How do FM radio channels work?

An FM station is a chain. In the studio, a radio jockey (RJ) talks into a microphone. A playout computer plays the songs, ads and jingles in the order planned for each hour, often drawn as a clock. The RJ blends it all on a mixing console.

Next comes the audio processor. It evens out quiet and loud parts and trims every peak, so the station sounds loud and punchy but never pushes the signal past its legal limit of ±75 kHz (chapter 3 shows what that means). This is why stations sound louder than your music app.

The studio is usually in an office in town, but the transmitter sits under a tall mast on a hill or a high building. A studio-to-transmitter link (STL), a small microwave dish or a fibre line, carries the audio there. The transmitter makes a strong radio signal at the station's frequency, and a stack of antenna bays sends it out towards the horizon. Your radio does the rest; RadioClear shows how a receiver works, and TVStationClear follows the same kind of chain for television.

"98.3" is the station's carrier frequency: its transmitter's radio wave wiggles 98.3 million times a second (98.3 MHz). Every FM station in India lives in the FM band, from 88 to 108 MHz. Tuning your radio means choosing which wiggle rate to listen to. A wave that fast is about 3 m long (λ = c / f), which is why FM antennas are around a metre or so.

A station isn't a single thin line. Its sound spreads its signal over about 200 kHz around the carrier (chapter 3 shows why). Frequencies in India are set on a 100 kHz grid, which is why numbers end in .1, .3, .9 and so on.

In one Indian city, private stations must be at least 800 kHz apart. That is why so many cities have 91.1, 91.9, 92.7, 93.5 and 94.3: steps of 0.8. The gap protects cheap radios. Next to one station's mast its signal can be a million times stronger than a station across town, and a nearby strong signal can swamp or mix into a weak one.

Far-away cities can reuse the same frequency. That's why one brand can be "98.3" in city after city. It works once the other station is so far away that its signal is at least 45 dB (about 30,000 times) weaker than yours at the edge of your area.

An FM station never changes how strong its wave is. It changes how fast it wiggles. When the music pushes the speaker cone out, the carrier speeds up a little; when the cone pulls in, it slows down. That's frequency modulation. At full volume a 98.3 MHz carrier swings between 98.225 and 98.375 MHz: a deviation of ±75 kHz. How far it swings is the loudness. How often it swings is the pitch.

Swinging the frequency spreads the station over a band of frequencies. Engineers estimate it with Carson's rule: bandwidth ≈ 2 × (deviation + highest audio frequency) = 2 × (75 + 15) = 180 kHz. That's why stations need the room from chapter 2.

Because the sound isn't in the wave's size, the crackle from a hair dryer's or room heater's motor and switches (see HairDryerClear and RoomHeaterClear), which mostly changes the size, barely bothers an FM radio. AM, which RadioClear compares, hears every spark. What noise is left in FM is worst at high pitches, so the transmitter boosts the treble first (pre-emphasis, 50 µs in India) and your radio turns it back down (de-emphasis), taking the hiss with it.

One more trick: if two stations share a frequency, an FM radio locks onto the stronger one and almost silences the other. This is the capture effect. AM radios hear both at once.

When FM stereo arrived in 1961, millions of mono radios already existed. The trick that kept them working: send L+R (left plus right) as the normal sound, which a mono radio plays as usual. The difference, L−R, rides higher up, on a 38 kHz subcarrier, where a mono radio never looks.

A stereo radio needs to know exactly where 38 kHz is. So the station also sends a quiet pilot tone at 19 kHz. The radio doubles it to get 38 kHz, recovers L−R, and then works out the two sides: (L+R) + (L−R) = 2L and (L+R) − (L−R) = 2R. When the pilot is there, the STEREO light comes on.

Above that, at 57 kHz (3 × 19), is the Radio Data System (RDS): 1,187.5 bits a second of text. It carries the 8-letter station name your car shows, song titles, traffic flags and the clock. Not every Indian station sends RDS, but many cars can show it.

L−R picks up much more hiss than L+R, so in a weak signal your radio slowly blends to mono. It gives up the stereo to keep the sound clean.

FM waves (about 3 m long, see WaveClear) travel mostly in straight lines. They don't follow the curve of the Earth the way long AM waves can. So the key limit is the radio horizon: how far the mast can "see" before the ground curves away. With normal air bending the waves a little, it is about 4.12 × (√h₁ + √h₂) km, with both heights in metres. A 150 m mast and a car antenna give about 55 km.

Power matters too, but less than you'd think. Stations quote ERP (effective radiated power): transmitter power × the antenna's gain. Stacking antenna bays squeezes the signal into a flat ring aimed at the horizon, instead of wasting it on the sky. Doubling the power adds only about 3 dB. Raising the mast often helps more.

Walls take a bite: about 10 dB indoors at 100 MHz, and much more underground, which is why FM fades in basements and tunnels (long tunnels need their own small transmitters). In a city, waves also bounce off buildings. The echoes add and cancel in spots about 1.5 m apart (half a wavelength), so a moving car hears a fast flutter: multipath.

Most phones have an FM chip, but no FM antenna (their cell-tower radio is a different system; see MobileClear). They use the earphone cord as the antenna, which is why FM on a phone needs wired earphones plugged in.

India's first FM broadcast went out from Madras in 1977. For years FM was Akashvani (All India Radio) only. Today it runs about 585 FM stations, and channels like FM Rainbow and FM Gold in big cities.

Private FM came in phases. The government auctions channels city by city. Phase I (licences in 2000, first station Radio City Bengaluru in July 2001) had high fixed fees, and only a few stations survived. Phase II (2005–06) switched to a fee of 4% of revenue, and FM spread to many more cities. Phase III (policy 2011, auctions from 2015) reaches smaller cities. About 388 private stations are on air.

Private stations earn money from ads. They pay the licence fee, rent space on shared towers, and pay music royalties to the companies and societies that own songs (such as PPL for recordings and IPRS for composers and lyricists). The rates have been fought over in courts for years. Private FM may not make its own news; since 2019 it may carry AIR news bulletins, unchanged. Community radio (about 480 stations, from Anna FM in 2004) is run by colleges and NGOs for local people, with limited ads.

FM costs nothing to receive, needs no data, and keeps working when mobile networks fail, so in 2023 the government advised phone makers to keep FM chips switched on. The future may be digital radio (DRM or DAB) and internet radio. Norway switched off national FM in 2017; India is still mostly analogue.