How do digital radio signals get transmitted

Radio

by Chris Woodford. Last updated: November 15, 2019.

Free music, news, and chat wherever you go! 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 catches radio waves sailing through the air, a bit like a baseball catcher's mitt, and converts them back into sounds your ears can hear. Radio was first developed in the late-19th century and reached the height of its popularity several decades later. Although radio broadcasting is not quite as popular as it once was, the basic idea of wireless communication remains hugely important: in the last few years, radio has become the heart of new technologies such as wireless Internet, cellphones (mobile phones), and RFID (radio frequency identification) chips. Meanwhile, radio itself has recently gained a new lease of life with the arrival of better-quality digital radio sets.

Photo: An antenna to catch waves, some electronics to turn them back into sounds, and a loudspeaker so you can hear them—that's pretty much all there is to a basic radio receiver like this. What's inside the case? Check out the 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's a method of transmitting electrical energy from one 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; the radio wave sent by a transmitter whizzes through the air—maybe from one side of the 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 energy passing by. Tune the radio into a station and an electronic circuit inside the radio selects only the program you want from all those that are broadcasting.

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 a mixture of electricity and magnetism, travels past you in waves like those on the surface of the ocean. These are called radio waves. Like ocean waves, radio waves have a certain speed, length, and frequency. The speed is simply how fast the wave travels between two places. The wavelength is the distance between one crest (wave peak) and the next, while the frequency is the number of waves that arrive each second. Frequency is measured with a unit called hertz, so if seven waves arrive in a second, we call that seven hertz (7 Hz). If you've ever watched ocean waves rolling in to the beach, you'll know they travel with a speed of maybe one meter (three feet) per second or so. The wavelength of ocean waves tends to be tens of meters or feet, and the frequency is about one wave every few seconds.

When your radio sits on a bookshelf trying to catch waves coming into your home, it's a bit like you standing by the beach watching the breakers rolling in. Radio waves are much faster, longer, and more frequent than ocean waves, however. Their wavelength is typically hundreds of meters—so that's the distance between one wave crest and the next. But their frequency can be in the millions of hertz—so millions of these waves arrive each second. If the waves are hundreds of meters long, how can millions of them arrive so often? It's simple. Radio waves travel unbelievably fast—at the speed of light (300,000 km or 186,000 miles per second).

Photo: A radio studio is essentially a soundproof box that converts sounds into high-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 the water move up and down. In much the same way, radio waves carry energy as an invisible, up-and-down movement of electricity and magnetism. This carries program signals from huge transmitter antennas, which are connected to the radio station, to the smaller antenna on your radio set. A program is transmitted by adding it to a radio wave called a carrier. This process is called modulation. Sometimes a radio program is added to the carrier in such a way that the 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 or smaller. Since the size of a wave is called its amplitude, this process is known as amplitude modulation (AM). Frequency modulation is how FM radio is broadcast; amplitude modulation is the technique used by AM radio stations.

What's the difference between AM and FM?

An example makes this clearer. Suppose I'm on a rowboat in the ocean pretending to be a radio transmitter and you're on the shore pretending to be a radio receiver. Let's say I want to send a distress signal to you. I could rock the boat up and down quickly in the water to send big waves to you. If there are already waves traveling past my boat, from the distant ocean to the shore, my movements are going to make those existing waves much bigger. In other words, I will be using the waves passing by as a carrier to send my signal and, because I'll be changing the height of the waves, I'll be transmitting my signal by amplitude modulation. Alternatively, instead of moving my boat up and down, I could put my hand in the water and move it quickly back and forth. Now I'll make the waves travel more often—increasing their frequency. So, in this case, my signal will travel to you by frequency modulation.

Sending information by changing the shapes of waves is an example of an analog process. This means the information you are trying to send is represented by a direct physical change (the water moving up and down or back and forth more quickly).

The trouble with AM and FM is that the program signal becomes part of the wave that carries it. So, if something happens to the wave en-route, part of the signal is likely to get lost. And if it gets lost, there's no way to get it back again. Imagine I'm sending my distress signal from the boat to the shore and a speedboat races in between. The waves it creates will quickly overwhelm the ones I've made and obliterate the message I'm trying to send. That's why analog radios can sound crackly, especially if you're listening in a car. Digital radio can help to solve that problem by sending radio broadcasts in a coded, numeric format so that interference doesn't disrupt the signal in the same way. We'll talk about that in a moment, but first let's see take a peek inside an analog radio.

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):

Band/useWavelengthFrequency
LW (Long wave)5km–1km60kHz–300kHz
AM/MW (Amplitude modulation / medium wave)600m–176m500kHz–1.7MHz
SW (Short wave)188m–10m1.6MHz–30MHz
VHF/FM (Very high frequency / frequency modulation)10m–6m100MHz–500MHz
FM (frequency modulation)3.4m–2.8m88MHz–125Mhz
Aircraft2.7m–2.2m108–135MHz
Cellphones80cm–15cm380–2000MHz
Radar100cm–3mm0.3–100GHz

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

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

  • 1888: German physicist Heinrich Hertz (1857–1894) made the first electromagnetic radio waves in his lab.
  • 1894: British physicist Sir Oliver Lodge (1851–1940) sent the first message using radio waves in Oxford, England.
  • 1897: Physicist Nikola Tesla (1856–1943) filed patents explaining how electrical energy could be transmitted without wires (US Patent 645,576 and US Patent 649,621) and later (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. White discusses 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 radio waves across the Atlantic, from Cornwall in England to Newfoundland.
  • 1902–1903: American physicist, mathematician, and inventor John Stone Stone (1869–1943) used his knowledge of electric telegraphs to make important advances in radio tuning that 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 makes radios smaller and more practical. This invention earned De Forest the nicknamed "the father of radio."
  • 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 by John Bardeen (1908–1991), Walter Brattain (1902–1987), and William Shockley (1910–1989) of Bell Labs made it possible to amplify radio signals with much more compact circuits.
  • 1954: The Regency TR-1, launched in October 1954, was the world's first commercially produced transistor radio. 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.

Find out more

On this site

On other sites

Books

General and technical

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.

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Woodford, Chris. (2006/2019) Radio. Retrieved from https://www.explainthatstuff.com/radio.html. [Accessed (Insert date here)]

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