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The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field. The 'resonant' cavity magnetron variant of the earlier magnetron tube was invented by Randall and Boot in 1940  Lenovo S10-2 Battery 5200mAh .

The high power of pulses from the cavity magnetron made centimetre-band radar practical. Shorter wavelength radars allowed detection of smaller objects. The compact cavity magnetron tube drastically reduced the size of radar sets so that they could be installed in anti-submarine aircraft and escort ships Lenovo Y730 Battery 5200mAh .

At present, cavity magnetrons are commonly used in microwave ovens and in various radar applications.

All cavity magnetrons consist of a hot cathode with a high (continuous or pulsed) negative potential by a high-voltage, direct-current power supply Lenovo Y710 Battery 5200mAh .

The cathode is built into the center of an evacuated, lobed, circular chamber. A magnetic field parallel to the filament is imposed by a permanent magnet. The magnetic field causes the electrons, attracted to the (relatively) positive outer part of the chamber, to spiral outward in a circular path rather than moving directly to this anode  Lenovo Y530 Battery 5200mAh .

Spaced around the rim of the chamber are cylindrical cavities. The cavities are open along their length and connect the common cavity space. As electrons sweep past these openings, they induce a resonant, high-frequency radio field in the cavity, which in turn causes the electrons to bunch into groups  Lenovo Y510 Battery 5200mAh .

A portion of this field is extracted with a short antenna that is connected to a waveguide (a metal tube usually of rectangular cross section). The waveguide directs the extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high-gain antenna in the case of radar Lenovo S10-3 Battery 5200mAh .

The sizes of the cavities determine the resonant frequency, and thereby the frequency of emitted microwaves. However, the frequency is not precisely controllable. The operating frequency varies with changes in load impedance, with changes in the supply current, and with the temperature of the tube Lenovo G450 Battery 5200mAh .

This is not a problem in uses such as heating, or in some forms of radar where the receiver can be synchronized with an imprecise magnetron frequency. Where precise frequencies are needed, other devices such as the klystron are used.

The magnetron is a self-oscillating device requiring no external elements other than a power supply Lenovo E30 Battery 5200mAh .

A well-defined threshold anode voltage must be applied before oscillation will build up; this voltage is a function of the dimensions of the resonant cavity, and the applied magnetic field. In pulsed applications there is a delay of several cycles before the oscillator achieves full peak power , and the build-up of anode voltage must be coordinated with the build-up of oscillator output Lenovo E40 Battery 5200mAh .

The magnetron is a fairly efficient device. In a microwave oven, for instance, a 1.1 kilowatt input will generally create about 700 watt of microwave power, an efficiency of around 65%. (The high-voltage and the properties of the cathode determine the power of a magnetron.) Lenovo E50 Battery 5200mAh

Large S-band magnetrons can produce up to 2.5 megawatts peak power with an average power of 3.75 kW. Large magnetrons can be water cooled. The magnetron remains in widespread use in roles which require high power, but where precise frequency control is unimportant Lenovo G460 Battery 5200mAh .

Radar

In radar devices the waveguide is connected to an antenna. The magnetron is operated with very short pulses of applied voltage, resulting in a short pulse of high power microwave energy being radiated Lenovo Y460 Battery 5200mAh .

As in all radar systems, the radiation reflected off a target is analyzed to produce a radar map on a screen.

Several characteristics of the magnetron's power output conspire to make radar use of the device somewhat problematic Lenovo Y450 Battery 5200mAh .

The first of these factors is the magnetron's inherent instability in its transmitter frequency. This instability is noted not only as a frequency shift from one pulse to the next, but also a frequency shift within an individual transmitter pulse Lenovo N200 Battery 5200mAh .

The second factor is that the energy of the transmitted pulse is spread over a wide frequency spectrum, which makes necessary its receiver to have a corresponding wide selectivity. This wide selectivity permits ambient electrical noise to be accepted into the receiver Lenovo T410 Battery 5200mAh ,

thus obscuring somewhat the received radar echoes, thereby reducing overall radar performance. The third factor, depending on application, is the radiation hazard caused by the use of high power electromagnetic radiation. In some applications, for example a marine radar mounted on a recreational vessel Lenovo T510 Battery 5200mAh ,

a radar with a magnetron output of 2 to 4 kilowatts is often found mounted very near an area occupied by crew or passengers. In practical use, these factors have been overcome, or merely accepted, and there are today thousands of magnetron aviation and marine radar units in service Lenovo SL410 Battery 5200mAh .

Recent advances in aviation weather avoidance radar and in marine radar have successfully implemented semiconductor transmitters that eliminate the magnetron entirely.

Heating

In microwave ovens the waveguide leads to a radio frequency-transparent port into the cooking chamber Lenovo SL510 Battery 5200mAh .

Lighting

In microwave-excited lighting systems, such as a sulfur lamp, a magnetron provides the microwave field that is passed through a waveguide to the lighting cavity containing the light-emitting substance (e.g., sulfur, metal halides, etc.) Lenovo C100 Battery 5200mAh

History

The first simple, two-pole magnetron was developed in 1920 by Albert Hull at General Electric's Research Laboratories (Schenectady, New York), as an outgrowth of his work on the magnetic control of vacuum tubes in an attempt to work around the patents held by Lee De Forest on electrostatic control Lenovo T400S Battery 5200mAh .

Hull's magnetron was not originally intended to generate VHF (very-high-frequency) electromagnetic waves. However, in 1924, Czech physicist August Žá?ek (1886–1961) and German physicist Erich Habann (1892–1968) independently discovered that the magnetron could generate waves of 100 megahertz to 1 gigahertz  Lenovo Y430 Battery 5200mAh .

Žá?ek, a professor at Prague's Charles University, published first; however, he published in a journal with a small circulation and thus attracted little attention. Habann, a student at the University of Jena, investigated the magnetron for his doctoral dissertation of 1924 Ibm ThinkPad G40 Battery 5200mAh .

Throughout the 1920s, Hull and other researchers around the world worked to develop the magnetron. Most of these early magnetrons were glass vacuum tubes with multiple anodes. However, the two-pole magnetron, also known as a split-anode magnetron, had relatively low efficiency  Ibm ThinkPad X200T Battery 5200mAh .

The cavity version (properly referred to as a resonant-cavity magnetron) proved to be far more useful.

While radar was being developed during World War II, there arose an urgent need for a high-power microwave generator that worked at shorter wavelengths (around 10 cm (3 GHz)) Apple A1280 battery 5200mAh

rather than the 150 cm (200 MHz) that was available from tube-based generators of the time. It was known that a multi-cavity resonant magnetron had been developed and patented in 1935 by Hans Hollmann inBerlin. However, the German military considered its frequency drift to be undesirable and based their radar systems on the klystron instead  Apple A1281 battery 5200mAh .

But klystrons could not achieve the high power output that magnetrons eventually reached. This was one reason that German night fighter radars were not a match for their British counterparts Samsung R522 battery 5200mAh .

In 1940, at the University of Birmingham in the United Kingdom, John Randall and Harry Boot produced a working prototype similar to Hollman's cavity magnetron, but added liquid cooling and a stronger cavity. Randall and Boot soon managed to increase its power output 100 fold Samsung P467 battery 5200mAh .

Instead of abandoning the magnetron due to its frequency instability, they sampled the output signal and synchronized their receiver to whatever frequency was actually being generated. In 1941, the problem of frequency instability was solved by coupling alternate cavities within the magnetron Hp Pavilion dm3 Battery 5400mAh .

Because France had just fallen to the Nazis and Britain had no money to develop the magnetron on a massive scale, Churchill agreed that Sir Henry Tizard should offer the magnetron to the Americans in exchange for their financial and industrial help (the Tizard Mission) Hp Pavilion dm4 Battery 5200mAh .

An early 6 kW version, built in England by the General Electric Company Research Laboratories, Wembley, London (not to be confused with the similarly named American company General Electric), was given to the US government in September 1940 Hp Pavilion cq62 Battery 5200mAh .

At the time the most powerful equivalent microwave producer available in the US (a klystron) had a power of only ten watts. The cavity magnetron was widely used during World War II in microwave radar equipment and is often credited with giving Allied radar a considerable performance advantage over German and Japanese radars Hp Pavilion cq42 Battery 5200mAh ,

thus directly influencing the outcome of the war. It was later described as "the most valuable cargo ever brought to our shores".

The Bell Telephone Laboratories made a producible version from the magnetron delivered to America by the Tizard Mission Hp Pavilion cq72 Battery 5200mAh ,

and before the end of 1940, the Radiation Laboratory had been set up on the campus of the Massachusetts Institute of Technology to develop various types of radar using the magnetron. By early 1941, portable centimetric airborne radars were being tested in American and British planes Hp cq20 Battery 2600mAh .

In late 1941, the Telecommunications Research Establishment in Great Britain used the magnetron to develop a revolutionary airborne, ground-mapping radar codenamed H2S. TheH2S radar was in part developed by Alan Blumlein and Bernard Lovell Hp 2230 Battery 2600mAh .

Centimetric radar, made possible by the cavity magnetron, allowed for the detection of much smaller objects and the use of much smaller antennas. The combination of small-cavity magnetrons, small antennas, and high resolution allowed small, high quality radars to be installed in aircraft Dell Inspiron Mini 12 battery 7800mAh .

They could be used by maritime patrol aircraft to detect objects as small as a submarine periscope, which allowed aircraft to attack and destroy submerged submarines which had previously been undetectable from the air. Centimetric contour mapping radars like H2S improved the accuracy of Allied bombers used in the strategic bombing campaign Dell XPS M1730 battery 7800mAh .

Centimetric gun-laying radars were likewise far more accurate than the older technology. They made the big-gunned Allied battleships more deadly and, along with the newly developed proximity fuze, made anti-aircraft guns much more dangerous to attacking aircraft Dell XPS M2010 battery 7800mAh .

The two coupled together and used by anti-aircraft batteries, placed along the flight path of German V-1 flying bombs on their way to London, are credited with destroying many of the flying bombs before they reached their target Dell Studio XPS 1340 battery 5200mAh/7800mAh .

Since then, many millions of cavity magnetrons have been manufactured; while some have been for radar the vast majority have been for microwave ovens. The use in radar itself has dwindled to some extent, as more accurate signals have generally been needed and developers have moved to klystron and traveling-wave tube systems for these needs Dell Inspiron 9100 battery 7800mAh .

Health hazards

Among more speculative hazards, at least one in particular is well known and documented. As the lens of the eye has no cooling blood flow, it is particularly prone to overheating when exposed to microwave radiation Dell Inspiron N4010 battery 5200mAh .

This heating can in turn lead to a higher incidence of cataracts in later life. A microwave oven with a warped door or poor microwave sealing can be hazardous.

There is also a considerable electrical hazard around magnetrons, as they require a high voltage power supply Dell Inspiron N3610 battery 5200mAh .

Some magnetrons have beryllium oxide (beryllia) ceramic insulators, which are dangerous if crushed and inhaled, or otherwise ingested. Single or chronic exposure can lead to berylliosis, an incurable lung condition. In addition, beryllia is listed as a confirmed human carcinogen by the IARC Dell Inspiron N5010 battery 5200mAh ;

therefore, broken ceramic insulators or magnetrons should not be directly handled.

A klystron is a specialized linear-beam vacuum tube (evacuated electron tube) Dell Inspiron N7010 battery 5200mAh .

Klystrons are used as amplifiers at microwave and radio frequencies to produce both low-power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for modern particle accelerators Dell Inspiron 1464 battery 5200mAh .

Klystron amplifiers have the advantage (over the magnetron) of coherently amplifying a reference signal so its output may be precisely controlled inamplitude, frequency and phase. Many klystrons have a waveguide for coupling microwave energy into and out of the device Dell Inspiron 1564 battery 5200mAh ,

although it is also quite common for lower power and lower frequency klystrons to use coaxial couplings instead. In some cases a coupling probe is used to couple the microwave energy from a klystron into a separate external waveguide Dell Inspiron 1764 battery 5200mAh .

All modern klystrons are amplifiers, since reflex klystrons, which were used as oscillators in the past, have been surpassed by alternative technologies.

The name klystron comes from the stem form ????- (klys) of a Greek verb referring to the action of waves breaking against a shore, and the end of the word electron Dell Inspiron 1320 battery 5200mAh .

History

The brothers Russell and Sigurd Varian of Stanford University are the inventors of the klystron. Their prototype was completed in August 1937. Upon publication in 1939, news of the klystron immediately influenced the work of US and UK researchers working on radar equipment Dell Inspiron 1320n battery 5200mAh .

The Varians went on to found Varian Associates to commercialize the technology (for example to make small linear accelerators to generate photons for external beamradiation therapy). In their 1939 paper, they acknowledged the contribution of A. Arsenjewa-Heil and Oskar Heil (wife and husband) for their velocity modulation theory in 1935 Dell Vostro 1710 battery 5200mAh .

The work of physicist W.W. Hansen was instrumental in the development of the klystron and was cited by the Varian brothers in their 1939 paper. His resonator analysis, which dealt with the problem of accelerating electrons toward a target, could be used just as well to decelerate electrons (i.e., transfer their kinetic energy to RF energy in a resonator) Dell Vostro 1720 battery 5200mAh .

During the second World War, Hansen lectured at the MIT Radiation labs two days a week, commuting to Boston from Sperry gyroscope company on Long Island. His resonator, called a "hohlraum" by nuclear physicists and coined "rhumbatron" by the Varian brothers, is used in 2009 in the National Ignition Facility investigating nuclear fusion Dell Studio XPS 1640 battery 5200mAh .

Hansen died in 1949 as a result of exposure to beryllium oxide (BeO).

During the second World War, the Axis powers relied mostly on (then low-powered) klystron technology for their radar system microwave generation  Dell Vostro A840 battery 5200mAh ,

while the Allies used the far more powerful but frequency-drifting technology of the cavity magnetron for microwave generation. Klystron tube technologies for very high-power applications, such as synchrotrons and radar systems, have since been developedDell Vostro A860 battery 5200mAh .

Explanation

Klystrons amplify RF signals by converting the kinetic energy in a DC electron beam into radio frequency power. A beam of electrons is produced by a thermionic cathode (a heated pellet of low work function material), and accelerated by high-voltage electrodes (typically in the tens of kilovolts) Dell Studio 1535 battery 5200mAh/7800mAh .

This beam is then passed through an input cavity. RF energy is fed into the input cavity at, or near, itsnatural frequency to produce a voltage which acts on the electron beam. The electric field causes the electrons to bunch: electrons that pass through during an opposing electric field are accelerated and later electrons are slowed Dell Studio 1735 battery 5200mAh/7800mAh ,

causing the previously continuous electron beam to form bunches at the input frequency. To reinforce the bunching, a klystron may contain additional "buncher" cavities. The RF current carried by the beam will produce an RF magnetic field, and this will in turn excite a voltage across the gap of subsequent resonant cavities Dell Vostro 1310 battery 5200mAh .

In the output cavity, the developed RF energy is coupled out. The spent electron beam, with reduced energy, is captured in a collector.

Two-cavity klystron amplifier

In the two-chamber klystron, the electron beam is injected into a resonant cavity Dell Latitude E6400 battery,10400mAh/7800mAh .

The electron beam, accelerated by a positive potential, is constrained to travel through a cylindrical drift tube in a straight path by an axial magnetic field. While passing through the first cavity, the electron beam is velocity modulated by the weak RF signal Dell Latitude E6400 Battery,10400mAh/7800mAh .

In the moving frame of the electron beam, the velocity modulation is equivalent to a plasma oscillation.Plasma oscillations are rapid oscillations of the electron density in conducting media such as plasmas ormetals.(The frequency only depends weakly on the wavelength) Dell Latitude E6400 Battery,10400mAh/7800mAh .

So in a quarter of one period of the plasma frequency, the velocity modulation is converted to density modulation, i.e. bunches of electrons. As the bunched electrons enter the second chamber they induce standing waves at the same frequency as the input signal. The signal induced in the second chamber is much stronger than that in the first Dell Latitude E6500 battery,10400mAh/7800mAh .

Two-cavity klystron oscillator

The two-cavity amplifier klystron is readily turned into an oscillator klystron by providing a feedback loop between the input and output cavities Dell Latitude E6500 battery,10400mAh/7800mAh .

Two-cavity oscillator klystrons have the advantage of being among the lowest-noisemicrowave sources available, and for that reason have often been used in the illuminator systems of missile targeting radars. The two-cavity oscillator klystron normally generates more power than the reflex klystron—typically watts of output rather than milliwatts Dell Latitude E6500 battery,10400mAh/7800mAh .

Since there is no reflector, only one high-voltage supply is necessary to cause the tube to oscillate, the voltage must be adjusted to a particular value. This is because the electron beam must produce the bunched electrons in the second cavity in order to generate output power Dell Latitude E6500 battery,10400mAh/7800mAh .

Voltage must be adjusted to vary the velocity of the electron beam (and thus the frequency) to a suitable level due to the fixed physical separation between the two cavities. Often several "modes" of oscillation can be observed in a given klystron HP Pavilion DV6-1210SA battery - 5200mAh/8800mAh .

Reflex klystron

In the reflex klystron (also known as a 'Sutton' klystron after its inventor), the electron beam passes through a single resonant cavity. The electrons are fired into one end of the tube by an electron gun  HP Pavilion DV7 battery - 7200mAh .

After passing through the resonant cavity they are reflected by a negatively charged reflector electrode for another pass through the cavity, where they are then collected. The electron beam is velocity modulated when it first passes through the cavity. The formation of electron bunches takes place in the drift space between the reflector and the cavity Ibm THINKPAD X200 Battery 8800mAh .

The voltageon the reflector must be adjusted so that the bunching is at a maximum as the electron beam re-enters the resonant cavity, thus ensuring a maximum of energy is transferred from the electron beam to the RF oscillations in the cavity Ibm THINKPAD X200 Battery 8800mAh .

The voltage should always be switched on before providing the input to the reflex klystron as the whole function of the reflex klystron would be destroyed if the supply is provided after the input. The reflector voltage may be varied slightly from the optimum value, which results in some loss of output power, but also in a variation in frequency Ibm THINKPAD X200 Battery 8800mAh .

This effect is used to good advantage for automatic frequency control in receivers, and in frequency modulation for transmitters. The level of modulation applied for transmission is small enough that the power output essentially remains constant. At regions far from the optimum voltage, no oscillations are obtained at all SONY VAIO VGN-FW21E Battery .

This tube is called a reflex klystron because it repels the input supply or performs the opposite function of a klystron.

There are often several regions of reflector voltage where the reflex klystron will oscillate; these are referred to as modes Asus M52N Battery .

The electronic tuning range of the reflex klystron is usually referred to as the variation in frequency between half power points—the points in the oscillating mode where the power output is half the maximum output in the mode. The frequency of oscillation is dependent on the reflector voltage Dell Latitude E6400 ATG Battery ,

and varying this provides a crude method of frequency modulating the oscillation frequency, albeit with accompanying amplitude modulation as well.

Modern semiconductor technology has effectively replaced the reflex klystron in most applications SONY VAIO VGN-FW21J Battery.

Multicavity klystron

In all modern klystrons, the number of cavities exceeds two. A larger number of cavities may be used to increase the gain of the klystron, or to increase the bandwidth  SONY VAIO VGN-FW21L Battery .

Tuning a klystron

Some klystrons have cavities that are tunable. Tuning a klystron is delicate work which, if not done properly, can cause damage to equipment or injury to the technician SONY VAIO VGN-FW21M Battery .

By adjusting the frequency of individual cavities, the technician can change the operating frequency, gain, output power, or bandwidth of the amplifier. The technician must be careful not to exceed the limits of the graduations, or damage to the klystron can result  SONY VAIO VGN-FW21Z Battery .

Manufacturers generally send a card with the unique calibrations for a klystron's performance characteristics, that lists the graduations to be set to attain any of a set of listed frequencies. No two klystrons are exactly identical (even when comparing like part/model number klystrons), and so every card is specific to the individual unit SONY VAIO VGN-FW25T/B Battery .

Klystrons have serial numbers on each of them to uniquely identify each unit, and for which manufacturers may (hopefully) have the performance characteristics in a database. If not, loss of the calibration card may be an economically insoluble problem, making the klystron unusable or perform marginally un-tuned SONY VAIO VGN-FW26T/B Battery .

Other precautions taken when tuning a klystron include using nonferrous tools. Some klystrons employ permanent magnets. If a technician uses ferrous tools, (which are ferromagnetic), and comes too close to the intense magnetic fields that contain the electron beam SONY VAIO VGN-FW27/B Battery ,

such a tool can be pulled into the unit by the intense magnetic force, smashing fingers, injuring the technician, or damaging the unit. Special lightweight nonmagnetic (aka diamagnetic) tools made ofberyllium alloy have been used for tuning U.S. Air Force klystronsSONY VAIO VGN-FW27/W Battery .

Precautions are routinely taken when transporting klystron devices in aircraft, as the intense magnetic field can interfere with magnetic navigation equipment. Special overpacks are designed to help limit this field "in the field," and thus allow such devices to be transported safely SONY VAIO VGN-FW31ZJ Battery .

Optical klystron

In an optical klystron the cavities are replaced with undulators. Very high voltages are needed. The electron gun, the drift tube and the collector are still used SONY VAIO VGN-FW27T/H Battery .

Floating drift tube klystron

The floating drift tube klystron has a single cylindrical chamber containing an electrically isolated central tube. Electrically, this is similar to the two cavity oscillator klystron with a lot of feedback between the two cavities SONY VAIO VGN-FW29/B Battery .

Electrons exiting the source cavity are velocity modulated by the electric field as they travel through the drift tube and emerge at the destination chamber in bunches, delivering power to the oscillation in the cavity. This type of oscillator klystron has an advantage over the two-cavity klystron on which it is based SONY VAIO VGN-FW290JRB Battery .

It only needs one tuning element to effect changes in frequency. The drift tube is electrically insulated from the cavity walls, and DC bias is applied separately. The DC bias on the drift tube may be adjusted to alter the transit time through it, thus allowing some electronic tuning of the oscillating frequency SONY VAIO VGN-FW290JTB Battery .

The amount of tuning in this manner is not large and is normally used for frequency modulation when transmitting.

Collector

After the RF energy has been extracted from the electron beam, the beam is destroyed in a collector SONY VAIO VGN-FW290JTH Battery .

Some klystrons include depressed collectors, which recover energy from the beam before collecting the electrons, increasing efficiency. Multistage depressed collectors enhance the energy recovery by "sorting" the electrons in energy bins SONY VAIO VGN-FW33G/E1 Battery .

Applications

Klystrons produce microwave power far in excess of that developed by solid state. In modern systems, they are used from UHF (hundreds of MHz) up through hundreds of gigahertz (as in the Extended Interaction Klystrons in the CloudSat satellite) SONY VAIO VGN-FW33G/W Battery.

Klystrons can be found at work in radar, satellite and wideband high-power communication (very common in television broadcasting and EHF satellite terminals), medicine (radiation oncology), and high-energy physics (particle accelerators and experimental reactors) SONY VAIO VGN-FW33GW Battery .

AtSLAC, for example, klystrons are routinely employed which have outputs in the range of 50 megawatts (pulse) and 50 kilowatts (time-averaged) at frequencies nearing 3 GHz Dell XPS M1210 Batteryhttp://www.hdd-shop.co.uk .

Popular Science's "Best of What's New 2007" described a company, Global Resource Corporation, using a klystron to convert the hydrocarbons in everyday materials, automotive waste, coal, oil shale, and oil sands into natural gas and diesel fuel Dell KM958 battery .

A free-electron laser, or FEL, is a laser that shares the same optical properties as conventional lasers such as emitting a beam consisting of coherentelectromagnetic radiation which can reach high power, but which uses some very different operating principles to form the beam Dell Studio 1555 battery .

Unlike gas, liquid, or solid-state lasers such as diode lasers, in which electrons are excited in bound atomic or molecular states, FELs use a relativistic electron beam as the lasing medium which moves freely through a magnetic structure, hence the term free electronSony VGP-BPS13 battery .

The free-electron laser has the widest frequency range of any laser type, and can be widely tunable, currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, toultraviolet, to X-rays.

Free-electron lasers were invented by John Madey in 1976 at Stanford University Sony VGP-BPS13/B battery .

The work emanates from research done by Hans Motz who proposed the wiggler magnetic configuration which is at the heart of a free electron laser. Madey used a 24 MeV electron beam and 5 m long wiggler to amplify a signal. Soon afterward, other laboratories with accelerators started developing such lasers Sony VGP-BPS13/S battery .

Beam creation

To create a FEL, a beam of electrons is accelerated to almost light speed. The beam passes through an FEL oscillator in the form of a periodic, transversemagnetic field, produced by arranging magnets with alternating poles within a laser cavity along the beam pathSony VGP-BPS13A/B battery .

This array of magnets is sometimes called anundulator, or a "wiggler", because it forces the electrons in the beam to follow a sinusoidal path. The acceleration of the electrons along this path results in the release of photons (synchrotron radiation) Sony VGP-BPS13B/B battery .

Since the electron motion is in phase with the field of the light already emitted, the fields add togethercoherently. Whereas conventional undulators would cause the electrons to radiate independently Sony VGP-BPL9 battery ,

instabilities in the electron beam resulting from the interactions of the oscillations of electrons in the undulators and the radiation they emit leads to a bunching of the electrons, which continue to radiate in phase with each other Sony VGP-BPS13B/B battery .

The wavelength of the light emitted can be readily tuned by adjusting the energy of the electron beam or the magnetic field strength of the undulators.

Accelerators

Today, a free-electron laser requires the use of an electron accelerator with its associated shielding, as accelerated electrons are a radiation hazard Sony VGP-BPL15 battery .

These accelerators are typically powered by klystrons, which require a high voltage supply. The electron beam must be maintained in a vacuum which requires the use of numerous vacuum pumps along the beam path. While this equipment is bulky and expensive, free-electron lasers can achieve very high peak powers Dell Inspiron E1505 battery ,

and the tunability of FELs makes them highly desirable in several disciplines, including medical diagnosis and non-destructive testing.

X-ray uses

The lack of suitable mirrors in the extreme ultraviolet and x-ray regimes prevents the operation of a FEL oscillator HP Pavilion dv6000 Battery ;

consequently, there must be suitable amplification over a single pass of the electron beam through the undulator to make the FEL worthwhile. X-ray free electron lasers use long undulators. The underlying principle of the intense pulses from the X-ray laser lies in the principle of self-amplified stimulated emission (SASE), which leads to the microbunching of the electrons HP Pavilion dv8000 battery .

Initially all electrons are distributed evenly and they emit incoherent spontaneous radiation only. Through the interaction of this radiation and the electrons' oscillations, they drift into microbunches separated by a distance equal to one radiation wavelength. Through this interaction, all electrons begin emitting coherent radiation in phase HP PAVILION DV6000 Battery,  8800mAh .

In other words, all emitted radiation can reinforce itself perfectly whereby wave crests and wave troughs are always superimposed on one another in the best possible way. This results in an exponential increase of emitted radiation power, leading to high beam intensities and laser-like properties HP PAVILION DV8000 Battery,  8800mAh .

Examples of facilities operating on the SASE FEL principle include the Free electron LASer (FLASH) in Hamburg, the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory, the European x-ray free electron laser (XFEL) in Hamburg, the SPring-8 Compact SASE Source (SCSS) and the SwissFEL at the Paul Scherrer Institute (Switzerland) SONY VAIO VGN-FZ Battery .

One problem with SASE FELs is the lack of temporal coherence due to a noisy startup process. To avoid this, one can "seed" an FEL with a laser tuned to the resonance of the FEL. Such a temporally coherent seed can be produced by more conventional means, such as by high-harmonic generation (HHG) using an optical laser pulse HP Pavilion DV6000 battery - 8800mAh .

This results in coherent amplification of the input signal; in effect, the output laser quality is characterized by the seed. While HHG seeds are available at wavelengths down to the extreme ultraviolet, seeding is not feasible at x-ray wavelengths due to the lack of conventional x-ray lasers HP Pavilion DV8000 battery - 7800mAh .

Medical uses

Research by Dr. Glenn Edwards and colleagues at Vanderbilt University's FEL Center in 1994 found that soft tissues like skin, cornea, and brain tissue could be cut, or ablated, using infrared FEL wavelengths around 6.45 micrometres with minimal collateral damage to adjacent tissue  HP Pavilion DV6000 battery - 8800mAh .

This led to further research and eventually surgeries on humans, the first ever using a free-electron laser. Starting in 1999, and using the Keck foundation funded FEL operating rooms at the Vanderbilt FEL Center, Dr. Michael Copeland and Dr. Pete Konrad of Vanderbilt performed three surgeries in which they resected meningioma brain tumors  HP Pavilion DV8000 battery - 6600mAh .

Beginning in 2000, Dr. Karen Joos and Dr. Louise Mawn performed five surgeries involving the cutting of a window in the sheath of the optic nerve, to test the efficacy for optic nerve sheath fenestration. These eight surgeries went as expected with results consistent with the routine standard of care and with the added benefit of laser surgery and minimal collateral damage SONY VAIO VGN-FZ18 Battery .

A review of FELs for medical uses is given in the 1st edition of Tunable Laser Applications.

Since these successful results, there have been several efforts to build small, clinical lasers tunable in the 6 to 7 micrometre range with pulse structure and energy to give minimal collateral damage in soft tissue  SONY VAIO VGN-FZ21E Battery .

At Vanderbilt, there exists a Raman shifted system pumped by an Alexandrite laser.

At the 2006 annual meeting of the American Society for Laser Medicine and Surgery (ASLMS), Dr. Rox Anderson of the Wellman Laboratory of Photomedicine of Harvard Medical School andMassachusetts General Hospital reported on the possible medical application of the free-electron laser in melting fats without harming the overlying skin SONY VAIO VGN-FW31J Battery 10400mAh.

It was reported that at infrared wavelengths, water in tissue was heated by the laser, but at wavelengths corresponding to 915, 1210 and 1720 nm, subsurface lipids were differentially heated more strongly than water SONY VAIO VGN-FW31J Battery10400mAh .

The possible applications of this selective photothermolysis (heating tissues using light) include the selective destruction of sebum lipids to treat acne, as well as targeting other lipids associated with cellulite and body fat as well as fatty plaques that form in arteries which can help treat atherosclerosis and heart disease SONY VAIO VGN-FW21E Battery .

Military uses

FEL technology is being evaluated by the US Navy as a good candidate for an anti aircraft and missile directed-energy weapon. Significant progress is being made in raising FEL power levels SONY VAIO VGN-NR11S/S Battery

(theThomas Jefferson National Accelerator Facility's FEL has demonstrated over 14 kW) and it should be possible to build compact multi-megawatt class FEL weapons. On June 9, 2009 the Office of Naval Research announced it had awarded Raytheon a contract to develop a 100 kW experimental FEL SONY VAIO VGN-NR11M/S Battery .

On March 18, 2010 Boeing Directed Energy Systems announced the completion of an initial design for U.S. Naval use. A prototype FEL system has since been demonstrated with a full-power prototype scheduled by 2018. As of February 2011, a 500 kilo-Volt electron-gun power supply for an FEL-laser has been implemented SONY VAIO VGN-NR11Z/S Battery ;

the power supply is to be used towards the Military goal of a practical 100kW laser; which is suspected to be operational on ships by 2020.

Electron wake is the disturbance left after a high-energy charged particle passes through condensed matter or plasma SONY VAIO VGN-NR11Z/T Battery .

Ions passing through can introduce periodic oscillations in the crystal lattice orplasma wave with the characteristic frequency of the crystal or plasma frequency. Interactions of the field created by these oscillations with the charged particle field alternate from constructive interference to destructive interference Sony VAIO VGN-FZ21E Battery ,

producing alternating waves of electric field and displacement. The frequency of the wake field is determined by the nature of the penetrated matter, and the period of the wake field is directly proportional to the speed of the incoming charged particle Sony VAIO VGN-FW21E Battery .

The amplitude of the first wake wave is the most important, as it produces a braking force on the charged particle, eventually slowing it down. Wake fields also can capture and guide light ions or positrons in the direction perpendicular to the wake  Sony VAIO VGN-NR11S/S Battery .

The larger the speed of the original charged particle, the larger the angle between the initial particle's velocity and the captured ion's velocity.

Electromagnetic radiation (often abbreviated E-M radiation or EMR) is a form of energy exhibiting wave-like behavior as it travels through space Sony VAIO VGN-NR11Z/S Battery .

EMR has both electric and magnetic field components, which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation.

Electromagnetic radiation is classified according to the frequency of its wave  Sony VAIO VGN-NR11M/S Battery .

In order of increasing frequency and decreasing wavelength, these are radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays (see Electromagnetic spectrum). The eyes of variousorganisms sense a small and somewhat variable window of frequencies called the visible spectrum  Sony VAIO VGN-NR11Z/T Battery .

The photon is the quantum of the electromagnetic interaction and the basic "unit" of light and all other forms of electromagnetic radiation and is also the force carrier for the electromagnetic force.

Electromagnetic radiation carries energy and momentum that may be imparted to matter with which it interacts SONY VAIO VGN-FZ180E Battery .

James Clerk Maxwell first formally postulated electromagnetic waves. These were subsequently confirmed by Heinrich Hertz. Maxwell derived a wave form of the electric and magnetic equations, thus uncovering the wave-like nature of electric and magnetic fields, and their symmetry SONY VAIO VGN-FZ220E Battery .

Because the speed of EM waves predicted by the wave equation coincided with the measured speed of light, Maxwell concluded that light itself is an EM wave.

According to Maxwell's equations, a time-varying electric field generates a time-varying magnetic field and vice versa SONY VAIO VGN-FZ340E Battery .

Therefore, as an oscillating electric field generates an oscillating magnetic field, the magnetic field in turn generates an oscillating electric field, and so on. These oscillating fields together form a propagating electromagnetic wave.

A quantum theory of the interaction between electromagnetic radiation and matter such as electrons is described by the theory of quantum electrodynamics  SONY VAIO VGN-FZ430E Battery .

Properties

The physics of electromagnetic radiation is electrodynamics. Electromagnetism is the physical phenomenon associated with the theory of electrodynamics. Electric and magnetic fields obey the properties of superposition SONY VAIO VGN-FZ460E Battery .

Thus, a field due to any particular particle or time-varying electric or magnetic field contributes to the fields present in the same space due to other causes. Further, as they are vector fields, all magnetic and electric field vectors add together according to vector addition SONY VAIO VGN-FZ480E Battery .

For example, in optics two or more coherent lightwaves may interact and by constructive or destructive interference yield a resultant irradiance deviating from the sum of the component irradiances of the individual lightwaves SONY VAIO VGN-FZ4000 Battery .

Since light is an oscillation it is not affected by travelling through static electric or magnetic fields in a linear medium such as a vacuum. However in nonlinear media, such as some crystals, interactions can occur between light and static electric and magnetic fields — these interactions include the Faraday effect and the Kerr effect SONY VAIO VGN-FZ31E Battery .

In refraction, a wave crossing from one medium to another of different density alters its speed and direction upon entering the new medium. The ratio of the refractive indices of the media determines the degree of refraction, and is summarized by Snell's law SONY VAIO VGN-FZ31B Battery .

Light disperses into a visible spectrum as light passes through a prism because of the wavelength dependent refractive index of the prism material (Dispersion).

EM radiation exhibits both wave properties and particle properties at the same time (see wave-particle duality)  SONY VAIO VGN-FZ31J Battery .

Both wave and particle characteristics have been confirmed in a large number of experiments. Wave characteristics are more apparent when EM radiation is measured over relatively large timescales and over large distances while particle characteristics are more evident when measuring small timescales and distances  SONY VAIO VGN-FZ31M Battery .

For example, when electromagnetic radiation is absorbed by matter, particle-like properties will be more obvious when the average number of photons in the cube of the relevant wavelength is much smaller than 1. Upon absorption of light, it is not too difficult to experimentally observe non-uniform deposition of energy  SONY VAIO VGN-FZ31Z Battery .

Strictly speaking, however, this alone is not evidence of "particulate" behavior of light, rather it reflects the quantum nature of matter.

There are experiments in which the wave and particle natures of electromagnetic waves appear in the same experiment, such as the self-interference of a single photon SONY VAIO VGN-FZ38M Battery .

True single-photon experiments (in a quantum optical sense) can be done today in undergraduate-level labs.[2] When a single photon is sent through an interferometer, it passes through both paths, interfering with itself, as waves do, yet is detected by a photomultiplier or other sensitive detector only once SONY VGP-BPS8 Battery .

Wave model

Electromagnetic radiation is a transverse wave meaning that the oscillations of the waves are perpendicular to the direction of energy transfer and travel. An important aspect of the nature of light isfrequency  SONY VGP-BPS13/S Battery .

The frequency of a wave is its rate of oscillation and is measured in hertz, the SI unit of frequency, where one hertz is equal to one oscillation per second. Light usually has a spectrum of frequencies which sum together to form the resultant wave. Different frequencies undergo different angles of refraction SONY VGP-BPS13A/B Battery .

A wave consists of successive troughs and crests, and the distance between two adjacent crests or troughs is called the wavelength. Waves of the electromagnetic spectrum vary in size, from very long radio waves the size of buildings to very short gamma rays smaller than atom nuclei  SONY VGP-BPS13B/B Battery .

Frequency is inversely proportional to wavelength, according to the equation:

\displaystyle v=f\lambda

where v is the speed of the wave (c in a vacuum, or less in other media), f is the frequency and ? is the wavelength. As waves cross boundaries between different media, their speeds change but their frequencies remain constant SONY VGP-BPS13A/S Battery .

Interference is the superposition of two or more waves resulting in a new wave pattern. If the fields have components in the same direction, they constructively interfere, while opposite directions cause destructive interference.

The energy in electromagnetic waves is sometimes called radiant energy SONY VGP-BPS13AS Battery .

Particle model

Because energy of an EM interaction is quantized, EM waves are emitted and absorbed as discrete packets of energy, or quanta, called photons. Because photons are emitted and absorbed by charged particles, they act as transporters of energy, and are associated with waves with frequency proportional to the energy carried Dell Inspiron 1320n Battery .

The energy per photon can be related to the frequency via thePlanck–Einstein equation:

\displaystyle E=hf

where E is the energy, h is Planck's constant, and f is frequency. The energy is commonly expressed in the unit of electronvolt (eV)Dell Studio 1450 Battery .

This photon-energy expression is a particular case of the energy levels of the more general electromagnetic oscillator, whose average energy, which is used to obtain Planck's radiation law, can be shown to differ sharply from that predicted by the equipartition principle at low temperature Dell Studio 1457 B

attery ,

thereby establishes a failure of equipartition due to quantum effects at low temperature.

As a photon is absorbed by an atom, it excites the atom, elevating an electron to a higher energy level. If the energy is great enough, so that the electron jumps to a high enough energy level  Dell Latitude D610 Battery ,

it may escape the positive pull of the nucleus and be liberated from the atom in a process called photoionisation. Conversely, an electron that descends to a lower energy level in an atom emits a photon of light equal to the energy difference. Since the energy levels of electrons in atoms are discrete  Toshiba NB100 Battery ,

each element emits and absorbs its own characteristic frequencies.

Together, these effects explain the emission and absorption spectra of light. The dark bands in the absorption spectrum are due to the atoms in the intervening medium absorbing different frequencies of the light Toshiba Satellite M65 battery .

The composition of the medium through which the light travels determines the nature of the absorption spectrum. For instance, dark bands in the light emitted by a distant star are due to the atoms in the star's atmosphere. These bands correspond to the allowed energy levels in the atoms  Toshiba Satellite M60 battery .

A similar phenomenon occurs for emission. As the electrons descend to lower energy levels, a spectrum is emitted that represents the jumps between the energy levels of the electrons. This is manifested in the emission spectrum of nebulae. Today, scientists use this phenomenon to observe what elements a certain star is composed of  Dell Latitude D830 Battery .

It is also used in the determination of the distance of a star, using the red shift.

Speed of propagation

Any electric charge which accelerates, or any changing magnetic field, produces electromagnetic radiation. Electromagnetic information about the charge travels at the speed of light  Dell Latitude D620 Battery .

Accurate treatment thus incorporates a concept known as retarded time (as opposed to advanced time, which is not physically possible in light of causality), which adds to the expressions for the electrodynamic electric field and magnetic field. These extra terms are responsible for electromagnetic radiation Dell Inspiron Mini 10 Battery .

When any wire (or other conducting object such as an antenna) conducts alternating current, electromagnetic radiation is propagated at the same frequency as the electric current. At the quantum level, electromagnetic radiation is produced when the wavepacket of a charged particle oscillates or otherwise accelerates  Sony VGN-FW11S Battery .

Charged particles in a stationary state do not move, but a superposition of such states may result in oscillation, which is responsible for the phenomenon of radiative transition between quantum states of a charged particle.

Depending on the circumstances, electromagnetic radiation may behave as a wave or as particles Sony VGN-FW11M Battery .

As a wave, it is characterized by a velocity (the speed of light), wavelength, and frequency. When considered as particles, they are known as photons, and each has an energy related to the frequency of the wave given by Planck's relation E = h?, where E is the energy of the photon, h = 6.626 × 10?34 J·s is Planck's constant, and ? is the frequency of the wave Sony VGN-FW139E/H battery .

One rule is always obeyed regardless of the circumstances: EM radiation in a vacuum always travels at the speed of light, relative to the observer, regardless of the observer's velocity. (This observation led to Albert Einstein's development of the theory of special relativity.) Dell Latitude E5400 Battery

In a medium (other than vacuum), velocity factor or refractive index are considered, depending on frequency and application. Both of these are ratios of the speed in a medium to speed in a vacuum.

Thermal radiation and electromagnetic radiation as a form of heat

The basic structure of matter involves charged particles bound together in many different ways Dell Latitude E4200 Battery .

When electromagnetic radiation is incident on matter, it causes the charged particles to oscillate and gain energy. The ultimate fate of this energy depends on the situation. It could be immediately re-radiated and appear as scattered, reflected, or transmitted radiation  Dell Inspiron 300M Battery .

It may also get dissipated into other microscopic motions within the matter, coming to thermal equilibrium and manifesting itself as thermal energy in the material. With a few exceptions such as fluorescence, harmonic generation,photochemical reactions and the photovoltaic effect Dell Studio 1737 battery ,

absorbed electromagnetic radiation simply deposits its energy by heating the material. This happens both for infrared and non-infrared radiation. Intense radio waves can thermally burn living tissue and can cook food. In addition to infrared lasers, sufficiently intense visible and ultraviolet lasers can also easily set paper afire  Dell Inspiron E1505 battery .

Ionizing electromagnetic radiation can create high-speed electrons in a material and break chemical bonds, but after these electrons collide many times with other atoms in the material eventually most of the energy gets downgraded to thermal energy, this whole process happening in a tiny fraction of a second Dell RM791 battery .

That infrared radiation is a form of heat and other electromagnetic radiation is not, is a widespread misconception in physics. Anyelectromagnetic radiation can heat a material when it is absorbed.

The inverse or time-reversed process of absorption is responsible for thermal radiation Toshiba Satellite P10 Battery .

Much of the thermal energy in matter consists of random motion of charged particles, and this energy can be radiated away from the matter. The resulting radiation may subsequently be absorbed by another piece of matter, with the deposited energy heating the material. Radiation is an important mechanism of heat transfer Acer Aspire One battery 10400mAh

The electromagnetic radiation in an opaque cavity at thermal equilibrium is effectively a form of thermal energy, having maximum radiation entropy. The thermodynamic potentials of electromagnetic radiation can be well-defined as for matter. Thermal radiation in a cavity has energy density (see Planck's Law) of SONY VGN-FZ210CE Battery

{U\over V} = \frac{8\pi^5(kT)^4}{15 (hc)^3},

Differentiating the above with respect to temperature, we may say that the electromagnetic radiation field has an effective volumetric heat capacity given by Dell Precision M70 Battery

\frac{32\pi^5 k^4 T^3}{15 (hc)^3},

Electromagnetic spectrum

Generally, EM radiation (the designation 'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays Toshiba Satellite L305 Battery .

Arbitrary electromagnetic waves can always be expressed by Fourier analysis in terms of sinusoidal monochromatic waves which can be classified into these regions of the spectrum.

The behavior of EM radiation depends on its wavelength Toshiba Satellite T4900 Battery .

Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm Toshiba PA3399U-2BRS battery .

A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. Detailed information about the physical properties of objects, gases, or even stars can be obtained from this type of device. It is widely used in astrophysics. For example,hydrogen atoms emit radio waves of wavelength 21.12 cm Sony VGP-BPS10 battery 10400mAh .

Soundwaves are not electromagnetic radiation. At the lower end of the electromagnetic spectrum, about 20 Hz to about 20 kHz, are frequencies that might be considered in the audio range. However, electromagnetic waves cannot be directly perceived by human ears  Sony VGP-BPS9 battery 10400mAh .

Sound waves are the oscillating compression of molecules. To be heard, electromagnetic radiation must be converted to air pressure waves, or if the ear is submerged, water pressure waves Sony VGP-BPS11 battery 10400mAh .

Light

EM radiation with a wavelength between approximately 400 nm and 700 nm is directly detected by the human eye and perceived as visiblelight. Other wavelengths, especially nearby infrared (longer than 700 nm) and ultraviolet (shorter than 400 nm) are also sometimes referred to as light, especially when visibility to humans is not relevant Sony VGP-BPS22 battery 10400mAh .

If radiation having a frequency in the visible region of the EM spectrum reflects off of an object, say, a bowl of fruit, and then strikes our eyes, this results in our visual perception of the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues Sony VGP-BPL8 battery  10400mAh ,

and through this not-entirely-understood psychophysical phenomenon, most people perceive a bowl of fruit.

At most wavelengths, however, the information carried by electromagnetic radiation is not directly detected by human senses Sony VGP-BPS14 battery 10400mAh .

Natural sources produce EM radiation across the spectrum, and our technology can also manipulate a broad range of wavelengths. Optical fibertransmits light which, although not suitable for direct viewing, can carry data that can be translated into sound or an image Sony VGP-BPS21 battery   10400mAh .

To be meaningful both transmitter and receiver must use some agreed-upon encoding system - especially so if the transmission is digital as opposed to the analog nature of the waves.

Radio waves

Radio waves can be made to carry information by varying a combination of the amplitude, frequency and phase of the wave within a frequency band Toshiba Satellite A200 Battery .

When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the electrons of the conducting material. This effect (the skin effect) is used in antennas Toshiba Satellite 1200 Battery .

EM radiation may also cause certain molecules to absorb energy and thus to heat up; this is exploited in microwave ovens. Radio waves are not ionizing radiation, as the energy per photon is too small.

The second of Maxwell's equations yields the magnetic field Toshiba Satellite M300 Battery .

The remaining equations will be satisfied by this choice of \mathbf{E},\mathbf{B}.

Not only are the electric and magnetic field waves traveling at the speed of light, but they have a special restricted orientation and proportional magnitudes, E0 = c0B0, which can be seen immediately from the Poynting vector SONY VGP-BPS13A/Q Battery .

The electric field, magnetic field, and direction of wave propagation are all orthogonal, and the wave propagates in the same direction as \mathbf{E} \times \mathbf{B}.

From the viewpoint of an electromagnetic wave traveling forward  SONY VGP-BPS13A/Q Battery ,

the electric field might be oscillating up and down, while the magnetic field oscillates right and left; but this picture can be rotated with the electric field oscillating right and left and the magnetic field oscillating down and up. This is a different solution that is traveling in the same direction  SONY VGP-BPS13B/Q Battery .

This arbitrariness in the orientation with respect to propagation direction is known as polarization. On a quantum level, it is described as photon polarization. The direction of the polarization is defined as the direction of the electric field SONY VGP-BPS13B/Q Battery .

More general forms of the second order wave equations given above are available, allowing for both non-vacuum propagation media and sources. A great many competing derivations exist, all with varying levels of approximation and intended applications  SONY VGP-BPS13/Q Battery .

One very general example is a form of the electric field equation,which was factorized into a pair of explicitly directional wave equations, and then efficiently reduced into a single uni-directional wave equation by means of a simple slow-evolution approximation  SONY VGP-BPS13/Q Battery .

An antenna (or aerial) is an electrical device which couples radio waves in free space to an electrical current used by a radio receiver or transmitter. In reception, the antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage that the radio receiver can amplify SONY VGP-BPS21A/B Battery .

Alternatively, a radio transmitter will produce a large radio frequency current that may be applied to the terminals of the same antenna in order to convert it into an electromagnetic wave (radio wave) radiated into free space. Antennas are thus essential to the operation of all radio equipment, both transmitters and receivers  SONY VGP-BPS21/S Battery .

They are used in systems such as radio andtelevision broadcasting, two-way radio, wireless LAN, mobile telephony, radar, and satellite communications.

Typically an antenna consists of an arrangement of metallic conductors (or "elements") with an electrical connection (often through a transmission line) to the receiver or transmitter  SONY VGP-BPS21B Battery .

A current forced through such a conductor by a radio transmitter will create an alternating magnetic field according to Ampère's law. Or the alternating magnetic field due to a distant radio transmitter will induce a voltage at the antenna terminals, according to Faraday's law, which is connected to the input of a receiver  SONY VGP-BPS21A Battery .

In the so-called far field, at a considerable distance away from the antenna, the oscillating magnetic field is coupled with a similarly oscillating electric field; together these define an electromagnetic wave which is capable of propagating great distances  SONY VGP-BPS21 Battery .

Light is one example of electromagnetic radiation, along with infrared and x-rays, while radio waves differ only in their much lower frequency (and much longer wavelength). Electronic circuits can operate at these lower frequencies, processing radio signals conducted through wires  Sony VGP-BPS21A/B Battery .

But it is only through antennas that those radio frequencyelectrical signals are converted to (and from) propagating radio waves. Depending on the design of the antenna, radio waves can be sent toward and received from all directions ("omnidirectional"), whereas a directional or beam antenna is designed to operate in a particular direction Sony VGP-BPS21/S Battery .

The first antennas were built in 1888 by Heinrich Hertz (1857–1894) in his pioneering experiments to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. Hertz placed dipole antennas at the focal point of parabolic reflectors for both transmitting and receiving  Sony VGP-BPS21A Battery .

He published his work and installation drawings in Annalen der Physik und Chemie (vol. 36, 1889).

Terminology

The words antenna (plural: antennas ) and aerial are used interchangeably; but usually a rigid metallic structure is termed an antenna and a wire format is called an aerial  Sony VGP-BPS21 Battery .

In the United Kingdom and other British English speaking areas the term aerial is more common, even for rigid types. The noun aerial is occasionally written with a diaeresis mark—aërial—in recognition of the original spelling of the adjective aërial from which the noun is derived Sony Vaio PCG-5G2L Battery .

The origin of the word antenna relative to wireless apparatus is attributed to Guglielmo Marconi. In 1895, while testing early radio apparatuses in the Swiss Alps at Salvan, Switzerland in the Mont Blancregion, Marconi experimented with early wireless equipmentSony Vaio PCG-5G3L Battery .

A 2.5 meter long pole, along which was carried a wire, was used as a radiating and receiving aerial element. In Italian a tent pole is known as l'antenna centrale, and the pole with a wire alongside it used as an aerial was simply called l'antenna. Until then wireless radiating transmitting and receiving elements were known simply as aerials or terminals Sony Vaio PCG-5J1L Battery .

Marconi's use of the word antenna (Italian for pole) would become a popular term for what today is uniformly known as the antenna.

In common usage, the word antenna may refer broadly to an entire assembly including support structure, enclosure (if any), etcSony Vaio PCG-5K2L Battery .

in addition to the actual functional components. Especially at microwave frequencies, a receiving antenna may include not only the actual electrical antenna but an integrated preamplifier and/or mixer.

Overview

Antennas are required by any radio receiver or transmitter in order to couple its electrical connection to the electromagnetic fieldSony Vaio PCG-5J2L Battery .

Radio waves are electromagnetic waves which carry signals through the air (or through space) at almost the speed of light with almost no transmission loss. Radio transmitters and receivers are used to convey signals (information) in systems including broadcast (audio) radio Sony Vaio PCG-5K1L Battery ,

television, mobile telephones, wi-fi (WLAN) data networks, trunk lines and point-to-point communications links (telephone, data networks), satellite links, many remote controlled devices such asgarage door openers, and wireless remote sensors, among many others Sony Vaio PCG-6S2L Battery .

Radio waves are also used directly for measurements in technologies including RADAR, GPS, and radio astronomy. In each and every case, the transmitters and receivers involved require antennas, although these are sometimes hidden (such as the antenna inside an AM radio or inside a laptop computer equipped with wi-fi) Sony Vaio PCG-6S3L Battery .

According to their applications and technology available, antennas generally fall in one of two catagories:

  1. Omnidirectional or only weakly directional antennas which receive or radiate more or less in all directions. These are employed when the relative position of the other station is unknown or arbitrary Sony Vaio PCG-6V1L Battery .
  2. They are also used at lower frequencies where a directional antenna would be too large, or simply to cut costs in applications where a directional antenna isn't required.
  3. Directional or beam antennas which are intended to preferentially radiate or receive in a particular direction or directional pattern Sony Vaio PCG-6W1L Battery . .

In common usage "omnidirectional" usually refers to all horizontal directions, typically with reduced performance in the direction of the sky or the ground (a truly isotropic radiator is not even possible). A "directional" antenna usually is intended to maximize its coupling to the electromagnetic field in the direction of the other station Sony Vaio PCG-6W2L Battery ,

or sometimes to cover a particular sector such as a 120° horizontal fan pattern in the case of a panel antenna at a cell site.

One example of omnidirectional antennas is the very common vertical antenna or whip antenna consisting of a metal rod (often, but not always, a quarter of a wavelength long) Sony Vaio PCG-6W3L Battery .

A dipole antenna is similar but consists of two such conductors extending in opposite directions, with a total length that is often, but not always, a half of a wavelength long. Dipoles are typically oriented horizontally in which case they are weakly directional  Sony Vaio PCG-7111L Battery :

signals are reasonably well radiated toward or received from all directions with the exception of the direction along the conductor itself; this region is called the antenna blind cone or null.

Both the vertical and dipole antennas are simple in construction and relatively inexpensive  Sony Vaio PCG-7112L Battery .

The dipole antenna, which is the basis for most antenna designs, is a balanced component, with equal but opposite voltages and currents applied at its two terminals through a balanced transmission line (or to a coaxial transmission line through a so-called balun)  Sony Vaio PCG-7113L Battery .

The vertical antenna, on the other hand, is amonopole antenna. It is typically connected to the inner conductor of a coaxial transmission line (or a matching network); the shield of the transmission line is connected to ground. In this way, the ground (or any large conductive surface) Sony Vaio PCG-7133L Battery .

plays the role of the second conductor of a dipole, thereby forming a complete circuit. Since monopole antennas rely on a conductive ground, a so-calledgrounding structure may be employed in order to provide a better ground contact to the earth or which itself acts as a ground plane to perform that function regardless of (or in absence of) an actual contact with the earth Sony Vaio PCG-7Z2L Battery .

Antennas fancier than the dipole or vertical designs are usually intended to increase the directivity and consequently the gain of the antenna. This can be accomplished in many different ways leading to a plethora of antenna designs Sony Vaio PCG-8Y1L Battery .

The vast majority of designs are fed with a balanced line (unlike a monopole antenna) and are based on the dipole antenna with additional components (or elements) which increase its directionality.

For instance, a phased array consists of two or more simple antennas which are connected together through an electrical networkSony Vaio PCG-8Y2L Battery .

This often involves a number of parallel dipole antennas with a certain spacing. Depending on the relative phase introduced by the network, the same combination of dipole antennas can operate as a "broadside array" (directional normal to a line connecting the elements) or as an "end-fire array" (directional along the line connecting the elements) Sony Vaio PCG-8Z1L Battery .

Antenna arrays may employ any basic (omnidirectional or weakly directional) antenna type, such as dipole, loop or slot antennas. These elements are often identical.

However a log-periodic dipole array consists of a number of dipole elements of different lengths in order to obtain a somewhat directional antenna having an extremely wide bandwidth Sony Vaio PCG-8Z2L Battery :

these are frequently used for television reception in fringe areas. The dipole antennas composing it are all considered "active elements" since they are all electrically connected together (and to the transmission line). On the other hand, a superficially similar dipole array  SONY VAIO VGN-FW31J Battery ,

the Yagi-Uda Antenna (or simply "Yagi"), has only one dipole element with an electrical connection; the other so-called parasitic elementsinteract with the electromagnetic field in order to realize a fairly directional antenna but one which is limited to a rather narrow bandwidth SONY VAIO VGN-FW31E Battery .

The Yagi antenna has similar looking parasitic dipole elements but which act differently due to their somewhat different lengths. There may be a number of so-called "directors" in front of the active element in the direction of propagation, and usually a single (but possibly more) "reflector" on the opposite side of the active element  SONY VAIO VGN-FW31B Battery .

Greater directionality can be obtained using beam-forming techniques such as a parabolic reflector or a horn. Since the size of a directional antenna depends on it being large compared to the wavelength, very directional antennas of this sort are mainly feasible at UHF and microwave frequencies SONY VAIO VGN-FW31M Battery .

On the other hand, at low frequencies (such as AM broadcast) where a practical antenna must be much smaller than a wavelength, significant directionality isn't even possible. A vertical antenna or loop antenna small compared to the wavelength is typically used, with the main design challenge being that of impedance matching  SONY VAIO VGN-FW31Z Battery .

With a vertical antenna a loading coil at the base of the antenna may be employed to cancel the reactive component of impedance; small loop antennas are tuned with parallel capacitors for this purpose.

An antenna lead-in is the transmission line (or feed line) which connects the antenna to a transmitter or receiver SONY VAIO VGN-FW38M Battery .

The antenna feed may refer to all components connecting the antenna to the transmitter or receiver, such as an impedance matching network in addition to the transmission line. In a so-called aperture antenna, such as a horn or parabolic dish, the "feed" may also refer to a basic antenna inside the entire system SONY VAIO VGN-AW41JF Battery

(normally at the focus of the parabolic dish or at the throat of a horn) which could be considered the one active element in that antenna system. A microwave antenna may also be fed directly from a waveguide in lieu of a (conductive) transmission line SONY VAIO VGN-AW41JF/H Battery .

An antenna counterpoise or ground plane is a structure of conductive material which improves or substitutes for the ground. It may be connected to or insulated from the natural ground. In a monopole antenna, this aids in the function of the natural ground SONY VAIO VGN-AW41MF Battery ,

particularly where variations (or limitations) of the characteristics of the natural ground interfere with its proper function. Such a structure is normally connected to the return connection of an unbalanced transmission line such as the shield of a coaxial cableSONY VAIO VGN-AW41MF/H Battery .

An electromagnetic wave refractor in some aperture antennas is a component which due to its shape and position functions to selectively delay or advance portions of the electromagnetic wavefront passing through it. The refractor alters the spatial characteristics of the wave on one side relative to the other side  SONY VAIO VGN-FW30B Battery .

It can, for instance, bring the wave to a focus or alter the wave front in other ways, generally in order to maximize the directivity of the antenna system. This is the radio equivalent of an optical lens.

An antenna coupling network is a passive network SONY VAIO VGN-FW50B Battery

(generally a combination of inductive and capacitive circuit elements) used for impedance matching in between the antenna and the transmitter or receiver. This may be used to improve the standing wave ratio in order to minimize losses in the transmission line (especially at higher frequencies and/or over longer distances)SONY VAIO VGN-FW51B/W Battery

and to present the transmitter or receiver with a standard resistive impedance (such as 75 ohms) that it expects to see for optimum operation.

Reciprocity

It is a fundamental property of antennas that the characteristics of an antenna described in the next section SONY VAIO VGN-FW51MF Battery ,

such as gain, radiation pattern, impedance, bandwidth, resonant frequency and polarization, are the same whether the antenna is transmitting or receiving. For example, the "receiving pattern" (sensitivity as a function of direction) of an antenna when used for reception is identical to theradiation pattern of the antenna when it is driven and functions as a radiator SONY VAIO VGN-FW51MF/H Battery .

This is a consequence of the reciprocity theorem of electromagnetics. Therefore in discussions of antenna properties no distinction is usually made between receiving and transmitting terminology, and the antenna can be viewed as either transmitting or receiving, whichever is more convenient  SONY VAIO VGN-FW51ZF Battery .

A necessary condition for the above reciprocity property is that the materials in the antenna and transmission medium are linear and reciprocal. Reciprocal (or bilateral) means that the material has the same response to an electric or magnetic field, or a current, in one direction, as it has to the field or current in the opposite direction SONY VAIO VGN-FW51ZF/H Battery .

Most materials used in antennas meet these conditions, but some microwave antennas use high-tech components such as isolators and circulators, made of nonreciprocal materials such as ferrite or garnet. These can be used to give the antenna a different behavior on receiving than it has on transmitting, which can be useful in applications like radar SONY VAIO VGN-FW52JB Battery .

Parameters

There are several critical parameters affecting an antenna's performance that can be adjusted during the design process. These are resonant frequency, impedance, gain, aperture or radiation pattern,polarization, efficiency and bandwidth SONY VAIO VGN-FW54FB Battery .

Transmit antennas may also have a maximum power rating, and receive antennas differ in their noise rejection properties. All of these parameters can bemeasured through various means.

Resonant frequency

Many types of antenna are tuned to work at one particular frequency, and are effective only over a range of frequencies centered on this frequency, called the resonant frequency  SONY VAIO VGN-FW70DB Battery .

These are calledresonant antennas. The antenna acts as an electrical resonator. When driven at its resonant frequency, large standing waves of voltage and current are excited in the antenna elements. These large currents and voltages radiate intense electromagnetic waves, so the power radiated by the antenna is maximum at the resonant frequency SONY VAIO VGN-FW71DB/W Battery .

In antennas made of thin linear conductive elements, the length of the driven element(s) determines the resonant frequency. To be resonant, the length of a driven element should typically be either half or a quarter of the wavelength at that frequency; these are called half-wave and quarter-wave antennas  SONY VAIO VGN-FW72JGB Battery .

The length referred to is not the physical length, but the electrical length of the element, which is the physical length divided by the velocity factor (the ratio of the speed of wave propagation in the wire to c0, the speed of light in a vacuum). Antennas are usually also resonant at multiples (harmonics) of the lowest resonant frequency SONY VAIO VGN-FW73JGB Battery .

Some antenna designs have multiple resonant frequencies, and some are relatively effective over a very broad range of frequencies. or bandwidth. One commonly known type of wide band antenna is the logarithmic or log-periodic antenna.

The resonant frequency also affects the impedance of the antenna  SONY VAIO VGN-FW74FB Battery .

At resonance, the equivalent circuit of an antenna is a pure resistance, with no reactive component. At frequencies other than the resonant frequencies, the antenna has capacitance or inductance as well as resistance. An antenna can be made resonant at other frequencies besides its natural resonant frequency bySONY VAIO VGN-FW81HS Battery .

compensating for these reactances by adding a loading coil or capacitor in series with it. Other properties of an antenna change with frequency, in particular the radiation pattern, so the antenna's operating frequency may be considerably different from the resonant frequency to optimize other important parameters  SONY VAIO VGN-FW81NS Battery .

Gain

Gain is a parameter which measures the degree of directivity of the antenna's radiation pattern. An antenna with a low gain emits radiation with about the same power in all directions, whereas a high-gain antenna will preferentially radiate in particular directionsSONY VAIO VGN-FW81S Battery .

Specifically, the antenna gain, directive gain, or power gain of an antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in the direction of its maximum output, at an arbitrary distance, divided by the intensity radiated at the same distance by a hypothetical isotropic antenna SONY VAIO VGN-FW82DS Battery .

The gain of an antenna is a passive phenomenon - power is not added by the antenna, but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. An antenna designer must take into account the application for the antenna when determining the gain SONY VAIO VGN-FW82JS Battery .

High-gain antennas have the advantage of longer range and better signal quality, but must be aimed carefully in a particular direction. Low-gain antennas have shorter range, but the orientation of the antenna is relatively inconsequential  SONY VAIO VGN-FW82XS Battery .

For example, a dish antenna on a spacecraft is a high-gain device that must be pointed at the planet to be effective, whereas a typical Wi-Fi antenna in a laptop computer is low-gain, and as long as the base station is within range, the antenna can be in any orientation in space  SONY VAIO VGN-FW83DS Battery .

It makes sense to improve horizontal range at the expense of reception above or below the antenna. Thus most antennas labelled "omnidirectional" really have some gain.

In practice, the half-wave dipole is taken as a reference instead of the isotropic radiator SONY VAIO VGN-FW83JS Battery .

The gain is then given in dBd (decibels over dipole):

NOTE: 0 dBd = 2.15 dBi. It is vital in expressing gain values that the reference point be included. Failure to do so can lead to confusion and error SONY VAIO VGN-FW83XS Battery .

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