by Chris Woodford. Last updated: July 29, 2016.
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
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
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
Frequency is measured with a unit called hertz,
so if seven
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
speed of maybe one meter (three feet) per second or so. The wavelength
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
so often? It's simple. Radio waves travel unbelievably fast—at
speed of light (300,000 km or 186,000 miles per second).
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 quickly—increasing their
frequency. So, in this case, my signal will travel to you by frequency
Sending information by changing the shapes of waves is
an example of an analog process. This means
the information you are trying to
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
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.
You're driving along the freeway and your favorite song comes on the radio. You go under a bridge and—buzz, hiss, crackle, pop—the
song disappears in a burst of static. Just as people have got used to
such niggles, inventors have come up with a new type of radio that
promises almost perfect sound. Digital radio, as it's called, sends
speech and songs through the air as strings of numbers. No matter
what comes between your radio and the transmitter, the signal almost
always gets through. That's why digital radio sounds better. But
digital technology also brings many more
stations and displays information about the program you're listening to
(such as the names of music tracks or programs).
Photo: A typical Roberts DAB digital radio. The big orange button in the middle lets
you pause a live radio broadcast and restart it later.
How is digital radio different from analog?
Let's go back to the earlier example of sending information from a boat to the shore—but this time
using a digital method. In case of emergency, I could store hundreds of plastic ducks on my boat,
each one carrying a number. If I get into trouble, as before, and want to send a distress signal, I could send
you an emergency coded message "12345" by releasing just the
ducks with those numbers. Let's suppose I do have a problem. I release ducks
with the numbers 1, 2, 3, 4, and 5—but instead of sending just five numbered ducks, I send maybe 10 or 20 of
each duck to increase the chances of the message arriving. Now, even if the sea is choppy
or a speedboat cuts through, there's still a high chance enough of the ducks
will get through. Eventually, waves will carry ducks with the numbers
1, 2, 3, 4, and 5 ashore. You collect the ducks together and work out
what I'm trying to say.
That's more or less how digital radio works!
- The transmitter sends program signals broken into fragments and coded in numbers (digits).
- The transmitter sends each fragment many times to increase the chances of it getting through.
- Even when things interrupt or delay some of the fragments, the receiver can still piece together fragments arriving from other places
and put them together to make an uninterrupted program signal.
To help avoid interference, a digital radio signal travels on a huge, broad band of radio frequencies about
1500 times wider than those used in analog radio. To return to our rowboat
example, if I could send a wave 1500 times wider, it would bypass any
speedboats that got in the way and get to the shore more easily. This
wide band allows a single digital signal to carry six stereo music
programs or 20 speech programs in one go. Blending signals together
in this way is called multiplexing. Part of
the signal might be music, while another part could be a stream of text information that
tells you what the music is, the name of the DJ, which radio station
you're listening to, and so on.
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):
|LW (Long wave)
|AM/MW (Amplitude modulation / medium wave)
|SW (Short wave)
|VHF/FM (Very high frequency / frequency modulation)
|FM (frequency modulation)
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.
- 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.
- 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.
Find out more
On this site
On other sites
General and technical
- 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. Lodge's autobiography gives some 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.
- The Victorian Internet by Tom Standage. Walker & Company, 2007. A more general history of how telecommunications changed during the 19th century with the development of electric power, telegraphs, and radio.
- 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.