Reactance
Understanding Reactance
Reactance is one of the two components of impedance (Z = R + jX). While resistance dissipates energy, reactance stores and returns energy. Understanding reactance is essential for impedance matching, filter design, and resonant circuit analysis.
Types of Reactance
- Inductive (XL): Positive reactance. Current lags voltage by 90 degrees. XL = 2 pi f L = omega L. Increases with frequency.
- Capacitive (XC): Negative reactance. Current leads voltage by 90 degrees. XC = -1/(2 pi f C) = -1/(omega C). Decreases (becomes less negative) with frequency.
Reactance in RF Design
- Matching networks: Use inductive and capacitive reactances to transform impedance.
- Filters: Frequency-dependent reactance creates frequency-selective behavior.
- Resonance: When XL = |XC|, they cancel, and impedance is purely resistive.
- Transmission lines: Lines shorter than lambda/4 act as open or shorted stubs with reactive impedance.
Capacitive reactance: XC = 1/(2 pi f C) (ohms)
Impedance: Z = R + jX
|Z| = sqrt(R^2 + X^2)
Resonance: XL = XC
f_res = 1/(2 pi sqrt(LC))
Frequently Asked Questions
What is reactance?
Reactance is the imaginary component of impedance caused by inductance or capacitance. It represents energy storage rather than dissipation. Inductive reactance increases with frequency; capacitive reactance decreases with frequency. At resonance, they cancel.
What is the difference between reactance and resistance?
Resistance dissipates energy as heat and is independent of frequency. Reactance stores energy in electric (capacitive) or magnetic (inductive) fields and varies with frequency. Both are measured in ohms but have different physical effects on the signal.
Why does reactance matter in RF?
Reactance determines how components behave at different frequencies. At RF, even short wire leads have significant inductive reactance, and small parasitic capacitances become important. Understanding reactance is essential for impedance matching, filter design, and avoiding unwanted resonances.