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Microwaves are a form of electromagnetic radiation with ranging from one meter to one millimeter; with between and .

(2018). 9781931504553, American Industrial Hygiene Assn.. .
(2018). 9788120349353, PHI Learning Pvt. Ltd. .
(2018). 9781136034107, Taylor & Francis. .
Pozar, David M. (1993). Microwave Engineering Addison–Wesley Publishing Company. .Sorrentino, R. and Bianchi, Giovanni (2010) Microwave and RF Engineering, John Wiley & Sons, p. 4, . Different sources define different frequency ranges as microwaves; the above broad definition includes both UHF and EHF () bands. A more common definition in radio engineering is the range between 1 and 100 GHz (wavelengths between 300 and 3 mm). In all cases, microwaves include the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum. Frequencies in the microwave range are often referred to by their IEEE radar band designations: , C, , , K, or , or by similar NATO or EU designations.

The in microwave is not meant to suggest a wavelength in the micrometer range. It indicates that microwaves are "small", compared to the used prior to microwave technology, in that they have shorter wavelengths. The boundaries between , terahertz radiation, microwaves, and ultra-high-frequency are fairly arbitrary and are used variously between different fields of study.

Microwaves travel by line-of-sight; unlike lower frequency radio waves they do not diffract around hills, follow the earth's surface as , or reflect from the , so terrestrial microwave communication links are limited by the visual horizon to about 40 miles (64 km). At the high end of the band they are absorbed by gases in the atmosphere, limiting practical communication distances to around a kilometer. Microwaves are extremely widely used in modern technology. They are used for point-to-point communication links, , microwave radio relay networks, , satellite and spacecraft communication, medical and cancer treatment, , , particle accelerators, , industrial heating, collision avoidance systems, garage door openers and keyless entry systems, and for cooking food in .


Electromagnetic spectrum
Microwaves occupy a place in the electromagnetic spectrum with frequency above ordinary , and below light:
> 62.1
124 keV – 124 eV
124 eV – 3 eV
3.2 eV – 1.7 eV
1.7 eV – 1.24
1.24 meV – 1.24

1.24 – 12.4
In descriptions of the electromagnetic spectrum, many sources classify microwaves as radio waves; microwaves are a subset of the radio wave band; while others classify microwaves and radio waves as distinct types of radiation. This is an arbitrary distinction.


Propagation
Microwaves travel solely by line-of-sight paths; unlike lower frequency radio waves, they do not travel as which follow the contour of the Earth, or reflect off the ().
(2018). 9780471743682, John Wiley and Sons. .
Although at the low end of the band they can pass through building walls enough for useful reception, usually rights of way cleared to the first are required. Therefore, on the surface of the Earth, microwave communication links are limited by the visual horizon to about . Microwaves are absorbed by moisture in the atmosphere, and the attenuation increases with frequency, becoming a significant factor () at the high end of the band. Beginning at about 40 GHz, atmospheric gases also begin to absorb microwaves, so above this frequency microwave transmission is limited to a few kilometers. A spectral band structure causes absorption peaks at specific frequencies (see graph at right). Above 100 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is in effect opaque, until the atmosphere becomes transparent again in the so-called and frequency ranges.


Troposcatter
In a microwave beam directed at an angle into the sky, a small amount of the power will be randomly scattered as the beam passes through the . A sensitive receiver beyond the horizon with a high gain antenna focused on that area of the troposphere can pick up the signal. This technique has been used at frequencies between 0.45 and 5 GHz in tropospheric scatter (troposcatter) communication systems to communicate beyond the horizon, at distances up to 300 km.


Antennas

The short of microwaves allow omnidirectional antennas for portable devices to be made very small, from 1 to 20 centimeters long, so microwave frequencies are widely used for such as , , and (Wifi) access for , and earphones. Antennas used include rubber ducky antennas, sleeve , , and increasingly the inverted F antenna (PIFA) used in cell phones.

Their short also allows narrow beams of microwaves to be produced by conveniently small antennas from a half meter to 5 meters in diameter. Therefore, beams of microwaves are used for point-to-point communication links, and for . An advantage of narrow beams is that they don't interfere with nearby equipment using the same frequency, allowing by nearby transmitters. Parabolic ("dish") antennas are the most widely used directive antennas at microwave frequencies, but , and antennas are also used. Flat microstrip antennas are being increasingly used in consumer devices. Another directive antenna practical at microwave frequencies is the , a computer-controlled array of antennas which produces a beam which can be electronically steered in different directions.

At microwave frequencies, the transmission lines which are used to carry lower frequency radio waves to and from antennas, such as and , have excessive power losses, so when low attenuation is required microwaves are carried by metal pipes called waveguides. Due to the high cost and maintenance requirements of waveguide runs, in many microwave antennas the output stage of the or the RF front end of the is located at the antenna.


Difference between microwave and radio frequency technology
The term microwave also has a more technical meaning in and . Apparatus and techniques may be described qualitatively as "microwave" when the frequencies used are high enough that wavelengths of signals are roughly the same as the dimensions of the circuit, so that lumped-element circuit theory is inaccurate, and instead distributed circuit elements and transmission-line theory are more useful methods for design and analysis. As a consequence, practical microwave circuits tend to move away from the discrete , , and used with lower-frequency . Open-wire and coaxial transmission lines used at lower frequencies are replaced by and , and lumped-element tuned circuits are replaced by cavity or . In turn, at even higher frequencies, where the wavelength of the electromagnetic waves becomes small in comparison to the size of the structures used to process them, microwave techniques become inadequate, and the methods of are used.


Microwave sources

High-power microwave sources use specialized to generate microwaves. These devices operate on different principles from low-frequency vacuum tubes, using the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the (used in ), , traveling-wave tube (TWT), and . These devices work in the modulated mode, rather than the modulated mode. This means that they work on the basis of clumps of electrons flying ballistically through them, rather than using a continuous stream of electrons.

Low-power microwave sources use solid-state devices such as the field-effect transistor (at least at lower frequencies), , , and . Microwave Oscillator notes by Herley General Microwave Low-power sources are available as benchtop instruments, rackmount instruments, embeddable modules and in card-level formats. A is a solid state device which amplifies microwaves using similar principles to the , which amplifies higher frequency light waves.

All warm objects emit low level microwave black-body radiation, depending on their , so in meteorology and microwave radiometers are used to measure the temperature of objects or terrain.

(2018). 9788122413380, New Age International. .
The sun
(2018). 9780124514515, Academic Press. .
and other astronomical radio sources such as emit low level microwave radiation which carries information about their makeup, which is studied by using receivers called . The cosmic microwave background radiation (CMBR), for example, is a weak microwave noise filling empty space which is a major source of information on cosmology's theory of the origin of the .


Microwave uses
Microwave technology is extensively used for point-to-point telecommunications (i.e. non-broadcast uses). Microwaves are especially suitable for this use since they are more easily focused into narrower beams than radio waves, allowing ; their comparatively higher frequencies allow broad bandwidth and high data transmission rates, and antenna sizes are smaller than at lower frequencies because antenna size is inversely proportional to transmitted frequency. Microwaves are used in spacecraft communication, and much of the world's data, TV, and telephone communications are transmitted long distances by microwaves between ground stations and communications satellites. Microwaves are also employed in and in technology.


Communication

Before the advent of transmission, most long-distance were carried via networks of microwave radio relay links run by carriers such as AT&T Long Lines. Starting in the early 1950s, frequency division multiplex was used to send up to 5,400 telephone channels on each microwave radio channel, with as many as ten radio channels combined into one antenna for the hop to the next site, up to 70 km away.

protocols, such as and the IEEE 802.11 specifications used for Wi-Fi, also use microwaves in the 2.4 GHz , although 802.11a uses and frequencies in the 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services have been used for almost a decade in many countries in the 3.5–4.0 GHz range. The FCC recently carved out spectrum for carriers that wish to offer services in this range in the U.S. — with emphasis on 3.65 GHz. Dozens of service providers across the country are securing or have already received licenses from the FCC to operate in this band. The WIMAX service offerings that can be carried on the 3.65 GHz band will give business customers another option for connectivity.

Metropolitan area network (MAN) protocols, such as (Worldwide Interoperability for Microwave Access) are based on standards such as IEEE 802.16, designed to operate between 2 and 11 GHz. Commercial implementations are in the 2.3 GHz, 2.5 GHz, 3.5 GHz and 5.8 GHz ranges.

Wireless Access (MBWA) protocols based on standards specifications such as IEEE 802.20 or ATIS/ANSI (such as ) operate between 1.6 and 2.3 GHz to give mobility and in-building penetration characteristics similar to mobile phones but with vastly greater spectral efficiency.

Some networks, like GSM, use the low-microwave/high-UHF frequencies around 1.8 and 1.9 GHz in the Americas and elsewhere, respectively. and use 1.452 to 1.492 GHz, while proprietary/incompatible in the U.S. uses around 2.3 GHz for DARS.

Microwave radio is used in and telecommunication transmissions because, due to their short wavelength, highly directional antennas are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies). There is also more bandwidth in the microwave spectrum than in the rest of the radio spectrum; the usable bandwidth below 300 MHz is less than 300 MHz while many GHz can be used above 300 MHz. Typically, microwaves are used in to transmit a signal from a remote location to a television station from a specially equipped van. See broadcast auxiliary service (BAS), remote pickup unit (RPU), and studio/transmitter link (STL).

Most satellite communications systems operate in the C, X, Ka, or Ku bands of the microwave spectrum. These frequencies allow large bandwidth while avoiding the crowded UHF frequencies and staying below the atmospheric absorption of EHF frequencies. either operates in the C band for the traditional fixed satellite service or Ku band for direct-broadcast satellite. Military communications run primarily over X or Ku-band links, with Ka band being used for .


Navigation
Global Navigation Satellite Systems (GNSS) including the Chinese Beidou, the American Global Positioning System (introduced in 1978) and the Russian broadcast navigational signals in various bands between about 1.2 GHz and 1.6 GHz.


Radar

is a technique in which a beam of radio waves emitted by a transmitter bounces off an object and returns to a receiver, allowing the location, range, speed, and other characteristics of the object to be determined. The short wavelength of microwaves causes large reflections from objects the size of motor vehicles, ships and aircraft. Also, at these wavelengths, the high gain antennas such as parabolic antennas which are required to produce the narrow beamwidths needed to accurately locate objects are conveniently small, allowing them to be rapidly turned to scan for objects. Therefore, microwave frequencies are the main frequencies used in radar. Microwave radar is widely used for applications such as air traffic control, weather forecasting, navigation of ships, and speed limit enforcement. Long distance radars use the lower microwave frequencies since at the upper end of the band atmospheric absorption limits the range, but are used for short range radar such as collision avoidance systems.


Radio astronomy
Microwaves emitted by astronomical radio sources; planets, stars, , and are studied in with large dish antennas called . In addition to receiving naturally occurring microwave radiation, radio telescopes have been used in active radar experiments to bounce microwaves off planets in the solar system, to determine the distance to the or map the invisible surface of through cloud cover.

A recently completed microwave radio telescope is the Atacama Large Millimeter Array, located at more than 5,000 meters (16,597 ft) altitude in Chile, observes the in the millimetre and submillimetre wavelength ranges. The world's largest ground-based astronomy project to date, it consists of more than 66 dishes and was built in an international collaboration by Europe, North America, East Asia and Chile.

A major recent focus of microwave radio astronomy has been mapping the cosmic microwave background radiation (CMBR) discovered in 1964 by radio astronomers and Robert Wilson. This faint background radiation, which fills the universe and is almost the same in all directions, is "relic radiation" from the , and is one of the few sources of information about conditions in the early universe. Due to the expansion and thus cooling of the Universe, the originally high-energy radiation has been shifted into the microwave region of the radio spectrum. Sufficiently sensitive can detected the CMBR as a faint signal that is not associated with any star, galaxy, or other object.

(2018). 052175576X, Cambridge University Press. 052175576X


Heating and power application
A passes microwave radiation at a frequency near through food, causing dielectric heating primarily by absorption of the energy in water. Microwave ovens became common kitchen appliances in Western countries in the late 1970s, following the development of less expensive . Water in the liquid state possesses many molecular interactions that broaden the absorption peak. In the vapor phase, isolated water molecules absorb at around 22 GHz, almost ten times the frequency of the microwave oven.

Microwave heating is used in industrial processes for drying and curing products.

Many semiconductor processing techniques use microwaves to generate for such purposes as reactive ion etching and plasma-enhanced chemical vapor deposition (PECVD).

Microwave frequencies typically ranging from 110 – 140 GHz are used in and tokamak experimental fusion reactors to help heat the fuel into a plasma state. The upcoming thermonuclear reactor is expected to range from 110–170 GHz and will employ electron cyclotron resonance heating (ECRH).

Microwaves can be used to transmit power over long distances, and post-World War II research was done to examine possibilities. worked in the 1970s and early 1980s to research the possibilities of using solar power satellite (SPS) systems with large solar arrays that would beam power down to the Earth's surface via microwaves.

weaponry exists that uses millimeter waves to heat a thin layer of human skin to an intolerable temperature so as to make the targeted person move away. A two-second burst of the 95 GHz focused beam heats the skin to a temperature of at a depth of . The United States Air Force and Marines are currently using this type of active denial system in fixed installations. Silent Guardian Protection System. Less-than-Lethal Directed Energy Protection. raytheon.com


Spectroscopy
Microwave radiation is used in electron paramagnetic resonance (EPR or ESR) spectroscopy, typically in the X-band region (~9 GHz) in conjunction typically with of 0.3 T. This technique provides information on unpaired in chemical systems, such as or ions such as Cu(II). Microwave radiation is also used to perform rotational spectroscopy and can be combined with as in microwave enhanced electrochemistry.


Microwave frequency bands
The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older use includes lower frequencies. Most common applications are within the 1 to 40 GHz range. One set of microwave frequency bands designations by the Radio Society of Great Britain (RSGB), is tabulated below:

+ Microwave frequency bands ! Designation !! Frequency range !! Wavelength range !! Typical uses
15 cm to 30 cmmilitary telemetry, GPS, mobile phones (GSM), amateur radio
7.5 cm to 15 cmweather radar, surface ship radar, and some communications satellites (microwave ovens, microwave devices/communications, radio astronomy, mobile phones, wireless LAN, Bluetooth, ZigBee, GPS, amateur radio)
3.75 cm to 7.5 cmlong-distance radio telecommunications
25 mm to 37.5 mmsatellite communications, radar, terrestrial broadband, space communications, amateur radio, molecular rotational spectroscopy
16.7 mm to 25 mmsatellite communications, molecular rotational spectroscopy
11.3 mm to 16.7 mmradar, satellite communications, astronomical observations, automotive radar, molecular rotational spectroscopy
5.0 mm to 11.3 mmsatellite communications, molecular rotational spectroscopy
6.0 mm to 9.0 mmsatellite communications, terrestrial microwave communications, radio astronomy, automotive radar, molecular rotational spectroscopy
5.0 mm to 7.5 mm
4.0 mm to 6.0 mmmillimeter wave radar research, molecular rotational spectroscopy and other kinds of scientific research
2.7 mm to 4.0 mmsatellite communications, millimeter-wave radar research, military radar targeting and tracking applications, and some non-military applications, automotive radar
2.1 mm to 3.3 mmSHF transmissions: Radio astronomy, microwave devices/communications, wireless LAN, most modern radars, communications satellites, satellite television broadcasting, DBS, amateur radio
1.8 mm to 2.7 mmEHF transmissions: Radio astronomy, high-frequency microwave radio relay, microwave remote sensing, amateur radio, directed-energy weapon, millimeter wave scanner

P band is sometimes used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521.For other definitions see Letter Designations of Microwave Bands.

When radars were first developed at K band during World War II, it was not known that there was a nearby absorption band (due to water vapor and oxygen in the atmosphere). To avoid this problem, the original K band was split into a lower band, Ku, and upper band, Ka.Skolnik, Merrill I. (2001) Introduction to Radar Systems, Third Ed., p. 522, McGraw Hill. 1962 Edition full text


Microwave frequency measurement

Microwave frequency can be measured by either electronic or mechanical techniques.

Frequency counters or high frequency systems can be used. Here the unknown frequency is compared with harmonics of a known lower frequency by use of a low frequency generator, a harmonic generator and a mixer. Accuracy of the measurement is limited by the accuracy and stability of the reference source.

Mechanical methods require a tunable resonator such as an absorption wavemeter, which has a known relation between a physical dimension and frequency.

In a laboratory setting, can be used to directly measure the wavelength on a transmission line made of parallel wires, the frequency can then be calculated. A similar technique is to use a slotted or slotted coaxial line to directly measure the wavelength. These devices consist of a probe introduced into the line through a longitudinal slot, so that the probe is free to travel up and down the line. Slotted lines are primarily intended for measurement of the voltage standing wave ratio on the line. However, provided a is present, they may also be used to measure the distance between the nodes, which is equal to half the wavelength. Precision of this method is limited by the determination of the nodal locations.


Effects on health
Microwaves do not contain sufficient energy to chemically change substances by ionization, and so are an example of radiation. The word "radiation" refers to energy radiating from a source and not to . It has not been shown conclusively that microwaves (or other electromagnetic radiation) have significant adverse biological effects at low levels. Some, but not all, studies suggest that long-term exposure may have a effect. This is separate from the risks associated with very high-intensity exposure, which can cause heating and burns like any heat source, and not a unique property of microwaves specifically.

During World War II, it was observed that individuals in the radiation path of radar installations experienced clicks and buzzing sounds in response to microwave radiation. This microwave auditory effect was thought to be caused by the microwaves inducing an electric current in the hearing centers of the brain.Philip L. Stocklin, , December 19, 1983 Research by in the 1970s has shown this to be caused by thermal expansion in parts of the inner ear. In 1955 Dr. James Lovelock was able to reanimate rats frozen at 0 °C using microwave diathermy.

When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in the body. Exposure to microwave radiation can produce by this mechanism, because the microwave heating denatures in the crystalline lens of the (in the same way that heat turns white and opaque). The lens and of the eye are especially vulnerable because they contain no that can carry away heat. Exposure to heavy doses of microwave radiation (as from an oven that has been tampered with to allow operation even with the door open) can produce heat damage in other tissues as well, up to and including serious that may not be immediately evident because of the tendency for microwaves to heat deeper tissues with higher moisture content.

Eleanor R. Adair conducted microwave health research by exposing herself, animals and humans to microwave levels that made them feel warm or even start to sweat and feel quite uncomfortable. She found no adverse health effects other than heat.


History and research
The existence of radio waves was predicted by James Clerk Maxwell in 1864 from his equations. In 1888, was the first to demonstrate the existence of radio waves by building a spark gap radio transmitter that produced 450 MHz microwaves, in the UHF region. The equipment he used was primitive, including a horse trough, a wrought iron point spark, and . He also built the first parabolic antenna, using a zinc gutter sheet. In 1894, Indian radio pioneer Jagdish Chandra Bose publicly demonstrated radio control of a bell using millimeter wavelengths, and conducted research into the propagation of microwaves.

Perhaps the first, documented, formal use of the term microwave occurred in 1931:

"When trials with wavelengths as low as 18 cm were made known, there was undisguised surprise that the problem of the micro-wave had been solved so soon." Telegraph & Telephone Journal XVII. 179/1

In 1943, the Hungarian engineer Zoltán Bay sent ultra-short radio waves to the moon, which, reflected from there, worked as a radar, and could be used to measure distance, as well as to study the moon.

Perhaps the first use of the word microwave in an astronomical context occurred in 1946 in an article "Microwave Radiation from the Sun and Moon" by and . This same article also made a showing in the New York Times issued in 1951. Ernst Weber pioneered microwave technologies.

In the history of electromagnetic theory, significant work specifically in the area of microwaves and their applications was carried out by researchers including:

+ Specific work on microwaves ! Names !! Area of work
Positive grid
Smooth bore
Velocity-modulated electron beam (→ tube)


See also
  • Block upconverter (BUC)
  • Cosmic microwave background
  • Electron cyclotron resonance
  • International Microwave Power Institute
  • Low-noise block converter (LNB)
  • Microwave auditory effect
  • Microwave chemistry
  • Microwave radio relay
  • Microwave transmission
  • Orthomode transducer (OMT)
  • Plasma-enhanced chemical vapor deposition
  • RF switch matrix
  • The Thing (listening device)
  • Tropospheric scatter


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