Radio and digital radio | How it works | AM and FM compared (2024)

by Chris Woodford. Last updated: May 20, 2022.

Free music, news, and chat wherever yougo! Until the Internet came along,nothing could rival the reach of radio—not even television.A radio is a box filled with electronic components that catchesradio waves sailing through the air, a bit like a baseball catcher's mitt, andconverts them back into sounds your ears can hear.Radio was first developed in the late-19th century and reached theheight of its popularity several decades later.Although radio broadcasting is not quite as popular as it once was, the basic idea ofwireless communication remains hugely important:in the last few years, radio has become the heartof new technologies such as wirelessInternet, cellphones (mobile phones),and RFID (radio frequency identification) chips.Meanwhile, radio itself has recently gained a new lease of life with thearrival of better-quality digital radio sets.

Photo: An antenna to catch waves, some electronics to turn them back into sounds, and a loudspeaker so youcan hear them—that's pretty much all there is to a basic radio receiver like this. What's inside the case? Check outthe photo in the box below!

What is radio?

You might think "radio" is a gadget you listen to, but it also means something else.Radio means sending energy with waves. In other words, it'sa method of transmitting electrical energy fromone place to another without using any kind of direct, wired connection. That's why it's often called wireless.The equipment that sends out a radio wave is known as a transmitter; theradio wave sent by a transmitter whizzes through the air—maybe from one side ofthe world to the other—and completes its journey when it reaches a second piece of equipment called a receiver.

When you extend the antenna (aerial) on a radio receiver, it snatches some of the electromagnetic energypassing by. Tune the radio into a station and an electronic circuit inside theradio selects only the program you want from all those that arebroadcasting.

Artwork: How radio waves travel from a transmitter to a receiver. 1) Electrons rush up and down the transmitter, shooting out radio waves. 2) The radio waves travel through the air at the speed of light. 3) When the radio waves hit a receiver, they make electrons vibrate inside it, recreating the original signal. This process can happen between one powerful transmitter and many receivers—which is why thousands or millions of people can pick up the same radio signal at the same time.

How does this happen? The electromagnetic energy, which is amixture of electricity and magnetism, travels past you in waveslikethose on the surface of the ocean. These are called radio waves. Likeocean waves, radio waves have a certain speed, length, and frequency.The speed is simply how fast the wave travels between two places. Thewavelength is the distance between one crest(wave peak) and the next,while the frequency is the number of wavesthat arrive eachsecond.Frequency is measured with a unit called hertz,so if sevenwavesarrive in a second, we call that seven hertz (7 Hz). If you've everwatched ocean waves rolling in to the beach, you'll know they travelwith aspeed of maybe one meter (three feet) per second or so. The wavelengthof oceanwaves tends to be tens of meters or feet, and the frequency is aboutone wave every few seconds.

When your radio sits on a bookshelf trying to catch waves cominginto your home, it's a bit like you standing by the beach watching thebreakers rolling in. Radio waves are muchfaster, longer, and more frequent than ocean waves, however. Theirwavelength is typically hundreds of meters—so that's the distancebetween one wave crest and the next. But their frequency can be inthe millions of hertz—so millions of these waves arrive eachsecond. If the waves are hundreds of meters long, how can millions ofthem arriveso often? It's simple. Radio waves travel unbelievably fast—atthespeed of light (300,000 km or 186,000 miles per second).

Photo: A radio studio is essentially a soundproof box that converts sounds intohigh-quality signals that can be broadcast using a transmitter.Credit: Photographs in the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.

Analog radio

Ocean waves carry energy by making thewater move up and down. In much the same way, radio waves carryenergy as an invisible, up-and-down movement of electricity andmagnetism. This carries program signals from huge transmitterantennas, which are connected to the radio station, to the smallerantenna on your radio set. A program is transmitted by adding it to aradio wave called a carrier. This process is called modulation.Sometimes a radio program is added to the carrier in such a way thatthe program signal causes fluctuations in the carrier's frequency.This is called frequency modulation (FM).Another way of sending a radio signal is to make the peaks of the carrier wave bigger orsmaller. Since the size of a wave is called its amplitude, thisprocess is known as amplitude modulation (AM).Frequency modulation is how FM radio is broadcast; amplitude modulation is the techniqueused by AM radio stations.

Radios like the one pictured above have circuits inside them called detectors whosejob is to convert modulated AM or FM radio signals back to copies of the sounds from which they were made.This process is the reverse of modulation, so it's called demodulation. Without getting into the technical details, you can probably imagine how it would work in an AM radio tuned to one frequency, but what about FM, where the frequency is varying? How can a station be broadcasting on a specific frequency if the frequency of the wavescoming out of the transmitter is constantly changing? Well it's not as random as that suggests: the frequency can vary only so much ("deviate") either side of the central, carrier frequency. FM radios use various kinds of detector circuits to convert that varying frequency back into a varying amplitude that recreates the original sounds.Exactly how these work is beyond the scope of this simple article.If you're interested, you can find out more in Wikipedia's article aboutdetectors in radios.

Digital radio

Photo: A typical Roberts DAB digital radio. The big orange button in the middle letsyou pause a live radio broadcast and restart it later.

You're driving along the freeway and your favorite song comes on the radio. You go under a bridge and—buzz, hiss, crackle, pop—thesong disappears in a burst of static. Just as people have got used tosuch niggles, inventors have come up with a new type of radio thatpromises almost perfect sound. Digital radio, as it's called, sendsspeech and songs through the air as strings of numbers. No matterwhat comes between your radio and the transmitter, the signal almostalways gets through. That's why digital radio sounds better. Butdigital technology also brings many morestations and displays information about the program you're listening to(such as the names of music tracks or programs).

Why don't radio waves all get mixed up?

From TV broadcasts to GPS satellite navigation, radio waves zap all kinds of handy information through the air,so you might be wondering why these very different signals don't get thoroughly mixed up? Now we have digital broadcasting, it's a lot easier to keep radio signals separate from one another using complex, mathematical codes; that's how people can use hundreds of cellphones simultaneously in a single city street without hearing one another's calls. But going back a few decades to the time when there was only analog radio, the only sensible way of stopping different types of signal from interfering with one another was to split the entire spectrum of radio frequencies into different bands with little or no overlap. Here are a few examples of the main radio broadcasting bands (don't take these as exact; definitions do vary somewhat around the world, some of the bands do overlap, and I have rounded some of the figures as well):

If you check out the US National Telecommunications & Information Administration website, you can find a very detailed poster called the United States Frequency Allocations: The Radio Spectrum Chart, showing all the different frequencies and what they're used for.

If you look at the table, you'll notice that the wavelength and the frequency move in opposite directions. As the wavelengths of radio waves get smaller (moving down the table), so their frequency gets bigger (higher). But if you multiply the frequency and wavelength of any of these waves, you'll find you always get the same result: 300 million meters per second, better known as the speed of light.

A brief history of radio

• 1888: German physicist Heinrich Hertz (1857–1894) madethe first electromagnetic radio waves in his lab.
• 1894: British physicist Sir Oliver Lodge (1851–1940) sentthe first message using radio waves in Oxford, England.
• 1897: Physicist Nikola Tesla (1856–1943) filed patents explaininghow electrical energy could be transmitted without wires (US Patent 645,576 and US Patent 649,621) andlater (following Marconi's work) realized they could be adapted for wireless communication as well (radio, in other words).The following year, Tesla was granted US Patent 613,809 for a radio-controlled boat. (Claims that he "invented" radio are disputed, however, as Thomas H. Whitediscusses at length in Nikola Tesla: The Guy Who DIDN'T 'Invent Radio'.)
• 1899: Italian inventor Guglielmo Marconi(1874–1937) sent radio waves across the English Channel. By 1901. Marconi had sent radiowaves across the Atlantic, from Cornwall in England to Newfoundland.

  

  Photo: Italian Radio pioneer Guglielmo Marconi.Photo courtesy of US Library of Congress

• 1902–1903: American physicist, mathematician, and inventor John Stone Stone(1869–1943) used his knowledge of electric telegraphs to make important advances in radio tuningthat helped overcome the problem of interference.
• 1906: Canadian-born engineer Reginald Fessenden(1866–1932) became the first person to transmit the human voice using radio waves.He sent a message 11 miles from a transmitter at Brant Rock,Massachusetts to ships with radio receivers in the Atlantic Ocean.
• 1906: American engineer Lee De Forest(1873–1961) invented the triode (audion) valve, an electronic component that makesradios smaller and more practical. This invention earned De Forest the nicknamed "the father of radio."

  

  Photo: Lee de Forest (1873–1961) pictured in the 1920s.Photo by Bain News Service courtesy of US Library of Congress.

• 1910: First public radio broadcast made from the Metropolitan Opera, New York City.
• 1920s: Radio began to evolve into television.
• 1947: The invention of the transistor byJohn Bardeen (1908–1991), Walter Brattain (1902–1987), and WilliamShockley (1910–1989) of Bell Labs made it possible to amplify radio signalswith much more compact circuits.
• 1954: The Regency TR-1, launched in October 1954, was the world's first commercially produced transistorradio. Around 1500 were sold the first year and, by the end of 1955, sales had reached 100,000.
• 1973: Martin Cooper of Motorola made the first ever telephone call by cellphone.
• 1981: German radio engineers began development of what's now called DAB (digital audio broadcasting) at the Institut für Rundfunktechnik in Munich.
• 1990: Radio experts came up with the original version of Wi-Fi (a way of connecting computers to one another and the Internet without wires).
• 1998: Bluetooth® (short-distance wireless communication for gadgets) was developed.

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Find out more

On this site

• Antennas and transmitters
• Cellphones
• History of communication
• Radar
• Television
• Wireless Internet

On other sites

• The Nobel Prize in Physics 1909: Marconi shared the 1909 prize for his radio work. You can read his own, firsthand account of radio's early development in his Nobel lecture.

Books

General and technical

• Keith's Radio Station: Broadcast, Satellite and Internet by John Allen Hendricks and Bruce Mims. Focal Press, 2015. The classic introduction to how radio stations work, fully updated for the Internet age.
• Build Your Own Transistor Radios: A Hobbyist's Guide to High-Performance and Low-Powered Radio Circuits Paperback by Ronald Quan. McGraw Hill (Tab), 2013. A detailed technical guide (not really suitable for beginners).
• How Radio Signals Work by Jim Sinclair. McGraw-Hill Professional, 1998. A look at the physics behind wireless communication with radio waves.

History

• The Victorian Internet by Tom Standage. Walker & Company, 2007/Bloomsbury, 2014. A more general history of how telecommunications changed during the 19th century with the development of electric power, telegraphs, and radio.
• Signor Marconi's Magic Box by Gavin Weightman. Da Capo Press, 2003. A readable biography of the best-known radio pioneer
• Past Years: An Autobiography by Oliver Lodge. Scribner's, 1932/Cambridge, 2012. Lodge's (somewhat dry) autobiography gives details about the early history of radio and confirms that he made key breakthroughs several years before Marconi's widely reported successes. Available secondhand or in modern reprints.
• Crystal Fire: The Invention of the Transistor and the Birth of the Information Age by Michael Riordan and Lillian Hoddeson. New York: W. W. Norton & Co., 1998. How the invention of the transistor led to the development of portable transistor radios.

For younger readers

• Make: Electronics by Charles Platt. O'Reilly, 2015. A great starting point for kids who want to learn how to build things from electronic components. The emphasis is on learning by doing with plentiful, practical examples. "Experiment 31: One Radio, No Solder, No Power" is an easy-to-build crystal radio that can pick up AM signals even without a battery.
• Radio Rescue by Lynne Barasch. Frances Foster, 2000. An illustrated (fictional) story book about a book who uses amateur (ham) radio to talk to people around the world.