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131-432: 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 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

262-415: A fuel , such as wood , coal , or natural gas , while many employ electricity . Microwave ovens heat materials by exposing them to microwave radiation , while electric ovens and electric furnaces heat materials using resistive heating . Some ovens use forced convection , the movement of gases inside the heating chamber, to enhance the heating process, or, in some cases, to change the properties of

393-587: A hearth used for heating and cooking. Ovens were used by cultures who lived in the Indus Valley and in pre-dynastic Egypt. By 3200 BC, each mud-brick house had an oven in settlements across the Indus Valley. Ovens were used to cook food and to make bricks. Pre-dynastic civilizations in Egypt used kilns around 5000–4000 BC to make pottery. Tandır ovens used to bake unleavened flatbread were common in Anatolia during

524-530: 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 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

655-411: 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

786-655: 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

917-630: A cooking tool was discovered by Percy Spencer in 1946, and with help from engineers, the microwave oven was patented. The microwave oven uses microwave radiation to excite water molecules in food, causing friction and thus producing heat. Some ovens can perform in multiple ways, sometimes at once. Combination ovens may be able to microwave and conventional heating such as baking or grilling simultaneously. Ovens are used as kitchen appliances for roasting and heating. Foods normally cooked in this manner include meat , casseroles and baked goods such as bread , cake and other desserts . In modern times,

1048-401: A desired temperature . Ovens are also used in the manufacturing of ceramics and pottery ; these ovens are sometimes referred to as kilns . Metallurgical furnaces are ovens used in the manufacturing of metals , while glass furnaces are ovens used to produce glass . There are many methods by which different types of ovens produce heat. Some ovens heat materials using the combustion of

1179-454: A few wavelengths of the antenna. More-distant near-field effects also involve energy transfer effects that couple directly to receivers near the antenna, affecting the power output of the transmitter if they do couple, but not otherwise. In a sense, the near field offers energy that is available to a receiver only if the energy is tapped, and this is sensed by the transmitter by means of responding to electromagnetic near fields emanating from

1310-478: A function of wavelength (or distance). However, these boundary regions are a fraction of one wavelength within the near field. The outer boundary of the reactive near-field region is commonly considered to be a distance of 1 2 π {\textstyle {\frac {1}{2\pi }}} times the wavelength (i.e., λ 2 π {\textstyle {\frac {\lambda }{2\pi }}} or approximately 0.159λ ) from

1441-401: A given current distribution radiating into space as is typical of microwave or optical devices . The actual values of the fields in space about the antenna are usually quite complex and can vary with distance from the antenna in various ways. However, in many practical applications, one is interested only in effects where the distance from the antenna to the observer is very much greater than

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1572-566: 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

1703-421: A higher temperature produces. However, such high heat sources can be added to microwave ovens in 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

1834-436: 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

1965-580: 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

2096-495: A hollow chamber and provide a means of heating the chamber in a controlled way. In use since antiquity , they have been used to accomplish a wide variety of tasks requiring controlled heating. Because they are used for a variety of purposes, there are many different types of ovens . These types differ depending on their intended purpose and based upon how they generate heat. Ovens are often used for cooking , usually baking , sometimes broiling ; they can be used to heat food to

2227-515: 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

2358-409: 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

2489-451: 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,

2620-452: 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

2751-521: 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

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2882-496: 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

3013-424: 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

3144-448: A relatively uniform wave pattern. The radiation zone is important because far fields in general fall off in amplitude by   1 r   . {\displaystyle \ {\tfrac {1}{r}}\ .} This means that the total energy per unit area at a distance r is proportional to   1 r 2   . {\displaystyle \ {\tfrac {1}{r^{2}}}\ .} The area of

3275-449: A wide variety of operating modes and special features including the use of a temperature probe to automatically shut the oven off when the food is completely cooked to the desired degree. Toaster ovens are essentially small-scale ovens and can be used to cook foods other than just toasting. A frontal door is opened, horizontally-oriented bread slices (or other food items) are placed on a rack that has heat elements above and below it, and

3406-417: 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 a wavelength that is small relative to

3537-462: Is being absorbed in the closest near-field zone (by a second antenna or some other object) and is forced to supply extra power to its antenna, and to draw extra power from its own power supply, whereas if no power is being absorbed there, the transmitter does not have to supply extra power. The near field itself is further divided into the reactive near field and the radiative near field. The reactive and radiative near-field designations are also

3668-401: Is contentious. The interaction with the medium can fail to return energy back to the source, but cause a distortion in the electromagnetic wave that deviates significantly from that found in free space, and this indicates the radiative near-field region, which is somewhat further away. Passive reflecting elements can be placed in this zone for the purpose of beam forming, such as the case with

3799-459: Is far enough from the antenna that back-coupling of the fields becomes out of phase with the antenna signal, and thus cannot efficiently return inductive or capacitive energy from antenna currents or charges. The energy in the radiative near field is thus all radiant energy , although its mixture of magnetic and electric components are still different from the far field. Further out into the radiative near field (one half wavelength to 1 wavelength from

3930-428: 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

4061-419: Is less useful (too many terms are required for an accurate description of the fields). Rather, in the near field, it is sometimes useful to express the contributions as a sum of radiating fields combined with evanescent fields , where the latter are exponentially decaying with r . And in the source itself, or as soon as one enters a region of inhomogeneous materials, the multipole expansion is no longer valid and

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4192-408: 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

4323-413: Is seen on the transmitter, resulting from the reactive near-field energy that is not returned. This effect shows up as a different impedance in the antenna, as seen by the transmitter. The reactive component of the near field can give ambiguous or undetermined results when attempting measurements in this region. In other regions, the power density is inversely proportional to the square of the distance from

4454-399: 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

4585-587: Is sometimes referred to as the Fraunhofer region . Other synonyms are far field , far zone , and radiation field . Any electromagnetic radiation consists of an electric field component E and a magnetic field component H . In the far field, the relationship between the electric field component E and the magnetic component H is that characteristic of any freely propagating wave, where E and H have equal magnitudes at any point in space (where measured in units where c = 1). In contrast to

4716-547: Is still central to rural food culture in this part of the world, reflected by the local folklore, in which a young man and woman sharing fresh tandır bread is a symbol of young love. However, the culture of traditional bread baking is changing with younger generations, especially with those who reside in towns and prefer modern conveniences. During the Middle Ages, instead of earth and ceramic ovens, Europeans used fireplaces in conjunction with large cauldrons. These were similar to

4847-540: 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

4978-565: Is the far-field distance. The far-field distance is the distance from the transmitting antenna to the beginning of the Fraunhofer region, or far field. The transition zone between these near and far field regions, extending over the distance from one to two wavelengths from the antenna, is the intermediate region in which both near-field and far-field effects are important. In this region, near-field behavior dies out and ceases to be important, leaving far-field effects as dominant interactions. (See

5109-458: Is thus activated, it then creates its own near-field regions, but the same conditions apply to them. The near field is remarkable for reproducing classical electromagnetic induction and electric charge effects on the EM field, which effects "die-out" with increasing distance from the antenna: The magnetic field component that’s in phase quadrature to electric fields is proportional to the inverse-cube of

5240-411: 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. Oven An oven is a tool that is used to expose materials to a hot environment. Ovens contain

5371-463: Is where the electric and magnetic parts of the radiated waves first balance out: The electric field of a linear antenna gains its corresponding magnetic field, and the magnetic field of a loop antenna gains its electric field. It can either be considered the furthest part of the near field, or the nearest part of the far field. It is from beyond this point that the electromagnetic wave becomes self-propagating. The electric and magnetic field portions of

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5502-454: The power intensity of electromagnetic radiation in the transmitted signal. By contrast, the near-field ' s E and B strengths decrease more rapidly with distance: The radiative field decreases by the inverse-distance squared , the reactive field by an inverse- cube law, resulting in a diminished power in the parts of the electric field by an inverse fourth-power and sixth-power, respectively. The rapid drop in power contained in

5633-437: The 1933 Chicago World's Fair , Westinghouse demonstrated the cooking of foods between two metal plates attached to 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

5764-525: The AGA cooker ) may not have any controls at all; the ovens simply run continuously at various temperatures. More conventional ovens have a simple thermostat which turns the oven on and off and selects the temperature at which it will operate. Set to the highest setting, this may also enable the broiler element. A timer may allow the oven to be turned on and off automatically at pre-set times. More sophisticated ovens may have complex, computer -based controls allowing

5895-517: The Dutch oven . After the Middle Ages, ovens underwent many changes over time from wood, iron, coal, gas, and even electric. Each design had its own motivation and purpose. The wood-burning stoves saw improvement through the addition of fire chambers that allowed better containment and release of smoke. Another recognizable oven would be the cast-iron stove. These were first used around the early 1700s, when they themselves underwent several variations including

6026-446: 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

6157-613: 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

6288-450: The Yagi–Uda antenna . Alternatively, multiple active elements can also be combined to form an antenna array, with lobe shape becoming a factor of element distances and excitation phasing. Another intermediate region, called the transition zone , is defined on a somewhat different basis, namely antenna geometry and excitation wavelength. It is approximately one wavelength from the antenna, and

6419-521: The phase relationship between E and H as well as the angle between the two vectors must also be known in every point of space. In this reactive region, not only is an electromagnetic wave being radiated outward into far space but there is a reactive component to the electromagnetic field, meaning that the strength, direction, and phase of the electric and magnetic fields around the antenna are sensitive to EM absorption and re-emission in this region, and respond to it. In contrast, absorption far from

6550-404: The regions attempt to characterize locations where the activity of the associated field components are the strongest. Mathematically, the distinction between field components is very clear, but the demarcation of the spatial field regions is subjective. All of the field components overlap everywhere, so for example, there are always substantial far-field and radiative near-field components in

6681-410: The square of distance , and absorption of the radiation does not feed back to the transmitter. In the far-field region, each of the electric and magnetic parts of the EM field is "produced by" (or associated with) a change in the other part, and the ratio of electric and magnetic field intensities is simply the wave impedance in the medium. Also known as the radiation-zone , the far field carries

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6812-455: The "Far Field" image above.) As far as acoustic wave sources are concerned, if the source has a maximum overall dimension or aperture width ( D ) that is large compared to the wavelength λ , the far-field region is commonly taken to exist at distances, when the Fresnel parameter S {\displaystyle S} is larger than 1: For a beam focused at infinity, the far-field region

6943-417: 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

7074-645: 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

7205-664: The Seljuk and Ottoman eras and have been found at archaeological sites distributed across the Middle East. The word tandır comes from the Akkadian tinuru , which becomes tanur in Hebrew and Arabic, and tandır in Turkish. Of the hundreds of bread varieties known from cuneiform sources, unleavened tinuru bread was made by adhering bread to the side walls of a heated cylindrical oven. This type of bread

7336-484: The Stewart Oberlin iron stove that was smaller and had its own chimney. In the early part of the 19th century, the coal oven was developed. It was cylindrical in shape and made of heavy cast iron. The gas oven saw its first use as early as the beginning of the 19th century as well. Gas stoves became very common household ovens once gas lines were available to most houses and neighborhoods. James Sharp patented one of

7467-755: 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

7598-516: 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

7729-533: The amplitude of different terms of the electric and magnetic field equations diminish as distance from the radiating element increases. The amplitudes of the far-field components fall off as 1 / r {\displaystyle 1/r} , the radiative near-field amplitudes fall off as 1 / r 2 {\displaystyle 1/r^{2}} , and the reactive near-field amplitudes fall off as 1 / r 3 {\displaystyle 1/r^{3}} . Definitions of

7860-447: The antenna conductors, or inside any polarizable media surrounding it, where the generation and emission of electromagnetic waves can be interfered with while the field lines remain electrically attached to the antenna, hence absorption of radiation in the near field by adjacent conducting objects detectably affects the loading on the signal generator (the transmitter). The electric and magnetic fields can exist independently of each other in

7991-419: The antenna current distributions and the observed far-field patterns. While far-field simplifications are very useful in engineering calculations, this does not mean the near-field functions cannot be calculated, especially using modern computer techniques. An examination of how the near fields form about an antenna structure can give great insight into the operations of such devices. The electromagnetic field in

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8122-399: The antenna has negligible effect on the fields near the antenna, and causes no back-reaction in the transmitter. Very close to the antenna, in the reactive region, energy of a certain amount, if not absorbed by a receiver, is held back and is stored very near the antenna surface. This energy is carried back and forth from the antenna to the reactive near field by electromagnetic radiation of

8253-445: The antenna in a regenerative way, so that it is not lost. A similar process happens as electric charge builds up in one section of the antenna under the pressure of the signal voltage, and causes a local electric field around that section of antenna, due to the antenna's self-capacitance . When the signal reverses so that charge is allowed to flow away from this region again, the built-up electric field assists in pushing electrons back in

8384-469: The antenna surface. The reactive near-field is also called the inductive near-field. The radiative near field (also called the Fresnel region ) covers the remainder of the near-field region, from λ 2 π {\textstyle {\frac {\lambda }{2\pi }}} out to the Fraunhofer distance. In the reactive near field (very close to the antenna), the relationship between

8515-462: The antenna, D , is not important, and the approximation is the same for all shorter antennas (sometimes idealized as so-called point antennas ). In all such antennas, the short length means that charges and currents in each sub-section of the antenna are the same at any given time, since the antenna is too short for the RF transmitter voltage to reverse before its effects on charges and currents are felt over

8646-415: The antenna. In the vicinity very close to the antenna, however, the energy level can rise dramatically with only a small decrease in distance toward the antenna. This energy can adversely affect both humans and measurement equipment because of the high powers involved. The radiative near field (sometimes called the Fresnel region ) does not contain reactive field components from the source antenna, since it

8777-420: The antenna. Thus, the near fields only transfer energy to very nearby receivers, and, when they do, the result is felt as an extra power draw in the transmitter. As an example of such an effect, power is transferred across space in a common transformer or metal detector by means of near-field phenomena (in this case inductive coupling ), in a strictly short-range effect (i.e., the range within one wavelength of

8908-459: The automatic-toasting settings, toaster ovens typically have settings and temperature controls to allow use of the appliance as a small oven. Extra features on toaster ovens can include: Outside the culinary world, ovens are used for a number of purposes: Near and far field Far-field E (electric) and B (magnetic) radiation field strengths decrease as the distance from the source increases, resulting in an inverse-square law for

9039-439: 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

9170-414: 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

9301-400: The center of the radiating part of the antenna, with the clear understanding that the values chosen are only approximate and will be somewhat inappropriate for different antennas in different surroundings. The choice of the cut-off numbers is based on the relative strengths of the field component amplitudes typically seen in ordinary practice. For antennas shorter than half of the wavelength of

9432-497: The closest-in near-field reactive region. The regions defined below categorize field behaviors that are variable, even within the region of interest. Thus, the boundaries for these regions are approximate rules of thumb , as there are no precise cutoffs between them: All behavioral changes with distance are smooth changes. Even when precise boundaries can be defined in some cases, based primarily on antenna type and antenna size, experts may differ in their use of nomenclature to describe

9563-406: The complicated effects in the near field can be conveniently ignored. The interaction with the medium (e.g. body capacitance) can cause energy to deflect back to the source feeding the antenna, as occurs in the reactive near field. This zone is roughly within ⁠ 1 / 6 ⁠ of a wavelength of the nearest antenna surface. The near field has been of increasing interest, particularly in

9694-604: The development of capacitive sensing technologies such as those used in the touchscreens of smart phones and tablet computers. Although the far field is the usual region of antenna function, certain devices that are called antennas but are specialized for near-field communication do exist. Magnetic induction as seen in a transformer can be seen as a very simple example of this type of near-field electromagnetic interaction. For example send / receive coils for RFID , and emission coils for wireless charging and inductive heating ; however their technical classification as "antennas"

9825-427: 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

9956-572: 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

10087-525: The distance ( 1 / r 3 {\displaystyle 1/r^{3}} ) and electric field strength proportional to inverse-square of distance ( 1 / r 2 {\displaystyle 1/r^{2}} ). This fall-off is far more rapid than the classical radiated far-field ( E and B fields, which are proportional to the simple inverse-distance ( 1 / r {\displaystyle 1/r} ). Typically near-field effects are not important farther away than

10218-548: The door is closed. The controls are set and actuated to toast the bread to the desired doneness, whereupon the heat elements are switched off. In most cases, the door must be opened manually, though there are also toaster ovens with doors that open automatically. Because the bread is horizontal, a toaster oven can be used to cook toast with toppings, like garlic bread , melt sandwiches , or toasted cheese . Toaster ovens are generally slower to make toast than pop-up toasters, taking 4–6 minutes as compared to 2–3 minutes. In addition to

10349-407: The electromagnetic field close to the antenna may be quite powerful, but, because of more rapid fall-off with distance than 1 / r {\displaystyle 1/r} behavior, they do not radiate energy to infinite distances. Instead, their energies remain trapped in the region near the antenna, not drawing power from the transmitter unless they excite a receiver in the area close to

10480-507: The energy levels still vary with distance and time. Such an angular energy distribution is usually termed an antenna pattern . Note that, by the principle of reciprocity , the pattern observed when a particular antenna is transmitting is identical to the pattern measured when the same antenna is used for reception. Typically one finds simple relations describing the antenna far-field patterns, often involving trigonometric functions or at worst Fourier or Hankel transform relationships between

10611-451: The entire antenna length. For antennas physically larger than a half-wavelength of the radiation they emit, the near and far fields are defined in terms of the Fraunhofer distance . Named after Joseph von Fraunhofer , the following formula gives the Fraunhofer distance : where D is the largest dimension of the radiator (or the diameter of the antenna ) and λ is the wavelength of

10742-439: The excitation "signal" voltage (a transmitter or other EM exciting potential). This generates an oscillating (or reversing) electrical dipole, which affects both the near field and the far field. The boundary between the near field and far field regions is only vaguely defined, and it depends on the dominant wavelength ( λ ) emitted by the source and the size of the radiating element. The near field refers to places nearby

10873-455: The far field, the diffraction pattern in the near field typically differs significantly from that observed at infinity and varies with distance from the source. In the near field, the relationship between E and H becomes very complex. Also, unlike the far field where electromagnetic waves are usually characterized by a single polarization type (horizontal, vertical, circular, or elliptical), all four polarization types can be present in

11004-419: The field, and returned to the antenna in every half-cycle, through self-induction. For even smaller r , terms proportional to 1 / r 3 {\displaystyle 1/r^{3}} become significant; this is sometimes called the electrostatic field term and can be thought of as stemming from the electrical charge in the antenna element. Very close to the source, the multipole expansion

11135-488: The first gas stoves in 1826. Other improvements to the gas stove included the AGA cooker , invented in 1922 by Gustaf Dalén . The first electric ovens were invented in the very late 19th century; however, like many electrical inventions destined for commercial use, mass ownership of electrical ovens could not be a reality until a better and more efficient use of electricity was available. Over time, ovens have become more high-tech in terms of cooking strategy. The microwave as

11266-447: 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,

11397-450: 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,

11528-441: The full solution of Maxwell's equations is generally required. If an oscillating electrical current is applied to a conductive structure of some type, electric and magnetic fields will appear in space about that structure. If those fields are lost to a propagating space wave the structure is often termed an antenna. Such an antenna can be an assemblage of conductors in space typical of radio devices or it can be an aperture with

11659-852: 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

11790-494: 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,

11921-416: 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

12052-416: The largest dimension of the transmitting antenna. The equations describing the fields created about the antenna can be simplified by assuming a large separation and dropping all terms that provide only minor contributions to the final field. These simplified distributions have been termed the "far field" and usually have the property that the angular distribution of energy does not change with distance, although

12183-603: 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 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

12314-404: 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

12445-443: 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

12576-600: The material being heated, such as in the Bessemer method of steel production. The earliest ovens were found in Central Europe and date back to 29,000 BC. They were roasting and boiling pits inside yurts used to cook mammoth . In Ukraine from 20,000 BC they used pits with hot coals covered in ashes. Food was wrapped in leaves and set on top, then covered with earth. In camps found in Mezhirich , each mammoth bone house had

12707-524: 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

12838-572: 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

12969-451: 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

13100-460: The near field, and one type of field can be disproportionately larger than the other, in different subregions. The near field is governed by multipole type fields , which can be considered as collections of dipoles with a fixed phase relationship . The general purpose of conventional antennas is to communicate wirelessly over long distances, well into their far fields, and for calculations of radiation and reception for many simple antennas, most of

13231-467: The near field. The near field is a region in which there are strong inductive and capacitive effects from the currents and charges in the antenna that cause electromagnetic components that do not behave like far-field radiation. These effects decrease in power far more quickly with distance than do the far-field radiation effects. Non-propagating (or evanescent) fields extinguish very rapidly with distance, which makes their effects almost exclusively felt in

13362-412: The near-field effects, especially that of focused antennas. Conversely, when a given antenna emits high frequency radiation, it will have a near-field region larger than what would be implied by a lower frequency (i.e. longer wavelength). Additionally, a far-field region distance d F must satisfy these two conditions. where D is the largest physical linear dimension of the antenna and d F

13493-432: The near-field ensures that effects due to the near-field essentially vanish a few wavelengths away from the radiating part of the antenna, and conversely ensure that at distances a small fraction of a wavelength from the antenna, the near-field effects overwhelm the radiating far-field. In a normally-operating antenna, positive and negative charges have no way of leaving the metal surface, and are separated from each other by

13624-418: The near-field region. Also, in the part of the near field closest to the antenna (called the reactive near field , see below ), absorption of electromagnetic power in the region by a second device has effects that feed back to the transmitter, increasing the load on the transmitter that feeds the antenna by decreasing the antenna impedance that the transmitter "sees". Thus, the transmitter can sense when power

13755-435: The necessary temperature to produce Maillard reactions . Exceptions occur in cases where 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

13886-426: The new direction of their flow, as with the discharge of any unipolar capacitor. This again transfers energy back to the antenna current. Because of this energy storage and return effect, if either of the inductive or electrostatic effects in the reactive near field transfer any field energy to electrons in a different (nearby) conductor, then this energy is lost to the primary antenna. When this happens, an extra drain

14017-596: 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

14148-419: 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 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,

14279-460: The oven from below. This is commonly used for baking and roasting. The oven may also be able to heat from the top to provide broiling (US) or grilling (UK/Commonwealth). A fan-assisted oven that uses a small fan to circulate the air in the cooking chamber, can be used. Both are also known as convection ovens . An oven may also provide an integrated rotisserie . Ovens also vary in the way that they are controlled. The simplest ovens (for example,

14410-451: The oven is used to cook and heat food in many households around the globe. Modern ovens are typically fueled by either natural gas or electricity , with bottle gas models available but not common. When an oven is contained in a complete stove, the fuel used for the oven may be the same as or different from the fuel used for the burners on top of the stove. Ovens usually can use a variety of methods to cook. The most common may be to heat

14541-402: The radiated field above. As one gets closer and closer to the source (smaller r ), approaching the near field, other powers of r become significant. The next term that becomes significant is proportional to 1 / r 2 {\displaystyle 1/r^{2}} and is sometimes called the induction term . It can be thought of as the primarily magnetic energy stored in

14672-464: The radiation they emit (i.e., electromagnetically "short" antennas), the far and near regional boundaries are measured in terms of a simple ratio of the distance r from the radiating source to the wavelength λ of the radiation. For such an antenna, the near field is the region within a radius r ≪ λ , while the far-field is the region for which r ≫ 2 λ . The transition zone is the region between r = λ and r = 2 λ   . The length of

14803-403: The radiative near field, forming a new radiating surface to consider. Depending on antenna characteristics and frequencies, such coupling may be far more efficient than simple antenna reception in the yet-more-distant far field, so far more power may be transferred to the secondary "antenna" in this region than would be the case with a more distant antenna. When a secondary radiating antenna surface

14934-451: The radio wave . Either of the following two relations are equivalent, emphasizing the size of the region in terms of wavelengths λ or diameters D : This distance provides the limit between the near and far field. The parameter D corresponds to the physical length of an antenna, or the diameter of a reflector ("dish") antenna. Having an antenna electromagnetically longer than one-half the dominated wavelength emitted considerably extends

15065-419: The receiver. Again, this is the same principle that applies in induction coupled devices, such as a transformer , which draws more power at the primary circuit, if power is drawn from the secondary circuit. This is different with the far field, which constantly draws the same energy from the transmitter, whether it is immediately received, or not. The amplitude of other components (non-radiative/non-dipole) of

15196-498: The region where r is large enough for these fields to dominate is the far field. In general, the fields of a source in a homogeneous isotropic medium can be written as a multipole expansion . The terms in this expansion are spherical harmonics (which give the angular dependence) multiplied by spherical Bessel functions (which give the radial dependence). For large r , the spherical Bessel functions decay as 1 / r {\displaystyle 1/r} , giving

15327-463: The regions. Because of these nuances, special care must be taken when interpreting technical literature that discusses far-field and near-field regions. The term near-field region (also known as the near field or near zone ) has the following meanings with respect to different telecommunications technologies: The most convenient practice is to define the size of the regions or zones in terms of fixed numbers (fractions) of wavelengths distant from

15458-406: 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

15589-403: The signal). Solving Maxwell's equations for the electric and magnetic fields for a localized oscillating source, such as an antenna, surrounded by a homogeneous material (typically vacuum or air ), yields fields that, far away, decay in proportion to 1 / r {\displaystyle 1/r} where r is the distance from the source. These are the radiating fields, and

15720-401: The source), the E and H field relationship is more predictable, but the E to H relationship is still complex. However, since the radiative near field is still part of the near field, there is potential for unanticipated (or adverse) conditions. For example, metal objects such as steel beams can act as antennas by inductively receiving and then "re-radiating" some of the energy in

15851-405: The sphere is proportional to r 2 {\displaystyle r^{2}} , so the total energy passing through the sphere is constant. This means that the far-field energy actually escapes to infinite distance (it radiates ). The separation of the electric and magnetic fields into components is mathematical, rather than clearly physical, and is based on the relative rates at which

15982-420: The strengths of the E and H fields is often too complicated to easily predict, and difficult to measure. Either field component ( E or H ) may dominate at one point, and the opposite relationship dominate at a point only a short distance away. This makes finding the true power density in this region problematic. This is because to calculate power, not only E and H both have to be measured but

16113-434: 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

16244-433: The type that slowly changes electrostatic and magnetostatic effects. For example, current flowing in the antenna creates a purely magnetic component in the near field, which then collapses as the antenna current begins to reverse, causing transfer of the field's magnetic energy back to electrons in the antenna as the changing magnetic field causes a self-inductive effect on the antenna that generated it. This returns energy to

16375-405: 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

16506-442: 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

16637-531: 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 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

16768-447: The wave are proportional to each other at a ratio defined by the characteristic impedance of the medium through which the wave is propagating. In contrast, the far field is the region in which the field has settled into "normal" electromagnetic radiation . In this region, it is dominated by transverse electric or magnetic fields with electric dipole characteristics. In the far-field region of an antenna, radiated power decreases as

16899-517: 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

17030-405: 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

17161-412: 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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