about Antenna

log-periodic-closeup

Short wave “curtain” antenna(Moosbrunn, Austria)

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An antenna (or aerial) is a transducer  that transmits or receives electromagnetic waves. In other  words, antennas convert electromagnetic radiation into electrical  current, or vice versa. Antennas generally deal in the transmission and  reception of radio waves, and are a  necessary part of all radio equipment. Antennas are used in systems such as radio and television  broadcasting, point-to-point radio communication, wireless  LAN, cell phones, radar, and spacecraft  communication. Antennas are most commonly employed in air or outer  space, but can also be operated under water or even through soil  and rock at certain frequencies for short distances.

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Physically, an antenna is an arrangement of one or more conductors, usually  called elements in this context. In transmission, an alternating current is created in the  elements by applying a voltage at the antenna terminals, causing the  elements to radiate an electromagnetic field. In reception,  the inverse occurs: an electromagnetic field from another source induces an alternating current in  the elements and a corresponding voltage at the antenna’s terminals.  Some receiving antennas (such as parabolic and horn types) incorporate shaped  reflective surfaces to collect the radio waves striking them and direct  or focus them onto the actual conductive elements.

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Some of the first rudimentary 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 the emitter  dipole  in the focal point of a parabolic reflector. He published his work and  installation drawings in Annalen der  Physik und Chemie (vol. 36, 1889

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Contents

  • 1 Terminology
  • 2 Overview
  • 3 Parameters
    • 3.1 Resonant frequency
    • 3.2 Gain
    • 3.3 Radiation pattern
    • 3.4 Impedance
    • 3.5 Efficiency
    • 3.6 Bandwidth
    • 3.7 Polarization
    • 3.8 Transmission and  reception
  • 4 Basic antenna models
  • 5 Practical antennas
  • 6 Effect of ground
  • 7 Mutual impedance and  interaction between antennas
  • 8 Antenna gallery
    • 8.1 Antennas and antenna  arrays
    • 8.2 Antennas and  supporting structures
    • 8.3 Diagrams as part of a  system

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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. 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.

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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  Blanc region, Marconi experimented with early wireless equipment. 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. Marconi’s use of the word antenna (Italian for pole) would become a popular term for  what today is uniformly known as the antenna.

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A Hertzian antenna is a  set of terminals that does not require the presence of a ground for its  operation (versus a Tesla antenna which is grounded[3]).  A loaded antenna is an active antenna having an elongated portion of  appreciable electrical  length and having additional inductance  or capacitance directly in series or shunt with the elongated portion so as to modify the standing  wave pattern existing along the portion or to change the effective  electrical length of the portion. An antenna grounding structure is a structure for  establishing a reference potential level for operating the active  antenna. It can be any structure closely associated with (or acting as)  the ground which is connected to the terminal of the signal receiver or  source opposing the active antenna terminal.

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In colloquial usage, the word antenna may refer broadly to an  entire assembly including support structure, enclosure (if any), etc. in  addition to the purely functional components.

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“Rabbit ears” dipole antenna for television reception

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Cell phone base station antennas

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Parabolic antenna for communicating with spacecraft, Canberra, Australia

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Yagi antenna used for mobile military communications station, Dresden,  Germany, 1955

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“Turnstile” type transmitting antenna for commercial radio broadcasting  station, Germany.

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Large Yagi antenna used by amateur  radio hobbyist

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A vertical mast radiator, Chapel Hill, North Carolina

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   Overview

 Antennas have practical uses for the transmission  and reception of radio frequency signals such as radio and television. In  air, those signals travel very quickly and with a very low transmission loss.  The signals are absorbed when moving through more conductive materials, such as  concrete walls or rock. When encountering an interface, the waves are  partially reflected and partially transmitted  through.

* A common antenna is a vertical rod a quarter of a wavelength long.  Such antennas are simple in construction, usually inexpensive, and both  radiate in and receive from all horizontal directions (omnidirectional).  One limitation of this antenna is that it does not radiate or receive  in the direction in which the rod points. This region is called the antenna blind cone or null.

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 There are two fundamental types of antenna directional patterns,  which, with reference to a specific two dimensional plane (usually  horizontal [parallel to the ground] or vertical [perpendicular to the  ground]), are either:

  1.  Omni-directional (radiates equally in all directions), such as a vertical rod (in the horizontal plane) or
  2. Directional (radiates more in one direction than in the other

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In colloquial usage “omnidirectional” usually refers to all  horizontal directions with reception above and below the antenna being  reduced in favor of better reception (and thus range) near the horizon. A  “directional” antenna usually refers to one focusing a narrow beam in a  single specific direction such as a telescope or satellite dish, or, at  least, focusing in a sector such as a 120° horizontal fan pattern in  the case of a panel antenna at a cell  site.

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All antennas radiate some energy in all directions in free space but  careful construction results in substantial transmission of energy in a  preferred direction and negligible energy radiated in other directions.  By adding additional elements (such as rods, loops or plates) and  carefully arranging their length, spacing, and orientation, an antenna  with desired directional properties can be created.

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An antenna array is two or more simple antennas combined to  produce a specific directional radiation pattern. In common usage an  array is composed of active elements, such as a linear array of parallel  dipoles fed as a “broadside array”. A slightly different feed method  could cause this same array of dipoles to radiate as an “end-fire  array”. Antenna arrays may be built up from any basic antenna type, such  as dipoles, loops or slots.

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The directionality of the array is due to the spatial relationships  and the electrical feed relationships between individual antennas.  Usually all of the elements are active (electrically fed) as in the log-periodic dipole array which offers  modest gain and broad bandwidth and is traditionally used for television  reception. Alternatively, a superficially similar dipole array, the Yagi-Uda Antenna (often abbreviated to  ”Yagi”), has only one active dipole element in a chain of parasitic  dipole elements, and a very different performance with high gain over a  narrow bandwidth.

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An active element is electrically connected to the antenna terminals  leading to the receiver or transmitter, as opposed to a parasitic element that modifies the  antenna pattern without being connected directly. The active element(s)  couple energy between the electromagnetic wave and the antenna  terminals, thus any functioning antenna has at least one active element.  A careful arrangement of parasitic elements, such as rods or coils, can  improve the radiation pattern of the active element(s). Directors and  reflectors are common parasitic elements.

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An antenna lead-in is the medium, for example, a transmission line or feed  line for conveying the signal energy between the signal source or  receiver and the antenna. The antenna  feed refers to the components between the antenna and an amplifier.

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An antenna counterpoise is a structure of conductive material most  closely associated with ground that may be insulated from or  capacitively coupled to the natural ground. It aids in the function of  the natural ground, particularly where variations (or limitations) of  the characteristics of the natural ground interfere with its proper  function. Such structures are usually connected to the terminal of a  receiver or source opposite to the antenna terminal.

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An antenna component is a portion of the antenna performing a  distinct function and limited for use in an antenna, as for example, a  reflector, director, or active antenna.

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An electromagnetic wave refractor is a structure which is shaped or  positioned to delay or accelerate transmitted electromagnetic waves,  passing through such structure, an amount which varies over the wave  front. The refractor alters the direction of propagation of the waves  emitted from the structure with respect to the waves impinging on the  structure. It can alternatively bring the wave to a focus or alter the  wave front in other ways, such as to convert a spherical wave front to a  planar wave front (or vice-versa). The velocity of the waves radiated  have a component which is in the same direction (director) or in the  opposite direction (reflector) as that of the velocity of the impinging  wave.

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A director is a parasitic element, usually a metallic  conductive structure, which re-radiates into free space impinging  electromagnetic radiation coming from or going to the active antenna,  the velocity of the re-radiated wave having a component in the direction  of the velocity of the impinging wave.

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A reflector is a parasitic element,  usually a metallic conductive structure (e.g., screen, rod or plate),  which re-radiates back into free space impinging electromagnetic  radiation coming from or going to the active antenna. The velocity of  the returned wave has a component in a direction opposite to the  direction of the velocity of the impinging wave. The reflector modifies  the radiation of the active antenna.

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An antenna coupling network is a passive network (which may be any  combination of a resistive, inductive or capacitive circuit(s)) for  transmitting the signal energy between the active antenna and a source  (or receiver) of such signal energy.
Typically, antennas are designed to operate in a relatively narrow frequency  range. The design criteria for receiving and transmitting antennas  differ slightly, but generally an antenna can receive and transmit  equally well. This property is called reciprocity.

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Parameters
Main article: Antenna measurement
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. Transmit antennas  may also have a maximum power rating, and receive antennas differ in  their noise rejection properties. All of these parameters can be measured through various means.

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Resonant frequency

The “resonant frequency” and “electrical resonance” is related to  the electrical  length of an antenna. The electrical length is usually the physical  length of the wire divided by its velocity factor (the ratio of the  speed of wave propagation in the wire to c0, the speed  of light in a vacuum). Typically an antenna is tuned for a specific  frequency, and is effective for a range of frequencies that are usually  centered on that resonant frequency. However, other properties of an  antenna change with frequency, in particular the radiation pattern and  impedance, so the antenna’s resonant frequency may merely be close to  the center frequency of these other more important properties.

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Antennas can be made resonant on harmonic  frequencies with lengths that are fractions of the target wavelength;  this resonance gives much better coupling to the electromagnetic wave,  and makes the aerial act as if it were physically larger.

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Some antenna designs have multiple resonant frequencies, and some are  relatively effective over a very broad range of frequencies. The most  commonly known type of wide band aerial is the logarithmic or log  periodic, but its gain is usually much lower than that of a specific or  narrower band aerial.

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Gain

Main article: Antenna  gain
Gain as a parameter measures the efficiency of a given antenna with  respect to a given norm, usually achieved by modification of its  directionality. 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 directions. Specifically, the 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 a given direction at an arbitrary distance  divided by the intensity radiated at the same distance by a hypothetical  isotropic antenna

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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. If an antenna has a gain greater than one in some directions,  it must have a gain less than one in other directions, since energy is  conserved by the antenna. An antenna designer must take into account the  application for the antenna when determining the gain. 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. 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. 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.[4]

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In practice, the half-wave dipole is taken as a reference instead of  the isotropic radiator. The gain is then given in dBd (decibels  over dipole):

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  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.  Radiation pattern

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polar plots of the horizontal cross sections of a (virtual)  Yagi-Uda-antenna. Outline connects points with 3db field power compared  to an ISO emitter.

The radiation pattern of an antenna is the  geometric pattern of the relative field strengths of the field emitted  by the antenna. For the ideal isotropic antenna, this would be a sphere.  For a typical dipole, this would be a toroid. The radiation pattern of an antenna is typically  represented by a three dimensional graph, or polar plots of the  horizontal and vertical cross sections. The graph should show sidelobes and backlobes, where the antenna’s  gain is at a minima or maxima.
See Antenna measurement: Radiation pattern  or Radiation pattern for more information

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Impedance

As an electro-magnetic wave travels through the different parts of  the antenna system (radio, feed line, antenna, free space) it may  encounter differences in impedance (E/H, V/I, etc.). At each interface,  depending on the impedance match, some fraction of the wave’s energy  will reflect back to the source,[5]  forming a standing wave in the feed line. The ratio of maximum power to  minimum power in the wave can be measured and is called the standing wave ratio (SWR). A SWR of 1:1 is ideal.  A SWR of 1.5:1 is considered to be marginally acceptable in low power  applications where power loss is more critical, although an SWR as high  as 6:1 may still be usable with the right equipment. Minimizing  impedance differences at each interface (impedance matching) will reduce SWR and maximize power  transfer through each part of the antenna system.

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Complex impedance of an antenna is related to the electrical  length of the antenna at the wavelength in use. The impedance of an  antenna can be matched to the feed line and radio by adjusting the  impedance of the feed line, using the feed line as an impedance transformer.  More commonly, the impedance is adjusted at the load (see below) with  an antenna tuner, a balun, a matching transformer, matching networks  composed of inductors and capacitors,  or matching sections such as the gamma match.

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Efficiency

Efficiency is the ratio of power  actually radiated to the power put into the antenna terminals. A dummy  load may have an SWR of 1:1 but an efficiency of 0, as it absorbs  all power and radiates heat but not RF energy, showing that SWR alone is  not an effective measure of an antenna’s efficiency. Radiation in an  antenna is caused by radiation resistance which can only be  measured as part of total resistance including loss resistance.  Loss resistance usually results in heat generation rather than  radiation, and reduces efficiency. Mathematically, efficiency is  calculated as radiation resistance divided by total resistance.

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Bandwidth

The bandwidth of an antenna is  the range of frequencies over which it is effective, usually centered  on the resonant frequency. The bandwidth of an antenna may be increased  by several techniques, including using thicker wires, replacing wires  with cages to simulate a thicker wire, tapering antenna  components (like in a feed horn), and combining multiple antennas into a  single assembly and allowing the natural impedance to select the  correct antenna. Small antennas are usually preferred for convenience,  but there is a fundamental limit relating bandwidth, size and  efficiency.

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[edit] Polarization

The polarization of an antenna is the  orientation of the electric field (E-plane) of the radio wave with respect to the  Earth’s surface and is determined by the physical structure of the  antenna and by its orientation. It has nothing in common with antenna  directionality terms: “horizontal”, “vertical” and “circular”. Thus, a  simple straight wire antenna will have one polarization when mounted  vertically, and a different polarization when mounted horizontally.  ”Electromagnetic wave polarization filters” are structures which can be  employed to act directly on the electromagnetic wave to filter out wave  energy of an undesired polarization and to pass wave energy of a desired  polarization.

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Reflections generally affect polarization. For radio waves the most  important reflector is the ionosphere  - signals which reflect from it will have their polarization changed  unpredictably. For signals which are reflected by the ionosphere,  polarization cannot be relied upon. For line-of-sight communications for  which polarization can be relied upon, it can make a large difference in  signal quality to have the transmitter and receiver using the same  polarization; many tens of dB difference are commonly seen and this is  more than enough to make the difference between reasonable communication  and a broken link.

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Polarization is largely predictable from antenna construction but,  especially in directional antennas, the polarization of side lobes can  be quite different from that of the main propagation lobe. For radio  antennas, polarization corresponds to the orientation of the radiating  element in an antenna. A vertical omnidirectional WiFi antenna will have vertical polarization  (the most common type). An exception is a class of elongated waveguide  antennas in which vertically placed antennas are horizontally polarized.  Many commercial antennas are marked as to the polarization of their  emitted signals.

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Polarization is the sum of the E-plane orientations over time  projected onto an imaginary plane perpendicular to the direction of  motion of the radio wave. In the most general case, polarization is elliptical,  meaning that the polarization of the radio waves varies over time. Two  special cases are linear polarization (the ellipse collapses into a  line) and circular polarization (in which the  two axes of the ellipse are equal). In linear polarization the antenna  compels the electric field of the emitted radio wave to a particular  orientation. Depending on the orientation of the antenna mounting, the  usual linear cases are horizontal and vertical polarization. In circular  polarization, the antenna continuously varies the electric field of the  radio wave through all possible values of its orientation with regard  to the Earth’s surface. Circular polarizations, like elliptical ones,  are classified as right-hand polarized or left-hand polarized using a  ”thumb in the direction of the propagation” rule. Optical researchers  use the same rule of thumb, but pointing it in the direction of the  emitter, not in the direction of propagation, and so are opposite to  radio engineers’ use.

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In practice, regardless of confusing terminology, it is important  that linearly polarized antennas be matched, lest the received signal  strength be greatly reduced. So horizontal should be used with  horizontal and vertical with vertical. Intermediate matchings will lose  some signal strength, but not as much as a complete mismatch.  Transmitters mounted on vehicles with large motional freedom commonly  use circularly polarized antennas so that there will never be a complete  mismatch with signals from other sources.

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 Transmission and  reception

All of the antenna parameters are expressed in terms of a transmission antenna, but  are identically applicable to a receiving antenna, due to reciprocity. Impedance,  however, is not applied in an obvious way; for impedance, the impedance  at the load (where the power is consumed) is most critical. For a  transmitting antenna, this is the antenna itself. For a receiving  antenna, this is at the (radio) receiver rather than at the antenna.  Tuning is done by adjusting the length of an electrically long linear  antenna to alter the electrical resonance of the antenna.

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Antenna tuning is done by adjusting an inductance or capacitance  combined with the active antenna (but distinct and separate from the  active antenna). The inductance or capacitance provides the reactance which  combines with the inherent reactance of the active antenna to establish a  resonance in a circuit including the active antenna. The established  resonance being at a frequency other than the natural electrical  resonant frequency of the active antenna. Adjustment of the inductance  or capacitance changes this resonance.

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Antennas used for transmission have a maximum power  rating, beyond which heating, arcing or sparking may occur in the  components, which may cause them to be damaged or destroyed. Raising  this maximum power rating usually requires larger and heavier  components, which may require larger and heavier supporting structures.  This is a concern only for transmitting antennas, as the power received  by an antenna rarely exceeds the microwatt range.

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Antennas designed specifically for reception might be optimized for noise rejection capabilities. An antenna  shield is a conductive or low reluctance structure (such as a wire,  plate or grid) which is adapted to be placed in the vicinity of an  antenna to reduce, as by dissipation through a resistance or by  conduction to ground, undesired electromagnetic radiation, or electric  or magnetic fields, which are directed toward the active antenna from an  external source or which emanate from the active antenna. Other methods  to optimize for noise rejection can be done by selecting a narrow bandwidth so that noise from other frequencies is  rejected, or selecting a specific radiation pattern to reject noise from  a specific direction, or by selecting a polarization different from the  noise polarization, or by selecting an antenna that favors either the  electric or magnetic field.

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For instance, an antenna to be used for reception of low frequencies  (below about ten megahertz) will be subject to both man-made noise from  motors and other machinery, and from natural sources such as lightning.  Successfully rejecting these forms of noise is an important antenna  feature. A small coil of wire with many turns is more able to reject  such noise than a vertical antenna. However, the vertical will radiate  much more effectively on transmit, where extraneous signals are not a  concern.

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 Basic antenna models

Typical US multiband TV antenna (aerial)

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There are many variations of antennas. Below are a few basic models.  More can be found in Category:Radio frequency  antenna types.

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  • The isotropic radiator is a purely  theoretical antenna that radiates equally in all directions. It is  considered to be a point in space with no dimensions and no mass. This  antenna cannot physically exist, but is useful as a theoretical model  for comparison with all other antennas. Most antennas’ gains are  measured with reference to an isotropic radiator, and are rated in dBi  (decibels with respect to an isotropic radiator).
  • The dipole antenna is simply two wires pointed in  opposite directions arranged either horizontally or vertically, with  one end of each wire connected to the radio and the other end hanging  free in space. Since this is the simplest practical antenna, it is also  used as a reference model for other antennas; gain  with respect to a dipole is labeled as dBd. Generally, the dipole is  considered to be omnidirectional in the plane  perpendicular to the axis of the antenna, but it has deep nulls in the directions of the axis. Variations of the  dipole include the folded dipole, the half wave antenna, the ground  plane antenna, the whip, and the J-pole.
  • The Yagi-Uda antenna is a directional variation  of the dipole with parasitic elements added which are  functionality similar to adding a reflector and lenses (directors) to  focus a filament light bulb.
  • The random wire antenna is simply a very  long (at least one quarter wavelength) wire with one end connected to  the radio and the other in free space, arranged in any way most  convenient for the space available. Folding will reduce effectiveness  and make theoretical analysis extremely difficult. (The added length  helps more than the folding typically hurts.) Typically, a random wire  antenna will also require an antenna  tuner, as it might have a random impedance that varies non-linearly  with frequency.
  • The horn is used where high gain is  needed, the wavelength is short (microwave)  and space is not an issue. Horns can be narrow band or wide band,  depending on their shape. A horn can be built for any frequency, but  horns for lower frequencies are typically impractical. Horns are also  frequently used as reference antennas.
  • The parabolic antenna consists of an active  element at the focus of a parabolic reflector to reflect the waves into a plane  wave. Like the horn it is used for high gain, microwave applications,  such as satellite dishes.
  • The patch antenna consists mainly of a square  conductor mounted over a groundplane. Another example of a planar  antenna is the tapered slot antenna (TSA), as the Vivaldi-antenna.

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Practical antennas

Very common “rabbit ears” set-top antenna

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Although any circuit can radiate if driven with a signal of high  enough frequency, most practical antennas are specially designed to  radiate efficiently at a particular frequency. An example of an  inefficient antenna is the simple Hertzian dipole antenna, which radiates over wide range of  frequencies and is useful for its small size. A more efficient variation  of this is the half-wave dipole, which radiates with high efficiency  when the signal wavelength is twice the electrical  length of the antenna.

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One of the goals of antenna design is to minimize the reactance of the device so that it  appears as a resistive load. An “antenna  inherent reactance” includes not only the distributed reactance of the  active antenna but also the natural reactance due to its location and  surroundings (as for example, the capacity relation inherent in the  position of the active antenna relative to ground). Reactance diverts  energy into the reactive field, which causes unwanted currents that heat  the antenna and associated wiring, thereby wasting energy without  contributing to the radiated output. Reactance can be eliminated by  operating the antenna at its resonant frequency, when its  capacitive and inductive reactances are equal and opposite, resulting in  a net zero reactive current. If this is not possible, compensating  inductors or capacitors can instead be added to the antenna to cancel  its reactance as far as the source is concerned.

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Once the reactance has been eliminated, what remains is a pure  resistance, which is the sum of two parts: the ohmic resistance of the  conductors, and the radiation resistance. Power absorbed by  the ohmic resistance becomes waste heat, and that absorbed by the  radiation resistance becomes radiated electromagnetic energy. The  greater the ratio of radiation resistance to ohmic resistance, the more  efficient the antenna.

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 Effect of ground

Antennas are typically used in an environment where other objects are  present that may have an effect on their performance. Height above  ground has a very significant effect on the radiation pattern of some  antenna types.

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At frequencies used in antennas, the ground behaves mainly as a dielectric.  The conductivity of ground at these frequencies is negligible. When an  electromagnetic wave arrives at the surface of an object, two waves are  created: one enters the dielectric and the other is reflected. If the  object is a conductor, the transmitted wave is negligible and the  reflected wave has almost the same amplitude as the incident one. When  the object is a dielectric, the fraction reflected depends (among others  things) on the angle of incidence. When the angle of incidence is small  (that is, the wave arrives almost perpendicularly) most of the energy  traverses the surface and very little is reflected. When the angle of  incidence is near 90° (grazing incidence) almost all the wave is  reflected.

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Most of the electromagnetic waves emitted by an antenna to the ground  below the antenna at moderate (say < 60°) angles of incidence enter  the earth and are absorbed (lost). But waves emitted to the ground at  grazing angles, far from the antenna, are almost totally reflected. At  grazing angles, the ground behaves as a mirror. Quality of reflection  depends on the nature of the surface. When the irregularities of the  surface are smaller than the wavelength reflection is good.

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The wave reflected by earth can be considered  as emitted by the image antenna

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This means that the receptor “sees” the real antenna and, under the  ground, the image of the antenna reflected by the ground. If the ground  has irregularities, the image will appear fuzzy.

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If the receiver is placed at some height above the ground, waves  reflected by ground will travel a little longer distance to arrive to  the receiver than direct waves. The distance will be the same only if  the receiver is close to ground.

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In the drawing at right, we have drawn the angle  far bigger than in reality. Distance between the antenna and its image  is .

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The situation is a bit more complex because the reflection of  electromagnetic waves depends on the polarization of the incident wave. As  the refractive index of the ground (average  value )  is bigger than the refractive index of the air (),  the direction of the component of the electric field parallel to the  ground inverses at the reflection. This is equivalent to a phase shift  of  radians or 180°. The vertical component of the electric field reflects  without changing direction. This sign inversion of the parallel  component and the non-inversion of the perpendicular component would  also happen if the ground were a good electrical conductor.

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The vertical component of the current reflects  without changing sign. The horizontahttp://enggate.net/admin/post.php?post=1364&action=editl component reverses sign at  reflection.

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This means that a receiving antenna “sees” the image antenna with the  current in the same direction if the antenna is vertical or with the  current inverted if the antenna is horizontal.

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For a vertical polarized emission antenna the far  electric field of the electromagnetic wave produced by the direct ray  plus the reflected ray is:

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The sign inversion for the parallel field case just changes a cosine  to a sine:

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In these two equations:

  •  is the electrical field radiated by the antenna if there were no  ground.
  •  is the wave number.
  •  is the wave length.
  •  is the distance between antenna and its image (twice the height of the  center of the antenna).

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Radiation patterns of antennas and their  images reflected by the ground. At left the polarization is vertical and  there is always a maximum for .  If the polarization is horizontal as at right, there is always a zero  for .

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For emitting and receiving antenna situated near the ground (in a  building or on a mast) far from each other, distances traveled by direct  and reflected rays are nearly the same. There is no induced phase  shift. If the emission is polarized vertically the two fields (direct  and reflected) add and there is maximum of received signal. If the  emission is polarized horizontally the two signals subtracts and the  received signal is minimum. This is depicted in the image at right. In  the case of vertical polarization, there is always a maximum at earth  level (left pattern). For horizontal polarization, there is always a  minimum at earth level. Note that in these drawings the ground is  considered as a perfect mirror, even for low angles of incidence. In  these drawings the distance between the antenna and its image is just a  few wavelengths. For greater distances, the number of lobes increases.

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Note that the situation is different–and more complex–if reflections  in the ionosphere occur. This happens over very long distances  (thousands of kilometers). There is not a direct ray but several  reflected rays that add with different phase shifts.

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This is the reason why almost all public address radio emissions have  vertical polarization. As public users are near ground, horizontal  polarized emissions would be poorly received. Observe household and  automobile radio receivers. They all have vertical antennas or  horizontal ferrite  antennas for vertical polarized emissions. In cases where the  receiving antenna must work in any position, as in mobile  phones, the emitter and receivers in base stations use circular polarized electromagnetic  waves.

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Classical (analog) television emissions are an exception. They are  almost always horizontally polarized, because the presence of buildings  makes it unlikely that a good emitter antenna image will appear.  However, these same buildings reflect the electromagnetic waves and can  create ghost images. Using horizontal  polarization, reflections are attenuated because of the low reflection  of electromagnetic waves whose magnetic field is parallel to the  dielectric surface near the Brewster’s angle. Vertically polarized analog television has  been used in some rural areas. In digital terrestrial television  reflections are less obtrusive, due to the inherent robustness of digital signalling and built-in error correction.

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 Mutual impedance  and interaction between antennas

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Mutual impedance between parallel  dipoles not staggered. Curves Re and Im are the resistive  and reactive parts of the impedance.

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Current circulating in any antenna induces currents in all others.  One can postulate a mutual impedance  between two antennas that has the same significance as the  in ordinary coupled inductors. The mutual impedance  between two antennas is defined as:

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where  is the current flowing in antenna 1 and  is the voltage that would have to be applied to antenna 2–with antenna 1  removed–to produce the current in the antenna 2 that was produced by  antenna 1.
From this definition, the currents and voltages applied in a set of  coupled antennas are:
where:

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  •  is the voltage applied to the antenna i
  •  is the impedance of antenna i
  •  is the mutual impedance between antennas i  and j

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 This is a consequence of Lorentz reciprocity. If some of the  elements are not fed (there is a short circuit instead a feeder cable),  as is the case in television antennas (Yagi-Uda antennas), the corresponding  are zero. Those elements are called parasitic elements. Parasitic elements  are unpowered elements that either reflect or absorb and reradiate RF  energy.

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In some geometrical settings, the mutual impedance between antennas  can be zero. This is the case for crossed dipoles used in circular  polarization antennas.

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 Antenna gallery

[edit] Antennas and  antenna arrays

A Yagi-Uda beam antenna.

A multi-band rotary directional antenna for amateur radio use.

Rooftop TV antenna.  It is actually three Yagi antennas. The longest  elements are for the low band, while the medium and short elements are  for the high and UHF band.

A terrestrial microwave radio antenna array.

Examples of US 136-174 MHz base station antennas.

Low cost LF time  signal receiver, antenna (left) and receiver (right).

Rotatable log-periodic array for VHF and UHF.

Shortwave antennas in Delano, California.

An old VHF-band Yagi-type television antenna.

A T2FD broadband antenna, covering the 5-30MHz band.

A US multiband “aerial” TV antenna.

“Rabbit ears” antenna

AM loop antenna

[edit]  Antennas  and supporting structures

A building rooftop supporting numerous dish and sectored mobile  telecommunications antennas (Doncaster, Victoria, Australia.

A water tower in Palmerston, Northern Territory with radio broadcasting and  communications antennas.

A three-sector telephone site in Mexico City.

Telephone site concealed as a palm tree.

Diagrams as part  of a system

Antennas may be connected through a multiplexing  arrangement in some applications like this trunked two-way  radio example.

Antenna network for an emergency medical services base station.[/QUOTE]