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Magnetron
The CAVITY MAGNETRON is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field while moving past a series of open metal cavities (cavity resonators ). Electrons
Electrons
pass by the openings to these cavities and cause radio waves to oscillate within, similar to the way a whistle produces a tone when excited by an air stream blown past its opening. The frequency of the microwaves produced, the resonant frequency , is determined by the cavities' physical dimensions. Unlike other vacuum tubes such as a klystron or a traveling-wave tube (TWT), the magnetron cannot function as an amplifier in order to increase the intensity of an applied microwave signal; the magnetron serves solely as an oscillator , generating a microwave signal from direct current electricity supplied to the vacuum tube. An early form of magnetron was invented by H. Gerdien in 1910
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Electromagnet
An ELECTROMAGNET is a type of magnet in which the magnetic field is produced by an electric current . The magnetic field disappears when the current is turned off. Electromagnets usually consist of insulated wire wound into a coil . A current through the wire creates a magnetic field which is concentrated in the hole in the center of the coil. The wire turns are often wound around a magnetic core made from a ferromagnetic or ferrimagnetic material such as iron ; the magnetic core concentrates the magnetic flux and makes a more powerful magnet. The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be quickly changed by controlling the amount of electric current in the winding. However, unlike a permanent magnet that needs no power, an electromagnet requires a continuous supply of current to maintain the magnetic field
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Hysteresis
HYSTERESIS is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field , depending on how the field changed in the past. Plots of a single component of the moment often form a loop or hysteresis curve, where there are different values of one variable depending on the direction of change of another variable. This history dependence is the basis of memory in a hard disk drive and the remanence that retains a record of the Earth\'s magnetic field magnitude in the past. Hysteresis
Hysteresis
occurs in ferromagnetic and ferroelectric materials, as well as in the deformation of rubber bands and shape-memory alloys and many other natural phenomena. In natural systems it is often associated with irreversible thermodynamic change such as phase transitions and with internal friction ; and dissipation is a common side effect
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Cyclotron Radiation
CYCLOTRON RADIATION is electromagnetic radiation emitted by accelerating charged particles deflected by a magnetic field . The Lorentz force
Lorentz force
on the particles acts perpendicular to both the magnetic field lines and the particles' motion through them, creating an acceleration of charged particles that causes them to emit radiation as a result of the acceleration they undergo as they spiral around the lines of the magnetic field. The name of this radiation derives from the cyclotron , a type of particle accelerator used since the 1930s to create highly energetic particles for study
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High Frequency
HIGH FREQUENCY (HF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) between 3 and 30 megahertz (MHz). It is also known as the DECAMETER BAND or DECAMETER WAVE as its wavelengths range from one to ten decameters (ten to one hundred metres). Frequencies immediately below HF are denoted medium frequency (MF), while the next band of higher frequencies is known as the very high frequency (VHF) band. The HF band is a major part of the shortwave band of frequencies, so communication at these frequencies is often called shortwave radio . Because radio waves in this band can be reflected back to Earth by the ionosphere layer in the atmosphere – a method known as "skip" or "skywave " propagation – these frequencies are suitable for long-distance communication across intercontinental distances
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Left-hand Rule
FLEMING\'S LEFT-HAND RULE FOR MOTORS is one of a pair of visual mnemonics , the other being Fleming\'s right-hand rule (for generators). They were originated by John Ambrose Fleming
John Ambrose Fleming
, in the late 19th century, as a simple way of working out the direction of motion in an electric motor , or the direction of electric current in an electric generator . When current flows through a conducting wire, and an external magnetic field is applied across that flow, the conducting wire experiences a force perpendicular both to that field and to the direction of the current flow (i.e they are mutually perpendicular) . A left hand can be held, as shown in the illustration, so as to represent three mutually orthogonal axes on the thumb, fore finger and middle finger. Each finger is then assigned to a quantity (mechanical force, magnetic field and electric current). The right and left hand are used for generators and motors respectively
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Horseshoe Magnet
A HORSESHOE MAGNET is a magnet made in the shape of a horseshoe . The magnet has two magnetic poles close together. This shape creates a strong magnetic field between the poles. It is one type of permanent magnet, meaning that it stays magnetized, as opposed to an electromagnet , whose magnetic field can be started and stopped. Magnetic field
Magnetic field
of a horseshoe magnet. The field is greatest where the lines are densest, around the poles (lower) Long U-shaped horseshoe magnets, on a high-power magneto used to power a lighthouse. The purpose of a horseshoe magnet's shape is to place the poles as close as possible together. The total magnetic flux is the same, but the field is greater, as it is spread over a smaller volume. A horseshoe is used, rather than a simpler C-shaped magnet, which is also used, because this places the maximum amount of magnetised material into the magnet, for given dimensions around the poles
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Clutter (radar)
CLUTTER is a term used for unwanted echoes in electronic systems, particularly in reference to radars . Such echoes are typically returned from ground, sea, rain, animals/insects, chaff and atmospheric turbulences , and can cause serious performance issues with radar systems. CONTENTS * 1 Backscatter coefficient * 2 Clutter-limited or noise-limited radar * 3 Volume clutter * 3.1 Problems in calculating signal to volume clutter ratio * 4 Surface clutter * 4.1 Beam filling * 4.2 Pulse length limited case * 4.3 Problems in calculating clutter for the pulse length limited case * 4.4 Beam width limited case * 4.5 General problems in calculating surface clutter * 5 See also * 6 References BACKSCATTER COEFFICIENTWhat one person considers to be clutter, another may consider to be a target. However, targets usually refer to point scatterers and clutter to extended scatterers (covering many range, angle, and Doppler cells)
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Cathode
A CATHODE is the electrode from which a conventional current leaves a polarized electrical device. (This definition can be recalled by using the mnemonic CCD for cathode current departs.) A conventional current describes the direction in which positive electronic charges move. Electrons have a negative charge, so the movement of electrons is opposite to the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode. Cathode
Cathode
polarity with respect to the anode can be positive or negative; it depends on how the device operates
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Anode
An ANODE is an electrode through which conventional current flows into a polarized electrical device. A common mnemonic is ACID for "anode current into device". The direction of (positive) electric current is opposite to the direction of electron flow: (negatively charged) electrons flow out the anode to the outside circuit. CONTENTS * 1 Charge flow * 2 Examples * 3 Etymology * 4 Electrolytic anode * 5 Battery or galvanic cell anode * 6 Vacuum tube anode * 7 Diode
Diode
anode * 8 Sacrificial anode
Sacrificial anode
* 9 Related antonym * 10 See also * 11 References * 12 External links CHARGE FLOWThe terms anode and cathode do not relate to the voltage polarity of those electrodes but the direction of the current: whether positive charge is flowing into or out of the device. Conventional current quantifies the flow of positive charge
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Lee De Forest
LEE DE FOREST (August 26, 1873 – June 30, 1961) was an American inventor, self-described "Father of Radio", and a pioneer in the development of sound-on-film recording used for motion pictures. He had over 180 patents, but also a tumultuous career—he boasted that he made, then lost, four fortunes. He was also involved in several major patent lawsuits, spent a substantial part of his income on legal bills, and was even tried (and acquitted) for mail fraud. His most famous invention, in 1906, was the three-element " Audion " (triode ) vacuum tube , the first practical amplification device. Although De Forest had only a limited understanding of how it worked, it was the foundation of the field of electronics , making possible radio broadcasting , long distance telephone lines, and talking motion pictures , among countless other applications
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Very High Frequency
VERY HIGH FREQUENCY (VHF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves ) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten to one meters. Frequencies immediately below VHF are denoted high frequency (HF), and the next higher frequencies are known as ultra high frequency (UHF). Common uses for VHF are FM radio broadcasting, television broadcasting, two way land mobile radio systems (emergency, business, private use and military), long range data communication up to several tens of kilometers with radio modems , amateur radio , and marine communications . Air traffic control communications and air navigation systems (e.g. VOR unlike in the HF band there is only some reflection at lower frequencies from the ionosphere (skywave propagation)
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Permanent Magnet
A MAGNET is a material or object that produces a magnetic field . This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials , such as iron , and attracts or repels other magnets. A PERMANENT MAGNET is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic ). These include iron , nickel , cobalt , some alloys of rare-earth metals , and some naturally occurring minerals such as lodestone
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Continuous-wave Radar
CONTINUOUS-WAVE RADAR is a type of radar system where a known stable frequency continuous-wave radio energy is transmitted and then received from any reflecting objects. Continuous-wave
Continuous-wave
(CW) radar uses Doppler , which renders the radar immune to interference from large stationary objects and slow moving clutter . CW radar systems are used at both ends of the range spectrum
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Vacuum
VACUUM is space devoid of matter . The word stems from the Latin adjective vacuus for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure . Physicists often discuss ideal test results that would occur in a perfect vacuum, which they sometimes simply call "vacuum" or FREE SPACE, and use the term PARTIAL VACUUM to refer to an actual imperfect vacuum as one might have in a laboratory or in space . In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is lower than atmospheric pressure. The Latin term IN VACUO is used to describe an object that is surrounded by a vacuum. The quality of a partial vacuum refers to how closely it approaches a perfect vacuum. Other things equal, lower gas pressure means higher-quality vacuum. For example, a typical vacuum cleaner produces enough suction to reduce air pressure by around 20%. Much higher-quality vacuums are possible
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Waveguide (electromagnetism)
In electromagnetics and communications engineering , the term WAVEGUIDE may refer to any linear structure that conveys electromagnetic waves between its endpoints. However, the original and most common meaning is a hollow metal pipe used to carry radio waves . This type of waveguide is used as a transmission line mostly at microwave frequencies, for such purposes as connecting microwave transmitters and receivers to their antennas , in equipment such as microwave ovens , radar sets, satellite communications , and microwave radio links. A DIELECTRIC WAVEGUIDE employs a solid dielectric rod rather than a hollow pipe. An optical fibre is a dielectric guide designed to work at optical frequencies. Transmission lines
Transmission lines
such as microstrip , coplanar waveguide , stripline or coaxial cable may also be considered to be waveguides
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