Module 7: AC Circuit Theory

AC circuit theory is where radio electronics truly begins. Every signal your transceiver transmits or receives is an alternating current — a continuously varying voltage that swings positive and negative many thousands or millions of times per second. To design, analyze, or repair radio equipment, you must understand how AC behaves differently from DC and why components that are simple in DC circuits become frequency-dependent in AC circuits. This module takes you from the fundamentals of AC waveforms through reactance, impedance, resonance, Q factor, and filters — all the building blocks your transceiver's RF circuitry is built from.

Module Overview

Module 6 gave you a thorough understanding of DC circuit theory — how resistors behave, how current divides, how Kirchhoff's laws govern every loop and node. All of that still applies in AC circuits, but two new components — capacitors and inductors — introduce frequency-dependent behavior that transforms the analysis entirely. Where a resistor's opposition to current is constant regardless of frequency, a capacitor's opposition decreases as frequency rises and an inductor's opposition increases. This frequency dependence is the key to everything in radio electronics.

The module opens by reviewing AC waveform fundamentals before introducing the concept of RMS voltage — the value your multimeter reads and the one that determines actual power delivered to a load. You will learn to calculate capacitive reactance and inductive reactance, then combine them with resistance to find the total impedance of a circuit at any frequency. Phasor diagrams provide a geometric tool for visualizing how voltages and currents relate in phase across different components.

The centerpiece of the module is resonance. When a capacitor and inductor are combined in a circuit, there is one specific frequency — the resonant frequency — at which their opposing reactances cancel exactly. At resonance, an LC circuit behaves in a dramatically different way: a series circuit presents its minimum impedance, while a parallel circuit presents its maximum. This phenomenon is exploited in every radio receiver to select a desired signal and reject all others, and in every transmitter's output network to present the correct load impedance to the final amplifier stage.

The Q factor quantifies how sharply a resonant circuit discriminates between frequencies. High-Q circuits select a narrow bandwidth; low-Q circuits pass a wider range. Understanding Q lets you predict exactly how selective a filter or tuned circuit will be, and lets you calculate the 3 dB bandwidth directly from the resonant frequency and Q value.

The final lessons cover four filter types — low-pass, high-pass, band-pass, and band-stop — and show you how to design simple first-order filters using resistors, capacitors, and inductors. The module ends with Pi and L networks, the impedance-matching circuits found at the output of virtually every HF transmitter and in countless antenna tuner designs.

Why This Module Matters for Ham Radio

AC circuit theory is not abstract — it is the direct explanation of why your radio works. The tuning dial on your transceiver selects a resonant frequency. The IF filter inside your receiver is a band-pass filter that selects a 2.4 kHz window of signals and rejects everything outside it. The antenna tuner in your shack is an L or Pi network that transforms the antenna's impedance to 50 ohms so your transmitter can deliver full power. The low-pass filter on your transmitter's output is specifically designed to pass your operating frequency while suppressing harmonics that would cause interference.

Every one of these functions — tuning, filtering, impedance matching — is a direct application of the theory in this module. A student who masters Module 7 can look at any RF schematic and immediately understand the purpose of each LC combination they see.

Lessons