Radio waves are a form of electromagnetic radiation with relatively long wavelengths and low frequencies. The radio section of the electromagnetic spectrum includes waves with frequencies ranging from about 10 kilohertz (thousands of cycles per second) to about 60,000 megahertz. This frequency range corresponds to wavelengths between 98,000 ft, or 30,000 m, and 0.2 in, or 0.5 cm. The commercial value of radio waves as a means of transmitting sounds was first appreciated by the Italian inventor Guglielmo Marconi in the 1890s. Marconi’s invention led to the wireless telegraph, the radio, and eventually to such variations as the AM radio, FM radio, and CB (citizen’s band) radio.
Radio waves travel by three different routes from their point of propagation to their point of detection. These three routes are through the troposphere, through the ground, and by reflection off the ionosphere. The first of these routes is the most direct. A radio wave generated and transmitted from point A may travel in a relatively straight line through the lower atmosphere to a second point, B, where its presence can be detected by a receiver. This “line of sight” propagation is similar to the transmission of a beam of light from one point to another on Earth’s surface. And, as with light, this form of radio wave propagation is limited by the curvature of Earth’s surface.
This description is, however, overly simplified. Radio waves are deflected in a number of ways as they move through the troposphere. For example, they may be reflected, refracted, or diffracted by air molecules through which they pass. As a consequence, radio waves can actually pass beyond Earth’s optical horizon and, to an extent, follow Earth’s curvature.
Line-of-sight transmission has taken on a new dimension with the invention of communications satellites. Today a radio wave can be aimed at an orbiting satellite traveling in the upper part of the atmosphere. That satellite can then retransmit the signal back to Earth’s surface, where it can be picked up by a number of receiving stations. Communications satellites can be of two types. One, a passive satellite, simply provides a surface off which the radio wave can be reflected. The other type, an active satellite, picks up the signal received from Earth’s surface, amplifies it, and then retransmits it to ground-based receiving stations.
Since radio waves are propagated in all directions from a transmitting antenna, some may reflect off the ground to the receiving antenna, where they can be detected. Such waves can also be transmitted along Earth’s surface in a form known as surface waves. Radio waves whose transmission takes place in connection with Earth’s surface may be modified because of changing ground conditions, such as irregularities in the surface or the amount of moisture in the ground.
Finally, radio waves can be transmitted by reflection from the ionosphere. When waves of frequencies up to about 25 megahertz (sometimes higher) are projected into the sky, they bounce off a region of the ionosphere known as the E layer. The E layer is a region of high electrondensity located about 50 mi (80 km) above earth’s surface. Some reflection occurs off the F layer of the ionosphere also, located about 120 mi (200 km) above Earth’s surface. Radio waves reflected by the ionosphere are also known as sky waves.
The radio wave that leaves a transmitting antenna originates as a sound spoken into a microphone. A microphone is a device for converting sound energy into electrical energy. A microphone accomplishes this transformation by any one of a number of mechanisms. In a carbon microphone, for example, sound waves entering the device cause a box containing carbon granules to vibrate. The vibrating carbon granules, in turn, cause a change in electrical resistance within the carbon box to vary, resulting in the production of an electrical current of varying strength.
A crystal microphone makes use of the piezoelectric effect, the production of a tiny electric current caused by the deformation of the crystal in the microphone. The magnitude of the current produced corresponds to the magnitude of the sound wave entering the microphone.
The electric current produced within the microphone then passes into an amplifier where the current strength is greatly increased. The current is then transmitted to an antenna, where the varying electrical field associated with the current initiates an electromagnetic wave in the air around the antenna. It is this radio wave that is then propagated through space by one of the mechanisms described above.
A radio wave can be detected by a mechanism that is essentially the reverse of the process described here. The wave is intercepted by the antenna, which converts the wave into an electrical signal that is transmitted to a radio or television set. Within the radio or television set, the electrical signal is converted to a sound wave that can be broadcast through speakers.
The simple transmission scheme outlined above cannot be used for commercial broadcasting. If a dozen stations all transmitted sounds by the mechanism described above, a receiving station would pick up a garbled combination of all transmissions. To prevent interference from a number of transmitting stations, all broadcast radio waves are first modulated.
Modulation is the process by which a sound wave is added to a basic radio wave known as the carrier wave. For example, an audio signal can be electronically added to a carrier signal to produce a new signal that has undergone amplitude modulation (AM). Amplitude modulation means that the amplitude (or size) of the wave of the original sound wave has been changed by adding it to the carrier wave.
Sound waves can also be modulated in such a way that their frequency is altered. For example, a sound wave can be added to a carrier signal to produce a signal with the same amplitude, but a different frequency. The sound wave has, in this case, undergone frequency modulation (FM).
Both AM and FM signals must be decoded at the receiving station. In either case, the carrier wave is electronically subtracted from the radio wave that is picked up by the receiving antenna. What remains after this process is the original sound wave, encoded, of course, as an electrical signal.
Antenna— An electrical conductor used to send out or receive radio waves.
Carrier wave— A radio wave with an assigned characteristic frequency for a given station to which is added a sound-generated electrical wave that carries a message.
Frequency— The number of vibrations, cycles, or waves that pass a certain point per second.
Hertz— A unit of measurement for frequency, abbreviated Hz. One hertz is one cycle per second.
Modulation— The addition of a sound-generated electrical wave to a carrier wave.
Piezoelectricity— A small electrical current produced when a crystal is deformed.
Propagation— The spreading of a wave from a common origin.
Troposphere— The layer of air up to 15 mi (24 km) above the surface of Earth, also known as the lower atmosphere.
Wavelength— The distance between two consecutive crests or troughs in a wave.
number of stations to operate in the same area without overlapping. Thus, two stations a few miles apart could both be sending out exactly the same program, but they would sound different (and have different electric signals) because each had been overlaid on a different carrier signal.
Receiving stations can detect the difference between these two transmissions because they can tune their equipment to pick up only one or the other carrier frequency. When you turn the tuning knob on your own radio, for example, you are adjusting the receiver to pick up carrier waves from station A, station B, or some other station. Your radio then decodes the signal it has received by subtracting the carrier wave and converting the remaining electric signal to a sound wave.
The identifying characteristics by which you recognize a radio station reflect its two important transmitting features. The frequency, such as 101.5 megahertz (or simply “101.5 on your dial”) identifies the carrier wave frequency, as described above. The power rating (“operating with 50,000 watts of power”) describes the power available to transmit its signal. The higher the power of the station, the greater the distance at which its signal can be picked up.
See also Wave motion.
Isaacs, April. Characteristics And Behavior Of Waves: Understanding Sound And Electromagnetic Wave. New York: Rosen Publishing Group, 2004.
Someda, Carlo G. Electromagnetic Waves. 2nd ed. Boca Raton, FL: CRC, 2006.
David E. Newton