Microwave

Microwave

Microwaves are electromagnetic waves with wavelengths shorter than one meter and longer than one millimeter, or frequencies between 300 megahertz and 300 gigahertz. ( UHF, SHF, EHF ) Apparatus and techniques may be described qualitatively as “ microwave ” when the wavelengths of signals are approximately the same as the dimensions of the equipment, so that lumped-element circuit theory is inaccurate. As a consequence, virtual microwave proficiency tends to move away from the discrete resistors, capacitors, and inductors used with lower frequency radio waves. alternatively, distributed circuit elements and transmission-line theory are more useful methods for design, analysis, and construction of microwave circuits. Open-wire and coaxial transmission lines give means to waveguides, and lumped-element tuned circuits are replaced by cavity resonators or resonant lines. Effects of reflection, polarization, scattering, diffraction, and atmospheric assimilation normally associated with visible abstemious are of practical meaning in the study of microwave propagation. The like equations of electromagnetic hypothesis apply at all frequencies. While the mention suggests a micron wavelength, it is beter sympathize as indicating wavelength very much smaller than those used in radio receiver circulate. The boundaries between far infrared idle, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study. The term microwave broadly refers to “ alternating current signals with frequencies between 300 MHz ( 3×108 Hz ) and 300 GHz ( 3×1011 Hz ). ” [ 1 ] ( UHF, SHF, EHF ) Both IEC standard 60050 and IEEE standard 100 define “ microwave ” frequencies starting at 1 GHz ( 30 centimeter wavelength ).

Reading: Microwave

electromagnetic waves longer ( lower frequency ) than microwaves are called “ radio waves ”. electromagnetic radiation with shorter wavelengths may be called “ millimeter waves ”, terahertz radiation or evening T-rays. Definitions differ for millimeter wave band, which the IEEE defines as 110GHz to 300GHz while military radar definitions use 30-300GHz .

Discovery

The universe of electromagnetic waves, of which microwaves are part of the frequency spectrum, was predicted by James Clerk Maxwell in 1864 from his equations. In 1888, Heinrich Hertz was the first to demonstrate the universe of electromagnetic waves by building an apparatus that produced and detected microwaves in the UHF region. The design inevitably used horse-and-buggy materials, including a horse bowl, a make iron point spark, Leyden jars, and a duration of zinc gutter whose parabolic cross-section worked as a mirror image antenna. In 1894 J. C. Bose publicly demonstrated radio receiver control of a bell using millimeter wavelengths, and conducted inquiry into the propagation of microwaves .

Frequency range

The microwave range includes ultra-high frequency ( UHF ) ( 0.3–3 GHz ), super high frequency ( SHF ) ( 3–30 GHz ), and extremely high frequency ( EHF ) ( 30–300 GHz ) signals. Above 300 GHz, the assimilation of electromagnetic radiation by Earth ‘s air is so great that it is effectively opaque, until the atmosphere becomes crystalline again in the alleged infrared and ocular window frequency ranges .

Microwave Sources

Vacuum tube based devices operate on the ballistic movement of electrons in a vacuum under the influence of controlling electric or charismatic fields, and include the magnetron, klystron, traveling wave tube ( TWT ), and gyrotron. These devices work in the density modulated modality, preferably than the stream modulated mode. This means that they work on the basis of clumps of electrons flying ballistically through them, preferably than using a continuous stream. A maser is a device exchangeable to a laser, except that it works at microwave frequencies.

Uses

  • A microwave oven works by passing microwave radiation, usually at a frequency of 2450 MHz (a wavelength of 12.24 cm), through the food. Water, fat, and sugar molecules in the food absorb energy from the microwave beam in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field induced by the microwave beam. This molecular movement creates heat as the rotating molecules hit other molecules and put them into motion. Microwave heating is most efficient on liquid water, and much less so on fats and sugars (which have less molecular dipole moment), and frozen water (where the molecules are not free to rotate). Microwave heating is sometimes incorrectly explained as a rotational resonance of water molecules: such resonance only occurs at much higher frequencies, in the tens of gigahertz. Moreover, large industrial/commercial microwave ovens operating in the 900 MHz range also heat water and food perfectly well.
    • A common misconception is that microwave ovens cook food from the “inside out”. In reality, microwaves are absorbed in the outer layers of food in a manner somewhat similar to heat from other methods. The rays from a microwave electrically manipulate water particles to cook food. It is actually the friction caused by the movement that creates heat and warms the food. The misconception arises because microwaves penetrate dry nonconductive substances at the surfaces of many common foods, and thus often deposit initial heat more deeply than other methods. Depending on water content the depth of initial heat deposition may be several centimeters or more with microwave ovens, in contrast to grilling (“broiling” in American English), which relies on infrared radiation, or the thermal convection of a convection oven, which deposit heat shallowly at the food surface. Depth of penetration of microwaves is dependent on food composition and the frequency, with lower microwave frequencies being more penetrating.
  • Microwave radio is used in broadcasting and telecommunication transmissions because, due to their short wavelength, highly directive antennas are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies). There 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 television news to transmit a signal from a remote location to a television station from a specially equipped van.
  • Microwave imaging; see Photoacoustic imaging in biomedicine
  • Before the advent of fiber optic transmission, most long distance telephone calls were carried via microwave point-to-point links through sites like the AT&T Long Lines. Starting in the early 1950’s, frequency division multiplex 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.
  • Radar also uses microwave radiation to detect the range, speed, and other characteristics of remote objects.
  • Wireless LAN protocols, such as Bluetooth and the IEEE 802.11 specifications, also use microwaves in the 2.4 GHz ISM band, although 802.11a uses ISM band and U-NII frequencies in the 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services can be found in many countries (but not the USA) in the 3.5–4.0 GHz range.
  • Metropolitan Area Networks: MAN protocols, such as WiMAX (Worldwide Interoperability for Microwave Access) based in the IEEE 802.16 specification. The IEEE 802.16 specification was designed to operate between 2 to 11 GHz. The commercial implementations are in the 2.5 GHz, 3.5 GHz and 5.8 GHz ranges.
  • Wide Area Mobile Broadband Wireless Access: MBWA protocols based on standards specifications such as IEEE 802.20 or ATIS/ANSI HC-SDMA (e.g. iBurst) are designed to 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.
  • Cable TV and Internet access on coax cable as well as broadcast television use some of the lower microwave frequencies. Some mobile phone networks, like GSM, also use the lower microwave frequencies.
  • Many semiconductor processing techniques use microwaves to generate plasma for such purposes as reactive ion etching and plasma-enhanced chemical vapor deposition (PECVD).
  • Microwaves can be used to transmit power over long distances, and post-World War II research was done to examine possibilities. NASA worked in the 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.
  • Most radio astronomy uses 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 the 95 GHz focused beam heats the skin to a temperature of 130 F (54 C) at a depth of 1/64th of an inch (0.4 mm). The United States Air Force and Marines are currently using this type of Active Denial System.[2]

Microwave frequency bands

The microwave spectrum is normally defined as electromagnetic department of energy ranging from approximately 1 GHz to 1000 GHz in frequency, but older custom includes lower frequencies. Most common applications are within the 1 to 40 GHz compass. Microwave Frequency Bands as defined by the Radio Society of Great Britain in the table below :
The above table reflects Radio Society of Great Britain ( RSGB ) usage. The term P band is sometimes used for Ku Band. For early definitions see letter Designations of Microwave Bands

Health effects

Main article: Electromagnetic radiation and health

Microwaves contain insufficient energy to directly chemically change substances by ionization, and therefore are an exemplar of nonionizing radiation. The parole “ radiation ” refers to the fact that energy can radiate, and not to the different nature and effects of unlike kinds of energy. A great number of studies have been undertaken in the last two decades, most conclude they are safe. It is understand that microwave radiation of a level that causes heat of life tissue is hazardous ( ascribable to the hypothesis of overheating and burns ) and most countries have standards limiting exposure, such as the Federal Communications Commission RF safety regulations. synthetic reviews of literature indicate the predomination of their base hit of habit. [ 3 ] [ 4 ]

History and research

possibly the first base, documented, courtly use of the term microwave occurred in 1931 :

“When trials with wavelengths as low as 18 cm were made known, there was undisguised surprise that the problem of the micro-wave had been solved so soon.” Telegraph & Telephone Journal XVII. 179/1

possibly the first use of the word microwave in an astronomic context occurred in 1946 in an article “ Microwave Radiation from the Sun and Moon ” by Robert Dicke and Robert Beringer. For some of the history in the development of electromagnetic theory applicable to modern microwave applications see the follow figures :
specific significant areas of research and exploit developing microwaves and their applications :

Specific work on microwaves
Work carried out by Area of work
Barkhausen and Kurz Positive grid oscillators
Hull Smooth bore magnetron
Varian Brothers Velocity modulated electron beam → klystron tube
Randall and Boot Cavity magnetron

References

  1. ^ Pozar, David M. (1993). Microwave Engineering Addison-Wesley Publishing Company. ISBN 0-201-50418-9.
  2. ^ Raytheon’s Silent Guardian millimeter wave weapon
  3. ^ Dugauquier C. – Effects of exposure to electromagnetic fields (microwaves) on mammalian pregnancy. Litterature review – Médecine et Armées, 2006; 34 (3): 215-218
  4. ^ Heynick C. et al. – Radio Frequency Electromagnetic Fields: Cancer,Mutagenesis, and Genotoxicity – Bioelectromagnetics Supplement, 2003; 6:S74-S100.

See also

Radio spectrum
ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF
3 Hz 30 Hz 300 Hz 3 kHz 30 kHz 300 kHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz
30 Hz 300 Hz 3 kHz 30 kHz 300 kHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz 300 GHz