FM broadcast radio in the United States occupies 87.8–108.0 MHz in the VHF band. The FCC defines 101 numbered channels (200 through 300), each 200 kHz wide, with carrier frequencies at the odd tenth (87.9, 88.1, 88.3, …, 107.9 MHz). The channel number and dial frequency are bidirectional via a simple linear formula.
Why 200 kHz spacing?
An FM stereo signal occupies ~160 kHz of bandwidth (75 kHz peak deviation on each side of the carrier, plus a few kHz of guard band for the SCA / RDS subcarriers). 200 kHz channel spacing gives a 40 kHz guard band between adjacent channels — enough for typical receiver filters to reject the neighbor with reasonable selectivity. The European FM band uses 100 kHz spacing in some markets (Eastern Europe historically) for denser packing at the cost of more interference.
Channel numbering origin
The "200 + " offset is FCC bookkeeping: channels 1–199 were assigned to other VHF allocations (TV, FM stereo experiments) before the modern FM band was finalized. So the lowest FM channel happens to be 200. Channels 201–220 (88.1–91.9 MHz) are reserved for non-commercial educational broadcasters — that's why college and NPR stations cluster at the low end of the dial.
Subcarrier services within the channel
An FM station's 200 kHz channel doesn't just carry mono+stereo audio. Several subcarriers live in there:
- Mono audio: baseband 30 Hz–15 kHz
- Pilot tone: 19 kHz (signals "stereo subcarrier present")
- Stereo subcarrier: 23–53 kHz (DSB-SC encoded L−R difference)
- RDS / RBDS: 57 kHz subcarrier carrying digital data (station name, song info)
- SCA subcarriers: 67 kHz and 92 kHz (optional, for muzak, reading services, etc.)
FM antenna size — surprisingly compact
At 100 MHz, a full wavelength is 3 m, half-wave is 1.5 m, quarter-wave whip is just 75 cm. That's why a typical car FM antenna is ~80 cm — a full quarter-wave at mid-band. Indoor "rabbit-ears" or dipole antennas at FM frequencies are ~1.4 m tip-to-tip. The exact length only matters for resonance / SWR; FM receivers tolerate a few percent mismatch easily.
Why are FM frequencies always odd tenths (88.1, not 88.0)?
By FCC convention. Channels are 200 kHz wide and centered on odd-tenth frequencies starting at 87.9. So 87.9 → 88.1 → 88.3 → ... → 107.9. Even-tenths (88.0, 88.2, …) are the GUARD BAND boundaries between channels. This convention applies in the US, Canada, and most countries that follow ITU Region 2.
Can I broadcast on any free channel?
No — even a "free" channel may be claimed by a station too distant to receive locally, or may conflict with adjacent-channel allocations. In the US, even a 100 mW Part 15 transmitter has to be careful about not interfering with licensed stations within ~200 ft. For higher power you need an FCC license. Use a tool like the FCC FM Query database to check actual assignments in your geographic area before transmitting on anything.
Why do my favorite stations cluster at certain frequencies?
Three factors: (1) Lower band (88.1–91.9) is reserved for non-commercial broadcasters — NPR, college radio. (2) Some frequencies are preferred for their "memorability" — 100.1, 104.7, etc. are easy to remember and find on the dial. (3) Local frequency reuse rules space stations 0.4 or 0.8 MHz apart in a single market to prevent receiver overload, so within any city you'll see clear gaps.
What's the FM antenna inside my phone?
In phones that support FM (rare in newer models — disabled in software despite the hardware being present), the antenna is your headphone cable. A standard 3.5 mm headphone cord is about 1.2 m long, which happens to be near a quarter-wavelength at FM band frequencies. Without headphones plugged in, no antenna, no FM. Phones with built-in FM (some Samsung models) include a tiny chip antenna and benefit dramatically from external wire.
Are there channels outside 87.9–107.9 MHz?
Yes — in other regions. Japan: 76–95 MHz with 100 kHz spacing. Eastern Europe (OIRT band): 65.8–74 MHz (legacy, mostly retired). ITU Region 1 (Europe, Africa): 87.5–108 MHz, same as US but with 100 kHz channel spacing in some countries. Brazil: 76–108 MHz (extended down). This tool covers the US band; the FCC channel formula applies only to the 200–300 channel scheme.
What's HD Radio / IBOC?
HD Radio (In-Band On-Channel) adds digital subcarriers in the GUARD bands at ±100–200 kHz from the analog carrier. So an HD Radio station occupies channel 261 + part of the guard bands on either side. The digital signal is at lower power than the analog carrier (~1% of analog power initially, now up to 10%) so it doesn't interfere significantly with adjacent analog stations. HD Radio receivers decode both analog and digital streams.