Radio waves

What are radio waves? | Live Science

Radio waves are a type of electromagnetic radiation best known for its use in communication technologies, such as television, cell phones, and radios. These devices receive radio waves and convert them into mechanical vibrations in the speaker to create sound waves.

The radio frequency spectrum is a relatively small part of the electromagnetic (EM) spectrum. The EM spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency, according to the University of Rochester. Common designations are radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays, and gamma rays.

Radio waves have the longest wavelengths of the EM spectrum, according to NASA, ranging from about 0.04 inch (1 millimeter) to over 62 miles (100 kilometers). They also have the lowest frequencies, from about 3,000 cycles per second, or 3 kilohertz, down to about 300 billion hertz, or 300 gigahertz.

Radio spectrum is a finite resource and is often compared to farmland. Just as farmers must organize their land to get the best harvest in terms of quantity and variety, radio spectrum must be distributed among users in the most efficient manner, according to the British Broadcasting Corp. (BBC). In the United States, the National Telecommunications and Information Administration of the United States Department of Commerce manages frequency allocations along the radio spectrum.

Discovery

Scottish physicist James Clerk Maxwell, who developed a unified theory of electromagnetism in the 1870s, predicted the existence of radio waves, according to the National Library of Scotland. In 1886, Heinrich Hertz, a German physicist, applied Maxwell’s theories to the generation and reception of radio waves. Hertz used simple homemade tools including an induction coil and a Leyden jar (one of the earliest types of capacitors that consisted of a glass jar with foil layers inside and out) to create electromagnetic waves. Hertz became the first person to transmit and receive controlled radio waves. The unit of frequency of an electromagnetic wave – one cycle per second – is called hertz, in his honor, according to the American Association for the Advancement of Science.

Radio wave bands

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

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

Low to medium frequencies

ELF radio waves, the lowest of all radio frequencies, have a long range and are useful for penetrating water and rock for communication with submarines and inside mines and caves. According to the Stanford VLF Group, the most powerful natural source of ELF / VLF waves is lightning. Waves produced by lightning can bounce between Earth and the ionosphere (the atmospheric layer with a high concentration of ions and free electrons), according to Phys.org. These lightning disturbances can distort important radio signals transmitted to satellites.

The LF and MF radio bands include marine and aeronautical radio, as well as commercial AM (amplitude modulation) radio, according to RF Page. AM radio frequency bands are between 535 kilohertz and 1.7 megahertz, according to How Stuff Works. AM radio has a long range, especially at night when the ionosphere refracts waves better back to earth, but it is prone to interference that affects sound quality. When a signal is partially blocked – for example, by a building with metal walls such as a skyscraper – the volume of the sound is reduced accordingly.

Higher frequencies

The HF, VHF and UHF bands include FM radio, broadcast television sound, public service radio, cell phones, and GPS (Global Positioning System). These bands typically use “frequency modulation” (FM) to encode or impress an audio or data signal on the carrier wave. In frequency modulation, the amplitude (maximum extent) of the signal remains constant while the frequency varies more or less high at a speed and an amplitude corresponding to the audio or data signal.

FM gives better signal quality than AM because environmental factors do not affect frequency as they affect amplitude, and the receiver ignores changes in amplitude as long as the signal remains above a minimum threshold. FM radio frequencies are between 88 megahertz and 108 megahertz, according to How Stuff Works.

Short wave radio

Shortwave radio uses frequencies in the HF band, from about 1.7 megahertz to 30 megahertz, according to the National Association of Shortwave Broadcasters (NASB). In this range, the shortwave spectrum is divided into several segments, some of which are dedicated to regular broadcasting stations, such as Voice of America, British Broadcasting Corp. and Voice of Russia. All over the world, there are hundreds of shortwave stations, according to the NASB. Shortwave stations can be heard for thousands of miles because the signals bounce off the ionosphere and bounce hundreds or thousands of miles from their point of origin.

Highest frequencies

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 satellite dishes (satellite dishes). This enables high-bandwidth, short-range communications between fixed locations.

The SHF, less affected by air than the EHF, is used for short range applications such as Wi-Fi, Bluetooth and wireless USB (universal serial bus). SHF can only work in line-of-sight paths, as the waves tend to bounce off objects such as cars, boats and airplanes, according to the RF page. And because the waves bounce off objects, SHF can also be used for radar.

Astronomical sources

Space is full of radio wave sources: planets, stars, clouds of gas and dust, galaxies, pulsars and even black holes. By studying them, astronomers can learn more about the movement and chemical composition of these cosmic sources as well as the processes that cause these emissions.

A radio telescope “sees” the sky very differently from what it appears in visible light. Instead of seeing point stars, a radio telescope picks up distant pulsars, regions of star formation, and supernova remnants. Radio telescopes can also detect quasars, short for a quasi-stellar radio source. A quasar is an incredibly bright galactic nucleus powered by a supermassive black hole. Quasars emit energy across the entire electromagnetic spectrum, but their name comes from the fact that the first identified quasars primarily emit radio energy. Quasars are very energetic; some emit 1,000 times more energy than the entire Milky Way.

According to the University of Vienna, radio astronomers often combine several small telescopes or satellite dishes in an array to create 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 36 kilometers in diameter.

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This article was updated on February 27, 2019 by Live Science contributor Traci Pedersen.