Microwave

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This page is about the radiation; for the appliance, see microwave oven.
File:8.4Ghz microwave image of galaxy 3C353.jpg
Microwave image of 3C353 galaxy at 8.4 GHz (36 mm). The overall linear size of the radio structure is 120 kpc.

Microwaves is the term used for electromagnetic radiation in the approximate wavelength range of 10 centimeters (cm) (frequency of 0.3 gigahertz (GHz)) to 1 millimeter (mm) (frequency 300 GHz). This places it between Terahertz radiation (which is at higher frequency) and radio waves (at lower frequency). It should, however, be noted that the boundaries between far infrared light, Terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and used variously between different fields of study.

The existence of electromagnetic waves, of which microwaves are a part, was predicted by James Clerk Maxwell in 1864, from his famous Maxwell's equations. In 1888, Heinrich Hertz was the first to demonstrate the existence of electromagnetic waves by building an apparatus that produced and detected microwaves in the UHF region. The design necessarily used horse-and-buggy materials, including a horse trough, a wrought iron point spark, Leyden jars, and a length of zinc gutter whose parabolic cross-section worked as a reflection antenna.

Microwave signals are often divided into three categories: ultra-high frequency (UHF) (0.3-3 GHz), super high frequency (SHF) (3-30 GHz), and extremely high frequency (EHF) (30-300 GHz). Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is effectively opaque. The atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.

Generation

Microwaves can be generated by a variety of means, generally divided into two types of devices: solid state and vacuum-tube based devices. Solid state microwave devices are based on semiconductors such as silicon or gallium arsenide. They include field-effect transistors (FETs), bipolar junction transistors (BJTs), Gunn diodes, and IMPATT diodes. Specialized versions of standard transistors, developed for higher speed, are commonly used in microwave applications.

Microwave devices based on the vacuum tube operate on the ballistic motion of electrons in a vacuum chamber under the influence of electric or magnetic fields. They include the magnetron, klystron, traveling wave tube (TWT), and gyrotron.

Uses

File:Atmospheric microwave transmittance at mauna kea(simulated).gif
Plot of the zenith atmospheric transmission on the summit of Mauna Kea throughout the entire gigahertz range of the electromagnetic spectrum at a precipitable water vapor level of 0.001 mm. (simulated)
  • A microwave oven uses a magnetron microwave generator to produce microwaves at a frequency of approximately 2.45 GHz for the purpose of cooking food. Microwave ovens cook food by causing molecules of water and other compounds in the food to vibrate or rotate. The vibrations generate heat, which warms the food. Given that food contains moisture, it is easily cooked by this method.
  • Microwaves pass easily through the earth's atmosphere with less interference than longer wavelengths. For this reason, microwaves are used in broadcasting transmissions. In addition, the microwave spectrum covers much more bandwidth than the rest of the radio spectrum. Typically, microwaves are used in television news coverage, to transmit signals from a remote location to a television station from a specially equipped van.
  • Radar also uses microwave radiation to detect the range, speed, and other characteristics of remote objects.
  • Wireless LAN protocols, such as Bluetooth and the IEEE 802.11g and 802.11b specifications, also use microwaves in the 2.4 GHz ISM band, although 802.11a uses an ISM band in the 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services can be found in many countries (but not the United States) in the 3.5–4.0 GHz range.
  • Metropolitan Area Networks - MAN protocols, such as WiMAX (Worldwide Interoperability for Microwave Access) based in the IEEE 802.16 specification. The IEEE 802.16 specification was designed to operate between 2 to 11 GHz. The commercial implementations are in the 2.5 GHz, 3.5 GHz and 5.8 GHz ranges.
  • Cable TV and Internet access on coax cable as well as broadcast television use some of the lower microwave frequencies. Some mobile phone networks, such as GSM (Global System for Mobile Communications), also use the lower microwave frequencies.
  • Microwaves are used to generate plasma for many semiconductor processing techniques, such as reactive ion etching and plasma-enhanced chemical vapor deposition (PECVD).
  • Microwaves can be used to transmit power over long distances. In the 1970s and early 1980s, NASA worked 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.
  • A maser is a device similar to a laser, except that it works at microwave frequencies.

Microwave frequency bands

The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 1000 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave Frequency Bands as defined by the Radio Society of Great Britain in the table below:

Microwave frequency bands
Designation Frequency range
L band 1 to 2 GHz
S band 2 to 4 GHz
C band 4 to 8 GHz
X band 8 to 12 GHz
Ku band 12 to 18 GHz
K band 18 to 26.5 GHz
Ka band 26.5 to 40 GHz
Q band 30 to 50 GHz
U band 40 to 60 GHz
V band 50 to 75 GHz
E band 60 to 90 GHz
W band 75 to 110 GHz
F band 90 to 140 GHz
D band 110 to 170 GHz

The above table reflects Radio Society of Great Britain (RSGB) usage. The term P band is sometimes used for UHF frequencies below L-band. For other definitions see Letter Designations of Microwave Bands

History and research

Perhaps the first 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

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.

For some of the history in the development of electromagnetic theory applicable to modern microwave applications see the following figures:

Specific significant areas of research and work developing microwaves and their applications:

Specific work on microwaves
Work carried out by Area of work
Barkhausen and Kurz Positive grid oscillators
Hull Smooth bore magnetron
Varian Brothers Velocity modulated electron beam → klystron tube
Randall and Boot Cavity magnetron

The Microwave integrated devices which are called MMIC (Monolithic Microwave Integrated Circuit) are manufactured by using mostly gallium arsenide (GaAs) wafers.

References
ISBN links support NWE through referral fees

  • Pozar, David M. (1993). Microwave Engineering Addison-Wesley Publishing Company. ISBN 0-201-50418-9.


See also

Radio spectrum
ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF
3 Hz 30 Hz 300 Hz 3 kHz 30 kHz 300 kHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz
30 Hz 300 Hz 3 kHz 30 kHz 300 kHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz 300 GHz


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