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Microwave usually refers to electromagnetic waves with a frequency range of 300MHz ~ 300GHz and a wavelength of about 1 meter to 1 millimeter, which can be further subdivided into sub-meter wave, centimeter wave and millimeter wave radar panels, whose corresponding frequencies are ultra-high frequency (UHF, ultra-high frequency), super-high frequency (SHF, super high frequency), and extremely high frequency (EHF, extremely high frequency). (UHF, ultra-high frequency), super high frequency (SHF, super high frequency), and extremely high frequency (EHF, extremely high frequency). With the development of modern microwave technology, the sub-millimeter wave with wavelength below 1 mm is also regarded as the scope of microwave, which is equivalent to further expanding the range of microwave frequency to higher frequency. This is equivalent to extending the frequency range of microwave to higher frequencies. In this regard, the frequency range of microwave is also defined as 300MHz~3000GHz in some literatures.
Microwaves usually exhibit three characteristics: penetration, reflection and absorption. For glass, plastic and porcelain, microwaves almost penetrate without being absorbed. For water and food, microwaves are absorbed and heat themselves up. For metals, microwaves are reflected. From the point of view of electronics and physics, microwave this section of the electromagnetic spectrum has different from other bands of the following important features:
1.Microwave penetration
Microwaves are longer than other electromagnetic waves used for radiation heating, such as infrared, far-infrared wavelengths, and so on, and have a better penetration of antiques. Microwave penetration into the medium, due to microwave energy and the medium of certain interactions, with a microwave frequency of 2450MHz, so that the molecules of the medium to produce 2.45 billion vibrations per second, the medium of the molecules of the friction between each other, caused by the medium of the temperature rise, so that the medium of the material inside, outside the almost simultaneous heating, the formation of the state of the body heat source, greatly shortened the conventional heating in the heat conduction time, and in the Condition for the dielectric loss factor and dielectric temperature is a negative correlation, the material inside and outside the heating is uniform.
2.Microwave selective heating
The ability of a material to absorb microwaves is mainly determined by its dielectric loss factor. Substances with large dielectric loss factor have strong microwave absorption ability, on the contrary, substances with small dielectric loss factor have weak microwave absorption ability. Due to the differences in the loss factor of each substance, microwave heating is characterized by selective heating.Different substances produce different heat effects. Water molecule is a polar molecule, the dielectric constant is larger, its dielectric loss factor is also large, the microwave has a strong absorption capacity. Protein, carbohydrates and other dielectric constant is relatively small, its microwave absorption capacity is much smaller than water.Therefore, for food, the amount of water content has a great impact on the microwave heating effect.
3.Microwave thermal inertia is small
Microwave heating of dielectric materials is transient heating, heating speed. On the other hand, the microwave output power can be adjusted at any time, the medium temperature rise can be changed without inertia, there is no “residual heat” phenomenon, extremely conducive to automatic control and continuous production needs.
4.Microwave like light
Microwave wavelength is very small, when the microwave irradiation to some objects,will produce significant reflection and refraction, and the light of the reverse, refraction. At the same time, the characteristics of microwave propagation and geometric optics are similar, like light, linear propagation and easy to concentrate,that is,light-like properties.
5.Microwave penetration
Microwave irradiation in the medium when the object, can penetrate into the internal characteristics of the object is called penetration. For example, microwaves are the only electromagnetic waves in the radio frequency spectrum that can penetrate the ionosphere (except for light waves).
6.Microwave Information
The information capacity of the microwave band is very large, even for a small relative bandwidth, the available bandwidth is also very wide, up to hundreds or even thousands of megahertz.
7.Microwave non-ionization
The quantum energy of microwaves is not large enough to change the internal structure of the molecules of the material or destroy the chemical bonds of the molecules, so the role of microwaves and the role of the material is non-ionizing.
A Brief History of Microwave Technology
Theoretical research on microwave and related experiments started in the early 20th century, but the early equipment could not meet the needs of experiments, mainly due to the lack of high-power signal generators and sensitive signal receivers, so the early research did not make substantial progress. To the 1930s of the 20th century, the emergence of high-frequency super-aberrant receivers and transistor mixers for the further development of microwave technology to provide the conditions for the development of microwave technology has made some progress.
In 1931, Italian scientist Marconi conducted a wireless communication experiment at a distance of 18 miles, in which 600MHz microwaves were used to transmit a better quality voice signal, with the signal source being a Barkhausen tube (Barkhausentube). This was the first time that microwaves above 300 MHz were used for wireless communication. In the following year, Marconi used 57 cm microwaves to establish radio communication between the Vatican and Gondolfi Castle, which were 15 miles apart, and to provide telephone and teleprinter services, from which microwave technology moved from the laboratory to real practical applications. During the same period, the concepts and theories related to radar were gradually developed, and by the mid-1930s, eight countries in the world were already conducting related research.
1.High-speed development of World War II
The Second World War is generally regarded as the golden age of microwave technology development, and the main driving force behind it came from the military's demand for radar. Radar played an important role in World War II, and the Allied Forces invested a lot of manpower and resources in radar research. In the United States, radar research was conducted at the Radiation Laboratory of the Massachusetts Institute of Technology (MIT), where the number of employees reached over 4,000 in 1945, and nine of the scientists who worked there were later awarded Nobel Prizes.
After the war, the Radiation Laboratory compiled the 28-volume Radiation Laboratory Series, published in 1947, which documented the extensive work of the Radiation Laboratory and related research organizations in the research and development of radar during World War II.
In 1886, Hertz had already discovered that electromagnetic waves would be reflected back by solids, which was actually the most basic principle of radar. In 1903, Hulsmeyer of Germany applied for a patent for his invention of a system for navigation, which utilized the reflection of radio waves to detect the presence of obstacles and navigate ships. In 1933, the U.S. Naval Laboratory demonstrated a radar transmitting 3 MHz radio waves that was able to detect the presence of airplanes at a range of 50 miles. In 1934, Watson Watt, a British physicist and expert in radar technology, set up an experimental radar station, and in 1938 organized an air defense radar network on the east coast of the United Kingdom, which consisted of 25 MHz radars. The air defense radar network played a major role in the British victory in World War II. In 1939, the U.S. Army equipped the 105MHz SCR 270 radar as a remote warning system, which successfully detected Japanese aircraft in advance during the Battle of Pearl Harbor. In 1943, the Radiation Laboratory developed a microwave radar utilizing cavity magnetrons - the 3GHz SCR 584 radar, which was put into production in the same year and widely used in the European and Pacific theaters. This radar was able to effectively determine the location of airborne objects and carry out fire control, and with its help, 85% of the V-1 missiles of the Germans in the Battle of London were destroyed by anti-aircraft fire.
During World War II, microwave technology in other applications also continued to develop. In wireless communications, in 1943 AT&T developed the AN/TRC-6 multi-band microwave radio, the equipment utilized pulse position modulation and digital modulation technology, the U.S. Army and the British Army equipped with this model of radio equipment. Microwave measurement technologies were also developed in the 1940s, including energy detection, energy measurement, frequency measurement, phase measurement, and voltage standing wave ratio.
Reflectometer and impedance measurement technologies were first introduced by Meyers, Charles and Julian in 1944. Microwave technology for biological and medical applications was also developed during World War II.
In 1938, the German Hollman and the Americans Hemingway and Stenstrom jointly proposed the use of microwaves in therapeutic applications. The principle is that compared with other electromagnetic waves, microwaves can better concentrate energy for heat treatment. Moreover, the use of microwaves in heating deep tissues will not cause the surface of the skin to become too hot.
2.Post-War Development of Microwave Technology
Microwave technology spread rapidly in the decade after World War II and was emphasized by various countries. Many countries set up microwave technology research programs in universities, large corporations or research institutes with the financial support of the government. Especially during the Cold War, in order to provide support for new weapon systems, a large number of microwave technology related research was carried out, and these researches greatly contributed to the development of microwave technology. New waveguide structures were proposed to replace the overly large rectangular and circular waveguides. Many new types of microwave tubes were introduced. The size of signal generators and receivers was also drastically reduced to reduce the cost of producing the entire microwave system and to increase stability. These technological advances have contributed to the development of microwave integrated circuit technology.
Radar technology also continues to progress,in 1955 Page invented the single pulse radar, the United States radio company in 1958 produced a high-precision single pulse tracer instrument radar AN/FPS-16, which is the first to reach 0.1-meter accuracy of the radar.
After the war,microwave communication technology developed rapidly, and in 1946, the U.S. Army successfully received microwave signals from the radar that were reflected off the surface of the moon. AT&T established a 3.7-4.2 GHz repeater-based line-of-sight radio system between New York and Boston in 1947, capable of transmitting 500 telephone lines or the programs of one black-and-white television channel.
The first commercial geosynchronous communications satellite, Early Bird, was developed in 1965.
In microwave measurement technology, Fellgett proposed the Fourier transform spectroscopy method in 1951. The first caesium-beam atomic clock applying the method of separated oscillatory fields was introduced in 1952 at the National Bureau of Standards of the United States. Subsequently, in 1955, Edson et al. from the National Physical Laboratory of the United Kingdom developed an ultra-precise microwave induced mobility caesium-beam atomic clock, and Cutler and Bagley introduced the atomic clock as the atomic frequency standard in 1960, which made the invention and application of atomic clocks a milestone in modern time measurement technology. In 1966, Weinert invented the radio frequency vector voltmeter.
Microwave technology in biological and medical applications of research in the post-war period also continued to develop, in 1950, Gessler, McCarty and Parkinson three bit physicists for the first time the use of microwave energy to treat cancer trials, and the use of microwave radiation with a frequency of 2,450 MHz to cure breast cancer in white rats. In 1955, Allen used a mixture of microwave and X-ray radiation to cure an S180 tumor in a rat, and a large amount of research on hybrid radiation therapy was subsequently carried out.
3.Transformation from military to civilian use
After World War II, the research and application of microwave technology gradually from meeting military needs to the civilian field of transition, representative of the invention of microwave ovens. Starting from the 1940s, there are many patents on the use of microwave heating technology, which marks the beginning of microwave heating technology. Westinghouse and GE focused on industrial applications such as drying textiles, tires, and wood, while Raytheon focused on cooking and heating food. In 1949, Raytheon patented some of the basics of the microwave oven. However, Raytheon focused on the commercial microwave oven market and did not venture into home microwave ovens. By 1960, under Raytheon's license, several large American companies, such as Hotpoint, Westinghouse, and Whirlpool, began to produce microwave ovens for the average consumer. However, the application of microwave heating technology in industrial processes grew slower than expected, and consumer demand in the home microwave oven market was limited, primarily because of the high cost of the magnetron, the main component of the microwave oven.
Historical turnaround occurred in 1967, Raytheon subordinate Amana introduced a home countertop microwave oven RR - 1, due to the use of a new type of cost reduction of the new magnetron, so that the price of the whole machine is less than half of the previous microwave oven. RR - 1 type of microwave oven once launched will quickly promote the development of the home microwave oven market, not only the United States domestic companies quickly followed, and attracted Toshiba, Sharp, Hitachi and other Japanese companies also invested in this market. In the 1960s, the United States of America's annual sales of microwave ovens only about 10,000 units, and by 1975, the number of more than 1 million units, in 1985 was as high as 5 million units. Among them, the Japanese company also in the household microwave oven market accounted for a large share, according to Sharp announced material, by 1977 Sharp has sold 2 million microwave ovens. Subsequently, in Japan, Europe and other places in the household microwave oven market also developed. A survey in 1976 showed that 17% of Japanese households used microwave ovens for cooking. Today, microwave ovens have become widely used around the world food processing and cooking appliances, many Americans even think that the microwave oven is the most important scientific and technological breakthroughs since the 1970s, in our country, microwave ovens in large and medium-sized cities, the popularity of the microwave oven has reached more than 80%.
Microwave technology industrialized applications
Modern microwave equipment using microwave technology and technology means in various industries for application and development:
1.Food, health products industry
The use of microwave food, health products can be sterilized, dehydration, drying, puffing, flavoring, deodorization, defrosting, promotion and preservation of fresh processing. Now has been used in milk powder, cheese, bone powder, ginseng pills, soybean powder, moon cakes, cakes, instant noodles, beef jerky, dried meat, meat strips, meat floss, dried fish flakes, fish floss, shellfish, ducks in brine, defrosted fish, poultry and eggs, sauces, potato flakes, cashews, peanuts, melon seeds, soybeans, white wine, liquor, beer, milk, oral liquid, Chinese herbs and so on in the production of.
2.Wood processing
The use of high-power microwave dryer for 2-10 centimeters thick wood drying, drying speed, wood does not crack, deformation of small, while killing the wood inside the eggworms and larvae, medium and high-grade furniture, flooring, packaging materials, the use of wood is most suitable for processing. Microwave can also be used for curing bamboo wood composite boards and splicing of the intersection of the curing process is also very ideal.
3.Insecticide and sterilization
Application of microwave heating technology can be in the lower humidity sterilization and insecticide, if the microwave treatment of food and materials, in the 50-80 degrees can play a role in killing pests and bacteria. Now widely used in rice, grain, beans, tobacco treatment, bamboo, wood, paper, food, medicine and other industries.
4.Rubber industry
Microwave technology should be used in the rubber industry in the rubber vulcanization process, combined with conventional heating and warming, greatly improving the rubber vulcanization time and efficiency. Currently in Hebei Hengshui area, Shandong area is widely used.
5.Ceramic industry
Microwave heating can be used in the ceramic industry, the combination of conventional heating and microwave heating methods, can greatly shorten the drying time of ceramics, while not affecting the yield of ceramic firing.
6.Coal drying
Coal mining is usually higher moisture content (about 35%), if the conventional drying method for drying, slow, low efficiency, drying is not complete, the microwave method of drying, can make the coal inside and outside the simultaneous heating, is the evaporation of water, dehydration drying, efficiency has been greatly improved.
7.Microwave Plasma Technology
Transistor production process has been used in microwave plasma technology, can be etched, sputtering, vapor deposition, oxidized silicon. Available for metals, alloys, non-metallic surface treatment. Can be used for high temperature sintering of ceramics. Can be used for plasma spectral analysis, can detect more than ten kinds of elements, and also used to decompose toxic compounds.
8.Microwave medical waste treatment technology
The use of microwave technology in the anaerobic or anoxic state, so that the microwave carrier quickly rise to high temperatures, so that the medical waste in a very short period of time quickly in a high-temperature state, direct ashing, greatly reduces the combustion process of toxic gases produced. The gas in the flue gas and then use the microwave plasma torch for high-temperature cracking, the entire treatment process and the treatment environment to achieve the full closure of the harmless treatment. The application of this technology is superior to conventional treatment methods, opening up a new field of microwave applications.
9.Sewage treatment
The use of microwave non-thermal effect and thermal effect to select the wave-absorbing “sensitizer” to microwave energy transfer to the pollutants in the water and induce chemical reactions, through physical and chemical effects on the pollutants in the water for degradation, transformation, accelerated solid and liquid separation in the fluid in order to achieve the purpose of wastewater purification, is a new revolution in the field of water treatment.
10.Microwave carbon production
The use of microwave technology for high-temperature cracking of bamboo, bamboo charcoal, greatly improving the production efficiency, but also can be selected to the bamboo vinegar liquid, bamboo tar and other products, has a higher economic value, better than the traditional method of making bamboo charcoal kiln, is now vigorously begin to promote the use of.
Current Developments in Microwave Communications
In the 21st century, microwave technology continues to play a great role in broadcasting, cable television, telephone and wireless communications, and also in other areas such as computer networking applications. In broadcasting and television, the current broadcasting and television programming is gradually moving towards digital, and digital programs need digital transmission means,so the SDH digital microwave network came into being, and the old analog microwave equipment compared to the transmission capacity has increased dramatically, the SDH digital microwave equipment can transmit dozens of sets of television programs and dozens of sets of sound broadcasting programs, and transmission distance is greater, the scope of the wider. The use of digital microwave digital environment, but also to build a two-way interactive information network,the realization of the integrated use of network functions and development.
In the field of communication, microwave, satellite and fiber optic cable are the three pillars of modern communication transmission. Microwave communication has its own unique advantages. First of all, microwave communication has good anti-disaster performance, and microwave communication is generally unaffected by natural disasters such as floods, winds and earthquakes. In the 1976 Tangshan earthquake,all the coaxial cables between Beijing and Tianjin were broken, but the six microwave channels still work normally.Secondly,the microwave transmission system is more rapidly organized. With the continuous expansion of the city and the occurrence of unexpected events, such as local water, road damage, local fire and fire, etc.,these occasions require the rapid establishment of local temporary communication system. The emergency response capability of microwave in the above occasions is far beyond other communication methods.
The rapid development of the network has brought about the urgent need for network communication line construction,although fiber optic cable has the advantage of large capacity,but also has a high cost, need to be paved in advance as well as susceptible to natural disasters and other weaknesses,in addition to fiber optic communication is not suitable for rural areas,islands and other remote areas and dedicated communication network. Microwave communications can be used as a trunk fiber optic transmission backup and supplement,to solve the problem of urban areas to lay cable resources difficult.Digital microwave access becomes another important pipeline to establish wide-area network connection, and is an effective supplement to the urban communications infrastructure.
Digital microwave technology will be the main direction of the future development of microwave technology, specific applications include microwave spread spectrum material transmission system, can be used in urban short-range feeder connection of high-frequency microwave, for future broadband business access to the local multi-distribution services, modern military digital microwave communication systems and so on.The key technologies to be researched are high-frequency transmission technologies above 10 GHz and compatible technologies in existing frequency bands,including high-efficiency modulation technologies,spread spectrum and frequency hopping anti-interference technologies, error correction technologies, etc. In addition, such as microwave monoliths, the microwave transmission technologies are also being developed.In addition,technologies such as microwave monolithic integration,full bit processing, and digital specialized integrated circuits to improve reliability and reduce cost also need to be further researched.