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 are electromagnetic waves with wavelengths longer than those of infrared light, but shorter than those of radio waves.

Microwaves have wavelengths approximately in the range of 30 cm (frequency = 1 GHz) to 1 mm (300 GHz). However, the boundaries between far infrared light, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study. The existence of electromagnetic waves, of which microwaves are part of the higher frequency spectrum, 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 apparatus to produce radio waves.

The microwave range include ultra-high frequency (UHF) (0.3-3 GHz), super high frequency (SHF) (3-30 GHz), and extremely high frequency (EHF) (30-300 GHz) signals.

Note: above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that the atmosphere is effectively opaque to higher frequencies of electromagnetic radiation, until 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 categories: solid state devices and vacuum-tube based devices. Solid state microwave devices are based on semiconductors such as silicon or gallium arsenide, and include field-effect transistors (FET's), bipolar junction transistors (BJT's), Gunn diodes, and IMPATT diodes. Specialized versions of standard transistors have been developed for higher speed which are commonly used in microwave applications. Microwave variants of BJT's include the heterojunction bipolar transistor (HBT), and microwave variants of FET's include the MESFET, the HEMT (also known as HFET), and LDMOS transistor. Vacuum tube based devices operate on the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the magnetron, klystron, travelling 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. Microwaves cook food by causing molecules of water and other compounds to vibrate or rotate. The vibration creates heat which warms the food. Since organic matter is made up primarily of water, food is easily cooked by this method.
  • Microwaves are used in broadcasting transmissions because microwaves pass easily through the earth's atmosphere with less interference than longer wavelengths. There is also much more bandwidth in the microwave spectrum than in the rest of the radio spectrum. Typically, microwaves are used in television news to transmit a signal 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 b 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 USA) 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 5.8G Hz ranges.
  • Cable TV and Internet access on coax cable as well as broadcast television use some of the lower microwave frequencies. Some cellphone networks also use the lower microwave frequencies.
  • 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 are defined 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 GHz
Ka band 26 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

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.

See also

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