G1 · Published 20 August 2026 · By The Ham Desk

Reactance, Resonance and the Components That Produce Them

At radio frequencies a coil is not just a coil and a capacitor is not just a capacitor: both oppose current in a way that depends on frequency. This unit works the two reactance formulas, the resonance condition they create together, and the matching and filtering jobs that follow from them.

Band 40 m
Study time 18
Level Core
questions 10

Waveform and antenna diagram for Circuits, Components and Measurements — Circuits, Components and Measurements — waveform overlay and radiation pattern

Pre-flight briefing

Two reactances, opposite signs

An inductor's reactance is XL = 2 pi f L, so it rises with frequency and blocks high frequencies. A capacitor's is XC = 1 / (2 pi f C), so it falls with frequency and passes them. At 3.5 MHz a 2 millihenry choke has about 44,000 ohms of reactance, while a 100 picofarad capacitor has only about 455 ohms - which is why small capacitors are short circuits at RF and why a series inductor makes a decent RF choke in a power lead.

Because the two reactances oppose each other in sign, they cancel at one frequency. That frequency is resonance: f = 1 / (2 pi times the square root of L times C). In a series resonant circuit the impedance collapses to the resistance alone and current peaks; in a parallel resonant circuit the impedance peaks and current in the tank is at its lowest for a given applied voltage. Antennas, traps, tank circuits and filters are all versions of this one idea.

Peak, RMS and why meters disagree

A sine wave of 100 volts peak has 70.7 volts RMS, because RMS is peak divided by the square root of 2. Mains-style average-reading meters and true RMS meters give the same number for a sine wave and different numbers for anything else, which matters when you measure a digital mode's output. A wattmeter calibrated for a sine wave will misreport the power of an SSB signal unless it is a peak-reading type.

Transformers, filters and matching

A transformer changes voltage in proportion to its turns ratio and impedance in proportion to the square of that ratio: a 4:1 turns ratio steps 50 ohms up to 800 ohms, or down from 800 to 50. That square law is what makes a 4:1 balun different from a 1:1 current choke. Filters follow from reactance too: a low-pass filter passes everything below a cut-off and attenuates harmonics above it; a high-pass does the reverse; a band-pass passes one range, which is what a tuned front end needs to reject a nearby broadcast station.

Terms you must be able to define
Term	What the examiner wants to hear
Reactance	Opposition to alternating current by an inductor or capacitor, measured in ohms and dependent on frequency.
Resonance	The frequency at which inductive and capacitive reactance cancel: f = 1 / (2 pi sqrt(LC)).
Turns ratio	The ratio of primary to secondary windings; impedance changes as the square of it.
Low-pass filter	Passes frequencies below a cut-off, attenuates harmonics above it.

Question deck 10 questions

G2A01 Core

What is the reactance of a 2 millihenry inductor at 3.5 MHz?

G2A02 Core

What is the reactance of a 100 picofarad capacitor at 3.5 MHz?

G2A03 Recall

Which formula gives the resonant frequency of an LC circuit?

G2A04 Core

In a series LC circuit at resonance, what happens to the impedance?

G2A05 Recall

A sine wave has a peak voltage of 100 volts. What is its RMS voltage?

G2A06 Core

A transformer with a 4:1 turns ratio is used to match a 50 ohm transmitter to a higher impedance antenna. What impedance does the antenna see?

G2A07 Core

Which filter would you use to pass the 20 metre band while rejecting both a 41 metre shortwave broadcast signal and 10 metre harmonics?

G2A08 Core

How must an ammeter be connected to measure the current drawn by a transmitter?

G2A09 Stretch

An inductor and capacitor are in a parallel tank circuit. If the capacitance is increased, what happens to the resonant frequency?

G2A10 Stretch

Why is a ferrite core often slipped over a cable carrying RF current?

Reference sheet

The explanations for all ten questions sit on this sheet. Pick an answer on the left and the matching card lights up.

G2A01 Explanation 1

Key B — About 44,000 ohms

XL = 2 pi f L = 2 x 3.1416 x 3.5 x 10^6 x 0.002 = 43,982 ohms. Doubling the inductance or the frequency doubles the reactance, so the halved answers correspond to 1 mH or 1.75 MHz.

Rule Fundamentals - inductive reactance

G2A02 Explanation 2

Key A — About 455 ohms

XC = 1 / (2 pi f C) = 1 / (2 x 3.1416 x 3.5 x 10^6 x 100 x 10^-12) = 455 ohms. Because capacitance is in the denominator, a larger capacitor has less reactance.

Rule Fundamentals - capacitive reactance

G2A03 Explanation 3

Key D — f = 1 / (2 pi sqrt(L C))

The resonant frequency is inversely proportional to the square root of the product of inductance and capacitance. Note the shape of the relationship: quadrupling either L or C halves the resonant frequency.

Rule Fundamentals - resonance

G2A04 Explanation 4

Key C — It falls to a minimum, equal to the circuit resistance

At series resonance the reactances cancel, leaving only resistance and giving maximum current. A parallel resonant circuit behaves the opposite way, presenting a high impedance at resonance.

Rule Fundamentals - series resonance

G2A05 Explanation 5

Key B — 70.7 volts

RMS is peak divided by the square root of 2, so 100 / 1.414 = 70.7 V. Going the other way, 120 V RMS mains has a peak of about 170 V.

Rule Fundamentals - RMS and peak

G2A06 Explanation 6

Key A — 800 ohms

Impedance changes as the square of the turns ratio: 50 x 4^2 = 800 ohms. Multiplying by 4 rather than 16 gives the 200 ohm trap answer.

Rule Fundamentals - transformer impedance transformation

G2A07 Explanation 7

Key D — A band-pass filter

Only a band-pass rejects on both sides of a wanted range. A low-pass would admit the lower-frequency broadcast signal, a high-pass would admit harmonics, and a notch would remove the band you want.

Rule 47 CFR §97.307 - harmonic suppression

G2A08 Explanation 8

Key C — In series with the circuit

Current is measured by putting the meter in the current path, so the same current flows through it - in series. Connecting an ammeter across a supply shorts it, because an ideal ammeter has almost no resistance.

Rule Fundamentals - measurements

G2A09 Explanation 9

Key B — It decreases, in proportion to the square root of the capacitance

Since f = 1 / (2 pi sqrt(LC)), capacitance is under the square root in the denominator: quadrupling C halves f. That is exactly how a variable capacitor tunes a receiver across a band.

Rule Fundamentals - resonance

G2A10 Explanation 10

Key B — It adds impedance to common-mode current on the shield without affecting the wanted differential signal

A clamp-on ferrite is a lossy inductance for current flowing on the outside of the shield - the common-mode path that causes RF in the shack and distorted patterns. Signal current inside the coax, flowing in opposite directions on centre and shield, largely cancels in the core.

Rule 47 CFR §97.13(c) - RF exposure and good engineering practice

How to use this unit

Work the ten questions in one sitting with the reference sheet covered, then uncover it and read every explanation - including the ones you got right. The wording of the real exam is different, so learn the reasoning rather than the answer letters.