Microwave
This page is about the radiation; for the appliance, see microwave oven. |
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
Terahertz (THz) wavelengths, but relatively short for
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, Terahertz radiation, microwaves, and ultra-high-frequency
radio waves are fairly arbitrary and are used variously between different fields of study. The term microwave generally refers to "alternating current signals with frequencies between 300 MHz (3 x 10
8 Hz) and 300 GHz (3 x 10
11 Hz)."
[Pozar, David M. (1993). Microwave Engineering Addison-Wesley Publishing Company. ISBN 0-201-50418-9.]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 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.
The microwave range includes
ultra-high frequency (UHF) (0.3-3 GHz),
super high frequency (SHF) (3-30 GHz), and
extremely high frequency (EHF) (30-300 GHz) signals.
Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is effectively opaque , until the atmosphere becomes transparent again in the so-called infrared and
optical window frequency ranges.
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 (FETs),
bipolar junction transistors (BJTs),
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 BJTs include the
heterojunction bipolar transistor (HBT), and microwave variants of FETs include the
MESFET, the
HEMT (also known as HFET), and
LDMOS transistor.
Microwaves can be generated and processed using integrated circuits, which are often called
MMIC (Monolithic Microwave Integrated Circuits). They are usually manufactured using
gallium arsenide (GaAs) wafers, though
silicon germanium (SiGe) and heavy-dope
silicon are increasingly used.
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,
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.
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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 works by passing microwave
radiation, usually at a
frequency of 2450
MHz (a
wavelength of 12.24
cm), through the food.
Water,
fat, and
sugar molecules in the food absorb
energy from the microwave beam in a process called
dielectric heating. Many molecules (such as those of water) are electric
dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field induced by the microwave beam. This molecular movement creates
heat as the rotating molecules hit other molecules and put them into motion. Microwave heating is most efficient on liquid water, and much less so on fats and sugars (which have less molecular
dipole moment), and frozen water (where the molecules are not free to rotate). Microwave heating is sometimes incorrectly explained as a rotational
resonance of water molecules, but this is incorrect: such resonance only occurs at much higher frequencies, in the tens of
gigahertz. Moreover, large industrial/commercial microwave ovens operating in the 900 MHz range also heat water and food perfectly well.
A common misconception is that microwave ovens cook food from the "inside out". In reality, microwaves are absorbed in the outer layers of food in a manner somewhat similar to heat from other methods. The misconception arises because microwaves penetrate dry nonconductive substances at the surfaces of many common foods, and thus often deposit initial heat more deeply than other methods. Depending on water content the depth of initial heat deposition may be several centimeters or more with microwave ovens, in contrast to grilling (which relies on infra-red radiation and is known as broiling in american english) or convection heating, which deposit heat thinly at the food surface. Depth of penetration of microwaves is dependent on food composition and the frequency, with lower microwave frequencies being more penetrating.
* 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 5.8 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, like
GSM, also use the lower microwave frequencies.
* Many
semiconductor processing techniques use microwaves to generate
plasma for such purposes as
reactive ion etching and plasma-enhanced
chemical vapor deposition (PECVD).
* 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.
* A
maser is a device similar to a
laser, except that it works at microwave frequencies.
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:
The above table reflects
Radio Society of Great Britain (RSGB) usage. The term
P band is sometimes used for Ku Band. For other definitions see
Letter Designations of Microwave BandsPerhaps 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:
*
Hans Christian Ă˜rsted.
*
Jagdish Chandra Bose.
*
Michael Faraday.
*
James Clerk Maxwell.
*
Heinrich Hertz.
*
Nikola Tesla.
*
Guglielmo Marconi.
*
Samuel Morse.
* Sir
William Thomson, later Lord Kelvin.
*
Oliver Heaviside.
*
Lord Rayleigh.
*
Oliver Lodge.
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 â†' klystron tube |
| Randall and Boot | Cavity magnetron |
*
Cosmic microwave background radiation*
Electron cyclotron resonance*
Home appliances
*
Microwave ovens
*
Microwave auditory effect*
Radio*
Optics*
Microwave chemistry*
Microwave radio relay
