RF and Microwave Passive and Active Technologies
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A spectral band structure causes absorption peaks at specific frequencies see graph at right. 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.
This technique has been used at frequencies between 0. The short wavelengths of microwaves allow omnidirectional antennas for portable devices to be made very small, from 1 to 20 centimeters long, so microwave frequencies are widely used for wireless devices such as cell phones , cordless phones , and wireless LANs Wi-Fi access for laptops , and Bluetooth earphones.
Antennas used include short whip antennas , rubber ducky antennas , sleeve dipoles , patch antennas , and increasingly the printed circuit inverted F antenna PIFA used in cell phones. Their short wavelength also 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. 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. The term microwave also has a more technical meaning in electromagnetics and circuit theory. 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. 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.
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.
Microwave technology is extensively used for point-to-point telecommunications i. 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. The FCC recently [ when? Dozens of service providers across the country are securing or have already received licenses from the FCC to operate in this band. Commercial implementations are in the 2. 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.
Typically, microwaves are used in television news to transmit a signal from a remote location to a television station from a specially equipped van. Most satellite communications systems operate in the C, X, K a , or K u 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 K u band for direct-broadcast satellite. Military communications run primarily over X or K u -band links, with K a band being used for Milstar. 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. 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.
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. A major recent focus of microwave radio astronomy has been mapping the cosmic microwave background radiation CMBR discovered in 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.
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Sufficiently sensitive radio telescopes can detected the CMBR as a faint signal that is not associated with any star, galaxy, or other object. A microwave oven passes microwave radiation at a frequency near 2. Microwave ovens became common kitchen appliances in Western countries in the late s, following the development of less expensive cavity magnetrons.
Water in the liquid state possesses many molecular interactions that broaden the absorption peak. 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. Microwaves are used in stellarators and tokamak experimental fusion reactors to help break down the gas into a plasma, and heat it to very high temperatures. 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 s and early s 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.
The United States Air Force and Marines are currently using this type of active denial system in fixed installations. 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.
Bands of frequencies in the microwave spectrum are designated by letters. Unfortunately, there are several incompatible band designation systems, and even within a system the frequency ranges corresponding to some of the letters vary somewhat between different application fields. P band is sometimes used for K u Band. 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, K u , and upper band, K a.
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.
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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.
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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. Microwaves do not contain sufficient energy to chemically change substances by ionization, and so are an example of non-ionizing radiation. 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. 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.
In Dr. 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,  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.
Microwaves were first generated in the s in some of the earliest radio experiments by physicists who thought of them as a form of "invisible light". In , German physicist Heinrich Hertz was the first to demonstrate the existence of radio waves using a primitive spark gap radio transmitter. They concentrated on producing short wavelength radio waves in the UHF and microwave ranges, with which they could duplicate classic optics experiments, using quasioptical components such as prisms and lenses made of paraffin , sulfur and pitch and wire diffraction gratings , to refract and diffract radio waves like light rays.
Experiment by John Ambrose Fleming in showing refraction of 1. In , Oliver Lodge and Augusto Righi generated 1.
However, since microwaves were limited to line of sight paths, they could not communicate beyond the visual horizon, and the low power of the spark transmitters then in use limited their practical range to a few miles. The subsequent development of radio communication after employed lower frequencies, which could travel beyond the horizon as ground waves and by reflecting off the ionosphere as skywaves , and microwave frequencies were not further explored at this time.
Practical use of microwave frequencies did not occur until the s and s due to a lack of adequate sources, since the triode vacuum tube valve electronic oscillator used in radio transmitters could not produce frequencies above a few hundred megahertz due to excessive electron transit time and interelectrode capacitance.
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A word was needed to distinguish these new shorter wavelengths, which had previously been lumped into the " short wave " band, which meant all waves shorter than meters. The terms quasi-optical waves and ultrashort waves were used briefly, but didn't catch on. The first usage of the word micro-wave apparently occurred in The development of radar , mainly in secrecy, before and during World War 2 , resulted in the technological advances which made microwaves practical. It was found that conventional transmission lines used to carry radio waves had excessive power losses at microwave frequencies, and George Southworth at Bell Labs and Wilmer Barrow at MIT independently invented waveguide in In a microwave receiver , a nonlinear component was needed that would act as a detector and mixer at these frequencies, as vacuum tubes had too much capacitance.
To fill this need researchers resurrected an obsolete technology, the point contact crystal detector cat whisker detector which was used as a demodulator in crystal radios around the turn of the century before vacuum tube receivers. The first modern silicon and germanium diodes were developed as microwave detectors in the s, and the principles of semiconductor physics learned during their development led to semiconductor electronics after the war.
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Southworth at left demonstrating waveguide at IRE meeting in , showing 1. The first modern horn antenna in with inventor Wilmer L. The MIT Radiation Laboratory established secretly at Massachusetts Institute of Technology in to research radar, produced much of the theoretical knowledge necessary to use microwaves.
The first microwave relay systems were developed by the Allied military near the end of the war and used for secure battlefield communication networks in the European theater. After World War 2, microwaves were rapidly exploited commercially. In the s and 60s transcontinental microwave relay networks were built in the US and Europe to exchange telephone calls between cities and distribute television programs.dioknivfagring.tk
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In the new television broadcasting industry, from the s microwave dishes were used to transmit backhaul video feed from mobile production trucks back to the studio, allowing the first remote TV broadcasts. The first communications satellites were launched in the s, which relayed telephone calls and television between widely separated points on Earth using microwave beams. In , Arno Penzias and Robert Woodrow Wilson while investigating noise in a satellite horn antenna at Bell Labs , Holmdel, New Jersey discovered cosmic microwave background radiation.
Microwave radar became the central technology used in air traffic control , maritime navigation , anti-aircraft defense , ballistic missile detection, and later many other uses. Radar and satellite communication motivated the development of modern microwave antennas; the parabolic antenna the most common type , cassegrain antenna , lens antenna , slot antenna , and phased array. The ability of short waves to quickly heat materials and cook food had been investigated in the s by I.
He investigated cooking with microwaves and invented the microwave oven , consisting of a magnetron feeding microwaves into a closed metal cavity containing food, which was patented by Raytheon on 8 October Microwave heating became widely used as an industrial process in industries such as plastics fabrication, and as a medical therapy to kill cancer cells in microwave hyperthermy. The gyrotron tube family developed in Russia could produce megawatts of power up into millimeter wave frequencies, and is used in industrial heating and plasma research, and to power particle accelerators and nuclear fusion reactors.
The development of semiconductor electronics in the s led to the first solid state microwave devices which worked by a new principle; negative resistance some of the prewar microwave tubes had also used negative resistance. The tunnel diode invented in by Japanese physicist Leo Esaki could produce a few milliwatts of microwave power. Its invention set off a search for better negative resistance semiconductor devices for use as microwave oscillators, resulting in the invention of the IMPATT diode in by W. They specialise in RF through millimeter-wave circuits.
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