microwave

(mī'krə-wāv', -krō-)

1. An electromagnetic wave with a wavelength between that of infrared and short waves (one millimeter to one meter).
2. Informal. A microwave oven.

View more Technology videos

For more information on microwave, visit Britannica.com.

Electromagnetic energy with wavelengths in free space ranging roughly from 0.3 to 30 cm. Corresponding frequencies range from 1 to 100 GHz. Frequency and wavelength are related by fλ = c, where f is the frequency, λ is the free-space wavelength, and c is the velocity of light in vacuum, approximately 3×10⁸ m/s. See also Electromagnetic radiation; Frequency (wave motion); Wave motion; Wavelength.

Characteristic transmission media for microwaves are hollow-pipe waveguides, where the cross-sectional dimensions are of the order of the wavelength and thus are of convenient size. Coaxial transmission lines are also used, however, especially in the lower-frequency bands, and various stripline techniques are used on microwave integrated circuits. Resonant cavities are commonly used as circuit elements, and radiation or reception of the energy is typically by horns, parabolic reflectors, or arrays. See also Antenna (electromagnetism); Cavity resonator; Coaxial cable; Electromagnetic wave transmission; Transmission lines; Waveguide.

Generation

For most applications, microwaves are generated in electronic devices that produce oscillations at microwave frequencies. The devices may be single-frequency or tunable, and continuous-wave (cw) or pulsed. Vacuum-tube generators include klystrons, magnetrons, and backward-wave oscillators; solid-state generators include tunnel diodes, Gunn diodes, IMPATT diodes, transistor oscillators, masers, and harmonic generators using varactor diodes. The vacuum-tube generators are used to produce higher powers, which can be as much as thousands of kilowatts. Solid-state generators were formerly limited in power to a few watts, but their power capabilities are continually increasing and now may reach hundreds of watts. See also Gyrotron; Klystron; Magnetron; Microwave solid-state devices; Microwave tube; Traveling-wave tube.

Circuit elements

Physical elements which produce specific effects on microwaves are called circuit elements.

The most common method of microwave transmission within a system is through hollow circular or rectangular metal tubes of uniform cross section called waveguides. The microwave energy is confined within these tubes and guided along them.

Various forms of stripline are used in interconnecting components on a dielectric or semiconductor substrate when microwave devices are integrated. One important example is the microstrip, in which a metallic strip or ribbon is placed on a thin dielectric, which is in turn backed by a conducting ground plane. Striplines have more losses than hollow-pipe waveguides but are generally used over very short distances.

Filters are needed in communication or information-processing systems for blocking of high frequencies (low-pass), blocking of low frequencies (high-pass), elimination of undesired bands (band elimination), or passing of desired bands while attenuating others (band pass). All of these may be made for microwaves by adding periodic perturbations, such as posts, irises, diaphragms, or dimensional variations, to the wave-guide or other transmission system. See also Electric filter; Microwave filter.

A thin sheet of plastic can be used to alter the amplitude or phase of microwaves. If the sheet is coated with powdered carbon with appropriate electrical conductivity and placed in a waveguide with the lossy material parallel to the lines of electrical intensity, it will absorb microwave power. Variable attenuation can be achieved by mechanically inserting more or less of the lossy strip into the path of the wave. See also Attenuation (electricity).

A phase shifter changes the phase of a microwave without changing its amplitude. It can be constructed in the same manner as an attenuator without the lossy material.

An attenuator which has a very large loss and is closed at one end is called a termination; it absorbs all the power transmitted into it, reflecting none.

The most common microwave detector is a silicon diode designed for high frequencies and mounted in a waveguide or a stripline. The diode rectifies the microwave signal, producing an average current which can be indicated by a direct-current meter connected between the diode terminals. If the microwave signal is modulated in amplitude, the modulation will appear in the output current. See also Amplitude modulation; Semiconductor diode.

The bolometer is a detector which absorbs microwave power, causing a temperature increase and a corresponding change in resistance. The bolometer does not respond fast enough to detect high-frequency modulation. It is often used as one arm of a resistance bridge circuit in microwave power meters. See also Bolometer.

A transmitting antenna takes microwave power from a waveguide and converts it into a plane wave that propagates through space to a distant receiving antenna. Two important characteristies of antennas are efficiency and directivity, efficiency being the ratio of the power delivered into space to the power available in the waveguide. High directivity is accomplished by large antennas which focus the microwave energy in the same way a searchlight focuses a beam of light.

The gyrator is a lossless, nonreciprocal, two-port circuit which has 180° more phase shift in one direction than in the other. This principle is used in the broadband microwave circulator. The three-port circulator has the property that all the power into port 1 exits at port 2, all the power into port 2 exits at port 3, and all the power into port 3 exits at port 1. The nonreciprocal phase shift is achieved in a magnetic ferrite placed in the waveguide junction and magnetized with an external permanent magnet. An isolator is a circulator with one port terminated, resulting in a circuit which transmits power in one direction and not the other. The input circuit is thus isolated from the output circuit. When the terminated port is internal, the isolator appears to be a two-port element. See also Gyrator.

A varactor is a solid-state diode whose capacitance changes with applied voltage. Varactors are used as harmonic generators to obtain microwave power efficiently from lower-frequency sources such as quartz crystal-controlled oscillators in the 10–100-MHz range. Varactors are also used in up-converters and down-converters performing the same functions as similar circuits using resistive diodes, but more efficiently and at the expense of narrower bandwidth. See also Oscillator; Varactor.

A microwave amplifier converts a low-power input signal to a higher-power output signal while preserving one or more characteristics. A linear amplifier preserves the amplitude, frequency, and phase of the input signal. When a linear amplifier is overloaded, it becomes saturated: the output amplitude tends to remain constant and the envelope of the input signal becomes distorted.

High power output is achieved with klystron and traveling-wave tube (TWT) amplifiers, both of which can be operated in the linear or saturated modes. Moderate power can be achieved with transistor amplifiers, and these are continually being extended in their frequency range of usefulness. Very low noise levels are achieved in the maser and the parametric amplifier, both of which require power at a single frequency to pump the active element. See also Amplifier; Maser; Parametric amplifier.

Microwave integrated circuits

Microwave integrated circuits (MICs) are of two types, hybrid MICs and monolithic microwave integrated circuits (MMICs). In the hybrid circuits, some or all of the active and passive devices are added to a dielectric or semiconducting substrate and interconnected by striplines as discussed above. For MMICs, all of the active and passive devices of the functional unit are formed on the substrate, and interconnected by striplines through a variety of microfabrication techniques, including photolithography, epitaxy, ion implantation, etching, diffusion, sputtering, and evaporation. See also Integrated circuits.

Receiver

The first active element in nearly all microwave receivers is a silicon diode operated as a down-converter. In this type of receiver, a strong, continuous-wave local oscillator signal is used to pump the diode over its nonlinear resistance range. In this manner, the local oscillator and the input signal are mixed, shifting the input signal down to an intermediate frequency, which is the difference between the frequencies of the local oscillator signal and the received signal. Intermediate frequencies of a few tens of megahertz are common. Frequency, phase, or amplitude modulation on the received signal appears in the detector output at the intermediate frequency. A bandpass intermediate-frequency amplifier, providing most of the gain of the receiver, follows the detector, after which a demodulator converts the modulation on the intermediate-frequency signal to usable form, for example, an audio or a television signal. See also Radio receiver.

Transmitter

The main components of a microwave transmitter are a microwave power source, a modulator, and, if necessary, a power amplifier. The modulation can be done directly at microwave frequencies or it can be performed at intermediate frequency and shifted to the microwave frequency in an up-converter, which is very much like a down-converter.

Propagation

In free space, microwaves travel in straight lines as do optical waves. Near the Earth, however, the atmosphere has an index of refraction which normally decreases with distance above the Earth and causes the wave to travel in a circular path which bends slightly toward the Earth. Microwaves are reflected and refracted by objects just as are optical waves. See also Microwave optics.

Occasionally during the summer, atmospheric conditions cause microwaves transmitted from an antenna to travel to a receiver via two or more paths. These waves interfere at the receiver and may cause large decreases in the received signal amplitude. This phenomenon, called multipath fading, is a serious problem in microwave transmission parallel to the surface of the Earth. Other atmospheric conditions can result in what is known as earth-bulge fading. When microwaves are directed well above the horizon, neither of these two problems occurs. Thus satellite microwave systems do not suffer from either multipath or earth-bulge fading.

At frequencies above about 10 GHz, rain absorbs microwave energy, resulting in large signal losses. Both satellite and point-to-point microwave systems are seriously affected by rain attenuation. For most frequencies the attenuation of microwaves by the Earth's atmosphere is very small. See also Radio-wave propagation.

Applications

Areas in which microwave radiation is applied include radar, communications, radiometry, medicine, physics, chemistry, and cooking food.

Radar is used in military applications, commercial aviation, remote sensing of the atmosphere, and astronomy. The high antenna directivity and the excellent propagation characteristics of microwaves in the atmosphere make this the preferred band for radar applications. Microwaves are also used in electronic countermeasures to radar. See also Electronic warfare; Radar.

There is at least 100 times as much frequency space available for communications in the microwave band as in the entire spectrum below microwaves. In addition, the high directivity obtainable at microwave frequencies allows reuse of these frequencies many times in the same area, a practice not possible at lower frequencies. The high directivity also makes possible communication to satellites and deep-space probes. See also Radio spectrum allocations; Space communications.

All objects, including liquids and gases, emit electromagnetic radiation in the form of noise, the amount of the noise being proportional to the absolute temperature of the object. A noise temperature can be assigned to the object corresponding to the amount of noise radiating from it. A microwave radiometer is a sensitive receiver which measures the noise power received by an antenna; from this measurement, the noise temperature of the source object can be determined. Radiometers are used extensively for remote sensing. Microwave radiometers are used to study astronomical sources of noise and to observe planets from deep space probes. See also Passive radar; Radiometry; Remote sensing.

Applications of microwaves in medicine include (1) thermography, the measurement of tissue temperature; (2) hyperthermia, microwave heating used in the treatment of cancer and in the treatment of hypothermic subjects; and (3) biomedical imaging, the use of microwaves to study the structure of tissue beneath the skin. See also Radiology.

Physics and chemistry

Microwave energy is used in large particle accelerators to accelerate charged particles such as electrons and protons to very high energies and cause them to collide. Knowledge of the structure of matter is also obtained from microwave spectroscopy. See also Microwave spectroscopy; Particle accelerator.

Microwave energy is absorbed in most foods and has been found to be a source of quick, uniform heating or cooking. Microwave ovens based upon this principle are now widely used. Microwaves are also used for the industrial heating of foodstuffs and other materials.

A radio signal in the frequency range from 1 to 40 GHz or from 1 to 300 GHz, depending on the rating system. Numerous transmission systems use microwaves including line-of-sight between buildings and across vast distances, as well as communications satellites, cellular systems and wireless LANs. See spectrum.

Early Microwave Tower

Download Computer Desktop Encyclopedia to your PC, iPhone or Android.

Band of very short wavelength radio waves within the UHF, SHF and EHF bands.

LearnThatWord.com is a free vocabulary and spelling program where you only pay for results!

Electromagnetic waves with a wavelength on the order of a few inches. Microwaves are longer than infrared radiation and shorter than radio waves. Microwaves are used extensively for communication, both in satellite television and for the transmission of long-distance telephone signals. In a microwave oven, food is cooked by the heat generated when the water in the food absorbs microwaves.

A wave typical of electromagnetic radiation between far infrared and radiowaves.

• m. acupuncture — utilizes deeply penetrating heat, produced by a microwave generator and transmitted via acupuncture needles.

• Electricity and Magnetism - microwave: radio wave of a few centimeters’ wavelength
• Electricity and Electronics - microwave: electromagnetic radiation with wavelength between infrared and radio waves
• Cooking Techniques - microwave: (vb) cook in a microwave oven

Dansk (Danish)
n. - mikrobølge
v. tr. - lave mad i mikrobølgeovn

idioms:

• microwave oven    mikrobølgeovn

Nederlands (Dutch)
magnetron, korte elektromagnetische golf, opwarmen/koken in de magnetron

Français (French)
n. - micro-ondes
v. tr. - passer au four à micro-ondes

idioms:

• microwave oven    four à micro-ondes

Deutsch (German)
n. - Mikrowelle, Mikrowellenherd
v. - im Mikrowellenherd kochen

idioms:

• microwave oven    Mikrowellenherd

Ελληνική (Greek)
n. - (φυσ.) μικροκύμα
v. - μικροκύμα

idioms:

• microwave oven    φούρνος μικροκυμάτων

Italiano (Italian)
microonde, mettere nel microonde

idioms:

• microwave oven    forno a microonde

Português (Portuguese)
n. - microonda (f)

idioms:

• microwave oven    forno de microondas (m)

Русский (Russian)
микроволновый

idioms:

• microwave oven    микроволновая печь

Español (Spanish)
n. - microonda, onda ultracorta
v. tr. - cocinar en el horno microondas

idioms:

• microwave oven    horno de microondas

Svenska (Swedish)
n. - mikrovåg
v. - laga (mat) i en mikrovågsugn

中文（简体）(Chinese (Simplified))
微波, 微波炉, 用微波炉热, 用微波炉烹调

idioms:

• microwave oven    微波炉

中文（繁體）(Chinese (Traditional))
n. - 微波, 微波爐
v. tr. - 用微波爐熱, 用微波爐烹調

idioms:

• microwave oven    微波爐

한국어 (Korean)
n. - 마이크로파
v. tr. - 전자레인지로 요리하다

日本語 (Japanese)
n. - マイクロ波, 極超短波

idioms:

• microwave oven    電子レンジ

العربيه (Arabic)
‏(الاسم) الموجه الصغرى, فرن ميكرويف يعمل بالموجات القصيرة جدا (فعل) يطبخ بالفرن المايكروويف‏

עברית (Hebrew)
n. - ‮מיקרוגל - גל אלקטרומגנטי קצר, תנור לחימום אוכל באמצעות מיקרוגלים‬
v. tr. - ‮בישל בתנור מיקרוגל‬

If you are unable to view some languages clearly, click here.

To select your translation preferences click here.