Microwave

A telecommunications tower with a variety of dish antennas for microwave relay links on Frazier Peak, Ventura County, California.
The atmospheric attenuation of microwaves and far infrared radiation in dry air with a precipitable water vapor level of 0.001 mm. The downward spikes in the graph correspond to frequencies at which microwaves are absorbed more strongly. This graph includes a range of frequencies from 0 to 1 THz; the microwaves are the subset in the range between 0.3 and 300 gigahertz.

Microwaves are a form of electromagnetic radiation with wavelengths ranging from one meter to one millimeter; with frequencies between 300 MHz (100 cm) and 300 GHz (0.1 cm).[1][2][3][4][5] Different sources define different frequency ranges as microwaves; the above broad definition includes both UHF and EHF (millimeter wave) bands. A more common definition in radio engineering is the range between 1 and 100 GHz (300 and 3 mm).[2] 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: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations.

The prefix micro- in microwave is not meant to suggest a wavelength in the micrometer range. It indicates that microwaves are "small", compared to the radio waves used prior to microwave technology, in that they have shorter wavelengths. The boundaries between far infrared, terahertz radiation, microwaves, and ultra-high-frequency radio waves 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 ground waves, or reflect from the ionosphere, 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, wireless networks, microwave radio relay networks, radar, satellite and spacecraft communication, medical diathermy and cancer treatment, remote sensing, radio astronomy, particle accelerators, spectroscopy, industrial heating, collision avoidance systems, garage door openers and keyless entry systems, and for cooking food in microwave ovens.

Electromagnetic spectrum

Microwaves occupy a place in the electromagnetic spectrum between ordinary radio waves and infrared light:

Electromagnetic spectrum
Name Wavelength Frequency (Hz) Photon energy (eV) Range width (Bel)
Gamma ray < 0.02 nm > 15 EHz > 62.1 keV infinite
X-ray 0.01 nm – 10 nm 30 EHz – 30 PHz 124 keV – 124 eV 3
Ultraviolet 10 nm – 400 nm 30 PHz – 750 THz 124 eV – 3 eV 1.6
Visible light 390 nm – 750 nm 770 THz – 400 THz 3.2 eV – 1.7 eV 0.3
Infrared 750 nm – 1 mm 400 THz – 300 GHz 1.7 eV – 1.24 meV 3.1
Microwave 1 mm – 1 m 300 GHz – 300 MHz 1.24 meV – 1.24 µeV 3
Radio 1 mm – 100 km 300 GHz – 3 kHz 1.24 µeV – 12.4 feV 8

Propagation

Microwaves travel solely by line-of-sight paths; unlike lower frequency radio waves they do not travel as ground waves which follow the contour of the Earth, or reflect off the ionosphere (skywaves).[6] 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 Fresnel zone are required. Therefore, on the surface of the Earth microwave communication links are limited by the visual horizon to about 30 – 40 miles (48 – 64 km). Microwaves are absorbed by moisture in the atmosphere, and the attenuation increases with frequency, becoming a significant factor (rain fade) 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 infrared and optical window 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 troposphere.[6] 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

Waveguide is used to carry microwaves. Example of waveguides and a diplexer in an air traffic control radar

Their short wavelength allows narrow beams of microwaves to be produced by conveniently small high gain antennas from a half meter to 5 meters in diameter. Therefore, beams of microwaves are used for point-to-point communication links, and for radar. An advantage of narrow beams is that they don't interfere with nearby equipment using the same frequency, allowing frequency reuse by nearby transmitters. Parabolic ("dish") antennas are the most widely used directive antennas at microwave frequencies, but horn antennas, slot antennas and dielectric lens antennas are also used. Flat microstrip antennas are being increasingly used in consumer devices. Another directive antenna practical at microwave frequencies is the phased array, a computer-controlled array of antennas which produces a beam which can be electronically steered in different directions. Where omnidirectional antennas are required, for example in wireless devices and Wifi routers for wireless LANs, small monopoles, such as the inverted F antenna (PIFA) in cell phones, dipole, or patch antennas are used.

At microwave frequencies, the transmission lines which are used to carry lower frequency radio waves to and from antennas, such as coaxial cable and parallel wire lines, 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 transmitter or the RF front end of the receiver is located at the antenna.

Difference between microwave and radio frequency technology

The term microwave also has a more technical meaning in electromagnetics and circuit theory. 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 resistors, capacitors, and inductors used with lower-frequency radio waves. Open-wire and coaxial transmission lines used at lower frequencies are replaced by waveguides and stripline, and lumped-element tuned circuits are replaced by cavity resonators or resonant stubs. 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 optics are used.

Microwave sources

Cutaway view inside a cavity magnetron as used in a microwave oven (left). Antenna splitter: microstrip techniques become increasingly necessary at higher frequencies (right).
Disassembled radar speed gun. The grey assembly attached to the end of the copper-colored horn antenna is the Gunn diode which generates the microwaves.

High-power microwave sources use specialized vacuum tubes 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 magnetron (used in microwave ovens), klystron, traveling-wave tube (TWT), and gyrotron. These devices work in the density modulated mode, rather than the current 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), tunnel diodes, Gunn diodes, and IMPATT diodes.[7] Low-power sources are available as benchtop instruments, rackmount instruments, embeddable modules and in card-level formats. A maser is a solid state device which amplifies microwaves using similar principles to the laser, which amplifies higher frequency light waves.

All warm objects emit low level microwave black-body radiation, depending on their temperature, so in meteorology and remote sensing microwave radiometers are used to measure the temperature of objects or terrain.[8] The sun[9] and other astronomical radio sources such as Cassiopeia A emit low level microwave radiation which carries information about their makeup, which is studied by radio astronomers using receivers called radio telescopes.[8] 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 Big Bang theory of the origin of the Universe.

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 frequency reuse; 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 microwave ovens and in radar technology.

Communication

A satellite dish on a residence, which receives satellite television over a Ku band 12–14 GHz microwave beam from a direct broadcast communications satellite in a geostationary orbit 35,700 kilometres (22,000 miles) above the Earth.

Before the advent of fiber-optic transmission, most long-distance telephone calls 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.

Wireless LAN protocols, such as Bluetooth and the IEEE 802.11 specifications used for Wi-Fi, also use microwaves in the 2.4 GHz ISM band, although 802.11a uses ISM band and U-NII 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 WiMAX (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.

Mobile Broadband Wireless Access (MBWA) protocols based on standards specifications such as IEEE 802.20 or ATIS/ANSI HC-SDMA (such as iBurst) 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.[10]

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

Microwave radio is used in broadcasting 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 television news 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. Satellite TV either operates in the C band for the traditional large dish 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 Milstar.

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

Radar

The parabolic antenna (lower curved surface) of an ASR-9 airport surveillance radar which radiates a narrow vertical fan-shaped beam of 2.7–2.9 GHz (S band) microwaves to locate aircraft in the airspace surrounding an airport.

Radar is a radiolocation 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 millimeter waves are used for short range radar such as collision avoidance systems.

Some of the dish antennas of the Atacama Large Millimeter Array (ALMA) a radio telescope located in northern Chile. It receives microwaves in the millimeter wave range, 31 – 1000 GHz.
Maps of the cosmic microwave background radiation (CMBR), showing the improved resolution which has been achieved with better microwave radio telescopes

Radio astronomy

Microwaves emitted by astronomical radio sources; planets, stars, galaxies, and nebulas are studied in radio astronomy with large dish antennas called radio telescopes. 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 Moon or map the invisible surface of Venus 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 universe 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.[11][12]

A major recent focus of microwave radio astronomy has been mapping the cosmic microwave background radiation (CMBR) discovered in 1964 by radio astronomers Arno Penzias and Robert Wilson. This faint background radiation, which fills the universe and is almost the same in all directions, is "relic radiation" from the Big Bang, 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 radio telescopes can detected the CMBR as a faint signal that is not associated with any star, galaxy, or other object.[13]

Heating and power application

Small microwave oven on a kitchen counter
Microwaves are widely used for heating in industrial processes. A microwave tunnel oven for softening plastic rods prior to extrusion.

A microwave oven passes microwave radiation at a frequency near 2.45 GHz (12 cm) 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 cavity magnetrons. 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 plasma 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 stellarators and tokamak experimental fusion reactors to help heat the fuel into a plasma state. The upcoming ITER thermonuclear reactor[14] is expected to range from 110–170 GHz and will employ electron cyclotron resonance heating (ECRH).[15]

Microwaves can be used to transmit power over long distances, and post-World War II research was done to examine possibilities. NASA 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.

Less-than-lethal 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 54 °C (129 °F) at a depth of 0.4 millimetres (1⁄64 in). The United States Air Force and Marines are currently using this type of active denial system in fixed installations.[16]

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 magnetic fields of 0.3 T. This technique provides information on unpaired electrons in chemical systems, such as free radicals or transition metal ions such as Cu(II). Microwave radiation is also used to perform rotational spectroscopy and can be combined with electrochemistry as in microwave enhanced electrochemistry.

Microwave frequency bands

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation. Microwaves are strongly absorbed at wavelengths shorter than about 1.5 cm (above 20 GHz) by water and other molecules in the air.

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
L band 1 to 2 GHz 15 cm to 30 cm military telemetry, GPS, mobile phones (GSM), amateur radio
S band 2 to 4 GHz 7.5 cm to 15 cm weather radar, surface ship radar, and some communications satellites (microwave ovens, microwave devices/communications, radio astronomy, mobile phones, wireless LAN, Bluetooth, ZigBee, GPS, amateur radio)
C band 4 to 8 GHz 3.75 cm to 7.5 cm long-distance radio telecommunications
X band 8 to 12 GHz 25 mm to 37.5 mm satellite communications, radar, terrestrial broadband, space communications, amateur radio, molecular rotational spectroscopy
Ku band 12 to 18 GHz 16.7 mm to 25 mm satellite communications, molecular rotational spectroscopy
K band 18 to 26.5 GHz 11.3 mm to 16.7 mm radar, satellite communications, astronomical observations, automotive radar, molecular rotational spectroscopy
Ka band 26.5 to 40 GHz 5.0 mm to 11.3 mm satellite communications, molecular rotational spectroscopy
Q band 33 to 50 GHz 6.0 mm to 9.0 mm satellite communications, terrestrial microwave communications, radio astronomy, automotive radar, molecular rotational spectroscopy
U band 40 to 60 GHz 5.0 mm to 7.5 mm
V band 50 to 75 GHz 4.0 mm to 6.0 mm millimeter wave radar research, molecular rotational spectroscopy and other kinds of scientific research
W band 75 to 110 GHz 2.7 mm to 4.0 mm satellite communications, millimeter-wave radar research, military radar targeting and tracking applications, and some non-military applications, automotive radar
F band 90 to 140 GHz 2.1 mm to 3.3 mm SHF transmissions: Radio astronomy, microwave devices/communications, wireless LAN, most modern radars, communications satellites, satellite television broadcasting, DBS, amateur radio
D band 110 to 170 GHz 1.8 mm to 2.7 mm EHF 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.[17][18][19]

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.[20]

Microwave frequency measurement

Absorption wavemeter for measuring in the Ku band.

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

Frequency counters or high frequency heterodyne 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, Lecher lines 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 waveguide 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 standing wave 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 non-ionizing radiation.[21] The word "radiation" refers to energy radiating from a source and not to radioactivity. It has not been shown conclusively that microwaves (or other non-ionizing electromagnetic radiation) have significant adverse biological effects at low levels. Some, but not all, studies suggest that long-term exposure may have a carcinogenic effect.[22] 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.[23] Research by NASA 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.[24]

When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in the body. Exposure to microwave radiation can produce cataracts by this mechanism,[25] because the microwave heating denatures proteins in the crystalline lens of the eye (in the same way that heat turns egg whites white and opaque). The lens and cornea of the eye are especially vulnerable because they contain no blood vessels 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 burns 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

Electromagnetic spectrum (visible-light range highlighted).

The existence of radio waves was predicted by James Clerk Maxwell in 1864 from his equations. In 1888, Heinrich Hertz 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 Leyden jars. 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.[26]

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 Robert Dicke and Robert Beringer. 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
Barkhausen and Kurz Positive grid oscillators
Hull Smooth bore magnetron
Russell and Sigurd Varian Velocity-modulated electron beam (→ klystron tube)
Randall and Boot Cavity magnetron

See also

References

  1. ↑ Hitchcock, R. Timothy (2004). Radio-frequency and Microwave Radiation. American Industrial Hygiene Assn. p. 1. ISBN 1931504555.
  2. 1 2 Kumar, Sanjay; Shukla, Saurabh (2014). Concepts and Applications of Microwave Engineering. PHI Learning Pvt. Ltd. p. 3. ISBN 8120349350.
  3. ↑ Jones, Graham A.; Layer, David H.; Osenkowsky, Thomas G. (2013). National Association of Broadcasters Engineering Handbook, 10th Ed. Taylor & Francis. p. 6. ISBN 1136034102.
  4. ↑ Pozar, David M. (1993). Microwave Engineering Addison–Wesley Publishing Company. ISBN 0-201-50418-9.
  5. ↑ Sorrentino, R. and Bianchi, Giovanni (2010) Microwave and RF Engineering, John Wiley & Sons, p. 4, ISBN 047066021X.
  6. 1 2 Seybold, John S. (2005). Introduction to RF Propagation. John Wiley and Sons. pp. 55–58. ISBN 0471743682.
  7. ↑ Microwave Oscillator notes by Herley General Microwave
  8. 1 2 Sisodia, M. L. (2007). Microwaves : Introduction To Circuits, Devices And Antennas. New Age International. pp. 1.4–1.7. ISBN 8122413382.
  9. ↑ Liou, Kuo-Nan (2002). An introduction to atmospheric radiation. Academic Press. p. 2. ISBN 0-12-451451-0. Retrieved 12 July 2010.
  10. ↑ "IEEE 802.20: Mobile Broadband Wireless Access (MBWA)". Official web site. Retrieved August 20, 2011.
  11. ↑ "ALMA website". Retrieved 2011-09-21.
  12. ↑ "Welcome to ALMA!". Retrieved 2011-05-25.
  13. ↑ Wright, E.L. (2004). "Theoretical Overview of Cosmic Microwave Background Anisotropy". In W. L. Freedman. Measuring and Modeling the Universe. Carnegie Observatories Astrophysics Series. Cambridge University Press. p. 291. ISBN 0-521-75576-X. arXiv:astro-ph/0305591 Freely accessible.
  14. ↑ "The way to new energy". ITER. 2011-11-04. Retrieved 2011-11-08.
  15. ↑ "Electron Cyclotron Resonance Heating (ECRH)". Ipp.mpg.de. Retrieved 2011-11-08.
  16. ↑ Silent Guardian Protection System. Less-than-Lethal Directed Energy Protection. raytheon.com
  17. ↑ "eEngineer – Radio Frequency Band Designations". Radioing.com. Retrieved 2011-11-08.
  18. ↑ PC Mojo – Webs with MOJO from Cave Creek, AZ (2008-04-25). "Frequency Letter bands – Microwave Encyclopedia". Microwaves101.com. Retrieved 2011-11-08.
  19. ↑ For other definitions see Letter Designations of Microwave Bands.
  20. ↑ Skolnik, Merrill I. (2001) Introduction to Radar Systems, Third Ed., p. 522, McGraw Hill. 1962 Edition full text
  21. ↑ Nave, Rod. "Interaction of Radiation with Matter". HyperPhysics. Retrieved 20 October 2014.
  22. ↑ Goldsmith, JR (December 1997). "Epidemiologic evidence relevant to radar (microwave) effects". Environmental Health Perspectives. 105 (Suppl. 6): 1579–1587. JSTOR 3433674. PMC 1469943 Freely accessible. PMID 9467086. doi:10.2307/3433674.
  23. ↑ Philip L. Stocklin, U.S. Patent 4,858,612, December 19, 1983
  24. ↑ Andjus, R.K.; Lovelock, J.E. (1955). "Reanimation of rats from body temperatures between 0 and 1 °C by microwave diathermy". The Journal of Physiology. 128 (3): 541–546. PMC 1365902 Freely accessible. PMID 13243347. doi:10.1113/jphysiol.1955.sp005323.
  25. ↑ "Resources for You (Radiation-Emitting Products)". US Food and Drug Administration home page. U.S. Food and Drug Administration. Retrieved 20 October 2014.
  26. ↑ Emerson, D.T. (February 1998). "The work of Jagdish Chandra Bose: 100 years of MM-wave research". National Radio Astronomy Observatory.
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