RADIOS work by using the electromagnetic spectrum to send information. When an electrical current travels through a wire it creates an electromagnetic field, sending out waves in all directions—rather like light, which is also a part of the spectrum but at much higher frequencies. An antenna and extra power allow the signal to be transmitted over long distances. The frequency of the wave can be changed so that signals do not interfere with one another, allowing more of the spectrum to be used.

Radio waves exist in nature, from sound and light to cosmic rays in space. The man-made sort can do things like transmit music or heat food in a microwave. At the low end of the spectrum are the frequencies used for things like television and mobile phones. The lower the frequency of the wave, the farther it can travel or penetrate physical objects. Thus FM radio at relatively high frequencies is used for local stations but does not travel far, whereas short-wave radio, at low frequencies, can stretch around the world.

Wireless technology can be split into five main sorts, roughly ranked by the distance that the signals travel. The top travellers are satellite communications such as the Global Positioning System (GPS). This is a constellation of 24 satellites, managed by the American armed forces, that constantly send out signals to devices on the ground. But the signals travel only one way, from satellite to device.

A little closer to home, and with signals going both ways, are “wide-area” mobile-phone technologies such as GSM or CDMA. Advanced “third-generation” (3G) versions include HSDPA and LTE, developed by the mobile-phone industry. A promising rival is WiMax, based on the internet standard and supported by the computer industry. A third category takes in shorter-range signals used to connect things in a building or room. Examples include the popular Wi-Fi standard to access the internet in hotels or airports, and Zigbee to link sensors. A new advance is ultra-wideband (UWB) technology, which uses very high frequencies at very short range to transmit huge amounts of data, as in sending video from an iPod-like device to a TV screen.

A fourth type connects things in a “personal-area network” or PAN. An example is Bluetooth, which is used to link mobile phones with earpieces. The last is near-field communications (NFC), where contact needs to be close, as in passes for buildings and public transport. A variant is radio-frequency identification (RFID) tags, used by retailers and others. When passed in front of a reader, the tags send back data stored on them. These radio systems are as different from each other as light is from sound, so satellites cannot track RFID tags, for instance. This means that some privacy worries are misplaced.

Radio has been cramming ever more information into the transmission, using techniques such as spreading it across the spectrum, dividing it up into portions and using “smart” antennae. As radio increasingly moves onto microchips, “software-defined radios” will be able to switch standards and frequencies as they go.

The proliferation of wireless systems has pushed up the demand for spectrum and increased its value. There are hopes that “cognitive” radios may eventually alleviate the problem by sharing huge swaths of spectrum intelligently—but they have yet to be invented.