How does a radio work?

For many Indian families in the 1970s and 80s, the transistor radio was the first electronic thing in the house. It sat on a shelf in a wooden or leatherette case, ran on four big D cells, and was always on for the news, film songs and the cricket.

Open one up and there are surprisingly few parts. At the top lies a black ferrite rod wrapped in copper wire: that is the antenna. The tuning knob turns a gang capacitor, a stack of metal plates that slide between each other. Together, the coil and the capacitor pick one station out of dozens. Small metal cans called IF transformers filter the signal, six or seven transistors amplify it, a tiny diode pulls the sound out of the radio wave, and the speaker turns it back into air pressure.

A radio today can be a single DSP chip a few millimetres across. It turns the radio wave into numbers and does the tuning and filtering with maths. The ideas inside are the same ones you will see in the old set.

Push electrons up and down a metal rod, fast. Each moving charge drags its electric field with it, and moving charge makes a magnetic field. The changes cannot spread instantly, so they ripple outwards at the speed of light, about 300,000 km every second. That ripple of electric and magnetic field, always at right angles to each other, is a radio wave. It is the same stuff as light, just with much longer waves (see WaveClear).

The frequency is how many times the charges swing each second. The wavelength is how far the wave travels in one swing: λ = c / f. AIR's medium-wave stations sit between 531 and 1,602 kHz, spaced 9 kHz apart in India, with waves 190 to 560 metres long. FM (88 to 108 MHz) has waves about 3 metres long. Phones use waves a few centimetres long (see MobileClear).

Wavelength decides how a wave travels. Long medium waves hug the ground as a ground wave for hundreds of kilometres. By day a low layer of the ionosphere, the D layer, soaks up any medium wave that heads for the sky. At night the D layer fades, and the higher E layer bounces medium waves back down, a thousand kilometres away or more. That is why a radio picks up far-off AIR stations after dark. Shortwaves bounce off the even higher F layer, hop after hop, round the world. FM goes straight through the ionosphere into space, so it only reaches as far as the horizon.

Your voice is a slow wobble, a few hundred to a few thousand times a second. Waves that slow would need an antenna kilometres long, and every station would talk over every other. So a radio station makes a fast, steady carrier wave on its own frequency, say 819 kHz, and makes the voice change it. This is modulation.

In AM (amplitude modulation) the voice changes the carrier's size. Trace the tops of the peaks and you see the voice: this outline is the envelope. In FM (frequency modulation) the size stays fixed and the voice nudges the frequency up and down, by up to 75 kHz for FM broadcasts. The waves bunch up and spread out in time with the sound.

Modulation spreads a station's signal into sidebands either side of the carrier. An AM station needs only 9 kHz of room, so there is space for many of them. An FM station takes about 200 kHz but sounds better. Lightning, motors and switches make bursts of static that change the size of the wave. An AM set hears every burst as a crackle. An FM set ignores the size and listens only to the timing, so most of the static vanishes. How FM stations share the band is the story of FMClear.

Every station in range is hitting your antenna at the same time, each on its own frequency. How does a radio pick just one? With resonance, the same effect that lets you pump a swing higher by pushing in time with it (see WaveClear).

A coil (inductor, L) and a capacitor (C) wired together make a tuned circuit. Charge sloshes back and forth between them, from electric field in the capacitor to magnetic field in the coil and back, at one natural frequency: f = 1 / (2π√LC). Signals at that frequency build up big. Signals at other frequencies do not.

Turning the tuning knob swings the gang capacitor's plates in or out. More overlap means more capacitance and a lower frequency. With a 230 µH coil, 40 to 400 pF tunes the whole medium-wave band.

How sharp the peak is depends on the circuit's Q (quality factor): the peak is about f / Q wide. A sharp peak gives good selectivity: it can split two stations only 18 kHz apart. But one tuned circuit is never quite sharp enough on its own, as you will see. That is why real radios add the trick in the next chapter.

Almost every radio since the 1930s is a superheterodyne, or "superhet", an idea Edwin Armstrong worked out in 1918. Follow the signal from left to right.

The antenna picks up every station at once. A tuned RF amplifier favours the one you want. Then comes the clever part. A local oscillator inside the radio makes its own wave, always 455 kHz above the station you tune (for AM). The mixer multiplies the two together, which makes their sum and their difference. The difference is always 455 kHz, whatever station you picked. This fixed intermediate frequency (IF) goes through sharp, fixed filters (the IF transformer cans) and most of the amplification. So the radio gets the same sharpness on every station.

The detector then recovers the sound. For AM it is a diode and a capacitor, which follow the envelope. An FM set uses 10.7 MHz as its IF and a detector that turns frequency changes into voltage. An audio amplifier drives the speaker.

The simplest radio of all, the crystal set, has no battery. An antenna, a coil, a capacitor, a crystal diode and earphones are enough, powered only by the radio wave itself. Build one and hear why the diode matters.

In India, radio reaches places nothing else does. All India Radio, called Akashvani ("voice from the sky"), is heard by about 99% of the population across some 92% of the country, in 23 languages. Generations grew up on Vividh Bharati's film songs, the news at fixed hours, and cricket commentary that made you see every ball. Since 2014 the Prime Minister's monthly Mann Ki Baat has gone out on the same network.

Radio also saves lives. When a cyclone forms over the Bay of Bengal or the Arabian Sea, the India Meteorological Department issues bulletins every three hours and AIR broadcasts them. Medium wave travels hundreds of kilometres over the sea, far beyond FM, and a battery radio keeps working when power lines and phone towers are down. After the 2004 tsunami, amateur ham radio operators visiting Port Blair kept the Andaman Islands in touch with the mainland. Shortwave bounced off the sky, so they needed no towers at all.

Community radio stations, run by colleges, NGOs and farm universities, broadcast with just 100 watts to villages about 12 km around. And DRM (Digital Radio Mondiale) puts digital, near-FM sound into AIR's old medium-wave channels. For FM stations and how they share the band, see FMClear. For television, see TVStationClear and TVClear.