An antenna is a transducer designed to transmit or receive electromagnetic waves. A transducer is a device usually electrical, electronic, Electro-mechanical, Electromagnetic, Photonic, or Photovoltaic For biologic transmitters see Transmitter substance. A transmitter is an electronic device which usually with the aid of an antenna Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. In other words, antennas convert electromagnetic waves into electrical currents and vice versa. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless LAN, radar, and space exploration. Radio is the transmission of signals by Modulation of electromagnetic waves with frequencies below those of visible Light. Television ( TV) is a widely used Telecommunication medium for sending ( Broadcasting) and receiving moving Images, either monochromatic A wireless LAN or WLAN is a Wireless Local area network, which is the linking of two or more computers or devices without using wires Radar is a system that uses electromagnetic waves to identify the range altitude direction or speed of both moving and fixed objects such as Aircraft, ships History First orbital flights The first successful orbital launch was of the Soviet unmanned Sputnik Antennas usually work in air or outer space, but can also be operated under water or even through soil and rock at certain frequencies for short distances. Outer space, often simply called space, comprises the relatively empty regions of the Universe outside the escape velocities of Celestial bodies.
Physically, an antenna is an arrangement of conductors that generate a radiating electromagnetic field in response to an applied alternating voltage and the associated alternating electric current, or can be placed in an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals. In Science and engineering, a conductor is a material which contains movable Electric charges. The electromagnetic field is a physical field produced by electrically charged objects. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. For the common use of RF induction process of heating a metal object by electromagnetic induction see Induction heating Radio-frequency induction Some antenna devices (parabolic antenna, Horn Antenna) just adapt the free space to another type of antenna. The parabolic antenna is a high-gain reflector antenna used for radio television and data communications and also for radiolocation ( RADAR) on the UHF and The Horn Antenna, at Bell Telephone Laboratories in Holmdel New Jersey, is listed as a National Historic Landmark because of its association with
Thomas Edison used antennas by 1885. Edison patented his system in . Antennas were also used in 1888 by Heinrich Hertz (1857-1894) to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. Heinrich Rudolf Hertz ( February 22, 1857 – January 1, 1894) was a German physicist who clarified and expanded the electromagnetic theory Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. James Clerk Maxwell (13 June 1831 &ndash 5 November 1879 was a Scottish mathematician and theoretical physicist. Hertz placed the emitter dipole in the focal point of a parabolic reflector. An emitter is a device used to emit any signal, Beacon, Light, Odor, Liquid, Fragrance, or any other type of In physics there are two kinds of dipoles ( Hellènic: di(s- = two- and pòla = pivot hinge An electric dipole is a A parabolic reflector (or dish or mirror) is a Parabola -shaped reflective device used to collect or distribute Energy such as He published his work and installation drawings in Annalen der Physik und Chemie (vol. Annalen der Physik is one of the best-known and oldest (since 1790 Physics journals worldwide 36, 1889).
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 United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,is a Sovereign state located British English or UK English ( BrE, BE, en-GB) is the broad term used to distinguish the forms of the English language used in the The noun aerial is occasionally written with a diaresis mark — aërial — in recognition of the original spelling of the adjective aërial from which the noun is derived.
The origin of the word antenna relative to wireless apparatus is attributed to Guglielmo Marconi. Marchese Guglielmo Marconi mar'koni (25 April 1874 – 20 July 1937 was an Italian inventor best known for his development of a Radiotelegraph system In 1895, while testing early radio apparatus in the Swiss Alps at Salvan, Switzerland in the Mont Blanc region, Marconi experimented with early wireless equipment. The Swiss Alps (Schweizer Alpen Alpes suisses Alpi svizzere Alps svizras are the central portion of the Alps Mountain range that lies within Salvan is a municipality in the district of Saint-Maurice, in the Canton of Valais, Switzerland. Mont Blanc Massif The Mont Blanc ( French for white mountain) or Monte Bianco ( Italian 'White Mountain' also 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. Italian ( or lingua italiana) is a Romance language spoken by about 63 million people as a First language, primarily in Italy.  .
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. Heinrich Rudolf Hertz ( February 22, 1857 – January 1, 1894) was a German physicist who clarified and expanded the electromagnetic theory A Tesla coil is a type of resonant transformer circuit invented by Serbian-American scientist Nikola Tesla around 1891 ) 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. In Telecommunications, the electrical length is any of A Transmission medium, its length expressed as a multiple or submultiple of the Wavelength In Electrical circuits, any Electric current i produces a Magnetic field and hence generates a total Magnetic flux \Phi acting Capacitance is a measure of the amount of Electric charge stored (or separated for a given Electric potential. A standing wave, also known as a stationary wave, is a Wave that remains in a constant position 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, (i. e. , the signal receiver or source is interposed between the active antenna and this structure
Antennas have practical uses for the transmission and reception of radio frequency signals (radio, TV, etc. For biologic transmitters see Transmitter substance. A transmitter is an electronic device which usually with the aid of an antenna This article is about a radio receiver for other uses see Radio (disambiguation. Radio frequency ( RF) is a Frequency or rate of Oscillation within the range of about 3 Hz to 300 GHz ). In air, those signals travel close to the speed of light in vacuum and with a very low transmission loss. In Physics, absorption of electromagnetic radiation is the process by which the Energy of a Photon is taken up by matter typically the electrons of an The signals are absorbed when propagating through more conducting materials, such as concrete walls, rock, etc. In Physics, absorption of electromagnetic radiation is the process by which the Energy of a Photon is taken up by matter typically the electrons of an Radio propagation is a term used to explain how Radio waves behave when they are Transmitted, or are propagated from one point on the Earth When encountering an interface, the waves are partially reflected and partially transmitted through. Reflection is the change in direction of a Wave front at an interface between two different media so that the wave front returns into the medium from which
The vast majority of antennas are simple vertical rods 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. In Telecommunications, antenna blind cone (sometimes called a cone of silence) is the volume of space usually approximately conical with its vertex at the antenna In Radio Electronics, a null is an area or vector in an antenna's Radiation pattern where the signal cancels out almost
There are two fundamental types of antennas, which, with reference to a specific three dimensional (usually horizontal or vertical) plane are either:
In colloquial usage omni-directional usually refers to all horizontal directions with reception above and below the antenna being reduced in favor of better reception (and thus range) in other directions. Also directional antennas are usually meant to refer to one targeting a single specific direction such as a telescope, satellite dish, or possibly a 120° horizontal reception and transmission area.
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 conducting rods or coils (called elements) and varying their length, spacing, and orientation (or changing the direction of the antenna beam), an antenna with specific desired properties can be created, such as a Yagi-Uda Antenna (often abbreviated to "Yagi"). A Yagi-Uda Antenna, commonly known simply as a Yagi antenna or Yagi, is a Directional antenna system consisting of an array of a dipole
An antenna array is two or more antennas coupled to a common source or load to produce a specific directional radiation pattern. The spatial relationship between individual antennas contributes to the directivity of the antenna.
The term active element is intended to describe an element whose energy output is modified due to the presence of a source of energy in the element (other than the mere signal energy which passes through the circuit) or an element in which the energy output from a source of energy is controlled by the signal input.
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. A transmission line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy The Feed line in a Radio Transmission, Reception or Transceiver system is the physical cabling that carries the RF signal to and/or The antenna feed refers to the components between the antenna and an amplifier. The antenna feed refers to the components between an antenna and an amplifier ( HPA and LNB or LNA) Generally an amplifier or simply amp, is any device that changes usually increases the amplitude of a signal.
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.
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.
Parasitic elements are usually metallic conductive structures which reradiate into free space impinging electromagnetic radiation coming from or going to the active antenna. A passive radiator or parasitic element is a Radio antenna element which does not have any Wired Input.
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.
A director is usually a metallic conductive structure which reradiates into free space impinging electromagnetic radiation coming from or going to the active antenna, the velocity of the reradiated wave having a component in the direction of the velocity of the impinging wave. The director modifies the radiation pattern of the active antenna and there is no significant potential relationship between the active antenna and this conductive structure.
A reflector is usually a metallic conductive structure (e. An antenna reflector is a device that reflects Electromagnetic waves It is often a part of an antenna assembly g. , screen, rod or plate) which reradiates back into free space impinging electromagnetic radiation coming from or going to the active antenna. The velocity of the returned wave having 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. There is no significant potential relationship between the active antenna and this conductive structure.
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. Frequency is a measure of the number of occurrences of a repeating event per unit Time. 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. This page is about reciprocity theorems in classical electromagnetism
There are several critical parameters that affect an antenna's performance and can be adjusted during the design process. Antenna measurement techniques refers to the testing of antennas to ensure that the antenna meets specifications or simply to characterize it These are resonant frequency, impedance, gain, aperture or radiation pattern, polarization, efficiency and bandwidth. In Physics, resonance is the tendency of a system to Oscillate at maximum Amplitude at certain frequencies, known as the system's Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal Alternating current (AC In Electronics, gain is a measure of the ability of a circuit (often an Amplifier) to increase the power or Amplitude of a As a receiver, antenna aperture can be visualised as the Area of a Circle constructed broadside to incoming Radiation where all radiation passing In the field of antenna design the term radiation pattern most commonly refers to the directional (angular dependence of radiation from the antenna or other Polarization ( ''Brit'' polarisation) is a property of Waves that describes the orientation of their oscillations 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. Antenna measurement techniques refers to the testing of antennas to ensure that the antenna meets specifications or simply to characterize it
The "resonant frequency" and "electrical resonance" is related to the electrical length of the antenna. In Physics, resonance is the tendency of a system to Oscillate at maximum Amplitude at certain frequencies, known as the system's Electrical resonance occurs in an electric circuit at a particular resonance frequency when the impedance between the input and output of the In Telecommunications, the electrical length is any of A Transmission medium, its length expressed as a multiple or submultiple of the Wavelength 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 usually centered on that resonant frequency. However, the other properties of the antenna (especially radiation pattern and impedance) change with frequency, so the antenna's resonant frequency may merely be close to the center frequency of these other more important properties.
Antennas can be made resonant on harmonic frequencies with lengths that are fractions of the target wavelength. In Acoustics and Telecommunication, the harmonic of a Wave is a component Frequency of the signal that is an Integer 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.
Gain as a parameter measures the directionality of a given antenna. Antenna Gain is defined as the ratio of the Radiation intensity of an antenna in a given direction to the intensity of the same antenna as it radiates in all directions 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 an hypothetical isotropic antenna. In Physics, intensity is a measure of the time-averaged Energy Flux. An isotropic radiator is a theoretical Point source of waves which exhibits the same magnitude or properties when measured in all directions
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 greater than one gain in some directions, it must have a less than one gain 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 inconsequential. For example, a dish antenna on a spacecraft is a high-gain device (must be pointed at the planet to be effective), while a typical WiFi antenna in a laptop computer is low-gain (as long as the base station is within range, the antenna can be in an any orientation in space). Wi-Fi (ˈwaɪfaɪ is the trade name for the popular wireless technology used It makes sense to improve horizontal range at the expense of reception above or below the antenna. Thus most antennas labeled "omnidirectional" really have some gain. 
Sometimes, the half-wave dipole is taken as a reference instead of the isotropic radiator. The gain is then given in dBd (decibels over dipole):
See Antenna measurement: Gain for more information. Antenna measurement techniques refers to the testing of antennas to ensure that the antenna meets specifications or simply to characterize it
The radiation pattern of an antenna is the geometric pattern of the relative field strengths of the field emitted by the antenna. In the field of antenna design the term radiation pattern most commonly refers to the directional (angular dependence of radiation from the antenna or other For the ideal isotropic antenna, this would be a sphere. "Globose" redirects here See also Globose nucleus. A sphere (from Greek σφαίρα - sphaira, "globe 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. In antenna engineering side lobes are the lobes of the far field Radiation pattern that are not the main beam, where the terms "beam"
See Antenna measurement: Radiation pattern or Radiation pattern for more information. Antenna measurement techniques refers to the testing of antennas to ensure that the antenna meets specifications or simply to characterize it In the field of antenna design the term radiation pattern most commonly refers to the directional (angular dependence of radiation from the antenna or other
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). Radio is the transmission of signals by Modulation of electromagnetic waves with frequencies below those of visible Light. The Feed line in a Radio Transmission, Reception or Transceiver system is the physical cabling that carries the RF signal to and/or In Classical physics, free space is a concept of Electromagnetic theory, corresponding to a theoretically "perfect" Vacuum, and sometimes At each interface, depending on the impedance match, some fraction of the wave's energy will reflect back to the source, 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). In Telecommunications standing wave ratio ( SWR) is the Ratio of the Amplitude of a partial Standing wave at an antinode (maximum 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. Impedance matching is the electronics design practice of setting the Output impedance ( Z S of a signal source equal to the Input impedance (
Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength in use. Complex plane In Mathematics, the complex numbers are an extension of the Real numbers obtained by adjoining an Imaginary unit, denoted In Telecommunications, the electrical length is any of A Transmission medium, its length expressed as a multiple or submultiple of the Wavelength 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. A transformer is a device that transfers Electrical energy from one circuit to another through inductively coupled Electrical conductors 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. An antenna tuner, transmatch or antenna tuning unit (ATU matches a transceiver with a fixed impedance (typically 50 Ohms for modern transceivers to a load (feed A balun, ("bal-un" is a passive electronic device that converts between '''bal'''anced and '''un'''balanced Electrical An inductor is a passive electrical component designed to provide Inductance in a circuit A capacitor is a passive electrical component that can store Energy in the Electric field between a pair of conductors
"Efficiency" is the ratio of power actually radiated to the power put into the antenna terminals. The efficiency of an entity (a device, component, or System) in Electronics and Electrical engineering is defined as useful 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. A dummy load is a device used to simulate an electrical load usually for testing purposes Radiation in an antenna is caused by radiation resistance which can only be measured as part of total resistance including loss resistance. Radiation resistance is that part of an antenna 's feedpoint resistance that is caused by the Radiation of Electromagnetic waves from the antenna Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is Loss resistance usually results in heat generation rather than radiation, and reduces efficiency. Mathematically, efficiency is calculated as radiation resistance divided by total resistance.
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. In Satellite dish and antenna design parlance a feedhorn (or feed horn) is a Horn antenna used to convey Radio waves between the Small antennas are usually preferred for convenience, but there is a fundamental limit relating bandwidth, size and efficiency.
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. Polarization ( ''Brit'' polarisation) is a property of Waves that describes the orientation of their oscillations The E-plane and H-plane are reference planes for linearly polarized antennas. 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.
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. The ionosphere is the uppermost part of the atmosphere, distinguished because it is Ionized by solar radiation 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. Line-of-sight propagation refers to Electro-magnetic radiation including light emissions traveling in a straight line
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 omnidirectional antenna is an antenna system which radiates power uniformly in one plane with a directive pattern shape in a perpendicular plane Wi-Fi (ˈwaɪfaɪ is the trade name for the popular wireless technology used 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.
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 (the projection is oblong), meaning that the antenna varies over time in the polarization of the radio waves it is emitting. Two special cases are linear polarization (the ellipse collapses into a line) and circular polarization (in which the ellipse varies maximally). In Electrodynamics, linear polarization or plane polarization of Electromagnetic radiation is a confinement of the Electric field vector or In Electrodynamics, circular polarization (also circular polarisation) of Electromagnetic radiation is a Polarization such that the tip of the 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.
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. In the case of radar, this is often reflections from rain drops.
All of the antenna parameters are expressed in terms of a transmission antenna, but are identically applicable to a receiving antenna, due to reciprocity. In Telecommunications transmission is the process of sending propagating and receiving an analogue or digital information signal over a physical point-to-point or This page is about reciprocity theorems in classical electromagnetism 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.
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.
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. In Electrical engineering, the power rating of a device is a guideline set by the manufacturer as a maximum power to be used with that device 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.
Antennas designed specifically for reception might be optimized for noise rejection capabilities. In Science, and especially in Physics and Telecommunication, noise is fluctuations in and the addition of external factors to the stream of target 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. Band rejection is a phenomenon in Waveform signals, where a certain Frequency or range of frequencies are lost or removed from a source signal
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. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. 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.
There are many variations of antennas. Below are a few basic models. More can be found in Category:Radio frequency antenna types.
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 dipole antenna, developed by Heinrich Rudolph Hertz around 1886, is an antenna with a center- fed Driven element for transmitting 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. In Telecommunications, the electrical length is any of A Transmission medium, its length expressed as a multiple or submultiple of the Wavelength
One of the goals of antenna design is to minimize the reactance of the device so that it appears as a resistive load. Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is 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. In Physics, resonance is the tendency of a system to Oscillate at maximum Amplitude at certain frequencies, known as the system's 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.
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. Radiation resistance is that part of an antenna 's feedpoint resistance that is caused by the Radiation of Electromagnetic waves from the antenna 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.
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.
At frequencies used in antennas, the ground behaves mainly as a dielectric. A dielectric is a nonconducting substance ie an insulator. The term was coined by William Whewell in response to a request from Michael Faraday. 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. Angle of incidence is a measure of deviation of something from "straight on" for example in the approach of a ray to a surface or the angle 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.
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.
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.
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.
In the drawing at right, we have drawn the angle far bigger than in reality. Distance between the antenna and its image is .
The situation is a bit more complex because the reflection of electromagnetic waves depends on the polarization of the incident wave. Polarization ( ''Brit'' polarisation) is a property of Waves that describes the orientation of their oscillations 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. The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium 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.
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.
For a vertical polarized emission antenna the far electric field of the electromagnetic wave produced by the direct ray plus the reflected ray is:
The sign inversion for the parallel field case just changes a cosine to a sine:
In these two equations:
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.
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.
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. The term base station can be used in the context of Land surveying, Wireless computer networking, and Wireless communications. In Electrodynamics, circular polarization (also circular polarisation) of Electromagnetic radiation is a Polarization such that the tip of the
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. See also Television interference In Television, a ghost is an unwanted Image on the screen appearing Superimposed on the desired 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. Brewster's angle (also known as the polarization angle) is an Angle of incidence at which light with a particular Polarization is perfectly transmitted Vertically polarized analog television has been used in some rural areas. In digital terrestrial television reflections are less annoying because of the type of modulation. Digital Terrestrial Television ( DTTV or DTT) is an implementation of digital technology to provide a greater number of channels and/or better quality of picture and
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. In Electrical circuits, any Electric current i produces a Magnetic field and hence generates a total Magnetic flux \Phi acting The mutual impedance between two antennas is defined as:
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:
Note that, as is the case for mutual inductances,
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. A Yagi-Uda Antenna, commonly known simply as a Yagi antenna or Yagi, is a Directional antenna system consisting of an array of a dipole Those elements are called parasitic elements. A passive radiator or parasitic element is a Radio antenna element which does not have any Wired Input. Parasitic elements are unpowered elements that either reflect or absorb and reradiate RF energy.
In some geometrical settings, the mutual impedance between antennas can be zero. This is the case for crossed dipoles used in circular polarization antennas.
A Yagi-Uda beam antenna. A Yagi-Uda Antenna, commonly known simply as a Yagi antenna or Yagi, is a Directional antenna system consisting of an array of a dipole
A multi-band rotary directional antenna for amateur radio use.
Rooftop television antenna. It is actually three Yagi antennas in one. A Yagi-Uda Antenna, commonly known simply as a Yagi antenna or Yagi, is a Directional antenna system consisting of an array of a dipole The longest elements are for the low band (channels 2-6 (1-6 in the UK)) the medium-length elements are for the high band (channels 7-13) and the shortest elements are for the UHF band (channels 14-69 (21-68 in the UK)).
A terrestrial microwave radio antenna array.
Examples of US 136-174 MHz base station antennas.
A building rooftop supporting numerous dish and sectored mobile telecommunications antennas (Doncaster, Victoria, Australia. Doncaster is a Suburb of Melbourne, Victoria, Australia. It is in the Local Government Area of the City of Manningham For a topic outline on this subject see List of basic Australia topics.
A three-sector telephone site in Mexico City.
Telephone site concealed as a palm tree.
Antennas may be connected through a multiplexing arrangement in some applications like this trunked two-way radio example. For multiplexing in electronics and signal processing see Multiplexer. trunked radio system is a complex type of computer-controlled radio system two-way radio is a radio that can both transmit and receive (a Transceiver) unlike a broadcast receiver which only receives content
Antenna network for an emergency medical services base station.