Microwaves are forms of electromagnetic radiation with wavelengths ranging from one meter to one millimeter; with the frequency of between 300Ã, MHz (100Ã, cm) and 300Ã, GHz (0.1 cm). Different sources define different frequency ranges as microwaves; the above broad definition includes UHF band and EHF (millimeter wave). The 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 all SHF bands (3 to 30 GHz, or 10 to 1 cm) at a minimum. Frequencies within the microwave range are often referred to by the determination of their IEEE radar bands: S, C, X, K u , K, or K a bands, or by a similar NATO or designation European Union.
Microwave prefix - in microwave is not intended to suggest wavelengths within the micrometer range. This shows that microwaves are "small", compared to radio waves used before microwave technology, because they have shorter wavelengths. The boundaries between far infrared, terahertz radiation, microwaves, and high frequency radio waves are quite arbitrary and are used diversely among the various fields of study.
Microwaves travel in line-of-sight; unlike the low-frequency radio waves, they are not diffracted around the hills, following the earth's surface as ground waves, or reflecting off the ionosphere, so that the terrestrial microwave-radio link is limited by the visual horizon to about 40 miles (64 km). At the high end of the band they are absorbed by gas in the atmosphere, limiting the distance of practical communication to about one kilometer. Microwaves are widely used in modern technology. They are used for point-to-point communication links, wireless networks, microwave radio relay networks, radar, satellites and spacecraft communications, medical diathermy and cancer treatments, remote sensing, radio astronomy, particle accelerators, spectroscopy, industrial heating, collision, garage door opener and keyless entry system, and to cook food in a microwave oven.
Video Microwave
Electromagnetic spectrum
Microwaves occupy a place in the electromagnetic spectrum with frequencies above ordinary radio waves, and under infrared light:
In the description of the electromagnetic spectrum, many sources classify microwaves as radio waves, part of radio waves; while others classify microwaves and radio waves as different types of radiation. This is an arbitrary difference.
Maps Microwave
Propagation
Microwaves travel only with line of sight; unlike low-frequency radio waves, they do not travel as ground waves that follow the contours of the Earth, or reflect the ionosphere (wave). Although at the lower end of the band they can pass through enough building walls for useful reception, usually the right way of being cleaned into the first Fresnel zone is required. Therefore, on the surface of the Earth, the microwave communication connection is limited by the visual horizon to about 30-40 miles (48-64 km). Microwaves are absorbed by water vapor in the atmosphere, and attenuation increases with frequency, becoming a significant factor (faded rain) at the high end of the band. Starting around 40 GHz, atmospheric gas also begins to absorb microwaves, so that above the microwave transmission this frequency is limited to several kilometers. The spectrum band structure causes peak absorption at certain frequencies (see graph on the right). Above 100 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it does not work, until the atmosphere becomes transparent again in the frequency range of infrared and optical windows.
Troposcatter
In a microwave beam directed at an angle to the sky, a small amount of power will be dispersed randomly as the emission passes through the troposphere. A sensitive receiver beyond the horizon with a high gain antenna focuses on a troposphere area that can pick up signals. This technique has been used at frequencies between 0.45 and 5 GHz in the troposphere communications distribution system (troposcatter) to communicate beyond the horizon, at distances up to 300 km.
Antenna
The microwave shortwave allows omnidirectional antennas for portable devices to be made very small, from 1 to 20 centimeters, so that microwave frequencies are widely used for wireless devices such as cell phones, cordless phones and wireless LAN access (WiFi) for laptops, and Bluetooth earphones. The antennas used include rubber duck antennas, arm dipoles, patch antennas, and inverted F antennas (PIFAs) used in mobile phones.
Their short wavelengths also enable narrow microwaves to be produced by high-income antennas small in size from half a meter to 5 meters in diameter. Therefore, microwave beams are used for point-to-point communication links, and for radar. The advantage of narrow beams is that they do not interfere with nearby equipment using the same frequency, allowing frequencies reused by the nearest transmitter. The parabolic antenna ("plate") is the most widely used directive antenna on microwave frequencies, but horn antennas, slot antennas and dielectric lens antennas are also used. Flat microstrip antennas are increasingly being used in consumer devices. Another practical directive antenna on microwave frequency is a gradual array, computer-controlled antenna array that produces electronically directed rays in different directions.
At microwave frequencies, the transmission lines used to carry radio frequency waves lower to and from the antenna, such as coaxial cables and parallel wires, have excessive power losses, so that when low attenuation microwaves are required to carry by metal pipes called waveguides. Because of the high cost and waveguide maintenance requirements running, in many microwave antennas, the transmitter output stage or the RF front end of the receiver are located in the antenna.
The difference between microwaves and radio frequency technology
The term microwave also has a more technical meaning in electromagnetics and circuit theory. Apparatus and technique can be described qualitatively as "microwaves" when the frequency used is high enough that the wavelength of the signal is approximately equal to the dimension of the circuit, so the theory of lumped element circuits is inaccurate, and instead distributing circuit elements and transmission line theory is more useful for design and analysis. As a result, practical microwave circuits tend to move away from discrete resistors, capacitors, and inductors used with low frequency radio waves. Open wire and coaxial transmission lines used at lower frequencies are replaced by waveguide and stripline, and tuned-element circuits are replaced by cavity resonators or resonance stubs. In turn, at higher frequencies, where the electromagnetic wavelength becomes smaller than the size of the structure used to process it, microwave engineering becomes inadequate, and optical methods are used.
Microwaves sources
High power microwave resources use special vacuum tubes to produce microwaves. This device operates on a different principle than a low-frequency vacuum tube, using electron ballistic movements in a vacuum under the influence of electric or magnetic field control, and includes magnetrons (used in microwave ovens), klystron, wave tube (TWT), and gyrotron. This device works in density modulation mode, not current modulation mode. This means that they work on the basis of electron clumps that fly ballistically through them, rather than using continuous electron flow.
Low power microwave resources use solid-state devices such as field effect transistors (at least at lower frequencies), tunnel diodes, Gunn diodes, and IMPATT diodes. Low resources are available as benchtop instruments, rackmount instruments, embedded modules, and card-level formats. Maser is a solid state device that amplifies microwaves using the same principle as lasers, which amplify higher frequency light waves.
All warm objects emit low-level black-body microwave radiation, depending on their temperature, so in meteorology and remote sensing microwave radiometers are used to measure the temperature of an object or field. The sun and other astronomical radio sources such as Cassiopeia A emit low-level microwave radiation that carries information about their makeup, which radio astronomers are studying using a receiver called a radio telescope. Cosmic microwave background radiation (CMBR), for example, is a weak microwaved sound that fills in the blank spaces that are the main source of information about Big Bang's theory of the origin of the Universe.
Microwave uses
Microwave technology is widely used for point-to-point communications (ie non-broadcast use). Microwaves are particularly suited for this use because they are more easily focused on a narrower beam than radio waves, allowing frequency reuse; their relatively higher frequencies allow for wide bandwidth and high data transmission rates, and smaller antenna sizes than at lower frequencies because the antenna size is inversely proportional to the transmitted frequency. Microwaves are used in spacecraft communications, and much of the world's data, TV, and telephone communications are transmitted remotely by microwaves between earth stations and communications satellites. Microwaves are also used in microwave ovens and radar technology.
Communications
Prior to the advent of fiber optic transmission, most long-distance telephone calls were made through a microwave radio relay network run by operators such as AT & amp; T Long Lines. Beginning in the early 1950s, the multiplex frequency division was used to deliver up to 5,400 telephone lines on every microwave radio channel, with as many as ten radio channels combined into one antenna for hop to the next site, up to 70 km away.
Wireless LAN protocols, such as Bluetooth and IEEE 802.11 specifications used for Wi-Fi, also use microwaves in the 2.4 GHz ISM band, although 802.11a uses ISM bands and U-NII frequencies in the 5 GHz range. Licensed remote service (up to approximately 25 km) Wireless Access has been used for almost a decade in many countries in the 3.5-4.0 GHz range. The FCC recently carved out a spectrum for carriers who want to offer services within this range in the US - with an emphasis at 3.65 GHz. Dozens of service providers across the country are securing or have received licenses from the FCC to operate in the band. WIMAX service offerings that can be made 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 implementation is in the range of 2.3 GHz, 2.5 GHz, 3.5 GHz, and 5.8 GHz.
Mobile Broadband Wireless Access (MBWA) protocols based on standard specifications such as IEEE 802.20 ATIS/ANSI HC-SDMA (such as iBurst) operate between 1.6 and 2.3 GHz to provide mobility and in-building penetration characteristics similar to mobile phones but with much greater spectral efficiency.
Some cellular phone networks, such as GSM, use low-microwave/high-UHF frequencies around 1.8 and 1.9 GHz in America and elsewhere, respectively. DVB-SH and S-DMB use 1,452 to 1,492Ã, GHz, while proprietary/incompatible satellite radio in the US uses about 2.3 GHz for DARS.
Microwave radios are used in telecommunication broadcasting and transmission because, because of their short wavelengths, highly directional antennas are smaller and therefore more practical than longer wavelengths (lower frequencies). There is also more bandwidth in the microwave spectrum than in other radio spectrums; The bandwidth that can be used under 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 signals from remote locations to television stations from specially equipped vans. View broadcast help (BAS), remote pickup (RPU), and studio/transmitter (STL) connections.
Most satellite communications systems operate in C, X, K a , or K u microwave spectrum bands. This frequency enables large bandwidth while avoiding crowded UHF frequencies and remains below atmospheric EHF absorption frequency. Satellite TV operates either in C bands for traditional large fixed fixed satellite services or K u bands for direct broadcast satellites. Military communications mainly run on X or K u links, with band K a used for Milstar.
The Global Navigation Satellite System (GNSS) includes Beidou China, the American Global Positioning System (introduced in 1978) and Russian GLONASS broadcast navigation signals in various bands between about 1.2 GHz and 1.6 GHz.
Radar
Radar is a radiolocation technique in which a beam of radio waves emitted by the transmitter bounces the object and returns to the receiver, allowing location, range, velocity, and other characteristics of the object to be determined. Microwaves short waves cause large reflections of motor vehicle, ship and plane sizes. Also, at these wavelengths, high gain antennas such as parabolic antennas are required to produce the narrow beamwidth required to accurately locate small objects, enabling them to rapidly change to scan objects. Therefore, the microwave frequency is the main frequency used in the radar. Microwave radars are widely used for applications such as air traffic control, weather forecasting, ship navigation, and speed limit enforcement. Long-range radar uses lower microwave frequencies because at the upper end of the atmospheric absorption bands limit the range, but millimeter waves are used for short-range radar such as collision avoidance systems.
Radio astronomy
Microwaves emitted by astronomical radio sources; planets, stars, galaxies, and nebulae are studied in radio astronomy with large dish dishes called radio telescopes. In addition to receiving naturally occurring microwave radiation, radio telescopes have been used in active radar experiments to reflect microwaves from planets in the solar system, to determine the distance to the Moon or mapping the surface of Venus that is not visible through cloud cover.
A recently completed microwave radio telescope is the Atacama Large Millimeter Array, located at an altitude of more than 5,000 meters (16,597 feet) in Chile, observing the universe in the wavelength range of millimeters and submillimetre. The world's largest terrestrial astronomy project to date, comprises over 66 plates and is built in international collaborations by Europe, North America, East Asia and Chile.
A recent major 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 vague background radiation, which fills the universe and almost the same in all directions, is the "relational radiation" of the Big Bang, and is one of the few sources of information about conditions in the early universe. Due to the expansion and thus the cooling of the universe, the high energy radiation had originally shifted to the microwave region of the radio spectrum. Simply sensitive radio telescopes can detect CMBRs as weak signals that are unrelated to stars, galaxies, or other objects.
Heating and power applications
Microwave ovens pass microwave radiation at a frequency near 2.45 GHz (12 cm) through food, causing dielectric heating mainly by absorption of energy in water. Microwave ovens became common kitchen appliances in Western countries in the late 1970s, following the development of cheaper magnetron cavities. Water in a liquid state has many molecular interactions that expand the absorption peak. In the vapor phase, isolated water molecules absorb about 22 GHz, nearly ten times the microwave oven frequency.
Microwave heating is used in industrial processes for drying and preserving products.
Many semiconductor processing techniques use microwaves to produce plasmas for purposes such as abrasion of reactive ions and plasma-enhanced chemical vapor deposition (PECVD).
Microwave frequencies typically start from 110 - 140 GHz are used in experimental fusion stellarators and tokamak fumes to help heat the fuel into the plasma state. The upcoming ITon thermonuclear reactor is expected to range from 110-170 GHz and will use electron cyclone heating resonance (ECRH).
Microwaves can be used to transmit power over long distances, and post-World War II research is done to test the possibilities. NASA worked in the 1970s and early 1980s to examine the possibility of using a solar satellite system (SPS) with large solar panels that would deliver power to the Earth's surface through microwaves.
Non-lethal weapons exist that use millimeter waves to heat a thin layer of human skin to unbearable temperatures that keep targeted people moving. A two-second burst from the 95 GHz focus beam heats up the skin to a temperature of 54Ã, Â ° C (129Ã, Â ° F) at a depth of 0.4 millimeters ( 1 / 64 in). The US Air Force and Marines currently use this type of active rejection system in a fixed installation.
Spectroscopy
Microwave radiation is used in electron resonance spectroscopy (EPR or ESR), usually in the X-band region (~ 9Ã,®zz) in relation to 0.3 T magnetic field. This technique provides information about 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 enhanced electrochemical microwaves.
Microwaves
The microwave spectrum is usually defined as electromagnetic energy that ranges from about 1 GHz to 100 GHz in frequency, but older use includes lower frequencies. The most common applications are in the range of 1 to 40 GHz. A set of microwave frequency bands by Radio Society of Great Britain (RSGB), tabulated below:
P band is sometimes used for K u Band. "P" for "previous" is the radar band used in the UK from 250 to 500 MHz and is now obsolete per IEEE Std 521.
When the radar was first developed in the K band during World War II, it is unknown that there is a nearby infiltration band (due to water vapor and oxygen in the atmosphere). To avoid this problem, the original band K is divided into the lower band, K u , and the top band, K a .
Microwave frequency measurement
The microwave frequency can be measured by electronic or mechanical techniques.
A frequency counter or a high frequency heterodyne system can be used. Here the unknown frequency is compared to the known low frequency harmonics by using a low frequency generator, harmonic generator and mixer. The measurement accuracy is limited by the accuracy and stability of the reference source.
The mechanical method requires a tunable resonator such as an absorbent wavemeter, which has a known relationship between physical and frequency dimensions.
In the laboratory setting, the Lecher line can be used to measure the wavelength directly on the transmission line made of parallel cable, then the frequency can be calculated. The same technique is to use a slotted waveguide or slotted coaxial line to measure the wavelength directly. This device consists of a probe inserted into the line through a longitudinal slot, so the probe is free to travel up and down the line. The slotted line is primarily intended for the measurement of standing wavefront voltage across the channel. However, if there are standing waves, they can also be used to measure the distance between vertices, which equals half the wavelength. The precision of this method is limited by the determination of the nodal location.
Health effects
Microwaves do not contain enough energy to chemically alter substances through ionization, and so are examples of non-ionizing radiation. The word "radiation" refers to energy radiating from the source and not to radioactivity. It has not been shown for certain 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. This is separate from the risks associated with high-intensity exposure, which can cause heating and burns like other heat sources, and not the unique properties of special microwaves.
During World War II, it was observed that individuals in radiation radar installation lines experience click and buzzing sounds in response to microwave radiation. The effect of microwave hearing is thought to be caused by microwaves that drive electric currents in the hearing centers of the brain. A study by NASA in 1970 has shown this to be due to thermal expansion in the inner parts of the inner ear. In 1955 Dr. James Lovelock was able to revive rats that were frozen at 0 ° C using microwave diathermy.
When injury from exposure to microwaves occurs, it is usually the result of induced dielectric heating in the body. Exposure to microwave radiation can produce cataracts by this mechanism, because microwave heating alters the properties of proteins in the crystal lens of the eye (in the same way as heat turns to egg white and opaque). Eye lenses and corneas are particularly vulnerable because they contain no blood vessels that can carry heat. Exposure to heavy doses of radiation (such as from damaged ovens to allow surgery even with open doors) may produce heat damage in other tissues as well, up to and including serious burns that may not be immediately proven because of the tendency of microwaves to heat deeper tissues with higher water content.
Eleanor R. Adair conducts micro-wave health research by exposing themselves, animals, and humans to microwave levels that make them feel warm or even begin to sweat and feel uncomfortable. He found no adverse health effects other than heat.
History and research
The existence of radio waves predicted by James Clerk Maxwell in 1864 from the equation. In 1888, Heinrich Hertz was the first to demonstrate radio waves by building a splash radio transmitter that produces 450 MHz microwaves, in the UHF region. The equipment he uses primitives, including horse troughs, wrought iron fire points, and Leiden jars. He also built the first dish antenna, using a sewer gutter. In 1894, Indian radio pioneer Jagdish Chandra Bose publicly demonstrated radio control over the bells using millimeter wavelengths, and conducted research on microwave spreading.
Perhaps the first, documented, formal use of the term "microwave" occurred in 1931:
- "When a test with a wavelength as low as 18 cm has been known, there is an unexpected surprise that the microwave problem has been solved so quickly." Telegraph & amp; Phone Journal XVII. 179/1
In 1943, the Hungarian engineer ZoltÃÆ'¡n Bay sent an ultra-short radio wave to the moon, which, reflected from there, worked as a radar, and could be used to measure distances, as well as to study the moon.
Perhaps the first use of the word microwave in the astronomical context occurred in 1946 in an article "Microwave Radiation from the Sun and the Moon" by Robert Dicke and Robert Beringer. This same article was also featured in the New York Times published in 1951. Ernst Weber pioneered microwave technology.
In the history of electromagnetic theory, work is particularly significant in the field of microwave and their application is carried out by researchers including:
See also
References
External links
- EM Talk, Tutorials and Microwave Engineering Tools
- Wave Millimeter and Waveguide Microwave dimension chart.
Source of the article : Wikipedia
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