What Are Radio Waves?
A radio tuner receives radio waves and converts them to mechanical vibrations in the speaker to create sound waves that can be heard.
Credit: Ensuper | Shutterstock

Radio waves are a type of electromagnetic radiation, as are microwaves, infrared radiation, X-rays and gamma-rays. The best-known use of radio waves is for communication; television, cellphones and radios all receive radio waves and convert them to mechanical vibrations in the speaker to create sound waves that can be heard. 

Electromagnetic radiation is transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic (EM) spectrum. The spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency. The common designations are radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays

Radio waves have the longest wavelengths in the EM spectrum, according to NASA, ranging from about 1 millimeter (0.04 inches) to more than 100 kilometers (62 miles). They also have the lowest frequencies, from about 3,000 cycles per second or 3 kilohertz (kHz) up to about 300 billion hertz, or 300 gigahertz (GHz). 

Scottish physicist James Clerk Maxwell, who developed a unified theory of electromagnetism in the 1870s, predicted the existence of radio waves. A few years later, Heinrich Hertz, a German physicist, applied Maxwell's theories to the production and reception of radio waves. The unit of frequency of an EM wave — one cycle per second — is named the hertz, in his honor. 

Hertz used a spark gap attached to an induction coil and a separate spark gap on a receiving antenna. When waves created by the sparks of the coil transmitter were picked up by the receiving antenna, sparks would jump its gap as well. Hertz showed in his experiments that these signals possessed all the properties of electromagnetic waves.

The National Telecommunications and Information Administration generally divides the radio spectrum into nine bands.

Band Frequency range Wavelength range
Extremely Low Frequency (ELF) <3 kHz >100 km
Very Low Frequency (VLF) 3 to 30 kHz 10 to 100 km
Low Frequency (LF) 30 to 300 kHz 1 m to 10 km
Medium Frequency (MF) 300 kHz to 3 MHz 100 m to 1 km
High Frequency (HF) 3 to 30 MHz 10 to 100 m
Very High Frequency (VHF) 30 to 300 MHz 1 to 10 m
Ultra High Frequency (UHF) 300 MHz to 3 GHz 10 cm to 1 m
Super High Frequency (SHF) 3 to 30 GHz 1 to 1 cm
Extremely High Frequency (EHF) 30 to 300 GHz 1 mm to 1 cm

According to the Stanford VLF Group, the most powerful natural source of ELF/VLF waves on Earth is lightning. Waves produced by lightning strikes can bounce back and forth between the Earth and the ionosphere, so they can travel around the world. Radio waves are also produced by artificial sources, including electrical generators, power lines, appliances and radio transmitters. ELF radio is useful because of its long range, and its ability to penetrate water and rock for communication with submarines and inside mines and caves. However, the carrier frequency is often lower than the frequency range of audible sound, which is considered to be 20 to 20,000 Hz. For this reason ELF radio cannot be modulated fast enough to reproduce sound, which is why it is only used for digital data at a very slow rate. 

LF and MF radio bands include marine and aviation radio, as well as commercial AM radio. Most radio in these bands uses amplitude modulation (AM) to impress an audible signal onto the radio carrier wave. The power, or amplitude, of the signal is varied, or modulated, at a rate corresponding to the frequencies of an audible signal such as voice or music. AM radio has a long range, particularly at night, but it is subject to interference that affects the sound quality. When a signal is partially blocked, the volume of the sound is reduced accordingly. 

HF, VHF and UHF bands include FM radio, broadcast television sound, public service radio, cellphones and GPS. These bands typically use frequency modulation to impress an audio or data signal onto the carrier wave. In this scheme, the amplitude of the signal remains constant while the frequency is varied slightly higher or lower at a rate and magnitude corresponding to the audio or data signal. This results in better signal quality than AM because environmental factors do not affect the frequency the way they affect amplitude, and the receiver ignores variations in amplitude as long as the signal remains above a minimum threshold. 

Shortwave radio uses frequencies in the HF band, from about 1.7 MHz to 30 MHz, according to the National Association of Shortwave Broadcasters (NASB). Within that range, the shortwave spectrum is divided into several segments, some of which are dedicated to regular broadcasting stations, such as the Voice of America, the British Broadcasting Corporation and the Voice of Russia. Throughout the world, there are hundreds of shortwave stations, according to the NASB. About 25 privately owned shortwave stations are licensed in the United States by the Federal Communications Commission.

Shortwave stations can be heard for thousands of miles because the signals bounce off the ionosphere and rebound back hundreds or thousands of miles from their point of origin, according to the NASB. 

As two-channel stereo music gained popularity, so did the demand for stereo radio broadcasting. However, one-channel (monaural, or mono) radios were already in wide use and were likely to remain so for the foreseeable future. The problem, then, was to create a system that could produce stereo music but still be compatible with existing mono receivers. 

The method adopted for FM stereo broadcasting was rather ingenious. Ryan Giedd, a professor of physics at Missouri State University, explained that the broadcaster combines the left and right channels as L + R and LR and broadcasts them on slightly different frequencies, A and B. A mono receiver can lock onto A and hear both channels. A stereo receiver, however, locks onto both frequencies and combines A and B as A + B and AB. A little algebra shows that A + B = (L + R) + (LR) = 2L, and AB = (L + R) − (LR) = 2R, thus effectively separating the left and right channels.

SHF and EHF represent the highest frequencies in the radio band and are sometimes considered to be part of the microwave band. Molecules in the air tend to absorb these frequencies, which limits their range and applications. However, their short wavelengths allow signals to be directed in narrow beams by parabolic dish antennas, so they can be effective for short-range high-bandwidth communications between fixed locations. SHF, which is affected less by the air than EHF, is used for short-range applications such as Wi-Fi, Bluetooth and wireless USB. Also, SHF waves tend to bounce off of objects like cars, boats and aircraft, so this band is often used for radar. 

Outer space is teeming with radio sources. These include planets, stars, gas and dust clouds, galaxies, pulsars, and even black holes. These sources allow astronomers to learn about the motion and chemical composition of these sources as well as the processes that cause these emissions. 

According to Robert Patterson, a professor of astronomy at Missouri State University, astronomers use large radio telescopes to map cold neutral hydrogen clouds in galaxies. These clouds are of particular interest because they line up along the spiral arms of galaxies such as the Milky Way, allowing scientists to map the clouds' structure. 

Specific radio frequencies corresponding to the resonant frequencies of common atoms and molecules have been reserved by the FCC for exclusive use by radio astronomers to prevent radio transmitters from interfering with observations by extremely sensitive radio telescopes. A list of these frequencies is available from the National Astronomy and Ionosphere Center website

According to NASA, radio astronomers often combine several smaller telescopes, or receiving dishes, into an array in order to make a clearer, or higher-resolution, radio image. For example, the Very Large Array (VLA) radio telescope in New Mexico consists of 27 antennas arranged in a huge "Y" pattern up to 22 miles (36 km) across.

A radio telescope "sees" the sky very differently than it appears in visible light, according to NASA. Instead of seeing point-like stars, such a telescope picks up distant pulsars, star-forming regions and supernova remnants.

Radio telescopes can also detect quasars, which is short for quasi-stellar radio source. A quasar is an incredibly bright galactic core powered by a supermassive black hole. Quasars radiate energy broadly across the EM spectrum, but the name comes from the fact that the first quasars to be identified emit mostly radio energy. Quasars are very energetic; some emit 1,000 times as much energy as the entire Milky Way. However, most quasars are blocked from view in visible light by dust in their surrounding galaxies.

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