How Radio Works: Sending Sound Through Thin Air

Radio feels like magic mainly because nothing about it is visible. A microphone turns your voice into a fluctuating electrical current; somehow that current ends up recreated, faithfully, in a speaker hundreds of kilometres away, with nothing physically connecting the two. The trick is that the signal never travels on its own. It hitches a ride on something that can actually cross that distance: a radio wave, which is simply a specific slice of the electromagnetic spectrum, the same family of radiation that includes visible light and X-rays, just at a much lower frequency.

Why you need a carrier wave at all

Sound itself, as an electrical signal, oscillates far too slowly to be broadcast usefully through the air. Human speech and music occupy roughly 20 to 20,000 cycles per second. A wave at that frequency would need an antenna many kilometres long to radiate efficiently, and different stations broadcasting simultaneously would hopelessly overlap since they'd all be using the same narrow slice of low frequencies.

The solution is to generate a separate, much higher-frequency wave called a carrier — commonly hundreds of kilohertz to hundreds of megahertz — and impose the audio signal onto it as a pattern of variation. Since each station is assigned its own carrier frequency, many stations can broadcast at once without interfering, and a receiver just needs to be tuned to pick out one carrier frequency from the crowd, which is exactly what turning a radio dial or pressing a station preset does.

AM: varying the wave's height

Amplitude modulation, AM, imposes the audio signal by varying the carrier wave's amplitude, or height, while keeping its frequency constant. A louder sound produces a bigger swing in the carrier's amplitude; a quieter sound produces a smaller one. The receiver then simply tracks the outer envelope of the amplitude variation to reconstruct the original audio.

AM's chief weakness is that a lot of natural and man-made interference, like lightning static or electrical motor noise, also shows up as amplitude variation, so it gets picked up right alongside the intended signal. That is why AM radio has a characteristic static and hiss that FM largely avoids.

FM: varying the wave's pitch

Frequency modulation, FM, takes the opposite approach: it holds the carrier's amplitude constant and instead varies its frequency slightly, in step with the audio signal. A louder or higher-pitched sound shifts the carrier frequency more; the receiver tracks those frequency shifts rather than amplitude changes to recover the audio.

Because most real-world interference affects a signal's amplitude rather than its frequency, an FM receiver can largely ignore that noise, which is why FM sounds cleaner and supports higher audio fidelity, including stereo separation. The tradeoff is range: FM signals don't travel as far for a given transmitter power, and they need a clear line of sight, since FM frequencies don't bend around the horizon or bounce off the atmosphere the way AM signals sometimes do.

Why AM travels farther, especially at night

AM radio's lower carrier frequencies can travel in two ways that FM generally can't: as a ground wave that follows the curvature of the earth for tens to over a hundred kilometres, and as a sky wave that bounces off the ionosphere, a layer of the upper atmosphere ionized by solar radiation, back down to earth far beyond the horizon.

During the day, the sun's radiation ionizes the lower part of the ionosphere strongly enough that it absorbs AM sky waves rather than reflecting them. After sunset, that lower layer recombines and thins out, and AM signals can reflect off a higher ionospheric layer instead, letting some stations be heard hundreds or even thousands of kilometres from their transmitter after dark, a phenomenon anyone who has scanned an AM dial late at night and picked up a distant station has experienced firsthand. Regulatory bodies including the Federal Communications Commission have historically required many AM stations to reduce power or go off the air at night specifically to limit this interference between stations sharing a frequency.

From carrier wave back to sound

A receiver's antenna picks up a mix of every radio wave washing over it at once, from every station in range plus general electromagnetic noise. A tuning circuit, built around components that resonate strongly at one particular frequency and largely ignore others, isolates the desired carrier from that mix. A demodulator then reverses whatever encoding was used at the transmitter — tracking amplitude for AM or frequency shifts for FM — to recover the original audio signal, which is amplified and sent to a speaker. The entire chain, from a voice in a studio to a sound from a dashboard speaker, typically takes a fraction of a millisecond, limited mostly by the processing electronics rather than the radio wave's travel time, since radio waves themselves move at the speed of light.