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Random Wire Antenna
A random wire antenna is a radio antenna consisting of a long wire suspended above the ground, whose length does not bear a particular relation to the wavelength of the radio waves used, but is typically chosen more for convenience. The wire may be straight or it may be strung back and forth between trees or walls just to get enough wire into the air. Due to the great variability of the (unplanned) antenna structure, effectiveness can vary wildly from one installation to another. Random wire antennas are typically fed at one end against a suitable counterpoise (such as earth ground). They are widely used as receiving antennas on the long wave, medium wave, and short wave bands, as well as transmitting antennas on these bands for small outdoor, temporary or emergency transmitting stations, as well as in situations where more permanent antennas cannot be installed. Random wire and long wire Often random wire antennas are also (inaccurately) referred to as long-wire antenna. Ac ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Axially Symmetric
In geometry, circular symmetry is a type of continuous symmetry for a Plane (geometry), planar object that can be rotational symmetry, rotated by any arbitrary angle and map onto itself. Rotational circular symmetry is isomorphic with the circle group in the complex plane, or the special orthogonal group SO(2), and unitary group U(1). Reflective circular symmetry is isomorphic with the orthogonal group O(2). Two dimensions A 2-dimensional object with circular symmetry would consist of concentric circles and Annulus (mathematics), annular domains. Rotational circular symmetry has all cyclic symmetry, Z''n'' as subgroup symmetries. Reflective circular symmetry has all dihedral symmetry, Dih''n'' as subgroup symmetries. Three dimensions In 3-dimensions, a surface of revolution, surface or solid of revolution has circular symmetry around an axis, also called cylindrical symmetry or axial symmetry. An example is a right circular cone. Circular symmetry in 3 dimensions has all ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Balun
A balun (from "balanced to unbalanced", originally, but now dated from "balancing unit") is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. A balun can take many forms and may include devices that also transform impedances but need not do so. Sometimes, in the case of transformer baluns, they use magnetic coupling but need not do so. Common-mode chokes are also used as baluns and work by eliminating, rather than rejecting, common mode signals. Types of balun Classical transformer type In classical transformers, there are two electrically separate windings of wire coils around the transformer's core. The advantage of transformer-type over other types of balun is that the electrically separate windings for input and output allow these baluns to connect circuits whose ground-level voltages are subject to ground loops or are otherwise electrically incompatible; for that reason t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Antenna Tuner
An antenna tuner (and any of the names in the list below) is a device that is inserted between a transmitter, radio transmitter and its antenna (radio), antenna; when placed close by the antenna and properly adjusted (tuned) it optimizes power transfer by Impedance matching, matching the Electrical impedance, impedance of the radio to the impedance of the end of the feedline connecting the antenna to the transmitter. Various alternate names are used for this device: antenna matching unit, Impedance matching, impedance matching unit, matchbox, matching network, transmatch, antenna match, antenna tuning unit (ATU), antenna coupler, feedline coupler. English language technical jargon makes no distinction between the terms. Antenna tuners are particularly important for use with transmitters. Transmitters are typically designed to feed power into a reactance (electronics), reactance-free, resistive load pull, load of a specific value: Radio transmitters built after the 1950s are alm ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Strain Insulator
A strain insulator is an electrical insulator that is designed to work in mechanical tension (strain), to withstand the pull of a suspended electrical wire or cable. They are used in overhead electrical wiring, to support radio antennas and overhead power lines. A strain insulator may be inserted between two lengths of wire to isolate them electrically from each other while maintaining a mechanical connection, or where a wire attaches to a pole or tower, to transmit the pull of the wire to the support while insulating it electrically. Strain insulators were first used in telegraph systems in the mid 19th century. Description and use A typical strain insulator is a piece of glass, porcelain, or fiberglass that is shaped to accommodate two cables or a cable shoe and the supporting hardware on the support structure (hook eye, or eyelet on a steel pole/tower). The shape of the insulator maximizes the distance between the cables while also maximizing the load-bearing transfer capa ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Wire Inverted L Antenna Construction
Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm A wire is a flexible strand of metal. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Wire gauges come in various standard sizes, as expressed in terms of a gauge number. Wires are used to bear mechanical loads, often in the form of wire rope. In electricity and telecommunications signals, a "wire" can refer to an electrical cable, which can contain a "solid core" of a single wire or separate strands in stranded or braided forms. Usually cylindrical in geometry, wire can also be made in square, hexagonal, flattened rectangular, or other cross-sections, either for decorative purposes, or for technical purposes such as high-efficiency voice coils in loudspeakers. Edge-wound coil springs, such as the Slinky toy, are made of special flattened wire. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Traveling Wave Antenna
In radio and telecommunication, a traveling-wave antenna is a class of antenna that uses a traveling wave on a guiding structure as the main radiating mechanism. Its distinguishing feature is that the radio-frequency current that generates the radio waves travels through the antenna in one direction. This is in contrast to a ''resonant antenna'', such as the monopole or dipole, in which the antenna acts as a resonator, with radio currents traveling in both directions, bouncing back and forth between the ends of the antenna. An advantage of traveling wave antennas is that since they are nonresonant they often have a wider bandwidth than resonant antennas. Common types of traveling wave antenna are the Beverage antenna, axial-mode helical antenna, and rhombic antenna. Traveling-wave antennas fall into two general categories: ''slow-wave antennas'', and ''fast-wave antennas''. Fast-wave antennas are often referred to as leaky wave antenna. See also * Axial mode helical ante ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Transmission Line
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmission must be taken into account. This applies especially to radio-frequency engineering because the short wavelengths mean that wave phenomena arise over very short distances (this can be as short as millimetres depending on frequency). However, the theory of transmission lines was historically developed to explain phenomena on very long telegraph lines, especially submarine telegraph cables. Transmission lines are used for purposes such as connecting radio transmitters and receivers with their antennas (they are then called feed lines or feeders), distributing cable television signals, trunklines routing calls between telephone switching centres, computer network connections and high speed computer data buses. RF engineers comm ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Null (radio)
In radio electronics, a null is a direction in an antenna's radiation pattern where the antenna radiates almost no radio waves, so the far field signal strength is a local minimum. Nulls occur because different parts of an antenna radiate radio waves of different phase. In directions at which the antenna radiates equal amplitude radio waves of opposite phase, the radio waves cancel, resulting in little or no radio power being radiated in that direction. In other directions the radio waves from different parts of the antenna are in phase and reinforce, resulting in a maximum signal strength in the radiation pattern, called a lobe. In transmitting antennas designed to provide broad coverage nulls can be a problem, preventing reception in a given area. Null fill in the vertical plane is used to prevent this. Nulling antenna Nulls can also be used to advantage. In a radio receiver the receiver's antenna can be adjusted so the direction of the interference source is located in ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Multipole Moments
A multipole expansion is a mathematical series representing a function that depends on angles—usually the two angles used in the spherical coordinate system (the polar and azimuthal angles) for three-dimensional Euclidean space, \R^3. Similarly to Taylor series, multipole expansions are useful because oftentimes only the first few terms are needed to provide a good approximation of the original function. The function being expanded may be real- or complex-valued and is defined either on \R^3, or less often on \R^n for some other Multipole expansions are used frequently in the study of electromagnetic and gravitational fields, where the fields at distant points are given in terms of sources in a small region. The multipole expansion with angles is often combined with an expansion in radius. Such a combination gives an expansion describing a function throughout three-dimensional space. The multipole expansion is expressed as a sum of terms with progressively finer angular featu ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Computational Electromagnetics
Computational electromagnetics (CEM), computational electrodynamics or electromagnetic modeling is the process of modeling the interaction of electromagnetic fields with physical objects and the environment. It typically involves using computer programs to compute approximate solutions to Maxwell's equations to calculate antenna performance, electromagnetic compatibility, radar cross section and electromagnetic wave propagation when not in free space. A large subfield is ''antenna modeling'' computer programs, which calculate the radiation pattern and electrical properties of radio antennas, and are widely used to design antennas for specific applications. Background Several real-world electromagnetic problems like electromagnetic scattering, electromagnetic radiation, modeling of waveguides etc., are not analytically calculable, for the multitude of irregular geometries found in actual devices. Computational numerical techniques can overcome the inability to derive closed f ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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