these values usually needs trimming. Use a variable capacitor or adjustable coil, and check it with a dip meter or NanoVNA. To wind the coil, see the toroid and coil calculator, and for filters, the low-pass filter calculator.

Questions