108-470: Hot Pockets is an American brand of microwaveable turnovers generally containing one or more types of cheese, meat, or vegetables. Hot Pockets was founded by Chef America Inc. Since April 20, 2002, they have been produced by Nestlé . Hot Pockets were developed by Paul Merage and his brother David through their company Chef America Inc. Chef America invented a packaging sleeve and dough formula to keep its calzone -like sandwiches crispy when cooked in
216-619: A band being used for Milstar . Global Navigation Satellite Systems (GNSS) including the Chinese Beidou , the American Global Positioning System (introduced in 1978) and the Russian GLONASS broadcast navigational signals in various bands between about 1.2 GHz and 1.6 GHz. Radar is a radiolocation technique in which a beam of radio waves emitted by a transmitter bounces off an object and returns to
324-573: A microwave . In 1980, Chef America introduced its first stuffed sandwich, the Tastywich, the predecessor of the Hot Pocket. Hot Pockets supplanted Tastywiches in 1983, first sold to restaurants because they were easier to break into than retail stores. On May 22, 2002, Chef America was sold to Nestlé. Hot Pocket products were "a $ 2 billion category of frozen sandwiches and snacks". Breakfast-style Hot Pockets were introduced in 2001. There are 50 varieties of
432-516: A standing wave is present, they may also be used to measure the distance between the nodes , which is equal to half the wavelength. The precision of this method is limited by the determination of the nodal locations. Microwaves are non-ionizing radiation, which means that microwave photons do not contain sufficient energy to ionize molecules or break chemical bonds, or cause DNA damage, as ionizing radiation such as x-rays or ultraviolet can. The word "radiation" refers to energy radiating from
540-454: A . Microwaves travel solely by line-of-sight paths; unlike lower frequency radio waves, they do not travel as ground waves which follow the contour of the Earth, or reflect off the ionosphere ( skywaves ). Although at the low end of the band they can pass through building walls enough for useful reception, usually rights of way cleared to the first Fresnel zone are required. Therefore, on
648-548: A 10 kW, 60 MHz shortwave transmitter . The Westinghouse team, led by I. F. Mouromtseff, found that foods like steaks and potatoes could be cooked in minutes. The 1937 United States patent application by Bell Laboratories states: This invention relates to heating systems for dielectric materials and the object of the invention is to heat such materials uniformly and substantially simultaneously throughout their mass. ... It has been proposed therefore to heat such materials simultaneously throughout their mass by means of
756-415: A United States patent application for Spencer's microwave cooking process, and an oven that heated food using microwave energy from a magnetron was soon placed in a Boston restaurant for testing. Another independent discovery of microwave oven technology was by British scientists, including James Lovelock , who in the 1950s used it to reanimate cryogenically frozen hamsters . In 1947, Raytheon built
864-440: A common kitchen appliance and are popular for reheating previously cooked foods and cooking a variety of foods. They rapidly heat foods which can easily burn or turn lumpy if cooked in conventional pans, such as hot butter, fats, chocolate, or porridge . Microwave ovens usually do not directly brown or caramelize food, since they rarely attain the necessary temperature to produce Maillard reactions . Exceptions occur in cases where
972-431: A computer-controlled array of antennas that produces a beam that 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
1080-656: A cooking power of between 600 and 1200 watts. Microwave cooking power, also referred to as output wattage, is lower than its input wattage, which is the manufacturer's listed power rating . The size of household microwave ovens can vary, but usually have an internal volume of around 20 liters (1,200 cu in; 0.71 cu ft), and external dimensions of approximately 45–60 cm (1 ft 6 in – 2 ft 0 in) wide, 35–40 cm (1 ft 2 in – 1 ft 4 in) deep and 25–35 cm (9.8 in – 1 ft 1.8 in) tall. Microwaves can be turntable or flatbed. Turntable ovens include
1188-530: A frequency near 2.45 GHz (12 cm) through food, causing dielectric heating primarily by absorption of the energy in water. Microwave ovens became common kitchen appliances in Western countries in the late 1970s, following the development of less expensive cavity magnetrons . Water in the liquid state possesses many molecular interactions that broaden the absorption peak. In the vapor phase, isolated water molecules absorb at around 22 GHz, almost ten times
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#17331059799951296-570: A glass plate or tray. Flatbed ones do not include a plate, so they have a flat and wider cavity. By position and type, US DOE classifies them as (1) countertop or (2) over the range and built-in (wall oven for a cabinet or a drawer model). A traditional microwave only has two power output levels, fully on and fully off. Intermediate heat settings are achieved using duty-cycle modulation and switch between full power and off every few seconds, with more time on for higher settings. An inverter type, however, can sustain lower temperatures for
1404-422: A harmonic generator and a mixer. The 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,
1512-437: A higher temperature than water (the temperature they require to vaporize is higher), so inside microwave ovens they normally reach higher temperatures – sometimes much higher. This can induce temperatures in oil or fatty foods like bacon far above the boiling point of water, and high enough to induce some browning reactions, much in the manner of conventional broiling (UK: grilling) , braising, or deep fat frying. The effect
1620-580: A higher temperature, or even if the inner layer is more thermally conductive than the outer layer making it feel hotter despite having a lower temperature. In most cases, however, with uniformly structured or reasonably homogeneous food item, microwaves are absorbed in the outer layers of the item at a similar level to that of the inner layers. Depending on water content, the depth of initial heat deposition may be several centimetres or more with microwave ovens, in contrast with broiling / grilling (infrared) or convection heating methods which thinly deposit heat at
1728-520: A lengthy duration without having to switch itself off and on repeatedly. Apart from offering superior cooking ability, these microwaves are generally more energy-efficient. As of 2020 , the majority of countertop microwave ovens (regardless of brand) sold in the United States were manufactured by the Midea Group . Domestic microwave ovens are typically marked with the microwave-safe symbol, next to
1836-409: A less expensive re-engineered magnetron. Several other companies joined in the market, and for a time most systems were built by defence contractors, who were most familiar with the magnetron. Litton was particularly well known in the restaurant business. While uncommon today, combination microwave-ranges were offered by major appliance manufacturers through much of the 1970s as a natural progression of
1944-453: A microwave oven in 1994, but that number had increased to 65% by 2004. Adoption has been slower in less-developed countries , as households with disposable income concentrate on more important household appliances like refrigerators and ovens. In India , for example, only about 5% of households owned a microwave oven in 2013, well behind refrigerators at 31% ownership. However, microwave ovens are gaining popularity. In Russia, for example,
2052-457: A microwave oven, up from only about 1% in 1971; the U.S. Bureau of Labor Statistics reported that over 90% of American households owned a microwave oven in 1997. In Australia, a 2008 market research study found that 95% of kitchens contained a microwave oven and that 83% of them were used daily. In Canada, fewer than 5% of households had a microwave oven in 1979, but more than 88% of households owned one by 1998. In France, 40% of households owned
2160-522: A motor driven mode stirrer in the top of the oven cavity rotated allowing the food to remain stationary. In the 1960s, Litton bought Studebaker 's Franklin Manufacturing assets, which had been manufacturing magnetrons and building and selling microwave ovens similar to the Radarange. Litton developed a new configuration of the microwave oven: the short, wide shape that is now common. The magnetron feed
2268-530: A place in the electromagnetic spectrum with frequency above ordinary radio waves , and below infrared light: In descriptions of the electromagnetic spectrum , some sources classify microwaves as radio waves, a subset of the radio wave band, while others classify microwaves and radio waves as distinct types of radiation. This is an arbitrary distinction. Bands of frequencies in the microwave spectrum are designated by letters. Unfortunately, there are several incompatible band designation systems, and even within
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#17331059799952376-499: A process known as dielectric heating . These molecules have a partial positive charge at one end and a partial negative charge at the other. In an alternating electric field, they will constantly spin around as they continually try to align themselves with the electric field. This can happen over a wide range of frequencies. The electric field's energy is absorbed by the dipole molecules as rotational energy. Then they hit non-dipole molecules, making them move faster as well. This energy
2484-425: A rapidly alternating electric field. The invention of the cavity magnetron made possible the production of electromagnetic waves of a small enough wavelength ( microwaves ). The cavity magnetron was a crucial component in the development of short wavelength radar during World War II . In 1937–1940, a multi-cavity magnetron was built by British physicist Sir John Turton Randall, FRSE and coworkers, for
2592-515: 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
2700-449: A source and not to radioactivity . The main effect of absorption of microwaves is to heat materials; the electromagnetic fields cause polar molecules to vibrate. 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
2808-558: A system the frequency ranges corresponding to some of the letters vary somewhat between different application fields. The letter system had its origin in World War 2 in a top-secret U.S. classification of bands used in radar sets; this is the origin of the oldest letter system, the IEEE radar bands. One set of microwave frequency bands designations by the Radio Society of Great Britain (RSGB),
2916-492: 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
3024-453: A wavelength that is small relative to the cavity (as in a modern microwave oven) is due to "far-field" effects that are due to classical electromagnetic radiation that describes freely propagating light and microwaves suitably far from their source. Nevertheless, the primary heating effect of all types of electromagnetic fields at both radio and microwave frequencies occurs via the dielectric heating effect, as polarized molecules are affected by
3132-566: Is also more bandwidth in the microwave spectrum than in the rest of the radio spectrum; the usable bandwidth below 300 MHz is less than 300 MHz while many GHz can be used above 300 MHz. Typically, microwaves are used in remote broadcasting of news or sports events as the backhaul link to transmit a signal from a remote location to a television station from a specially equipped van. See broadcast auxiliary service (BAS), remote pickup unit (RPU), and studio/transmitter link (STL). Most satellite communications systems operate in
3240-416: Is also used to perform rotational spectroscopy and can be combined with electrochemistry as in microwave enhanced electrochemistry . Microwave frequency can be measured by either electronic or mechanical techniques. 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,
3348-439: Is generally credited with developing and patenting the world's first commercial microwave oven post World War II from British radar technology developed before and during the war. Named the "RadaRange", it was first sold in 1947. Raytheon later licensed its patents for a home-use microwave oven that was introduced by Tappan in 1955, but it was still too large and expensive for general home use. Sharp Corporation introduced
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3456-429: Is glass, which can exhibit thermal runaway in a microwave oven to the point of melting if preheated. Additionally, microwaves can melt certain types of rocks, producing small quantities of molten rock. Some ceramics can also be melted, and may even become clear upon cooling. Thermal runaway is more typical of electrically conductive liquids such as salty water. Another misconception is that microwave ovens cook food "from
3564-555: Is inversely proportional to the 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. Before the advent of fiber-optic transmission, most long-distance telephone calls were carried via networks of microwave radio relay links run by carriers such as AT&T Long Lines . Starting in
3672-424: Is limited to a few kilometers. A spectral band structure causes absorption peaks at specific frequencies (see graph at right). Above 100 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so effective that it is in effect opaque , until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges. In a microwave beam directed at an angle into
3780-408: Is most often noticed by consumers from unexpected damage to plastic containers when microwaving foods high in sugar, starch, or fat generates higher temperatures. Foods high in water content and with little oil rarely exceed the boiling temperature of water and do not damage plastic. Cookware must be transparent to microwaves. Conductive cookware, such as metal pots, reflects microwaves, and prevents
3888-400: Is shared deeper into the substance as molecular rotations, vibrations or other movement signifying an increase in the temperature of the food. Once the electrical field's energy is initially absorbed, heat will gradually spread through the object similarly to any other heat transfer by contact with a hotter body. Microwave heating is more efficient on liquid water than on frozen water, where
3996-432: Is tabulated below: Other definitions exist. The term P band is sometimes used for UHF frequencies below the L band but is now obsolete per IEEE Std 521. When radars were first developed at K band during World War 2, 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
4104-541: Is that they should be in one of the industrial, scientific, and medical (ISM) frequency bands set aside for unlicensed purposes. For household purposes, 2.45 GHz has the advantage over 915 MHz in that 915 MHz is only an ISM band in some countries ( ITU Region 2) while 2.45 GHz is available worldwide. Three additional ISM bands exist in the microwave frequencies, but are not used for microwave cooking. Two of them are centered on 5.8 GHz and 24.125 GHz, but are not used for microwave cooking because of
4212-617: Is the range between 1 and 100 GHz (wavelengths between 30 cm and 3 mm), or between 1 and 3000 GHz (30 cm and 0.1 mm). The prefix micro- in microwave is not meant to suggest a wavelength in the micrometer range; rather, it indicates that microwaves are small (having shorter wavelengths), compared to the radio waves used in prior radio technology . The boundaries between far infrared , terahertz radiation , microwaves, and ultra-high-frequency (UHF) are fairly arbitrary and are used variously between different fields of study. In all cases, microwaves include
4320-680: Is typically offered, as either a power level or a separate function. Some models include pre-programmed settings for different food types, typically taking weight as input. In the 1990s, brands such as Panasonic and GE began offering models with a scrolling-text display showing cooking instructions. Microwaves Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequencies between 300 MHz and 300 GHz, broadly construed. A more common definition in radio-frequency engineering
4428-602: 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. Sufficiently sensitive radio telescopes can detect the CMBR as a faint signal that is not associated with any star, galaxy, or other object. A microwave oven passes microwave radiation at
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4536-446: The ISM (industrial, scientific, medical) bands , which are otherwise used for communication amongst devices that do not need a license to operate, so they do not interfere with other vital radio services. It is a common misconception that microwave ovens heat food by operating at a special resonance of water molecules in the food. Instead, microwave ovens heat by causing molecules to spin under
4644-655: The Rancho Feeding Corp was used to make Hot Pockets. The USDA described the food as "unfit for humans". This Rancho Feeding Corp meat recall was based out of a production facility in California, but the recalled Hot Pockets were distributed nationwide. The two types of Hot Pockets involved in the recall were the Philly Steak and Cheese and the Croissant Crust Philly Steak and Cheese. A full federal inspection
4752-614: The United States Department of Energy require less than 1 watt, or approximately 9 kWh per year, of standby power for most types of microwave ovens. A microwave oven generally consists of: In most ovens, the magnetron is driven by a linear transformer which can only feasibly be switched completely on or off. (One variant of the GE Spacemaker had two taps on the transformer primary, for high and low power modes.) Usually choice of power level does not affect intensity of
4860-577: The field-effect transistor (at least at lower frequencies), tunnel diodes , Gunn diodes , and IMPATT diodes . Low-power sources are available as benchtop instruments, rackmount instruments, embeddable modules and in card-level formats. 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
4968-419: The "Radarange", the first commercially available microwave oven. It was almost 1.8 metres (5 ft 11 in) tall, weighed 340 kilograms (750 lb) and cost about US$ 5,000 ($ 68,000 in 2023 dollars) each. It consumed 3 kilowatts, about three times as much as today's microwave ovens, and was water-cooled. The name was the winning entry in an employee contest. An early Radarange was installed (and remains) in
5076-438: The 2.3 GHz, 2.5 GHz, 3.5 GHz and 5.8 GHz ranges. Mobile Broadband Wireless Access (MBWA) protocols based on standards specifications such as IEEE 802.20 or ATIS/ANSI HC-SDMA (such as iBurst ) operate between 1.6 and 2.3 GHz to give mobility and in-building penetration characteristics similar to mobile phones but with vastly greater spectral efficiency. Some mobile phone networks, like GSM , use
5184-425: The 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services have been used for almost a decade in many countries in the 3.5–4.0 GHz range. The FCC recently carved out spectrum for carriers that wish to offer services in this range in the U.S. — with emphasis on 3.65 GHz. Dozens of service providers across the country are securing or have already received licenses from
5292-685: The 95 GHz focused beam heats the skin to a temperature of 54 °C (129 °F) at a depth of 0.4 millimetres ( 1 ⁄ 64 in). The United States Air Force and Marines are currently using this type of active denial system in fixed installations. Microwave radiation is used in electron paramagnetic resonance (EPR or ESR) spectroscopy, typically in the X-band region (~9 GHz) in conjunction typically with magnetic fields of 0.3 T. This technique provides information on unpaired electrons in chemical systems, such as free radicals or transition metal ions such as Cu(II). Microwave radiation
5400-648: The British and American military radar installations in World War II. A higher-powered microwave generator that worked at shorter wavelengths was needed, and in 1940, at the University of Birmingham in England, Randall and Harry Boot produced a working prototype. They invented a valve that could produce pulses of microwave radio energy at a wavelength of 10 cm, an unprecedented discovery. Sir Henry Tizard traveled to
5508-437: 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
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#17331059799955616-454: The FCC to operate in this band. The WIMAX service offerings that can be carried 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 implementations are in
5724-757: The US in late September 1940 to offer Britain's most valuable technical secrets including the cavity magnetron in exchange for US financial and industrial support (see Tizard Mission ). An early 6 kW version, built in England by the General Electric Company Research Laboratories, Wembley , London, was given to the U.S. government in September 1940. The cavity magnetron was later described by American historian James Phinney Baxter III as "[t]he most valuable cargo ever brought to our shores". Contracts were awarded to Raytheon and other companies for
5832-517: The additional component cost could better be absorbed compared with countertop units where pricing was increasingly market-sensitive. By 1972, Litton (Litton Atherton Division, Minneapolis) introduced two new microwave ovens, priced at $ 349 and $ 399, to tap into the market estimated at $ 750 million by 1976, according to Robert I Bruder, president of the division. While prices remained high, new features continued to be added to home models. Amana introduced automatic defrost in 1974 on their RR-4D model, and
5940-442: The average residential microwave oven consumes only 72 kWh per year. Globally, microwave ovens used an estimated 77 TWh per year in 2018, or 0.3% of global electricity generation. A 2000 study by Lawrence Berkeley National Laboratory found that the average microwave drew almost 3 watts of standby power when not being used, which would total approximately 26 kWh per year. New efficiency standards imposed in 2016 by
6048-490: The body. The lens and cornea of the eye are especially vulnerable because they contain no blood vessels that can carry away heat. 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). Exposure to heavy doses of microwave radiation (as from an oven that has been tampered with to allow operation even with
6156-400: The cause of the fallen sales, stating SNAP benefit recipients are "a big part of the consumption of this particular product." In 2014, Nestle USA recalled 238,000 cases of its Hot Pockets because they may have contained meat from a massive recall of about 8.7 million pounds (3,900,000 kilograms) of meat from "diseased and unsound" animals. Nestle stated that "a small quantity of meat" from
6264-428: The device's approximate IEC 60705 output power rating, in watts (typically either: 600W, 700W, 800W, 900W, 1000W), and a voluntary Heating Category (A-E). A microwave oven heats food by passing microwave radiation through it. Microwaves are a form of non-ionizing electromagnetic radiation with a frequency in the so-called microwave region (300 MHz to 300 GHz). Microwave ovens use frequencies in one of
6372-572: The dielectric loss produced in them when they are subjected to a high voltage, high frequency field. However, lower-frequency dielectric heating , as described in the aforementioned patent, is (like induction heating ) an electromagnetic heating effect, the result of the so-called near-field effects that exist in an electromagnetic cavity that is small compared with the wavelength of the electromagnetic field. This patent proposed radio frequency heating, at 10 to 20 megahertz (wavelength 30 to 15 meters, respectively). Heating from microwaves that have
6480-573: The dipole moments of their hydroxyl groups or ester groups . Microwave heating is less efficient on fats and sugars than on water because they have a smaller molecular dipole moment . Although fats and sugar typically absorb energy less efficiently than water, paradoxically their temperatures rise faster and higher than water when cooking: Fats and oils require less energy delivered per gram of material to raise their temperature by 1 °C than does water (they have lower specific heat capacity ) and they begin cooling off by "boiling" only after reaching
6588-508: 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. Microwaves were first generated in the 1890s in some of the earliest radio wave experiments by physicists who thought of them as a form of "invisible light". James Clerk Maxwell in his 1873 theory of electromagnetism , now called Maxwell's equations , had predicted that
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#17331059799956696-512: The early 1950s, frequency-division multiplexing was used to send up to 5,400 telephone channels on each microwave radio channel, with as many as ten radio channels combined into one antenna for the hop to the next site, up to 70 km away. Wireless LAN protocols , such as Bluetooth and the IEEE 802.11 specifications used for Wi-Fi, also use microwaves in the 2.4 GHz ISM band , although 802.11a uses ISM band and U-NII frequencies in
6804-819: The earth's surface as ground waves , or reflect from the ionosphere , so terrestrial microwave communication links are limited by the visual horizon to about 40 miles (64 km). At the high end of the band, they are absorbed by gases in the atmosphere, limiting practical communication distances to around a kilometer. Microwaves are widely used in modern technology, for example in point-to-point communication links, wireless networks , microwave radio relay networks, radar , satellite and spacecraft communication , medical diathermy and cancer treatment, remote sensing , radio astronomy , particle accelerators , spectroscopy , industrial heating, collision avoidance systems , garage door openers and keyless entry systems , and for cooking food in microwave ovens . Microwaves occupy
6912-528: The entire super high frequency (SHF) band (3 to 30 GHz, or 10 to 1 cm) at minimum. A broader definition includes UHF and extremely high frequency (EHF) ( millimeter wave ; 30 to 300 GHz) bands as well. Frequencies in the microwave range are often referred to by their IEEE radar band designations: S , C , X , K u , K , or K a band , or by similar NATO or EU designations. Microwaves travel by line-of-sight ; unlike lower frequency radio waves , they do not diffract around hills, follow
7020-521: The first microwave oven with a turntable between 1964 and 1966. The countertop microwave oven was introduced in 1967 by the Amana Corporation . After microwave ovens became affordable for residential use in the late 1970s, their use spread into commercial and residential kitchens around the world, and prices fell rapidly during the 1980s. In addition to cooking food, microwave ovens are used for heating in many industrial processes. Microwave ovens are
7128-448: The first microwave oven with a turntable, an alternative means to promote more even heating of food. In 1965, Raytheon, looking to expand their Radarange technology into the home market, acquired Amana to provide more manufacturing capability. In 1967, they introduced the first popular home model, the countertop Radarange, at a price of US$ 495 ($ 5,000 in 2023 dollars). Unlike the Sharp models,
7236-454: The food surface. Penetration depth of microwaves depends on food composition and the frequency, with lower microwave frequencies (longer wavelengths) penetrating deeper. In use, microwave ovens can be as low as 50% efficient at converting electricity into microwaves, but energy efficient models can exceed 64% efficiency. Stovetop cooking is 40-90% efficient depending on the type of appliance used. Because they are used fairly infrequently,
7344-580: The food to vibrate and produce thermal energy in a process known as dielectric heating . Microwave ovens heat foods quickly and efficiently because excitation is fairly uniform in the outer 25–38 mm (1–1.5 inches) of a homogeneous , high-water-content food item. The development of the cavity magnetron in the United Kingdom made possible the production of electromagnetic waves of a small enough wavelength ( microwaves ) to efficiently heat up water molecules. American electrical engineer Percy Spencer
7452-443: The form of a convection microwave oven. The exploitation of high-frequency radio waves for heating substances was made possible by the development of vacuum tube radio transmitters around 1920. By 1930 the application of short waves to heat human tissue had developed into the medical therapy of diathermy . At the 1933 Chicago World's Fair , Westinghouse demonstrated the cooking of foods between two metal plates attached to
7560-414: The frequency can then be calculated. 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. Slotted lines are primarily intended for measurement of the voltage standing wave ratio on the line. However, provided
7668-406: The frequency of the microwave oven. 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
7776-854: The galley of the nuclear-powered passenger/cargo ship NS Savannah . An early commercial model introduced in 1954 consumed 1.6 kilowatts and sold for US$ 2,000 to US$ 3,000 ($ 23,000 to $ 34,000 in 2023 dollars). Raytheon licensed its technology to the Tappan Stove company of Mansfield, Ohio in 1952. Under contract to Whirlpool, Westinghouse, and other major appliance manufacturers looking to add matching microwave ovens to their conventional oven line, Tappan produced several variations of their built-in model from roughly 1955 to 1960. Due to maintenance (some units were water-cooled), in-built requirement, and cost—US$ 1,295 ($ 15,000 in 2023 dollars)—sales were limited. Japan's Sharp Corporation began manufacturing microwave ovens in 1961. Between 1964 and 1966, Sharp introduced
7884-458: The gas into a plasma and heat it to very high temperatures. The frequency is tuned to the cyclotron resonance of the electrons in the magnetic field, anywhere between 2–200 GHz, hence it is often referred to as Electron Cyclotron Resonance Heating (ECRH). The upcoming ITER thermonuclear reactor will use up to 20 MW of 170 GHz microwaves. Microwaves can be used to transmit power over long distances, and post- World War 2 research
7992-456: 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 . Apparatus and techniques may be described qualitatively as "microwave" when the wavelengths of signals are roughly
8100-495: The influence of a constantly changing electric field, usually in the microwave frequencies range, and a higher wattage power of the microwave oven results in faster cooking times. Typically, consumer ovens work around a nominal 2.45 gigahertz (GHz) – a wavelength of 12.2 centimetres (4.80 in) in the 2.4 GHz to 2.5 GHz ISM band – while large industrial / commercial ovens often use 915 megahertz (MHz) – 32.8 centimetres (12.9 in). Among other differences,
8208-416: The inside out", meaning from the center of the entire mass of food outwards. This idea arises from heating behavior seen if an absorbent layer of water lies beneath a less absorbent drier layer at the surface of a food; in this case, the deposition of heat energy inside a food can exceed that on its surface. This can also occur if the inner layer has a lower heat capacity than the outer layer causing it to reach
8316-405: The longer wavelength of a commercial microwave oven allows the initial heating effects to begin deeper within the food or liquid, and therefore become evenly spread within its bulk sooner, as well as raising the temperature deep within the food more quickly. A microwave oven takes advantage of the electric dipole structure of water molecules , fats, and many other substances in the food, using
8424-572: The low-microwave/high-UHF frequencies around 1.8 and 1.9 GHz in the Americas and elsewhere, respectively. DVB-SH and S-DMB use 1.452 to 1.492 GHz, while proprietary/incompatible satellite radio in the U.S. uses around 2.3 GHz for DARS . Microwave radio is used in point-to-point telecommunications 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). There
8532-759: 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
8640-443: The mass production of the cavity magnetron. In 1945, the heating effect of a high-power microwave beam was independently and accidentally discovered by Percy Spencer , an American self-taught engineer from Howland, Maine . Employed by Raytheon at the time, he noticed that microwaves from an active radar set he was working on started to melt a Mr. Goodbar candy bar he had in his pocket. The first food deliberately cooked by Spencer
8748-528: The microwave radiation; instead, the magnetron is cycled on and off every few seconds, thus altering the large scale duty cycle . Newer models use inverter power supplies that use pulse-width modulation to provide effectively continuous heating at reduced power settings, so that foods are heated more evenly at a given power level and can be heated more quickly without being damaged by uneven heating. The microwave frequencies used in microwave ovens are chosen based on regulatory and cost constraints. The first
8856-572: The microwaves from reaching the food. Cookware made of materials with high electrical permittivity will absorb microwaves, resulting in the cookware heating rather than the food. Cookware made of melamine resin is a common type of cookware that will heat in a microwave oven, reducing the effectiveness of the microwave oven and creating a hazard from burns or shattered cookware. Microwave heating can cause localized thermal runaways in some materials with low thermal conductivity which also have dielectric constants that increase with temperature. An example
8964-452: The movement of molecules is more restricted. Defrosting is done at a low power setting, allowing time for conduction to carry heat to still frozen parts of food. Dielectric heating of liquid water is also temperature-dependent: At 0 °C, dielectric loss is greatest at a field frequency of about 10 GHz, and for higher water temperatures at higher field frequencies. Sugars and triglycerides (fats and oils) absorb microwaves due to
9072-599: The number of households with a microwave oven grew from almost 24% in 2002 to almost 40% in 2008. Almost twice as many households in South Africa owned microwave ovens in 2008 (38.7%) as in 2002 (19.8%). Microwave oven ownership in Vietnam in 2008 was at 16% of households, versus 30% ownership of refrigerators; this rate was up significantly from 6.7% microwave oven ownership in 2002, with 14% ownership for refrigerators that year. Consumer household microwave ovens usually come with
9180-515: The origin of the Universe . Microwave technology is extensively used for point-to-point telecommunications (i.e., non-broadcast uses). 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
9288-444: The oven is used to heat frying-oil and other oily items (such as bacon), which attain far higher temperatures than that of boiling water. Microwave ovens have a limited role in professional cooking, because the boiling-range temperatures of a microwave oven do not produce the flavorful chemical reactions that frying, browning, or baking at a higher temperature produces. However, such high heat sources can be added to microwave ovens in
9396-436: The previous year), according to Euromonitor International data. Paul Grimwood took over Nestlé SA's struggling U.S operations in 2011. In an attempt to bolster the failing brand by improving supply chain, Grimwood made the decision to drop the calzone version of Hot Pockets and the quesadillas Lean Pockets, reducing the number of doughs needed. Nestlé executive Chris Johnson points to an end of extended SNAP benefits in 2013 as
9504-425: 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 do not interfere with nearby equipment using
9612-427: The product will cause serious, adverse health consequences or death." Affected products include Premium Pepperoni Made With Pork , and Chicken & Beef Pizza with a Garlic Buttery Crust . Microwave oven A microwave oven or simply microwave is an electric oven that heats and cooks food by exposing it to electromagnetic radiation in the microwave frequency range. This induces polar molecules in
9720-406: The rim of the door, choke connections on the door edges act like metal-to-metal contact, at the frequency of the microwaves, to prevent leakage. The oven door usually has a window for easy viewing, with a layer of conductive mesh some distance from the outer panel to maintain the shielding. Because the size of the perforations in the mesh is much less than the microwaves' wavelength (12.2 cm for
9828-639: The same as the dimensions of the circuit, so that lumped-element circuit theory is inaccurate, and instead distributed circuit elements and transmission-line theory are more useful methods for design and analysis. 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 . In turn, at even higher frequencies, where
9936-406: 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 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 ,
10044-898: The sky, a small amount of the power will be randomly scattered as the beam passes through the troposphere . A sensitive receiver beyond the horizon with a high gain antenna focused on that area of the troposphere can pick up the signal. This technique has been used at frequencies between 0.45 and 5 GHz in tropospheric scatter (troposcatter) communication systems to communicate beyond the horizon, at distances up to 300 km. 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
10152-631: The solar system, to determine the distance to the Moon or map the invisible surface of Venus through cloud cover. A recently completed microwave radio telescope is the Atacama Large Millimeter Array , located at more than 5,000 meters (16,597 ft) altitude in Chile, which observes the universe in the millimeter and submillimeter wavelength ranges. The world's largest ground-based astronomy project to date, it consists of more than 66 dishes and
10260-433: The surface of the Earth, microwave communication links are limited by the visual horizon to about 30–40 miles (48–64 km). Microwaves are absorbed by moisture in the atmosphere, and the attenuation increases with frequency, becoming a significant factor ( rain fade ) at the high end of the band. Beginning at about 40 GHz, atmospheric gases also begin to absorb microwaves, so above this frequency microwave transmission
10368-434: The technology. Both Tappan and General Electric offered units that appeared to be conventional stove top/oven ranges, but included microwave capability in the conventional oven cavity. Such ranges were attractive to consumers since both microwave energy and conventional heating elements could be used simultaneously to speed cooking, and there was no loss of countertop space. The proposition was also attractive to manufacturers as
10476-464: The temperature of objects or terrain. The sun and other astronomical radio sources such as Cassiopeia A emit low level microwave radiation which carries information about their makeup, which is studied by radio astronomers using receivers called radio telescopes . The cosmic microwave background radiation (CMBR), for example, is a weak microwave noise filling empty space which is a major source of information on cosmology 's Big Bang theory of
10584-784: The traditional Hot Pocket, including breakfast, lunch and dinner varieties. Nestlé also offers Pretzel Bread Hot Pockets, Hot Pockets Croissant Crust (formerly called Croissant Pockets), Hot Pockets Breakfast items, Hot Pockets Breakfast/Snack Bites and Hot Pockets Side shots. Nestlé formerly produced Lean Pockets, Hot Pie Express, Hot Pocket Pizza Minis (originally called Hot Pockets Pizza Snacks), Hot Pockets Subs, Hot Pockets Calzones, Hot Pockets Panini and Hot Pockets Breakfast fruit pastries. Hot Pockets are viewed as "an after school staple". Individual Hot Pockets contain about 350 calories. Citing reduced sales, Nestlé announced in 2011 that it would cut employee numbers at its California factory. U.S. sales were about $ 610 million in 2010 (down $ 44 million from
10692-406: The usual 2.45 GHz), microwave radiation cannot pass through the door, while visible light (with its much shorter wavelength) can. Modern microwave ovens use either an analog dial-type timer or a digital control panel for operation. Control panels feature an LED , LCD or vacuum fluorescent display, buttons for entering the cook time and a power level selection feature. A defrost option
10800-444: The very high cost of power generation at these frequencies. The third, centered on 433.92 MHz, is a narrow band that would require expensive equipment to generate sufficient power without creating interference outside the band, and is only available in some countries. The cooking chamber is similar to a Faraday cage to prevent the waves from coming out of the oven. Even though there is no continuous metal-to-metal contact around
10908-410: The wavelength of the electromagnetic waves becomes small in comparison to the size of the structures used to process them, microwave techniques become inadequate, and the methods of optics are used. 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
11016-518: Was also unique. This resulted in an oven that could survive a no-load condition: an empty microwave oven where there is nothing to absorb the microwaves. The new oven was shown at a trade show in Chicago, and helped begin a rapid growth of the market for home microwave ovens. Sales volume of 40,000 units for the U.S. industry in 1970 grew to one million by 1975. Market penetration was even faster in Japan, due to
11124-416: 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 1964 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
11232-443: 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. 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. A two-second burst of
11340-409: Was not performed, and there were no illnesses reported in connection to this recall. Customers who bought the recalled products were refunded by contacting Nestle Consumer Service. In January 2021, some batches of Hot Pockets produced during November 13–16, 2020, were found to contain pieces of glass or hard plastic, posing a choking and laceration risk. The USDA issued a Class I recall, because "use of
11448-493: Was observed that individuals in the radiation path of radar installations experienced clicks and buzzing sounds in response to microwave radiation. Research by NASA in the 1970s has shown this to be caused by thermal expansion in parts of the inner ear. In 1955, Dr. James Lovelock was able to reanimate rats chilled to 0 and 1 °C (32 and 34 °F) using microwave diathermy. When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in
11556-407: Was popcorn, and the second was an egg, which exploded in the face of one of the experimenters. To verify his finding, Spencer created a high-density electromagnetic field by feeding microwave power from a magnetron into a metal box from which it had no way to escape. When food was placed in the box with the microwave energy, the temperature of the food rose rapidly. On 8 October 1945, Raytheon filed
11664-412: Was the first to offer a microprocessor controlled digital control panel in 1975 with their RR-6 model. The late 1970s saw an explosion of low-cost countertop models from many major manufacturers. Formerly found only in large industrial applications, microwave ovens increasingly became a standard fixture of residential kitchens in developed countries . By 1986, roughly 25% of households in the U.S. owned
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