Radio Waves: The Invisible Messengers

At this very moment the air around us is filled with invisible signals carrying music, voices and streams of data from one place to another. We cannot see, hear or feel them directly, yet without them there would be no radio, no television as we knew it, no mobile telephones and no wireless internet. These signals are radio waves, and although they seem almost magical in their effects, they obey the same simple laws of physics as the light we see every day. Learning a little about how they behave explains a great deal about the connected world we live in.

Radio waves belong to the same family as visible light. Both are forms of what scientists call electromagnetic waves, a family that also includes the warmth we feel from a fire and the rays that give us a suntan. What sets radio waves apart is their length. A wave of visible light is extremely short, far smaller than anything the eye could measure, whereas a radio wave may stretch from a few centimetres to many metres from one crest to the next. Despite this difference, radio waves travel at exactly the same speed as light, and, unlike light, they can pass straight through the walls of a building.

Radio waves are described by their frequency, which is the number of waves that pass a given point each second. Different radio stations and different services are given different frequencies so that their signals do not interfere with one another. The whole range of frequencies available for this purpose is known as the radio spectrum, and it is a limited resource that has to be shared out carefully by international agreement. A frequency handed to one broadcaster or one mobile network cannot be used freely by anyone else in the same area, and so this invisible space is treated as valuable property.

To send a broadcast, a station must first find a way of carrying sound upon a radio wave. On its own a steady radio wave, called a carrier wave, contains no sound at all. The station therefore alters the carrier in step with the sound it wishes to send, a process known as modulation, so that the pattern of the music or speech is stamped upon the wave. The altered wave is then fed to an antenna, a length of metal that radiates the signal outward into the surrounding air, where it spreads in every direction at the speed of light.

At the far end, a receiver reverses the process. Its own antenna picks up a great jumble of signals arriving from many different transmitters at once. A part of the receiver called the tuner selects just one frequency out of this crowd, ignoring all the rest, which is what a listener does when moving a dial from one station to another. The receiver then strips the sound information back off the carrier wave and feeds it to a loudspeaker, turning the invisible signal back into the music or the voice that was originally sent.

There is more than one way to stamp sound onto a carrier. In one older method the height of the wave is varied; in another the frequency itself is nudged up and down. The two methods have different strengths, one tending to travel further and the other to give a clearer, cleaner sound, and both have been used for broadcasting for many years. The choice between them is a trade-off, and engineers pick whichever suits the job in hand.

Broadcasting, however, is only the beginning. The same invisible waves carry telephone calls to and from the mobile phones in our pockets, link computers to the internet without any wires, and allow aircraft and ships to be tracked by radar. Spacecraft far out in the Solar System send their findings home across hundreds of millions of kilometres using nothing but faint radio signals. From the humble transistor set to the vast dishes that listen to distant probes, all of these depend on the same quiet physics: an invisible wave, shaped to carry a message, racing outward at the speed of light.