Microwaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequenciesbetween 300 MHz (0.3 GHz) and 300 GHz. This broad definition includes both UHF and EHF (millimeter waves), and various sources use different boundaries Lenovo S10-2 Battery 5200mAh .
In all cases, microwave includes the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum, with RF engineering often putting the lower boundary at 1 GHz (30 cm), and the upper around 100 GHz (3mm) Lenovo Y730 Battery 5200mAh .
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 Lenovo Y710 Battery 5200mAh .
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Open-wire and coaxial transmission lines give way to waveguides and stripline, and lumped-element tuned circuits are replaced by cavity resonators or resonant lines. Effects of reflection, polarization, scattering, diffraction and atmospheric absorption usually associated with visible light are of practical significance in the study of microwave propagation Lenovo Y510 Battery 5200mAh .
The same equations of electromagnetic theory apply at all frequencies.
While the name may suggest a micrometer wavelength, it is better understood as indicating wavelengths much shorter than those used in radio broadcasting Lenovo S10-3 Battery 5200mAh .
The boundaries between far infrared light, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study.
Electromagnetic waves longer (lower frequency) than microwaves are called "radio waves" Lenovo G450 Battery 5200mAh .
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 110 GHz to 300 GHz.
Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is effectively opaque Lenovo E30 Battery 5200mAh ,
until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.
Vacuum tube devices operate on the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include themagnetron, klystron, traveling-wave tube (TWT), and gyrotron Lenovo E40 Battery 5200mAh.
These devices work in the density modulated mode, rather than the current modulated mode. This means that they work on the basis of clumps of electrons flying ballistically through them, rather than using a continuous stream Lenovo E50 Battery 5200mAh .
Low power microwave sources use solid-state devices such as the field-effect transistor (at least at lower frequencies), tunnel diodes, Gunn diodes, andIMPATT diodes.
A maser is a device similar to a laser, which amplifies light energy by stimulating photons Lenovo G460 Battery 5200mAh .
The maser, rather than amplifying light energy, amplifies the lower frequency, longer wavelength microwaves and radio frequency emissions.
The sun also emits microwave radiation, and most of it is blocked by Earth's atmosphere Lenovo Y460 Battery 5200mAh .
The Cosmic Microwave Background Radiation (CMBR) is a source of microwaves that supports the science of cosmology's Big Bang theory of the origin of the Universe.
Before the advent of fiber-optic transmission, most long distance telephone calls were carried via networks of microwave radio relay links run by carriers such as AT&T Long Lines Lenovo Y450 Battery 5200mAh .
Starting in the early 1950s, 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 Lenovo N200 Battery 5200mAh .
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 have been used for almost a decade in many countries in the 3.5–4.0 GHz range Lenovo T410 Battery 5200mAh .
The FCC recently carved out spectrum for carriers that wish to offer services in this range in the U.S. — with emphasis on 3.65 GHz. Dozens of service providers across the country are securing or have already received licenses from the FCC to operate in this bandLenovo T510 Battery 5200mAh .
The WIMAX service offerings that can be carried on the 3.65 GHz band will give business customers another option for connectivity.
Metropolitan area networks: MAN protocols, such as WiMAX (Worldwide Interoperability for Microwave Access) based in the IEEE 802.16 specification Lenovo SL410 Battery 5200mAh .
The IEEE 802.16 specification was designed to operate between 2 to 11 GHz. The commercial implementations are in the 2.3 GHz, 2.5 GHz, 3.5 GHz and 5.8 GHz ranges.
Wide Area Mobile Broadband Wireless Access Lenovo SL510 Battery 5200mAh :
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 Lenovo C100 Battery 5200mAh .
Some mobile phone networks, like GSM, use the low-microwave/high-UHF frequencies around 1.8 and 1.9 GHz in the Americas and elsewhere, respectively. DVB-SH and S-DMB use 1.452 to 1.492 GHz, while proprietary/incompatible satellite radio in the U.S. uses around 2.3 GHz for DARS Lenovo T400S Battery 5200mAh .
Microwave radio is used in broadcasting and telecommunication transmissions because, due to their short wavelength, highly directional antennas are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies)Lenovo Y430 Battery 5200mAh .
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 Ibm ThinkPad G40 Battery 5200mAh .
See broadcast auxiliary service (BAS), remote pickup unit (RPU), and studio/transmitter link (STL).
Most satellite communications systems operate in the C, X, Ka, or Ku bands of the microwave spectrum Ibm ThinkPad X200T Battery 5200mAh .
These frequencies allow large bandwidth while avoiding the crowded UHF frequencies and staying below the atmospheric absorption of EHF frequencies. Satellite TV either operates in the C band for the traditional large dish fixed satellite service or Ku band for direct-broadcast satellite Apple A1280 battery 5200mAh .
Military communications run primarily over X or Ku-band links, with Ka band being used for Milstar.
Radar uses microwave radiation to detect the range, speed, and other characteristics of remote objects Apple A1281 battery 5200mAh .
Development of radar was accelerated during World War II due to its great military utility. Now radar is widely used for applications such as air traffic control, weather forecasting, navigation of ships, and speed limit enforcement.
A Gunn diode oscillator and waveguide are used as a motion detector for automatic door openers Samsung R522 battery 5200mAh .
Most radio astronomy uses microwaves. Usually the naturally-occurring microwave radiation is observed, but active radar experiments have also been done with objects in the solar system, such as determining the distance to the Moon or mapping the invisible surface of Venus through cloud cover Samsung P467 battery 5200mAh .
Global Navigation Satellite Systems (GNSS) including the Chinese Beidou, the American Global Positioning System (GPS) and the Russian GLONASSbroadcast navigational signals in various bands between about 1.2 GHz and 1.6 GHz Hp Pavilion dm3 Battery 5400mAh .
A microwave oven passes (non-ionizing) microwave radiation (at a frequency near 2.45 GHz) through food, causing dielectric heating by absorption of energy in the water, fats, and sugar contained in the food Hp Pavilion dm4 Battery 5200mAh .
Microwave ovens became common kitchen appliances in Western countries in the late 1970s, following development of inexpensive cavity magnetrons. Water in the liquid state possesses many molecular interactions which broaden the absorption peakHp Pavilion cq62 Battery 5200mAh .
In the vapor phase, isolated water molecules absorb at around 22 GHz, almost ten times the frequency of the microwave oven.
Microwave heating is used in industrial processes for drying and curing products Hp Pavilion cq42 Battery 5200mAh .
Many semiconductor processing techniques use microwaves to generate plasma for such purposes as reactive ion etching and plasma-enhanced chemical vapor deposition (PECVD).
Microwave frequencies typically ranging from 110 – 140 GHz are used in stellarators and more notably in tokamak experimental fusion reactors to help heat the fuel into a plasma state Hp Pavilion cq72 Battery 5200mAh .
The upcomingITER Thermonuclear Reactor is expected to range from 110–170 GHz and will employ Electron Cyclotron Resonance Heating (ECRH).
Microwaves can be used to transmit power over long distances, and post-World War II research was done to examine possibilities Hp cq20 Battery 2600mAh .
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 Hp 2230 Battery 2600mAh .
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) Dell Inspiron Mini 12 battery 7800mAh .
The United States Air Force and Marines are currently using this type of active denial system.
Microwave radiation is used in electron paramagnetic resonance (EPR or ESR) spectroscopy, typically in the X-band region (~9 GHz) in conjunction typically with magnetic fields of 0.3 T Dell XPS M1730 battery 7800mAh .
This technique provides information on unpaired electrons in chemical systems, such as free radicals or transition metal ions such as Cu(II). The microwave radiation can also be combined withelectrochemistry as in microwave enhanced electrochemistry Dell XPS M2010 battery 7800mAh .
Microwave frequency bands
The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies Dell Studio XPS 1340 battery 5200mAh/7800mAh .
Most common applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the Radio Society of Great Britain (RSGB), are shown in the table below:
Footnote (1): P band is sometimes incorrectly used for Ku Band Dell Inspiron 9100 battery 7800mAh .
"P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521, see  and. For other definitions see Letter Designations of Microwave Bands.
Footnote (2): When radars were first developed at K band during World War II, it was not realized that there was a nearby absorption band (due to water vapor and oxygen at the atmosphere) Dell Inspiron N4010 battery 5200mAh .
To avoid this problem, the original K band was split into a lower band, Ku, and upper band, Ka see .
Microwave frequency measurement
Microwave frequency can be measured by either electronic or mechanical techniques Dell Inspiron N3610 battery 5200mAh .
Frequency counters or high frequency heterodyne systems can be used. Here the unknown frequency is compared with harmonics of a known lower frequency by use of a low frequency generator, a harmonic generator and a mixer. Accuracy of the measurement is limited by the accuracy and stability of the reference source Dell Inspiron N5010 battery 5200mAh .
Mechanical methods require a tunable resonator such as an absorption wavemeter, which has a known relation between a physical dimension and frequency.
In a laboratory setting, Lecher lines can be used to directly measure the wavelength on a transmission line made of parallel wires, the frequency can then be calculated Dell Inspiron N7010 battery 5200mAh .
A similar technique is to use a slotted waveguide or slotted coaxial line to directly measure the wavelength. These devices consist of a probe introduced into the line through a longitudinal slot, so that the probe is free to travel up and down the line. Slotted lines are primarily intended for measurement of the voltage standing wave ratio on the line Dell Inspiron 1464 battery 5200mAh .
However, provided a standing wave is present, they may also be used to measure the distance between thenodes, which is equal to half the wavelength. Precision of this method is limited by the determination of the nodal locations Dell Inspiron 1564 battery 5200mAh .
Microwaves do not contain sufficient energy to chemically change substances by ionization, and so are an example of nonionizing radiation. The word "radiation" refers to the fact that energy can radiate. The term in this context is not to be confused with radioactivity Dell Inspiron 1764 battery 5200mAh .
It has not been shown conclusively that microwaves (or other nonionizing electromagnetic radiation) have significant adverse biological effects at low levels. Some but not all studies suggest that long-term exposure may have a carcinogenic effect. This is separate from the risks associated with very high intensity exposure Dell Inspiron 1320 battery 5200mAh ,
which can cause heating and burns like any heat source, and not a unique property of microwaves specifically.
During World War II, it was observed that individuals in the radiation path of radar installations experienced clicks and buzzing sounds in response to microwave radiation Dell Inspiron 1320n battery 5200mAh .
This microwave auditory effect was thought to be caused by the microwaves inducing an electric current in the hearing centers of the brain. Research by NASA in the 1970s has shown this to be caused by thermal expansion in parts of the inner ear Dell Vostro 1710 battery 5200mAh .
When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in the body. Exposure to microwave radiation can producecataracts by this mechanism, because the microwave heating denatures proteins in the crystalline lens of the eye Dell Vostro 1720 battery 5200mAh
(in the same way that heat turns egg whites white and opaque) faster than the lens can be cooled by surrounding structures. The lens and cornea of the eye are especially vulnerable because they contain no blood vessels that can carry away heat. Exposure to heavy doses of microwave radiation Dell Studio XPS 1640 battery 5200mAh
(as from an oven that has been tampered with to allow operation even with the door open) can produce heat damage in other tissues as well, up to and including serious burns which may not be immediately evident because of the tendency for microwaves to heat deeper tissues with higher moisture content Dell Vostro A840 battery 5200mAh .
History and research
The existence of electromagnetic waves 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 Dell Vostro A860 battery 5200mAh .
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 millimeter wavelengths, and conducted research into the propagation of microwaves Dell Studio 1535 battery 5200mAh/7800mAh .
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 JournalXVII. 179/1 Dell Studio 1735 battery 5200mAh/7800mAh
In 1943: the Hungarian engineer Zoltán Bay sent ultra-short radio waves to the moon, which, reflected from there worked as a radar, and could be used to measure distance, as well as to study the moon Dell Vostro 1310 battery 5200mAh .
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. This same article also made a showing in the New York Times issued in 1951Dell Latitude E6400 battery,10400mAh/7800mAh .
A block upconverter (BUC) is used in the transmission (uplink) of satellite signals. It converts a band (or "block") of frequencies from a lower frequency to a higher frequency. Modern BUCs convert from the L band to Ku band, C band and Ka band. Older BUCs convert from a 70 MHz intermediate frequency (IF) to Ku band or C band Dell Latitude E6400 Battery,10400mAh/7800mAh .
Most BUCs use phase-locked loop local oscillators and require an external 10 MHz frequency reference to maintain the correct transmit frequency.
BUCs used in remote locations are often 2 or 4 W in the Ku band and 5 W in the C band Dell Latitude E6400 Battery,10400mAh/7800mAh .
The 10 MHz reference frequency is usually sent on the same feedline as the main carrier. Many smaller BUCs also get their direct current (DC) over the feedline, using an internal DC block.
BUCs are generally used in conjunction with low-noise block converters (LNB) Dell Latitude E6500 battery,10400mAh/7800mAh .
The BUC, being an up-converting device, makes up the "transmit" side of the system, while the LNB is the down-converting device and makes up the "receive" side. An example of a system utilizing both a BUC and an LNB is a VSATsystem, used for bidirectional Internet access via satellite Dell Latitude E6500 battery,10400mAh/7800mAh .
The block upconverter is assembled with the LNB in association with an OMT, orthogonal mode transducer to the feed-horn that faces the reflector parabolic dish.
In cosmology, cosmic microwave background (CMB) radiation (also CMBR, CBR, MBR, and relic radiation) is thermal radiation filling the universealmost uniformly Dell Latitude E6500 battery,10400mAh/7800mAh .
With a traditional optical telescope, the space between stars and galaxies (the background) is completely dark. But a sufficiently sensitive radio telescopeshows a faint background glow, almost exactly the same in all directions, that is not associated with any star, galaxy, or other object Dell Latitude E6500 battery,10400mAh/7800mAh .
This glow is strongest in the microwave region of the radio spectrum. The CMB's serendipitous discovery in 1964 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s, and earned them the 1978 Nobel Prize HP Pavilion DV6-1210SA battery - 5200mAh/8800mAh .
Cosmic background radiation is well explained as radiation left over from an early stage in the development of the universe, and its discovery is considered a landmark test of the Big Bang model of the universe HP Pavilion DV7 battery - 7200mAh
When the universe was young, before the formation of stars and planets, it was smaller, much hotter, and filled with a uniform glow from its white-hot fog of hydrogen plasma. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled enough, stable atoms could form Ibm THINKPAD X200 Battery 8800mAh .
These atoms could no longer absorb the thermal radiation, and the universe became transparent instead of being an opaque fog. The photons that existed at that time have been propagating ever since, though growing fainter and less energetic, since exactly the same photons fill a larger and larger universe Ibm THINKPAD X200 Battery 8800mAh .
This is the source for the alternate term relic radiation.
Precise measurements of cosmic background radiation are critical to cosmology, since any proposed model of the universe must explain this radiation Ibm THINKPAD X200 Battery 8800mAh .
The CMBR has a thermal black body spectrum at a temperature of 2.725 K, thus the spectrum peaks in the microwave range frequency of 160.2 GHz, corresponding to a 1.9 mm wavelength. This holds if you measure the intensity per unit frequency, as in Planck's law SONY VAIO VGN-FW21E Battery .
If instead you measure it per unit wavelength, using Wien's law, the peak will be at 1.06 mm corresponding to a frequency of 283 GHz.
The glow is highly uniform in all directions, but shows a very specific pattern equal to that expected if a fairly uniformly distributed hot gas is expanded to the current size of the universe Asus M52N Battery .
In particular, the spatial power spectrum (how much difference is observed versus how far apart the regions are on the sky) contains small anisotropies, or irregularities, which vary with the size of the region examined. They have been measured in detail, and match what would be expected if small thermal variations Dell Latitude E6400 ATG Battery ,
generated by quantum fluctuations of matter in a very tiny space, had expanded to the size of the observable universe we see today. This is still a very active field of study, with scientists seeking both better data (for example, the Planck spacecraft) and better interpretations of the initial conditions of expansion SONY VAIO VGN-FW21J Battery .
Although many different processes might produce the general form of a black body spectrum, no model other than the Big Bang has yet explained the fluctuations. As a result, most cosmologists consider the Big Bang model of the universe to be the best explanation for the CMBR SONY VAIO VGN-FW21L Battery .
The cosmic microwave background (CMB) radiation is an emission of uniform, black body thermal energy coming from all parts of the sky. The radiation is isotropic to roughly one part in 100,000: the root mean square variations are only 18 µK, after subtracting out a dipole anisotropy from the Doppler shift of the background radiation SONY VAIO VGN-FW21M Battery .
The latter is caused by the peculiar velocity of the Earth relative to the comoving cosmic rest frame as the planet moves at some 627 km/s towards the constellation Virgo.
In the Big Bang model for the formation of the universe SONY VAIO VGN-FW21Z Battery ,
Inflationary Cosmology predicts that after about 10?37 seconds the nascent universe underwent exponential growth that smoothed out nearly all inhomogeneities. The remaining inhomogeneities were caused by quantum fluctuations in the inflaton field that caused the inflation event SONY VAIO VGN-FW25T/B Battery .
After 10?6 seconds, the early universe was made up of a hot, interactingplasma of photons, electrons, and baryons. As the universe expanded, adiabatic cooling caused the plasma to lose energy until it became favorable for electrons to combine with protons, forming hydrogen atoms SONY VAIO VGN-FW26T/B Battery .
This recombination event happened when the temperature was around 3000 K or when the universe was approximately 379,000 years old. At this point, the photons no longer interacted with the now electrically neutral atoms and began to travel freely through space, resulting in the decoupling of matter and radiation SONY VAIO VGN-FW27/B Battery .
The color temperature of the decoupled photons has continued to diminish ever since; now down to 2.725 K, their temperature will continue to drop as the universe expands. According to the Big Bang model, the radiation from the sky we measure today comes from a spherical surface called the surface of last scattering SONY VAIO VGN-FW27/W Battery .
This represents the set of locations in space at which the decoupling event is believed to have occurred and at a point in time such that the photons from that distance have just reached observers. Most of the radiation energy in the universe is in the cosmic microwave background, making up a fraction of roughly 6×10?5 of the total density of the universe SONY VAIO VGN-FW31ZJ Battery .
Two of the greatest successes of the Big Bang theory are its prediction of the almost perfect black body spectrum and its detailed prediction of the anisotropies in the cosmic microwave background. The CMB spectrum has become the most precisely measured black body spectrum in nature SONY VAIO VGN-FW27T/H Battery .
The cosmic microwave background was predicted in 1948 by George Gamow, Ralph Alpher, and Robert Herman. Alpher and Herman were able to estimate the temperature of the cosmic microwave background to be 5 K, though two years later they re-estimated it at 28 K SONY VAIO VGN-FW29/B Battery .
This high estimate was due to an mis-estimate of the Hubble constant by Alfred Behr, which could not be replicated and was later abandoned for the earlier estimate. Although there were several previous estimates of the temperature of space, these suffered from two flaws SONY VAIO VGN-FW290JRB Battery .
First, they were measurements of the effective temperature of space and did not suggest that space was filled with a thermal Planck spectrum. Next, they depend on our being at a special spot at the edge of the Milky Way galaxy and they did not suggest the radiation is isotropic SONY VAIO VGN-FW290JTB Battery .
The estimates would yield very different predictions if Earth happened to be located elsewhere in the Universe.
The 1948 results of Alpher and Herman were discussed in many physics settings through about 1955, when each left the Applied Physics Laboratory at Johns Hopkins University SONY VAIO VGN-FW290JTH Battery .
The mainstream astronomical community, however, was not intrigued at the time by cosmology. Alpher and Herman's prediction was rediscovered by Yakov Zel'dovich in the early 1960s, and independently predicted by Robert Dicke at the same time SONY VAIO VGN-FW33G/E1 Battery .
The first published recognition of the CMB radiation as a detectable phenomenon appeared in a brief paper by Soviet astrophysicists A. G. Doroshkevich and Igor Novikov, in the spring of 1964. In 1964,David Todd Wilkinson and Peter Roll, Dicke's colleagues at Princeton University SONY VAIO VGN-FW33G/W Battery ,
began constructing a Dicke radiometer to measure the cosmic microwave background. In 1965, Arno Penzias and Robert Woodrow Wilson at the Crawford Hill location of Bell Telephone Laboratories in nearby Holmdel Township, New Jersey had built a Dicke radiometer that they intended to use for radio astronomy and satellite communication experiments SONY VAIO VGN-FW33GW Battery .
Their instrument had an excess 3.5 K antenna temperature which they could not account for. After receiving a telephone call from Crawford Hill, Dicke famously quipped: "Boys, we've been scooped Dell XPS M1210 Battery) http://www.hdd-shop.co.uk ."
A meeting between the Princeton and Crawford Hill groups determined that the antenna temperature was indeed due to the microwave background. Penzias and Wilson received the 1978Nobel Prize in Physics for their discovery Dell KM958 battery .
The interpretation of the cosmic microwave background was a controversial issue in the 1960s with some proponents of the steady state theory arguing that the microwave background was the result of scattered starlight from distant galaxies. Using this model, and based on the study of narrow absorption line features in the spectra of stars Dell Studio 1555 battery ,
the astronomer Andrew McKellar wrote in 1941: "It can be calculated that the 'rotational temperature' of interstellar space is 2 K." However, during the 1970s the consensus was established that the cosmic microwave background is a remnant of the big bangSony VGP-BPS13 battery .
This was largely because new measurements at a range of frequencies showed that the spectrum was a thermal, black bodyspectrum, a result that the steady state model was unable to reproduce.
Harrison, Peebles, Yu and Zel'dovich realized that the early universe would have to have inhomogeneities at the level of 10?4or 10?5Sony VGP-BPS13/B battery .
Rashid Sunyaev later calculated the observable imprint that these inhomogeneities would have on the cosmic microwave background. Increasingly stringent limits on the anisotropy of the cosmic microwave background were set by ground based experiments during the 1980s Sony VGP-BPS13/S battery .
The NASA COBEmission clearly detected the primary anisotropy with the Differential Microwave Radiometer instrument, publishing their findings in 1992.The team received the Nobel Prize in physics for 2006 for this discovery Sony VGP-BPS13A/B battery .
Inspired by the COBE results, a series of ground and balloon-based experiments measured cosmic microwave background anisotropies on smaller angular scales over the next decade. The primary goal of these experiments was to measure the scale of the first acoustic peak, which COBE did not have sufficient resolution to resolve Sony VGP-BPS13B/B battery .
This peak corresponds to large scale density variations in the early universe that are created by gravitational instabilities, resulting in acoustical oscillations in the plasma. The first peak in the anisotropy was tentatively detected by the Toco experiment and the result was confirmed by the BOOMERanG and MAXIMA experiments Sony VGP-BPL9 battery .
These measurements demonstrated that the geometry of the Universe is approximately flat, rather than curved. They ruled out cosmic strings as a major component of cosmic structure formation and suggested cosmic inflation was the right theory of structure formation Sony VGP-BPS13B/B battery .
The second peak was tentatively detected by several experiments before being definitively detected by WMAP, which has also tentatively detected the third peak. As of 2010, several experiments to improve measurements of the polarization and the microwave background on small angular scales are ongoing Sony VGP-BPL15 battery .
These include DASI, WMAP, BOOMERanG, QUaD, Planck spacecraft, Atacama Cosmology Telescope, South Pole Telescope and the QUIET telescope.
Relationship to the Big Bang
The cosmic microwave background radiation and the cosmological redshift are together regarded as the best available evidence for the Big Bangtheory Dell Inspiron E1505 battery .
Measurements of the CMB have made the inflationary Big Bang theory the standard model of the earliest eras of the universe. The discovery of the CMB in the mid-1960s curtailed interest in alternatives such as the steady state theory HP Pavilion dv6000 Battery .
The Big Bang theory predicts that the initial conditions for the universe are originally random in nature, and inhomogeneities follow a roughlyGaussian probability distribution, which, when graphed in cross-section, form bell-shaped curves. By analyzing this distribution at different frequencies, a spectral density or power spectrum is generated HP Pavilion dv8000 battery .
The power spectrum of these fluctuations has been calculated, and agrees with the observations. The resulting standard model of the Big Bang uses a Gaussian random field with a nearly scale invariant or Harrison-Zel'dovichspectrum to represent the primeval inhomogeneities HP PAVILION DV6000 Battery, 8800mAh .
Certain observables, for example the overall amplitude of the fluctuations, are more or less free parameters of the cosmic inflation model.Therefore, meaningful statements about the inhomogeneities in the universe need to be statistical in nature HP PAVILION DV8000 Battery, 8800mAh .
This leads to cosmic variance in which the uncertainties in the variance of fluctuations at the largest scale observed are difficult to accurately compare to theory.
The CMB gives a snapshot of the universe when, according to standard cosmology SONY VAIO VGN-FZ Battery ,
the temperature dropped enough to allow electrons and protons to form hydrogen atoms, thus making the universe transparent to radiation. When it originated some 380,000 years after the Big Bang—this time is generally known as the "time of last scattering" or the period of recombination or decoupling—the temperature of the universe was about 3000 K HP Pavilion DV6000 battery - 8800mAh .
This corresponds to an energy of about 0.25 eV, which is much less than the 13.6 eV ionization energy of hydrogen.
Since decoupling, the temperature of the background radiation has dropped by a factor of roughly 1,100 due to the expansion of the universe HP Pavilion DV8000 battery - 7800mAh .
As the universe expands, the CMB photons areredshifted, making the radiation's temperature inversely proportional to a parameter called the universe's scale length. The temperature Tr of the CMB as a function of redshift, z, can be shown to be proportional to the temperature of the CMB as observed in the present day (2.725 K or 0.235 meV) HP Pavilion DV6000 battery - 8800mAh :
- Tr = 2.725(1 + z)
For details about the reasoning that the radiation is evidence for the Big Bang, see Cosmic background radiation of the Big Bang.
The anisotropy of the cosmic microwave background is divided into two sorts: primary anisotropy, due to effects which occur at the last scattering surface and before HP Pavilion DV8000 battery - 6600mAh ;
and secondary anisotropy, due to effects such as interactions of the background radiation with hot gas or gravitational potentials, which occur between the last scattering surface and the observer.
The structure of the cosmic microwave background anisotropies is principally determined by two effects SONY VAIO VGN-FZ18 Battery :
acoustic oscillations and diffusion damping (also called collisionless damping or Silk damping). The acoustic oscillations arise because of a competition in the photon–baryonplasma in the early universe. The pressure of the photons tends to erase anisotropies, whereas the gravitational attraction of the baryons SONY VAIO VGN-FZ21E Battery
—moving at speeds much slower than light—makes them tend to collapse to form dense haloes. These two effects compete to create acoustic oscillations which give the microwave background its characteristic peak structure SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh .
The peaks correspond, roughly, to resonances in which the photons decouple when a particular mode is at its peak amplitude.
The peaks contain interesting physical signatures. The angular scale of the first peak determines the curvature of the universe (but not thetopology of the universe) SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh .
The next peak—ratio of the odd peaks to the even peaks—determines the reduced baryon density. The third peak can be used to pull information about the dark matter density.
The locations of the peaks also give important information about the nature of the primordial density perturbations SONY VAIO VGN-FW21E Battery .
There are two fundamental brands of density perturbations—called adiabatic and isocurvature. A general density perturbation is a mixture of both, and different theories that purport to explain the primordial density perturbation spectrum predict different mixtures SONY VAIO VGN-NR11S/S Battery .
- Adiabatic density perturbations
- the fractional overdensity in each matter component (baryons, photons ...) is the same SONY VAIO VGN-NR11M/S Battery .
- That is, if there is 1% more energy in baryons than average in one spot, then with a pure adiabatic density perturbations there is also 1% more energy in photons, and 1% more energy in neutrinos, than average. Cosmic inflation predicts that the primordial perturbations are adiabatic SONY VAIO VGN-NR11Z/S Battery .
- Isocurvature density perturbations
- the sum of the fractional overdensities is zero. That is, a perturbation where at some spot there is 1% more energy in baryons than average, 1% more energy in photons than average, and 2% lessenergy in neutrinos than average, would be a pure isocurvature perturbation SONY VAIO VGN-NR11Z/T Battery .
- Cosmic strings would produce mostly isocurvature primordial perturbations.
The CMB spectrum is able to distinguish these two because these two brands of perturbations produce different peak locations Sony VAIO VGN-FZ21E Battery .
Isocurvature density perturbations produce a series of peaks whose angular scales (l-values of the peaks) are roughly in the ratio 1:3:5:..., while adiabatic density perturbations produce peaks whose locations are in the ratio 1:2:3:... Sony VAIO VGN-FW21E Battery O
bservations are consistent with the primordial density perturbations being entirely adiabatic, providing key support for inflation, and ruling out many models of structure formation involving, for example, cosmic strings Sony VAIO VGN-NR11S/S Battery .
Collisionless damping is caused by two effects, when the treatment of the primordial plasma as fluid begins to break down:
- the increasing mean free path of the photons as the primordial plasma becomes increasingly rarefied in an expanding universeSony VAIO VGN-NR11Z/S Battery
- the finite depth of the last scattering surface (LSS), which causes the mean free path to increase rapidly during decoupling, even while some Compton scattering is still occurring.
These effects contribute about equally to the suppression of anisotropies on small scales, and give rise to the characteristic exponential damping tail seen in the very small angular scale anisotropies Sony VAIO VGN-NR11M/S Battery .
The depth of the LSS refers to the fact that the decoupling of the photons and baryons does not happen instantaneously, but instead requires an appreciable fraction of the age of the Universe up to that era. One method to quantify exactly how long this process took uses the photon visibility function (PVF) Sony VAIO VGN-NR11Z/T Battery .
This function is defined so that, denoting the PVF by P(t), the probability that a CMB photon last scattered between time t and t+dt is given by P(t)dt.
The maximum of the PVF (the time where it is most likely that a given CMB photon last scattered) is known quite precisely SONY VAIO VGN-FZ180E Battery .
The first-year WMAP results put the time at which P(t) is maximum as372±14 ka. This is often taken as the "time" at which the CMB formed. However, to figure out how long it took the photons and baryons to decouple, we need a measure of the width of the PVFSONY VAIO VGN-FZ220E Battery .
The WMAP team finds that the PVF is greater than half of its maximum value (the "full width at half maximum", or FWHM) over an interval of 115±5 ka. By this measure, decoupling took place over roughly 115,000 years, and when it was complete, the universe was roughly 487,000 years old SONY VAIO VGN-FZ340E Battery.
Late time anisotropy
Since the CMB came into existence, it has apparently been modified by several subsequent physical processes, which are collectively referred to as late-time anisotropy, or secondary anisotropy SONY VAIO VGN-FZ430E Battery .
When the CMB photons became free to travel unimpeded, ordinary matter in the universe was mostly in the form of neutral hydrogen and helium atoms. However, observations of galaxies today seem to indicate that most of the volume of the intergalactic medium (IGM) consists of ionized material (since there are few absorption lines due to hydrogen atoms) SONY VAIO VGN-FZ460E Battery .
This implies a period of reionization during which some of the material of the universe was broken into hydrogen ions.
The CMB photons scatter off free charges such as electrons that are not bound in atoms. In an ionized universe, such charged particles have been liberated from neutral atoms by ionizing (ultraviolet) radiation SONY VAIO VGN-FZ480E Battery .
Today these free charges are at sufficiently low density in most of the volume of the Universe that they do not measurably affect the CMB. However, if the IGM was ionized at very early times when the universe was still denser, then there are two main effects on the CMB SONY VAIO VGN-FZ4000 Battery :
- Small scale anisotropies are erased. (Just as when looking at an object through fog, details of the object appear fuzzy.)
- The physics of how photons scatter off from free electrons (Thomson scattering) induces polarization anisotropies on large angular scales SONY VAIO VGN-FZ31E Battery .
- This broad angle polarization is correlated with the broad angle temperature perturbation.
Both of these effects have been observed by the WMAP spacecraft, providing evidence that the universe was ionized at very early times, at a redshift more than 17. The detailed provenance of this early ionizing radiation is still a matter of scientific debate SONY VAIO VGN-FZ31B Battery .
It may have included starlight from the very first population of stars (population III stars), supernovae when these first stars reached the end of their lives, or the ionizing radiation produced by the accretion disks of massive black holes SONY VAIO VGN-FZ31J Battery .
The time following the emission of the cosmic microwave background—and before the observation of the first stars—is semi-humorously referred to by cosmologists as the dark age, and is a period which is under intense study by astronomers (See 21 centimeter radiation) SONY VAIO VGN-FZ31M Battery .
Two other effects which occurred between reionization and our observations of the cosmic microwave background, and which appear to cause anisotropies, include the Sunyaev–Zel'dovich effect, where a cloud of high-energy electrons scatters the radiationSONY VAIO VGN-FZ31Z Battery ,
transferring some of its energy to the CMB photons, and the Sachs–Wolfe effect, which causes photons from the Cosmic Microwave Background to be gravitationally redshifted or blueshifted due to changing gravitational fields.
The cosmic microwave background is polarized at the level of a few microkelvins SONY VAIO VGN-FZ38M Battery .
There are two types of polarization, called E-modes and B-modes. This is in analogy to electrostatics, in which the electric field (E-field) has a vanishing curl and the magnetic field (B-field) has a vanishing divergence. The E-modes arise naturally from Thomson scattering in a heterogeneous plasma SONY VGP-BPS8 Battery .
The B-modes, which have not been measured and are thought to have an amplitude of at most a 0.1 µK, are not produced from the plasma physics alone. They are a signal fromcosmic inflation and are determined by the density of primordial gravitational wavesSONY VGP-BPS13/S Battery .
Detecting the B-modes will be extremely difficult, particularly given that the degree of foreground contamination is unknown, and the weak gravitational lensing signal mixes the relatively strongE-mode signal with the B-mode signal SONY VGP-BPS13A/B Battery .
Microwave background observations
Subsequent to the discovery of the CMB, hundreds of cosmic microwave background experiments have been conducted to measure and characterize the signatures of the radiation SONY VGP-BPS13B/B Battery .
The most famous experiment is probably the NASA Cosmic Background Explorer (COBE) satellite that orbited in 1989–1996 and which detected and quantified the large scale anisotropies at the limit of its detection capabilities. Inspired by the initial COBE results of an extremely isotropic and homogeneous background SONY VGP-BPS13A/S Battery ,
a series of ground- and balloon-based experiments quantified CMB anisotropies on smaller angular scales over the next decade. The primary goal of these experiments was to measure the angular scale of the first acoustic peak, for which COBE did not have sufficient resolution SONY VGP-BPS13AS Battery .
These measurements were able to rule out cosmic strings as the leading theory of cosmic structure formation, and suggested cosmic inflationwas the right theory. During the 1990s, the first peak was measured with increasing sensitivity and by 2000 the BOOMERanG experiment reported that the highest power fluctuations occur at scales of approximately one degree Dell Inspiron 1320n Battery .
Together with other cosmological data, these results implied that the geometry of the Universe is flat. A number of ground-based interferometers provided measurements of the fluctuations with higher accuracy over the next three years, including the Very Small Array Dell Studio 1450 Battery ,
Degree Angular Scale Interferometer (DASI), and the Cosmic Background Imager (CBI). DASI made the first detection of the polarization of the CMB and the CBI provided the first E-mode polarization spectrum with compelling evidence that it is out of phase with the T-mode spectrum Dell Studio 1457 Battery .
In June 2001, NASA launched a second CMB space mission, WMAP, to make much more precise measurements of the great scale anisotropies over the full sky. WMAP used symmetric, rapid-multi-modulated scanning, rapid switching radiometers to minimize non-sky signal noise Dell Latitude D610 Battery .
The first results from this
and are available in detail at NASA's data bank for Cosmic Microwave Background (CMB) (see links below). Although WMAP provided very accurate measurements of the great angular-scale fluctuations in the CMB (structures about as broad in the sky as the moon)Toshiba Satellite M65 battery ,
it did not have the angular resolution to measure the smaller scale fluctuations which had been observed by former ground-based interferometers.
A third space mission, the ESA (European Space Agency) Planck Surveyor Toshiba Satellite M60 battery ,
launched in May, 2009 and is currently performing an even more detailed investigation. Planck employs both HEMTradiometers as well as bolometer technology and will measure the CMB on smaller scales than WMAP. Its detectors got a trial run at the Antarctic Viper telescope as ACBAR (Arcminute Cosmology Bolometer Array Receiver) experiment Dell Latitude D830 Battery
—which has produced the most precise measurements at small angular scales to date—and at the Archeops balloon telescope.
Additional ground-based instruments such as the South Pole Telescope in Antarctica and the proposed Clover Project Dell Latitude D620 Battery ,
Atacama Cosmology Telescope and the QUIET telescope in Chile will provide additional data not available from satellite observations, possibly including the B-mode polarization.
Data reduction and analysis
Raw CMBR data coming down from the space vehicle (i.e., WMAP) contain foreground effects that completely obscure the fine-scale structure of the Cosmic Microwave background Dell Inspiron Mini 10 Battery .
The fine-scale structure is superimposed on the raw CMBR data but is too small to be seen at the scale of the raw data. The most prominent of the foreground effects is the dipole anisotropy caused by the Sun's motion relative to the CMBR background Sony VGN-FW11S Battery .
The dipole anisotropy and others due to Earth's annual motion relative to the Sun and numerous microwave sources in the galactic plane and elsewhere must be subtracted out to reveal the extremely tiny variations characterizing the fine-scale structure of the CMBR background Sony VGN-FW11M Battery .
The detail analysis of CMBR data to produce maps, an angular power spectrum, and ultimately cosmological parameters is a complicated, computationally difficult problem. Although computing a power spectrum from a map is in principle a simple Fourier transform Sony VGN-FW139E/H battery ,
decomposing the map of the sky into spherical harmonics, in practice it is hard to take the effects of noise and foreground sources into account. In particular, these foregrounds are dominated by galactic emissions such free-free, synchrotron, and dust that emit in the microwave band Dell Latitude E5400 Battery ;
in practice, the galaxy has to be removed resulting in a CMB map that is not a full-sky map. In addition, point sources like galaxies and clusters represent another source of foreground which must be removed lest they distort the short scale structure of the CMB power spectrum Dell Latitude E4200 Battery .
Constraints on many cosmological parameters can be obtained from their effects on the power spectrum, and results are often calculated using Markov Chain Monte Carlo sampling techniques Dell Inspiron 300M Battery .
CMBR dipole anisotropy
From the CMB data it is seen that our local group of galaxies (the galactic cluster that includes the Solar System's Milky Way Galaxy) appears to be moving at 627±22 km/s relative to the reference frame of the CMB Dell Studio 1737 battery
(also called the CMB rest frame, or the frame of reference in which there is no motion through the CMB) in the direction of galactic longitude l = 276±3°, b = 30±3°. This motion results in an anisotropy of the data (CMB appearing slightly warmer in the direction of movement than in the opposite direction) Dell Inspiron E1505 battery .
The standard interpretation of this temperature variation is a simple velocity redshift and blueshift due to motion relative to the CMB, but alternative cosmological models can explain some fraction of the observed dipole temperature distribution in the CMB Dell RM791 battery .
Low multipoles and other anomalies
With the increasingly precise data provided by WMAP, there have been a number of claims that the CMB suffers from anomalies, such as very great-scale anisotropies, anomalous alignments, and non-Gaussian distributions Toshiba Satellite P10 Battery .
The most longstanding of these is the low-l multipole controversy. Even in the COBE map, it was observed that the quadrupole (l=2 spherical harmonic) has a low amplitude compared to the predictions of the big bang. Some observers have pointed out that the anisotropies in the WMAP data did not appear to be consistent with the big bang picture Acer Aspire One battery 10400mAh .
In particular, the quadrupole and octupole (l=3) modes appear to have an unexplained alignment with each other and with the ecliptic plane, an alignment sometimes referred to as the axis of evil. A number of groups have suggested that this could be the signature of new physics at the greatest observable scales; other groups suspect systematic errors in the data SONY VGN-FZ210CE Battery .
Ultimately, due to the foregrounds and the cosmic variance problem, the greatest modes will never be as well measured as the small angular scale modes. The analyses were performed on two maps that have had the foregrounds removed as best as is possible Dell Precision M70 Battery :
the "internal linear combination" map of the WMAP collaboration and a similar map prepared by Max Tegmark and others. Later analyses have pointed out that these are the modes most susceptible to foreground contamination from synchrotron, dust, and free-free emission, and from experimental uncertainty in the monopole and dipole Toshiba Satellite L305 Battery .
A fullBayesian analysis of the WMAP power spectrum demonstrates that the quadrupole prediction of Lambda-CDM cosmology is consistent with the data at the 10% level and that the observed octupole is not remarkable Toshiba Satellite T4900 Battery .
Carefully accounting for the procedure used to remove the foregrounds from the full sky map further reduces the significance of the alignment by ~5%.
Electron cyclotron resonance is a phenomenon observed both in plasma physics and condensed matter physics Toshiba PA3399U-2BRS battery .
An electron in a static and uniform magnetic field will move in a circle due to theLorentz force. The circular motion may be superimposed with a uniform axial motion, resulting in a helix, or with a uniform motion perpendicular to the field, e.g., in the presence of an electrical or gravitational field, resulting in a cycloid Sony VGP-BPS10 battery 10400mAh .
The angular frequency (? = 2?f ) of this cyclotron motion for a given magnetic field strength B is given (in SI units) by
where e is the elementary charge and m is the mass of the electron Sony VGP-BPS9 battery 10400mAh .
For the commonly used microwave frequency 2.45 GHz and the bare electron charge and mass, the resonance condition is met when B = 875 G = 0.0875 T. For particles of charge q, rest mass m0 moving at relativistic speeds v, the formula needs to be adjusted according to the special theory of relativity to Sony VGP-BPS11 battery 10400mAh. :
In plasma physics
An ionized plasma may be efficiently produced or heated by superimposing a static magnetic field and a high-frequency electromagnetic field at the electron cyclotron resonance frequency Sony VGP-BPS22 battery 10400mAh .
In the toroidal magnetic fields used in magnetic fusion energy research, the magnetic field decreases with the major radius, so the location of the power deposition can be controlled within about a centimeter. Furthermore, the heating power can be rapidly modulated and is deposited directly into the electrons Sony VGP-BPL8 battery 10400mAh .
These properties make electron cyclotron heating a very valuable research tool for energy transport studies. In addition to heating, electron cyclotron waves can be used to drive current. The inverse process of electron cyclotron emission can be used as a diagnostic of the radial electron temperature profile Sony VGP-BPS14 battery 10400mAh .
ECR ion sources
Since the early 1980s, following the award-winning pioneering work done by Dr. Richard Geller, Dr. Claude Lyneis, and Dr. H. Postma; respectively from French Atomic Energy Commission,Lawrence Berkeley National Laboratory and the Oak Ridge National Laboratory Sony VGP-BPS21 battery 10400mAh .
the use of electron cyclotron resonance for efficient plasma generation, especially to obtain large numbers of multiply charged ions, has acquired a unique importance in various technological fields. Many diverse activities depend on electron cyclotron resonance technology, including Toshiba Satellite A200 Battery
- advanced cancer treatment, where ECR ion sources are crucial for proton therapy,
- advanced semiconductor manufacturing, especially for high density DRAM memories, through plasma etching or other plasma processing technologies Toshiba Satellite 1200 Battery .
- electric propulsion devices for spacecraft propulsion, where a broad range of devices (HiPEP, some ion thrusters, or electrodeless plasma thrusters),
- for particle accelerators, on-line mass separation and radioactive ion charge breeding, Toshiba Satellite M300 Battery
- and, as a more mundane example, painting of plastic bumpers for cars.
The ECR ion source makes use of the electron cyclotron resonance to heat a plasma SONY VGP-BPS13A/Q Battery .
Microwaves are injected into a volume, at the frequency corresponding to the electron cyclotron resonance defined by a magnetic field applied to a region inside the volume. The volume contains a low pressure gas SONY VGP-BPS13A/Q Battery .
The microwaves heat free electrons in the gas which in turn collide with the atoms or molecules of the gas in the volume and cause ionization. The ions produced correspond to the gas type used. The gas may be pure, a compound gas, or can be a vapour of a solid or liquid material SONY VGP-BPS13B/Q Battery .
ECR ion sources are able to produce singly charged ions with high intensities (e.g. H+ and D+ ions of more than 100 mA (electrical) in DC mode using a 2.45 GHz ECR ion source) SONY VGP-BPS13B/Q Battery .
For multiply-charged ions, the ECR ion source has the advantage that it is able to confine the ions for long enough for multiple collisions to take place (leading to multiple ionization) and that the low gas pressure in the source avoids recombination SONY VGP-BPS13/Q Battery .
The VENUS ECR ion source at Lawrence Berkeley National Laboratory has produced in intensity of 0.25 mA (electrical) of Bi29+.
Some of these industrial fields would not even exist without the use of this fundamental technology, which makes electron cyclotron resonance ion and plasma sources one of the enabling technologies of today's world SONY VGP-BPS13/Q Battery .
A low noise block-downconverter (or LNB) is the receiving device of a parabolic satellite dish antenna of the type commonly used for satellite TVreception. The device is sometimes called an LNA (for low noise amplifier), LNC (for low noise converter) or even LND (for low noise downconverter) SONY VGP-BPS21A/B Battery
but as block-downconversion is the principle function of the device, LNB is the preferred term, although this acronym is often incorrectly expanded to the incomplete descriptions, low noise block or low noise block converter SONY VGP-BPS21/S Battery .
It is functionally equivalent to the dipole antenna used for most terrestrial TV reception, although it is actually waveguide based. Inside the LNB waveguide a metal pin, or probe, protrudes into the waveguide at right angles to the axis and this acts as an aerial, collecting the signal travelling down the waveguide SONY VGP-BPS21B Battery .
The LNB is usually fixed on the satellite dish framework, at the focus of the reflector, and it derives its power from the connected receiver, sent "up" the same cable that carries the received signals "down" to the receiver. The corresponding component in the transmit link uplink to a satellite is called a Block upconverter (BUC) SONY VGP-BPS21A Battery .
Satellites use comparatively high radio frequencies (microwaves) to transmit their TV signals. As microwave satellite signals do not easily pass through walls, roofs, or even glass windows, satelliteantennas are required to be outdoors, and the signal needs to be passed indoors via cables SONY VGP-BPS21 Battery .
When radio signals are sent through coaxial cables, the higher the frequency, the more losses occur in the cable per unit of length. The signals used for satellite are of such high frequency (in the multiple gigahertz range) that special (costly) cable types or waveguides would be required and any significant length of cable leaves very little signal at the receiving end Sony VGP-BPS21A/B Battery .
The purpose of the LNB is to use the superheterodyne principle to take a block (or band) of relatively high frequencies and convert them to similar signals carried at a much lower frequency (called theintermediate frequency or IF) Sony VGP-BPS21/S Battery .
These lower frequencies travel through cables with much less attenuation, so there is much more signal left at the satellite receiver end of the cable. It is also much easier and cheaper to design electronic circuits to operate at these lower frequencies, rather than the very high frequencies of satellite transmission Sony VGP-BPS21A Battery .
The frequency conversion is performed by mixing a fixed frequency produced by a local oscillator inside the LNB with the incoming signal, to generate two signals equal to the sum of their frequencies and the difference. The frequency sum signal is filtered out and the frequency difference signal (the IF) is amplified and sent down the cable to the receiver Sony VGP-BPS21 Battery :
IF frequency = received frequency - local oscillator frequency
The local oscillator frequency determines what block of incoming frequencies is downconverted to the frequencies expected by the receiver Sony Vaio PCG-5G2L Battery .
For example, to downconvert the incoming signals fromAstra 1KR, which transmits in a frequency block of 10.70GHz-11.70GHz, to within a standard European receiver’s IF tuning range of 950MHz-2150MHz, a 9.75GHz local oscillator frequency is used, producing a block of signals in the band 950MHz-1950MHz Sony Vaio PCG-5G3L Battery .
For the block of higher transmission frequencies used by Astra 2A and 2B (11.70GHz-12.75GHz), a different local oscillator frequency converts the block of incoming frequencies. Typically, a local oscillator frequency of 10.60GHz is used to downconvert the block to 1100MHz-2150MH, which is, again, within the receiver’s 950MHz-2150MHz IF tuning range Sony Vaio PCG-5J1L Battery .
For the reception of wideband satellite television carriers, typically 27 MHz wide, the accuracy of the frequency of the LNB local oscillator need only be in the order of ±500 kHz, so low cost dielectric oscillators (DRO) may be used Sony Vaio PCG-5K2L Battery .
For the reception of narrow bandwidth carriers or ones using advanced modulation techniques, such as 16-QAM, highly stable and low phase noise LNB local oscillators are required. These use an internal crystal oscillator or an external 10 MHz reference from the indoor unit and a phase-locked loop (PLL) oscillator Sony Vaio PCG-5J2L Battery .
Amplification and Noise
To compensate for the inevitable attenuation of the extremely weak signal received, by its transmission along the cable to the receiver, and to keep introduced noise (unwanted signals) at a low level relative to the signal Sony Vaio PCG-5K1L Battery .
the LNB provides considerable amplification, both of the signal from the waveguide probe and of the converted IF signal. Special electronic engineering techniques are used to ensure the signal has less noise on the output than would be possible with less stringent engineering Sony Vaio PCG-6S2L Battery .
If low noise engineering techniques were not used, the sound and picture of satellite TV would be of very low quality, if it could be received at all without a much larger dish reflector.
The low noise quality of an LNB is expressed as the noise figure (or sometimes noise temperature) Sony Vaio PCG-6S3L Battery .
This is the ratio of the amount of noise in the output to the amount in the input, in decibels (dB). The ideal LNB would have a noise figure of 0dB. In fact, every LNB introduces some noise, although clever design, expensive components and even individual tweaking of the LNB after manufacture, can reduce noise levels to very low levels Sony Vaio PCG-6V1L Battery .
Every LNB off the production line has a different noise figure because of manufacturing tolerances. When you choose an LNB, the noise figure quoted in the specifications is important to determine its suitability but the figure quoted is usually representative of neither that particular LNB nor the performance across the whole frequency range Sony Vaio PCG-6W1L Battery .
The noise figure quoted for most LNBs is the typical figure and should be an average of the production batch. Even if it's an accurate average (only a few samples from the batch are actually measured), the sample range is not given and so you cannot tell how far from the quoted average your LNB may lie Sony Vaio PCG-6W2L Battery .
In addition, the figures measured are often at just one frequency so the quoted figure tells you nothing about the amount of noise introduced at other frequencies, which may be considerably higher . Sony Vaio PCG-6W3L Battery .
With the launch of the first DTH broadcast satellite in Europe (Astra 1A) by SES Astra in 1988, antenna design was simplified for the anticipated mass-market. In particular, the feedhorn Sony Vaio PCG-7111L Battery
(which gathers the signal and directs it to the LNB) and the polarizer (which selects between differently polarized signals) were combined with the LNB itself into a single unit, called an LNB-feed or LNB-feedhorn (LNBF), or even an "Astra type" LNB Sony Vaio PCG-7112L Battery .
The prevalence of these combined units has meant that today the term LNB is commonly used to refer to all antenna units that provide the block-downconversion function, with or without a feedhorn.
The probe inside the LNB waveguide collects signals that are polarized in the same plane as the probe Sony Vaio PCG-7113L Battery .
To maximise the strength of the wanted signals (and to minimise reception of unwanted signals of the opposite polarization), the probe is aligned with the polarization of the incoming signals. This is most simply achieved by adjusting the LNB's skew - its rotation about the waveguide axis Sony Vaio PCG-7133L Battery .
To remotely select between the two polarizations, and to compensate for inaccuracies of the skew angle, it used to be common to fit a polarizer in front of the LNB's waveguide mouth. This either rotated the incoming signal with an electromagnet around the waveguide (a magnetic polarizer) Sony Vaio PCG-7Z2L Battery
or rotated an intermediate probe within the waveguide using a servo motor (a mechanical polarizer) but such adjustable skew polarizers are rarely used today Sony Vaio PCG-8Y1L Battery .
The simplification of antenna design that accompanied the first Astra DTH broadcast satellites in Europe to produce the LNBF extended to a simpler approach to the selection between vertical and horizontal polarized signals too Sony Vaio PCG-8Y2L Battery .
Astra type LNBFs incorporate two probes in the waveguide, at right angles to one another so that, once the LNB has been skewed in its mount to match the local polarization angle, one probe collects horizontal signals and the other vertical, and an electronic switch (controlled by the voltage of the LNB's power supply from the receiver Sony Vaio PCG-8Z1L Battery
- 13 volts for vertical and 18 volts for horizontal) determines which polarization is passed on through the LNB for amplification and block-downconversion.
Such LNBs can receive all the transmissions from a satellite with no moving parts and with just one cable connected to the receiverSony Vaio PCG-8Z2L Battery ,
and have since become the most common type of LNB produced.
In Europe, as SES Astra launched more satellites to the Astra 19.2°E orbital position in the 1990s SONY VAIO VGN-FW31J Battery ,
the range of downlink frequencies used in the FSS band (10.70-11.70GHz) grew beyond that catered for by the standard LNBs and receivers of the time. Reception of signals from Astra 1D required an extension of receivers’ IF tuning range from 950-150MHz to 950-2150MHz and a change of LNBs’ local oscillator frequency from the usual 10GHz to 9.75GHz SONY VAIO VGN-FW31E Battery.
The launch of Astra 1E and subsequent satellites saw the first use by Astra of the BSS band of frequencies (11.70-12.75GHz) for new digital services and required the introduction of an LNB that would receive the whole frequency range 10.70-12.75GHz - the “Universal” LNB SONY VAIO VGN-FW31B Battery .
A Universal LNB has a switchable local oscillator frequency of 9.75/10.60GHz to provide two modes of operation – low band reception (10.70-11.70GHZ) and high band reception (11.70-12.75GHz). The local oscillator frequency is switched in response to a 22kHz signal superimposed on the supply voltage from the connected receiver SONY VAIO VGN-FW31M Battery .
Along with the supply voltage level used to switch between polarizations, this enables a Universal LNB to receive both polarizations (Vertical and Horizontal) and the full range of frequencies in the satellite Ku band under the control of the receiver, in four sub-bandsSONY VAIO VGN-FW31Z Battery :
An LNB with a single feedhorn but multiple outputs for connection to multiple tuners (in separate receivers or within the same receiver in the case of a twin-tuner PVR receiver). Typically, two, four or eight outputs are provided SONY VAIO VGN-FW38M Battery .
Each output responds to the tuner’s band and polarization selection signals independently of the other outputs and "appears" to the tuner to be a separate LNB. Such an LNB usually may derive its power from a receiver connected to any of the outputs. Unused outputs may be left unconnected (but waterproofed for the protection of the whole LNB) SONY VAIO VGN-AW41JF Battery .
Note: In the US, an LNB with two outputs is termed a "dual LNB" but in the UK, the term "dual LNB" describes antennas for reception from two satellite positions, using either two separate LNBs or a single Monoblock LNB with two feedhorns SONY VAIO VGN-AW41JF/H Battery .
In the UK, the term "twin-output LNB", or simply "twin LNB", is usually used for an LNB with a single feedhorn but two independent outputs.
A special type of LNB intended for use in a shared dish installation to deliver signals to any number of tuners SONY VAIO VGN-AW41MF Battery .
A quattro LNB has a single feedhorn and four outputs, which each supply just one of the Ku sub-bands (low band/horizontal polarization, high band/vertical polarization, low/vertical and high/horizontal) to a multiswitch or an array of multiswitches, which then delivers to each connected tuner whichever sub-band is required by that tuner SONY VAIO VGN-AW41MF/H Battery .
Although a quattro LNB typically looks similar to a quad LNB, it cannot (sensibly) be connected to receivers directly. Note again the difference between a quad and a quattro LNB: A quad LNB can drive four tuners directly, with each output providing signals from the entire Ku-band SONY VAIO VGN-FW30B Battery .
A quattro LNB is for connection to a multiswitch in a shared dish distribution system and each output provides only a quarter of the Ku-band signals.
A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission SONY VAIO VGN-FW50B Battery .
Historically, “maser” derives from the original, upper-case acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation". The lower-case usage arose from technological development having rendered the original denotation imprecise SONY VAIO VGN-FW51B/W Battery ,
because contemporary masers emit EM waves (microwave and radio frequencies) across a broader band of the electromagnetic spectrum; thus, the physicist Charles H. Townes’s suggested usage of “molecular” replacing “microwave”, for contemporary linguistic accuracy SONY VAIO VGN-FW51MF Battery .
In 1957, when the optical coherent oscillator was first developed, it was denominated optical maser, but usually called laser (Light Amplification by Stimulated Emission of Radiation), the acronymGordon Gould established in 1957 SONY VAIO VGN-FW51MF/H Battery .
Theoretically, the principle of the maser was described by Nikolay Basov and Alexander Prokhorov from Lebedev Institute of Physics at an All-Union Conference on Radio-Spectroscopy held by USSR Academy of Sciences in May 1952 SONY VAIO VGN-FW51ZF Battery .
They subsequently published their results in October 1954. Independently, Charles H. Townes, J. P. Gordon, and H. J. Zeiger built the first maser at Columbia University in 1953. The device used stimulated emission in a stream of energized ammonia molecules to produce amplification of microwavesat a frequency of 24 gigahertz SONY VAIO VGN-FW51ZF/H Battery .
Townes later worked with Arthur L. Schawlow to describe the principle of the optical maser, or laser, which Theodore H. Maiman first demonstrated in 1960. For their research in this field Townes, Basov, and Prokhorov were awarded the Nobel Prize in Physics in 1964 SONY VAIO VGN-FW52JB Battery .
The maser is based on the principle of stimulated emission proposed by Albert Einstein in 1917. When atoms have been put into an excited energy state, they can amplify radiation at the proper frequency SONY VAIO VGN-FW54FB Battery .
By putting such an amplifying medium in a resonant cavity, feedback is created that can produce coherent radiation.
Masers serve as high precision frequency references. These "atomic frequency standards" are one form of atomic clock SONY VAIO VGN-FW70DB Battery .
They are also used as electronic amplifiers in radio telescopes. Masers are being developed as directed-energy weapons.
Today, the most important type of maser is the hydrogen maser which is currently used as an atomic frequency standard SONY VAIO VGN-FW71DB/W Battery .
Together with other types of atomic clocks, they constitute the "Temps Atomique International" or TAI. This is the international time scale, which is coordinated by the Bureau International des Poids et Mesures, or BIPM.
It was Norman Ramsey and his colleagues who first realized this device SONY VAIO VGN-FW72JGB Battery .
Today's masers are identical to the original design. The maser oscillation relies on stimulated emission between two hyperfine levels of atomic hydrogen. Here is a brief description of how it works:
- First, a beam of atomic hydrogen is produced. This is done by submitting the gas at low pressure to an RF discharge (see the picture on this page) SONY VAIO VGN-FW73JGB Battery .
- The next step is "state selection"—in order to get some stimulated emission, it is necessary to create a population inversion of the atoms. This is done in a way that is very similar to the famous Stern-Gerlach experiment SONY VAIO VGN-FW74FB Battery.
- After passing through an aperture and a magnetic field, many of the atoms in the beam are left in the upper energy level of the lasing transition. From this state, the atoms can decay to the lower state and emit some microwave radiation SONY VAIO VGN-FW81HS Battery .
- A high quality factor microwave cavity confines the microwaves and reinjects them repeatedly into the atom beam. The stimulated emission amplifies the microwaves on each pass through the beam. This combination of amplification and feedback is what defines all oscillators SONY VAIO VGN-FW81NS Battery .
- The resonant frequency of the microwave cavity is exactly tuned to the hyperfine structure of hydrogen: 1420 405 751.768 Hz.
- A small fraction of the signal in the microwave cavity is coupled into a coaxial cable and then sent to a coherent receiver SONY VAIO VGN-FW81S Battery .
- The microwave signal coming out of the maser is very weak (a few pW). The frequency of the signal is fixed and extremelystable. The coherent receiver is used to amplify the signal and change the frequency. This is done using a series of phase-locked loops and a high performance quartz oscillator SONY VAIO VGN-FW82DS Battery .
Maser-like stimulated emission also occurs in nature in interstellar space, and is frequently called superradiant emission to distinguish it from laboratory masers SONY VAIO VGN-FW82JS Battery.
Such emission is observed from molecules such as water (H2O), hydroxyl radicals (OH), methanol (CH3OH), formaldehyde (CH2O), and silicon monoxide (SiO). Water molecules in star-forming regions can undergo a population inversion and emit radiation at 22 GHz, creating the brightest spectral line in the radio universe SONY VAIO VGN-FW82XS Battery .
Some water masers also emit radiation from a vibrational mode at 96 GHz.
Extremely powerful masers, associated with active galactic nuclei, are known as megamasers and are up to a million times more powerful than stellar masers SONY VAIO VGN-FW83DS Battery .
The meaning of the term maser has changed slightly since its introduction. Initially the acronym was universally given as "microwave amplification by stimulated emission of radiation," which described devices which emitted in the microwave region of the electromagnetic spectrum SONY VAIO VGN-FW83JS Battery .
The principle of stimulated emission has since been extended to more devices and frequencies, and so the original acronym is sometimes modified, as suggested by Charles H. Townes, to "molecular amplification by stimulated emission of radiation SONY VAIO VGN-FW83XS Battery .
" Some have asserted that Townes's efforts to extend the acronym in this way were primarily motivated by the desire to increase the importance of his invention, and his reputation in the scientific community SONY VAIO VGN-FW90NS Battery .
When the laser was developed, Townes and Schawlow and their colleagues at Bell Labs pushed the use of the term optical maser, but this was largely abandoned in favor of laser, coined by their rivalGordon Gould. In modern usage, devices that emit in the X-ray through infrared portions of the spectrum are typically called lasers SONY VAIO VGN-FW37J/B Battery ,
and devices that emit in the microwave region and below are commonly called masers, regardless of whether they emit microwaves or other frequencies.
Gould originally proposed distinct names for devices that emit in each portion of the spectrum, including grasers SONY VAIO VGN-FW37J Battery
(gamma ray lasers), xasers (x-ray lasers), uvasers (ultraviolet lasers), lasers (visiblelasers), irasers (infrared lasers), masers(microwave masers), and rasers (RF masers). Most of these terms never caught on, however, and all have now become (apart from in science fiction) obsolete except for maser and laser SONY VAIO VGN-FW378J/B Battery .