How Does FM Radio Work? | Signals And Sound

FM radio works by encoding audio as variations in a carrier wave’s frequency, which a tuner locks onto and converts back into sound.

For the full breakdown, see our best Battery Operated FM Radio guide.

When you spin the dial and a station snaps into place, you’re not catching particles in the air—you’re aligning a receiver with a specific frequency in a carefully governed slice of the electromagnetic spectrum. Understanding how FM radio works comes down to a few principles that haven’t changed since the 1930s, though the rules around them have been refined by the FCC to keep the band orderly.

This guide breaks down the physics, the U.S. band plan, and the licensing that keeps broadcasters from talking over each other.

The Core Physics: Frequency, Not Volume

The “FM” in FM radio stands for frequency modulation. A transmitter generates a steady carrier wave tuned to a specific frequency—say, 98.7 MHz—then slightly speeds up and slows down that wave’s frequency to mirror the audio signal. The loudness of the sound becomes the amount of frequency shift; the pitch becomes the rate of the shift.

That’s the critical distinction from AM radio, which encodes audio by varying a carrier wave’s amplitude, or strength. FM’s frequency-based encoding is why FM stations resist static and interference: most electrical noise changes a signal’s amplitude, not its frequency, so a receiver ignores it.

• The transmitter varies carrier frequency to match audio.
• The receiver locks onto one station and rejects adjacent ones.
• The demodulator extracts the audio from the shifting carrier.
• The amplifier boosts the recovered sound to your speaker.

The U.S. FM Band: 88 To 108 MHz

In the United States, the FM broadcast band runs from 88.0 MHz to 108.0 MHz. The FCC explains that FM channels are 200 kHz wide, with center frequencies spaced every 200 kHz. Standard center frequencies end in.1,.3,.5,.7, or.9 MHz—so 97.3, 97.5, and 97.7 are all valid, while 97.4 sits between channels.

That band isn’t open to everyone. The FCC states that only noncommercial educational stations are licensed in the reserved 88–92 MHz portion, while both commercial and noncommercial educational stations may operate in the 92–108 MHz range. FM commercial stations may be authorized on 92.1 to 107.9 MHz, corresponding to Channels 221 through 300.

LPFM: The Low-Power Exception

For smaller voices, the FCC created the low power FM service in January 2000. LPFM is available to noncommercial educational entities and public safety or travelers’ information entities—not individuals or commercial operations. A construction permit or license is required before an LPFM station begins building or broadcasting.

LPFM stations may operate at effective radiated powers between 1 watt and 100 watts, depending on factors the FCC specifies. That’s tiny compared to a full-power commercial station, but enough to cover a neighborhood or a small town.

Digital FM And Where To Go From Here

FM isn’t stuck in the analog era. The FCC has approved In-Band On-Channel (IBOC) digital radio for FM stations. It’s still broadcast on the same dial positions—just with a digital signal layered onto the analog one.

If you’re thinking about picking up a portable unit, a good battery-operated FM radio is still the easiest way to enjoy local stations on the go.