Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as high as 300 gigahertz (GHz) to as low as 30 hertz (Hz).[1] At 300 GHz, the corresponding wavelength is 1 mm, and at 30 Hz is 10,000 km. Like all other electromagnetic waves, radio waves travel at the speed of light. They are generated by electric charges undergoing acceleration, such as time varying electric currents.[2] Naturally occurring radio waves are emitted by lightning and astronomical objects.

To prevent interference between different users, the artificial generation and use of radio waves is strictly regulated by law, coordinated by an international body called the International Telecommunications Union (ITU), which defines radio waves as "electromagnetic waves of frequencies arbitrarily lower than 3 000 GHz, propagated in space without artificial guide".[3] The radio spectrum is divided into a number of radio bands on the basis of frequency, allocated to different uses.

Diagram of the electric fields (E) and magnetic fields (H) of radio waves emitted by a monopole radio transmitting antenna (small dark vertical line in the center). The E and H fields are perpendicular, as implied by the phase diagram in the lower right.
Animation of a half-wave dipole antenna radiating radio waves, showing the electric field lines. The antenna in the center is two vertical metal rods connected to a radio transmitter (not shown). The transmitter applies an alternating electric current to the rods, which charges them alternately positive (+) and negative (−). Loops of electric field leave the antenna and travel away at the speed of light; these are the radio waves. In this animation the action is shown slowed down enormously.

## Discovery and exploitation

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including radio waves.

Radio waves were first predicted by mathematical work done in 1867 by British mathematical physicist James Clerk Maxwell.[4] Maxwell noticed wavelike properties of light and similarities in electrical and magnetic observations. His mathematical theory, now called Maxwell's equations, described light waves and radio waves as waves of electromagnetism that travel in space, radiated by a charged particle as it undergoes acceleration. In 1887, Heinrich Hertz demonstrated the reality of Maxwell's electromagnetic waves by experimentally generating radio waves in his laboratory,[5] showing that they exhibited the same wave properties as light: standing waves, refraction, diffraction, and polarization. Radio waves, originally called "Hertzian waves",[6] were first used for communication in the mid 1890s by Guglielmo Marconi, who developed the first practical radio transmitters and receivers. The modern term "radio wave" replaced the original name "Hertzian wave" around 1912.

## Speed, wavelength, and frequency

Animated diagram of a half-wave dipole antenna receiving a radio wave. The antenna consists of two metal rods connected to a receiver R. The electric field (E, green arrows) of the incoming wave pushes the electrons in the rods back and forth, charging the ends alternately positive (+) and negative (−). Since the length of the antenna is one half the wavelength of the wave, the oscillating field induces standing waves of voltage (V, represented by red band) and current in the rods. The oscillating currents (black arrows) flow down the transmission line and through the receiver (represented by the resistance R).

Radio waves in a vacuum travel at the speed of light.[7][8] When passing through a material medium, they are slowed according to that object's permeability and permittivity. Air is thin enough that in the Earth's atmosphere radio waves travel very close to the speed of light.

The wavelength is the distance from one peak of the wave's electric field (wave's peak/crest) to the next, and is inversely proportional to the frequency of the wave. The distance a radio wave travels in one second, in a vacuum, is 299,792,458 meters (983,571,056 ft) which is the wavelength of a 1 hertz radio signal. A 1 megahertz radio signal has a wavelength of 299.8 meters (984 ft).

## Propagation

The study of radio propagation, how radio waves move in free space and over the surface of the Earth, is vitally important in the design of practical radio systems. Radio waves passing through different environments experience reflection, refraction, polarization, diffraction, and absorption. Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certain radio bands more useful for specific purposes than others. Practical radio systems mainly use three different techniques of radio propagation to communicate:[9]

• Line of sight: This refers to radio waves that travel in a straight line from the transmitting antenna to the receiving antenna. It does not necessarily require a cleared sight path; at lower frequencies radio waves can pass through buildings, foliage and other obstructions. This is the only method of propagation possible at frequencies above 30 MHz. On the surface of the Earth, line of sight propagation is limited by the visual horizon to about 64 km (40 mi). This is the method used by cell phones, FM, television broadcasting and radar. By using dish antennas to transmit beams of microwaves, point-to-point microwave relay links transmit telephone and television signals over long distances up to the visual horizon. Ground stations can communicate with satellites and spacecraft billions of miles from Earth.
• Indirect propagation: Radio waves can reach points beyond the line-of-sight by diffraction and reflection.[9] Diffraction allows a radio wave to bend around obstructions such as a building edge, a vehicle, or a turn in a hall. Radio waves also reflect from surfaces such as walls, floors, ceilings, vehicles and the ground. These propagation methods occur in short range radio communication systems such as cell phones, cordless phones, walkie-talkies, and wireless networks. A drawback of this mode is multipath propagation, in which radio waves travel from the transmitting to the receiving antenna via multiple paths. The waves interfere, often causing fading and other reception problems.
• Ground waves: At lower frequencies below 2 MHz, in the medium wave and longwave bands, due to diffraction vertically polarized radio waves can bend over hills and mountains, and propagate beyond the horizon, traveling as surface waves which follow the contour of the Earth. This allows mediumwave and longwave broadcasting stations to have coverage areas beyond the horizon, out to hundreds of miles. As the frequency drops, the losses decrease and the achievable range increases. Military very low frequency (VLF) and extremely low frequency (ELF) communication systems can communicate over most of the Earth, and with submarines hundreds of feet underwater.
• Skywaves: At medium wave and shortwave wavelengths, radio waves reflect off conductive layers of charged particles (ions) in a part of the atmosphere called the ionosphere. So radio waves directed at an angle into the sky can return to Earth beyond the horizon; this is called "skip" or "skywave" propagation. By using multiple skips communication at intercontinental distances can be achieved. Skywave propagation is variable and dependent on atmospheric conditions; it is most reliable at night and in the winter. Widely used during the first half of the 20th century, due to its unreliability skywave communication has mostly been abandoned. Remaining uses are by military over-the-horizon (OTH) radar systems, by some automated systems, by radio amateurs, and by shortwave broadcasting stations to broadcast to other countries.

In radio communication systems, information is carried across space using radio waves. At the sending end, the information to be sent, in the form of a time-varying electrical signal, is applied to a radio transmitter.[10] The information signal can be an audio signal representing sound from a microphone, a video signal representing moving images from a video camera, or a digital signal representing data from a computer. In the transmitter, an electronic oscillator generates an alternating current oscillating at a radio frequency, called the carrier wave because it serves to "carry" the information through the air. The information signal is used to modulate the carrier, altering some aspect of it, "piggybacking" the information on the carrier. The modulated carrier is amplified and applied to an antenna. The oscillating current pushes the electrons in the antenna back and forth, creating oscillating electric and magnetic fields, which radiate the energy away from the antenna as radio waves. The radio waves carry the information to the receiver location.

At the receiver, the oscillating electric and magnetic fields of the incoming radio wave push the electrons in the receiving antenna back and forth, creating a tiny oscillating voltage which is a weaker replica of the current in the transmitting antenna.[10] This voltage is applied to the radio receiver, which extracts the information signal. The receiver first uses a bandpass filter to separate the desired radio station's radio signal from all the other radio signals picked up by the antenna, then amplifies the signal so it is stronger, then finally extracts the information-bearing modulation signal in a demodulator. The recovered signal is sent to a loudspeaker or earphone to produce sound, or a television display screen to produce a visible image, or other devices. A digital data signal is applied to a computer or microprocessor, which interacts with a human user.

The radio waves from many transmitters pass through the air simultaneously without interfering with each other. They can be separated in the receiver because each transmitter's radio waves oscillate at a different rate, in other words each transmitter has a different frequency, measured in kilohertz (kHz), megahertz (MHz) or gigahertz (GHz). The bandpass filter in the receiver consists of a tuned circuit which acts like a resonator, similarly to a tuning fork.[10] It has a natural resonant frequency at which it oscillates. The resonant frequency is set equal to the frequency of the desired radio station. The oscillating radio signal from the desired station causes the tuned circuit to oscillate in sympathy, and it passes the signal on to the rest of the receiver. Radio signals at other frequencies are blocked by the tuned circuit and not passed on.

## Biological and environmental effects

Since the heating effect is in principle no different from other sources of heat, most research into possible health hazards of exposure to radio waves has focused on "nonthermal" effects; whether radio waves have any effect on tissues besides that caused by heating. Electromagnetic radiation has been classified by the International Agency for Research on Cancer (IARC) as "Possibly carcinogenic to humans".[16][17] The conceivable evidence of cancer risk via Personal exposure to RF-EMF with mobile telephone use was identified.[18]

Radio waves can be shielded against by a conductive metal sheet or screen, an enclosure of sheet or screen is called a Faraday cage. A metal screen shields against radio waves as well as a solid sheet as long as the holes in the screen are smaller than about 1/20 of wavelength of the waves.[19]

## Measurement

Since radio frequency radiation has both an electric and a magnetic component, it is often convenient to express intensity of radiation field in terms of units specific to each component. The unit volts per meter (V/m) is used for the electric component, and the unit amperes per meter (A/m) is used for the magnetic component. One can speak of an electromagnetic field, and these units are used to provide information about the levels of electric and magnetic field strength at a measurement location.

Another commonly used unit for characterizing an RF electromagnetic field is power density. Power density is most accurately used when the point of measurement is far enough away from the RF emitter to be located in what is referred to as the far field zone of the radiation pattern.[20] In closer proximity to the transmitter, i.e., in the "near field" zone, the physical relationships between the electric and magnetic components of the field can be complex, and it is best to use the field strength units discussed above. Power density is measured in terms of power per unit area, for example, milliwatts per square centimeter (mW/cm2). When speaking of frequencies in the microwave range and higher, power density is usually used to express intensity since exposures that might occur would likely be in the far field zone.

## Notes

1. ^ Altgelt, CA (2005). "The World's Largest "Radio" Station" (PDF). hep.wisc.edu. High Energy Physics @ UW Madison. Retrieved 9 Jan 2019.
2. ^ Ellingson SW (2016). Radio Systems Engineering. Cambridge University Press. pp. 16–17. ISBN 1316785165.
3. ^ "Ch. 1: Terminology and technical characteristics - Terms and definitions". Radio Regulations (PDF). Geneva: ITU. 2016. p. 7. ISBN 9789261191214.
4. ^ Harman PM (1998). The natural philosophy of James Clerk Maxwell. Cambridge, England: Cambridge University Press. p. 6. ISBN 0-521-00585-X.
5. ^ Rubin, J. "Heinrich Hertz: The Discovery of Radio Waves". Juliantrubin.com. Retrieved 8 Nov 2011.
6. ^ White, TH. "United States Early Radio History. Section 22: Word Origins". earlyradiohistory.us. Retrieved 9 Jan 2019.
7. ^ "Electromagnetic Frequency, Wavelength and Energy Ultra Calculator". 1728.org. 1728 Software Systems. Retrieved 15 Jan 2018.
8. ^ "How Radio Waves Are Produced". NRAO. Archived from the original on 28 Mar 2014. Retrieved 15 Jan 2018.
9. ^ a b Seybold JS (2005). "1.2 Modes of Propagation". Introduction to RF Propagation. John Wiley and Sons. pp. 3–10. ISBN 0471743682.
10. ^ a b c Brain, M (7 Dec 2000). "How Radio Works". HowStuffWorks.com. Retrieved 11 Sep 2009.
11. ^ Kitchen R (2001). RF and Microwave Radiation Safety Handbook (2nd ed.). Newnes. pp. 64–65. ISBN 0750643552.
12. ^ VanderVorst A, Rosen A, Kotsuka Y (2006). RF/Microwave Interaction with Biological Tissues. John Wiley & Sons. pp. 121–122. ISBN 0471752045.
13. ^ Graf RF, Sheets W (2001). Build Your Own Low-power Transmitters: Projects for the Electronics Experimenter. Newnes. p. 234. ISBN 0750672447.
14. ^ Elder JA, Cahill DF (1984). "Biological Effects of RF Radiation". Biological Effects of Radiofrequency Radiation. US EPA. pp. 5.116–5.119.
15. ^ Hitchcock RT, Patterson RM (1995). Radio-Frequency and ELF Electromagnetic Energies: A Handbook for Health Professionals. Industrial Health and Safety Series. John Wiley & Sons. pp. 177–179. ISBN 9780471284543.
16. ^ "IARC Classifies Radiofrequency Electromagnetic Fields as Possibly Carcinogenic to Humans" (PDF). www.iarc.fr (Press Release). WHO. 31 May 2011. Retrieved 9 Jan 2019.
17. ^ "Agents Classified by the IARC Monographs, Volumes 1–123". monographs.iarc.fr. IARC. 9 Nov 2018. Retrieved 9 Jan 2019.
18. ^ Baan, R; Grosse, Y; Lauby-Secretan, B; et al. (2014). "Radiofrequency Electromagnetic Fields: evaluation of cancer hazards" (PDF). monographs.iarc.fr (conference poster). IARC. Retrieved 9 Jan 2019.
19. ^ Kimmel WD, Gerke D (2018). Electromagnetic Compatibility in Medical Equipment: A Guide for Designers and Installers. Routledge. p. 6.67. ISBN 9781351453370.
20. ^ National Association of Broadcasters (1996). Antenna & Tower Regulation Handbook. NAB, Science and Technology Department. p. 186. ISBN 9780893242367. Archived from the original on 1 May 2018.

## References

Alexander Stepanovich Popov

Alexander Stepanovich Popov (sometimes spelled Popoff; Russian: Алекса́ндр Степа́нович Попо́в; March 16 [O.S. March 4] 1859 – January 13 [O.S. December 31, 1905] 1906) was a Russian physicist who is acclaimed in his homeland and some eastern European countries as the inventor of radio.Popov's work as a teacher at a Russian naval school led him to explore high frequency electrical phenomena. On May 7, 1895 he presented a paper on a wireless lightning detector he had built that worked via using a coherer to detect radio noise from lightning strikes. This day is celebrated in the Russian Federation as Radio Day. In a March 24, 1896 demonstration, he used radio waves to transmit a message between different campus buildings in St. Petersburg. His work was based on that of another physicist – Oliver Lodge, and contemporaneous with the work of Guglielmo Marconi. Marconi had just registered a patent with the description of the device two months after first transmission of radio signals made by Popov.

Atmospheric diffraction

Atmospheric diffraction is manifested in the following principal ways:

Optical atmospheric diffraction

Radio wave diffraction is the scattering of radio frequency or lower frequencies from the Earth's ionosphere, resulting in the ability to achieve greater distance radio broadcasting.

Sound wave diffraction is the bending of sound waves, as the sound travels around edges of geometric objects. This produces the effect of being able to hear even when the source is blocked by a solid object. The sound waves bend appreciably around the solid object.However, if the object has a diameter greater than the acoustic wavelength, a 'sound shadow' is cast behind the object where the sound is inaudible. (Note: some sound may be propagated through the object depending on material).

Denpa teki na Kanojo

Denpa teki na Kanojo (電波的な彼女, lit. The Radio Wave-Like Girlfriend, translated as Electromagnetic Girlfriend) is a Japanese light novel series by Kentarō Katayama, with illustrations by Yamato Yamamoto. In 2009, a two-episode OVA adaptation was exclusively bundled with the pre-order editions of Kure-nai DVD volumes 3 and 4. Kure-nai is another anime by the same author. Under the title Denpa Biyori (電波日和, lit. Radio Wave Weather), the first story in the series won an honorable mention at the Super Dash Novel Rookie of the Year Awards in 2003. The Japanese term denpa originally referred to radio waves, but in slang usage it refers to people who perceive things ordinary people can't. Three novels were published between 2004 and 2005 with an additional short piece of writing being included with the OVA. As of 2016, the series has yet to be formally concluded.

Electrolytic detector

Electronic pest control

Electronic pest control is the name given to any of several types of electrically powered devices designed to repel or eliminate pests, usually rodents or insects. Since these devices are not regulated under the Federal Insecticide, Fungicide, and Rodenticide Act in the United States, the EPA does not require the same kind of efficacy testing that it does for chemical pesticides. Studies on ultrasound pest control devices have been described as ineffective, a waste of money and potentially harmful to users and their efforts to deter insects and prevent disease.

Hydrogen line

The hydrogen line, 21-centimeter line or H I line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420405751.7667±0.0009 Hz, which is equivalent to the vacuum wavelength of 21.1061140542 cm in free space. This wavelength falls within the microwave region of the electromagnetic spectrum, and it is observed frequently in radio astronomy, since those radio waves can penetrate the large clouds of interstellar cosmic dust that are opaque to visible light.

The microwaves of the hydrogen line come from the atomic transition of an electron between the two hyperfine levels of the hydrogen 1s ground state that have an energy difference of ≈ 5.87433 µeV. It is called the spin-flip transition. The frequency, ν, of the quanta that are emitted by this transition between two different energy levels is given by the Planck–Einstein relation E = hν. According to that relation, the photon energy of a 1,420,405,751.7667 Hz photon is ≈ 5.87433 µeV. The constant of proportionality, h, is known as the Planck constant.

The Japan Amateur Radio League (JARL) (in Japanese, 日本アマチュア無線連盟) is a national non-profit organization for amateur radio enthusiasts in Japan. JARL was founded in 1926 by Japanese radio communication enthusiasts whose stated aim was to promote the development and utilization of radio wave technology as a medium. JARL says its current membership comprises the largest number of radio stations in the world, and credits its growth to "the devoted efforts of pioneering hams, who took the history of amateur radio to heart and guided it through the changing and challenging winds of technology and radio regulations". JARL is the national member society representing Japan in the International Amateur Radio Union.

Microquasar

A microquasar, the smaller version of a quasar, is a compact region surrounding a black hole with a mass several times that of our sun, and its companion star. The matter being pulled from the companion star forms an accretion disk around the black hole. This accretion disk may become so hot, due to friction, that it begins to emit X-rays. The disk also projects narrow streams or "jets" of subatomic particles at near-light speed, generating a strong radio wave emission.

NGC 7196

NGC 7196 is an elliptical galaxy registered in the New General Catalogue. It is located in the direction of the Indus constellation, at a distance of circa 150 million light years. It was discovered by the English astronomer John Herschel in 1834 using a 47.5 cm (18.7 inch) reflector. NGC 7196 appears slightly distorted, with asymmetric outer isophotes. Asymmetry is also observed near the centre. The inner luminosity pattern resembles that of lenticular galaxies with circumscribing dust lanes, except that the feature is extremely close to the center. A shell has been observed around the galaxy. Shells are generally considered to have formed after the accretion of a smaller galaxy by a massive one. It has weak radio wave emission.NGC 7196 is the foremost member of a galaxy group known as the NGC 7196 group, which also includes NGC 7200 and some dwarf elliptical and irregular galaxies. In the same galaxy cloud lies NGC 7168. NGC 7196 lies in the foreground of galaxy cluster known as Abell S0989.

Noise (video)

Noise, in analog video and television, is a random dot pixel pattern of static displayed when no transmission signal is obtained by the antenna receiver of television sets and other display devices. The random pattern superimposed on the picture, visible as a random flicker of "dots" or "snow", is the result of electronic noise and radiated electromagnetic noise accidentally picked up by the antenna. This effect is most commonly seen with analog TV sets or blank VHS tapes.

There are many sources of electromagnetic noise which cause the characteristic display patterns of static. Atmospheric sources of noise are the most ubiquitous, and include electromagnetic signals prompted by cosmic microwave background radiation, or more localized radio wave noise from nearby electronic devices.The display device itself is also a source of noise, due in part to thermal noise produced by the inner electronics. Most of this noise comes from the first transistor the antenna is attached to.UK viewers used to see "snow" on black after sign-off, instead of "bugs" on white, a purely technical artifact due to old 405-line British senders using positive rather than the negative video modulation used in Canada, the U.S., and (currently) the UK as well. Since one impression of the "snow" is of fast-flickering black bugs on a white background, the phenomenon is often called myrornas krig in Swedish, myrekrig in Danish, hangyák háborúja in Hungarian, and semut bertengkar in Indonesian, which all translate to "war of the ants". It is also known as ekran karıncalanması in Turkish, meaning "ants on the screen", hangyafoci in Hungarian which means "ant football", and in Romanian, purici, which translates into "fleas".

Most modern televisions automatically change to a blue or black screen, or turn to standby after some time if static is present.

Applications of radio waves which do not involve transmitting the waves significant distances, such as RF heating used in industrial processes and microwave ovens, and medical uses such as diathermy and MRI machines, are not usually called radio. The noun radio is also used to mean a broadcast radio receiver.

Radio waves were first identified and studied by German physicist Heinrich Hertz in 1886. The first practical radio transmitters and receivers were developed around 1895-6 by Italian Guglielmo Marconi, and radio began to be used commercially around 1900. To prevent interference between users, the emission of radio waves is strictly regulated by law, coordinated by an international body called the International Telecommunications Union (ITU), which allocates frequency bands in the radio spectrum for different uses.

Radio-frequency engineering, or RF engineering, is a subset of electrical and electronic engineering involving the application of transmission line, waveguide, antenna and electromagnetic field principles to the design and application of devices that produce or utilize signals within the radio band, the frequency range of about 20 kHz up to 300 GHz.It is incorporated into almost everything that transmits or receives a radio wave, which includes, but is not limited to, mobile phones, radios, Wi-Fi, and two-way radios.

RF engineering is a highly specialized field that typically includes the following areas of expertise:

Design of antenna systems to provide radiative coverage of a specified geographical area by an electromagnetic field or to provide specified sensitivity to an electromagnetic field impinging on the antenna.

Design of coupling and transmission line structures to transport RF energy without radiation.

Application of circuit elements and transmission line structures in the design of oscillators, amplifiers, mixers, detectors, combiners, filters, impedance transforming networks and other devices.

Verification and measurement of performance of radio frequency devices and systems.To produce quality results, the RF engineer needs an in-depth knowledge of mathematics, physics and general electronics theory as well as specialized training in areas such as wave propagation, impedance transformations, filters and microstrip printed circuit board design to name a few.

Radio Wave 96.5 is a British Independent Local Radio station that serves the Blackpool and Fylde coast areas of Lancashire, owned and operated by Bauer Radio. The station broadcasts from a specially-constructed transmitter aerial atop Blackpool Tower. It was originally called Radio Wave and has also been known as The Wave 96.5.

Radio Wave Surfer is the sixth album by Holger Czukay, released in 1991 through Virgin Records.

Radio propagation is the behavior of radio waves as they travel, or are propagated, from one point to another, or into various parts of the atmosphere. As a form of electromagnetic radiation, like light waves, radio waves are affected by the phenomena of reflection, refraction, diffraction, absorption, polarization, and scattering. Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for international shortwave broadcasters, to designing reliable mobile telephone systems, to radio navigation, to operation of radar systems.

Several different types of propagation are used in practical radio transmission systems. Line-of-sight propagation means radio waves which travel in a straight line from the transmitting antenna to the receiving antenna. Line of sight transmission is used to medium range radio transmission such as cell phones, cordless phones, walkie-talkies, wireless networks, FM radio and television broadcasting and radar, and satellite communication, such as satellite television. Line-of-sight transmission on the surface of the Earth is limited to the distance to the visual horizon, which depends on the height of transmitting and receiving antennas. It is the only propagation method possible at microwave frequencies and above. At microwave frequencies, moisture in the atmosphere (rain fade) can degrade transmission.

At lower frequencies in the MF, LF, and VLF bands, due to diffraction radio waves can bend over obstacles like hills, and travel beyond the horizon as surface waves which follow the contour of the Earth. These are called ground waves. AM broadcasting stations use ground waves to cover their listening areas. As the frequency gets lower, the attenuation with distance decreases, so very low frequency (VLF) and extremely low frequency (ELF) ground waves can be used to communicate worldwide. VLF and ELF waves can penetrate significant distances through water and earth, and these frequencies are used for mine communication and military communication with submerged submarines.

At medium wave and shortwave frequencies (MF and HF bands) radio waves can refract from a layer of charged particles (ions) high in the atmosphere, called the ionosphere. This means that radio waves transmitted at an angle into the sky can be reflected back to Earth beyond the horizon, at great distances, even transcontinental distances. This is called skywave propagation. It is used by amateur radio operators to talk to other countries, and shortwave broadcasting stations that broadcast internationally. Skywave communication is variable, dependent on conditions in the upper atmosphere; it is most reliable at night and in the winter. Due to its unreliability, since the advent of communication satellites in the 1960s, many long range communication needs that previously used skywaves now use satellites.

In addition, there are several less common radio propagation mechanisms, such as tropospheric scattering (troposcatter) and near vertical incidence skywave (NVIS) which are used in specialized communication systems.

A radio propagation model, also known as the radio wave propagation model or the radio frequency propagation model, is an empirical mathematical formulation for the characterization of radio wave propagation as a function of frequency, distance and other conditions. A single model is usually developed to predict the behavior of propagation for all similar links under similar constraints. Created with the goal of formalizing the way radio waves are propagated from one place to another, such models typically predict the path loss along a link or the effective coverage area of a transmitter.

The information produced by the receiver may be in the form of sound, moving images (television), or data. A radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. Radio receivers are very widely used in modern technology, as components of communications, broadcasting, remote control, and wireless networking systems.

Scintillometer

A scintillometer is a scientific device used to measure small fluctuations of the refractive index of air caused by variations in temperature, humidity, and pressure. It consists of an optical or radio wave transmitter and a receiver at opposite ends of an atmospheric propagation path. The receiver detects and evaluates the intensity fluctuations of the transmitted signal, called scintillation.

The magnitude of the refractive index fluctuations is usually measured in terms of ${\displaystyle C_{n}^{2}}$, the structure constant of refractive index fluctuations, which is the spectral amplitude of refractive index fluctuations in the inertial subrange of turbulence. Some types of scintillometers, such as displaced-beam scintillometers, can also measure the inner scale of refractive index fluctuations, which is the smallest size of eddies in the inertial subrange.

Scintillometers also allow measurements of the transfer of heat between the Earth's surface and the air above, called the sensible heat flux. Inner-scale scintillometers can also measure the dissipation rate of turbulent kinetic energy and the momentum flux.

Traveling-wave tube

A traveling-wave tube (TWT, pronounced "twit") or traveling-wave tube amplifier (TWTA, pronounced "tweeta") is a specialized vacuum tube that is used in electronics to amplify radio frequency (RF) signals in the microwave range. The TWT belongs to a category of "linear beam" tubes, such as the klystron, in which the radio wave is amplified by absorbing power from a beam of electrons as it passes down the tube. Although there are various types of TWT, two major categories are:

Helix TWT - in which the radio waves interact with the electron beam while traveling down a wire helix which surrounds the beam. These have wide bandwidth, but output power is limited to a few hundred watts.

Coupled cavity TWT - in which the radio wave interacts with the beam in a series of cavity resonators through which the beam passes. These function as narrowband power amplifiers.A major advantage of the TWT over some other microwave tubes is its ability to amplify a wide range of frequencies, a wide bandwidth. The bandwidth of the helix TWT can be as high as two octaves, while the cavity versions have bandwidths of 10–20%. Operating frequencies range from 300 MHz to 50 GHz. The power gain of the tube is on the order of 40 to 70 decibels, and output power ranges from a few watts to megawatts.TWTs account for over 50% of the sales volume of all microwave vacuum tubes. They are widely used as the power amplifiers and oscillators in radar systems, communication satellite and spacecraft transmitters, and electronic warfare systems.

Visible (optical)
Microwaves