Microwave




Microwaves are UHF, SHF, EHF)

Apparatus and techniques may be described qualitatively as "microwave" when the wavelengths of signals are roughly the same as the dimensions of the equipment, so that lumped-element circuit theory is inaccurate. As a consequence, practical microwave technique tends to move away from the discrete resistors, capacitors, and inductors used with lower frequency absorption usually associated with visible light are of practical significance in the study of microwave propagation. The same equations of electromagnetic theory apply at all frequencies.

While the name suggests a micrometer wavelength, it is beter understood as indicating wavelengths very much smaller than those used in radio broadcasting. The boundaries between far infrared light, UHF, SHF, EHF)Both IEC standard 60050 and IEEE standard 100 define "microwave" frequencies starting at 1 GHz (30 cm wavelength).

Electromagnetic waves longer (lower frequency) than microwaves are called "radio waves". Electromagnetic radiation with shorter wavelengths may be called "millimeter waves", terahertz radiation or even 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 existence 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 existence of electromagnetic waves by building an apparatus that produced and detected microwaves in the UHF region. The design necessarily used horse-and-buggy materials, including a horse trough, a wrought iron point spark, Leyden jars, and a length of zinc gutter whose parabolic cross-section worked as a reflection antenna. In 1894 J. C. Bose publicly demonstrated radio control of a bell using millimetre wavelengths, and conducted research into the propagation of microwaves.

 

Frequency range

The microwave range includes extremely high frequency (EHF) (30–300 GHz) signals.

Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is effectively opaque, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.

Microwave Sources

Vacuum tube based devices operate on the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include 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.

A laser, except that it works at microwave frequencies.

Uses

 

  • A 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.
  • 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 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.
  • Microwaves can be used to 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 usually defined as electromagnetic energy ranging from approximately 1 GHz to 1000 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave Frequency Bands as defined by the Radio Society of Great Britain in the table below:

Microwave frequency bands
DesignationFrequency range
L band1 to 2 GHz
S band2 to 4 GHz
C band4 to 8 GHz
X band8 to 12 GHz
Ku band12 to 18 GHz
K band18 to 26.5 GHz
Ka band26.5 to 40 GHz
Q band30 to 50 GHz
U band40 to 60 GHz
V band50 to 75 GHz
E band60 to 90 GHz
W band75 to 110 GHz
F band90 to 140 GHz
D band110 to 170 GHz

The above table reflects Radio Society of Great Britain (RSGB) usage. The term P band is sometimes used for Ku Band. For other 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 so are an example of nonionizing radiation. The word "radiation" refers to the fact that energy can radiate, and not to the different nature and effects of different kinds of energy.

A great number of studies have been undertaken in the last two decades, most concluding they are safe. It is understood that microwave radiation of a level that causes heating of living tissue is hazardous (due to the possibility 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 predominance of their safety of use. [3] [4]

History and research

Perhaps the first, documented, formal 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

Perhaps the first use of the word microwave in an astronomical 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 following figures:

Specific significant areas of research and work developing microwaves and their applications:

Specific work on microwaves
Work carried out byArea of work
Barkhausen and KurzPositive grid oscillators
HullSmooth bore magnetron
Varian BrothersVelocity modulated electron beam → klystron tube
Randall and BootCavity 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 .
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Wireless Video and Data Distribution Methods

Advanced Wireless Services  · Amateur television  · Analog television  · Digital radio  · Digital television  · Digital television in Europe  · Digital terrestrial television (DTT or DTTV)  ·
Digital Video Broadcasting: ( Terrestrial - Satellite - Handheld )  · DVB-MS  · Ku band  · Local Multipoint Distribution Service (LMDS)  · Microwave  · Mobile Pay TV  · Multichannel Multipoint Distribution Service (MMDS)  · MVDS  · MVDDS  · Satellite Internet access  · Satellite radio  · Satellite television  · Wi-Fi  · WiMAX  · Wireless local loop

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

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Longwave
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Microwave". A list of authors is available in Wikipedia.