G5A: Reactance and Impedance – Ham Radio General License Study Guide

G5A covers the fundamental AC circuit concepts of reactance, impedance, admittance, and resonance. Understanding how inductors and capacitors oppose AC current — and how that opposition changes with frequency — is essential for understanding how antennas, tuners, filters, and matching networks work.

The exam draws from topics including what reactance is, which component types produce reactance, how inductive reactance changes with frequency, how capacitive reactance changes with frequency, what impedance is, what admittance is, what units are used for reactance, what letter symbols represent these quantities, what happens at resonance in an LC circuit, and which devices can be used for impedance matching at radio frequencies.

What Is Reactance?

Reactance is the opposition to the flow of alternating current caused by capacitance or inductance. Like resistance, it is measured in ohms. Unlike resistance, reactance does not dissipate energy as heat — it stores energy in electric or magnetic fields and releases it back to the circuit. Reactance is represented by the letter X.

The key distinction is that reactance applies to AC circuits only. Direct current does not change direction, so capacitors and inductors behave differently under DC (blocking or passing freely) than under AC. In AC circuits, both components create a frequency-dependent opposition.

Inductive vs. Capacitive Reactance

An inductor (coil of wire) resists changes in current flow. As alternating current changes direction, the inductor's magnetic field collapses and rebuilds. The faster the current changes — that is, the higher the frequency — the greater the opposition. Therefore, inductive reactance increases as frequency increases. At very high frequencies, an inductor becomes an open circuit. At DC (zero frequency), it has no reactance at all.

A capacitor stores charge on its plates and resists changes in voltage. At low frequencies, the voltage has time to build up across the plates and the capacitor charges up, blocking current. At high frequencies, the voltage reverses before the capacitor fully charges, and more current flows through the circuit. Therefore, capacitive reactance decreases as frequency increases. At very high frequencies, a capacitor approaches a short circuit. At DC, it is a complete open circuit.

Impedance and Admittance

Impedance (Z) is the total opposition to alternating current in a circuit, combining both resistance (R) and reactance (X). It is defined as the ratio of voltage to current — just as resistance is for DC circuits. Impedance is measured in ohms.

Admittance (Y) is the inverse of impedance, just as conductance is the inverse of resistance. High admittance means low impedance (current flows easily); low admittance means high impedance (current is strongly opposed).

Resonance in LC Circuits

In a circuit containing both an inductor and a capacitor, there is a specific frequency called the resonant frequency at which inductive reactance and capacitive reactance are equal. When XL = XC, the two reactances cancel each other out. This is resonance.

In a series LC circuit, at resonance the total reactance is zero, leaving only resistance. Since impedance is minimized, the circuit allows maximum current to flow at the resonant frequency. This is why series resonant circuits are used as frequency-selective elements in receivers and transmitters — they present minimum impedance to the desired frequency and high impedance to others.

Impedance Matching

Many RF systems require impedance matching — connecting a source with one impedance to a load with a different impedance so that maximum power transfers between them. Several methods accomplish this at radio frequencies:

• A transformer — steps voltage and current up or down according to the turns ratio, transforming impedance by the square of the turns ratio
• A Pi-network — a network of inductors and capacitors shaped like the Greek letter π, commonly used in vacuum tube RF amplifier output stages
• A length of transmission line — a quarter-wave transformer or other transmission line section can transform impedance at a specific frequency

All three methods are valid techniques for impedance matching at radio frequencies.

G5A Practice Questions