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Radio

Last updated: June 19, 2007.

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 simply a box of electronic components that snatches passing radio waves from the air, much like a baseball catcher's mitt, and turns them back into sounds you can hear. Radio was first developed in the late-19th century and reached the height of its popularity several decades later. Although it's not quite as popular as it once was, radio remains a hugely important form of communication. In the last few years, it's also become the heart of such new technologies as wireless Internet, cellphones (mobile phones), and RFID (radio frequency identification) chips.

Radio is sending energy with waves

Radio is a way of sending electrical energy between two places without using wires. That's why it's often called wireless. The piece of equipment that sends a radio wave is called a transmitter; the radio wave ends its journey at another piece of equipment called a receiver.

When you pull up the antenna (aerial) on an ordinary radio receiver, it catches some of the electromagnetic energy rushing 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.

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).

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.

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 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 radios can help to solve that problem.

How analog radio works

Let's lift the lid on an old-style analog transistor radio and see what we can find inside!

Photo of a transistor radio with the cover removed, showing the various components inside.

External, telescopic FM antenna: The one on this radio extends to about 30 cm (1 ft), which is plenty long enough to catch a good range of FM broadcasts. You can extend and swivel the telescopic antenna for better reception. Generally speaking, the longer the antenna (known as an aerial in the UK), the more signals you can pick up.
Battery compartment: This radio is either battery or AC powered. When you plug in an AC lead, a switch automatically cuts out the battery power.
Loudspeaker: There is only one loudspeaker, so this radio can reproduce only mono sounds. Generally, the bigger the loudspeaker the louder the radio (and the better the quality of sound it will make).
AC power input: A cable plugs into this socket so you can run the radio economically from a domestic power supply (mains electricity socket).
Transformer: The radio's electronic components operate on small voltages, but the power that comes in from the AC outlet is typically 110 volts (in the USA), 240 volts (in the UK), or similar. The transformer's job is to scale down the AC voltage so it's safe and appropriate for the radio's delicate components.
Internal AM antenna: When you're listening to an AM (also known as MW or medium wave) broadcast, the external FM antenna is redundant. Instead, signals are picked up by this tightly coiled AM antenna concealed inside the case. If you're listening on AM, you have to turn the entire radio to reorient the built-in antenna and improve your signal reception.
Transformer: A series of smaller transformers help the radio hone in on just the station you want by blocking out other, nearby stations.
Amplifier: This small chip boosts the signal strength so it's powerful enough to drive the loudspeaker.
Earphone socket: You can plug a small mono earphone in here to listen in privacy. If you plug stereo headphones into the mono socket, you'll hear sound in only one of the two earpieces.
Volume control: This is the back of the volume knob. Turning the volume knob adjusts an electronic component called a variable resistor or potentiometer, which increases or decreases the electric current flowing to the loudspeaker. A bigger current makes a louder sound with more volume; a smaller current makes a quieter sound with less volume.
Tuning control: This is a variable capacitor that tunes the radio in to a specific station.

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A brief history of radio

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: American 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.