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125-478: An antenna is an array of conductors ( elements ), electrically connected to the receiver or transmitter. Antennas can be designed to transmit and receive radio waves in all horizontal directions equally ( omnidirectional antennas ), or preferentially in a particular direction ( directional , or high-gain, or "beam" antennas). An antenna may include components not connected to the transmitter, parabolic reflectors , horns , or parasitic elements , which serve to direct

250-408: A sphere . Many nondirectional antennas, such as monopoles and dipoles , emit equal power in all horizontal directions, with the power dropping off at higher and lower angles; this is called an omnidirectional pattern and when plotted looks like a torus or donut. Conductor (material) In physics and electrical engineering , a conductor is an object or type of material that allows

375-423: A beam. This type is called an aperture antenna . A parabolic dish is an example of this type of antenna. A second technique is to use multiple antennas which are fed from the same transmitter or receiver; this is called an array antenna, or antenna array. For a transmitting antenna the electromagnetic wave received at any point is the vector sum of the electromagnetic waves from each of the antenna elements. If

500-405: A boom; the boom is only for support and not involved electrically. Only one of the elements is electrically connected to the transmitter or receiver, while the remaining elements are passive. The Yagi produces a fairly large gain (depending on the number of passive elements) and is widely used as a directional antenna with an antenna rotor to control the direction of its beam. It suffers from having

625-454: A conductor of uniform cross section, therefore, can be computed as where ℓ {\displaystyle \ell } is the length of the conductor, measured in metres [m], A is the cross-section area of the conductor measured in square metres [m ], σ ( sigma ) is the electrical conductivity measured in siemens per meter (S·m ), and ρ ( rho ) is the electrical resistivity (also called specific electrical resistance ) of

750-549: A conductor; metals, characteristically, possess a delocalized sea of electrons which gives the electrons enough mobility to collide and thus affect a momentum transfer. As discussed above, electrons are the primary mover in metals; however, other devices such as the cationic electrolyte (s) of a battery , or the mobile protons of the proton conductor of a fuel cell rely on positive charge carriers. Insulators are non-conducting materials with few mobile charges that support only insignificant electric currents. The resistance of

875-405: A current of 1 Ampere will require 63 Volts, and the antenna will radiate 63 Watts (ignoring losses) of radio frequency power. Now consider the case when the antenna is fed a signal with a wavelength of 1.25 m; in this case the current induced by the signal would arrive at the antenna's feedpoint out-of-phase with the signal, causing the net current to drop while the voltage remains

1000-463: A current will reflect when there are changes in the electrical properties of the material. In order to efficiently transfer the received signal into the transmission line, it is important that the transmission line has the same impedance as its connection point on the antenna, otherwise some of the signal will be reflected backwards into the body of the antenna; likewise part of the transmitter's signal power will be reflected back to transmitter, if there

1125-494: A fashion are known to be harmonically operated . Resonant antennas usually use a linear conductor (or element ), or pair of such elements, each of which is about a quarter of the wavelength in length (an odd multiple of quarter wavelengths will also be resonant). Antennas that are required to be small compared to the wavelength sacrifice efficiency and cannot be very directional. Since wavelengths are so small at higher frequencies ( UHF , microwaves ) trading off performance to obtain

1250-400: A feed-point impedance that matches that of a transmission line; a matching network between antenna terminals and the transmission line will improve power transfer to the antenna. A non-adjustable matching network will most likely place further limits the usable bandwidth of the antenna system. It may be desirable to use tubular elements, instead of thin wires, to make an antenna; these will allow

1375-418: A few hundred amperes. Aside from the geometry of the wire, temperature also has a significant effect on the efficacy of conductors. Temperature affects conductors in two main ways, the first is that materials may expand under the application of heat. The amount that the material will expand is governed by the thermal expansion coefficient specific to the material. Such an expansion (or contraction) will change

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1500-402: A fixed radiation pattern, we may consider that the feed network is a part of the antenna array. Thus, the antenna array has a single port. Narrow beams can be formed, provided the phasing of each element of the array is appropriate. If, in addition, the amplitude of the excitation received by each element (during emission) is also well chosen, it is possible to synthesize a single-port array having

1625-433: A flux of 1 pW / m (10 Watts per square meter) and an antenna has an effective area of 12 m, then the antenna would deliver 12 pW of RF power to the receiver (30 microvolts RMS at 75 ohms). Since the receiving antenna is not equally sensitive to signals received from all directions, the effective area is a function of the direction to the source. Due to reciprocity (discussed above)

1750-495: A given conductor depends on the material it is made of, and on its dimensions. For a given material, the resistance is inversely proportional to the cross-sectional area. For example, a thick copper wire has lower resistance than an otherwise-identical thin copper wire. Also, for a given material, the resistance is proportional to the length; for example, a long copper wire has higher resistance than an otherwise-identical short copper wire. The resistance R and conductance G of

1875-403: A greater bandwidth. Or, several thin wires can be grouped in a cage to simulate a thicker element. This widens the bandwidth of the resonance. Amateur radio antennas that operate at several frequency bands which are widely separated from each other may connect elements resonant at those different frequencies in parallel. Most of the transmitter's power will flow into the resonant element while

2000-450: A long Beverage antenna can have significant directivity. For non directional portable use, a short vertical antenna or small loop antenna works well, with the main design challenge being that of impedance matching . With a vertical antenna a loading coil at the base of the antenna may be employed to cancel the reactive component of impedance ; small loop antennas are tuned with parallel capacitors for this purpose. An antenna lead-in

2125-603: A number of parallel dipole antennas with a certain spacing. Depending on the relative phase introduced by the network, the same combination of dipole antennas can operate as a "broadside array" (directional normal to a line connecting the elements) or as an "end-fire array" (directional along the line connecting the elements). Antenna arrays may employ any basic (omnidirectional or weakly directional) antenna type, such as dipole, loop or slot antennas. These elements are often identical. Log-periodic and frequency-independent antennas employ self-similarity in order to be operational over

2250-427: A proper resonant antenna at the trap frequency. At substantially higher or lower frequencies the trap allows the full length of the broken element to be employed, but with a resonant frequency shifted by the net reactance added by the trap. The bandwidth characteristics of a resonant antenna element can be characterized according to its Q where the resistance involved is the radiation resistance , which represents

2375-515: A pure resistance. Sometimes the resulting (lower) electrical resonant frequency of such a system (antenna plus matching network) is described using the concept of electrical length , so an antenna used at a lower frequency than its resonant frequency is called an electrically short antenna For example, at 30 MHz (10 m wavelength) a true resonant ⁠ 1  / 4 ⁠  wave monopole would be almost 2.5 meters long, and using an antenna only 1.5 meters tall would require

2500-402: A radiation pattern that closely approximates a specified pattern. Many methods have been developed for array pattern synthesis. Additional issues to be considered are matching, radiation efficiency and bandwidth. The design of an electronically steerable antenna array is different, because the phasing of each element can be varied, and possibly also the relative amplitude for each element. Here,

2625-504: A rather limited bandwidth, restricting its use to certain applications. Rather than using one driven antenna element along with passive radiators, one can build an array antenna in which multiple elements are all driven by the transmitter through a system of power splitters and transmission lines in relative phases so as to concentrate the RF power in a single direction. What's more, a phased array can be made "steerable", that is, by changing

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2750-417: A reducing atmosphere, then oxygen-free high conductivity copper (CW008A or ASTM designation C10100) may be used. Because of its ease of connection by soldering or clamping, copper is still the most common choice for most light-gauge wires. Silver is 6% more conductive than copper, but due to cost it is not practical in most cases. However, it is used in specialized equipment, such as satellites , and as

2875-619: A separate antenna element or group of elements. An antenna array can achieve higher gain ( directivity ), that is a narrower beam of radio waves, than could be achieved by a single element. In general, the larger the number of individual antenna elements used, the higher the gain and the narrower the beam. Some antenna arrays (such as military phased array radars) are composed of thousands of individual antennas. Arrays can be used to achieve higher gain, to give path diversity (also called MIMO ) which increases communication reliability, to cancel interference from specific directions, to steer

3000-443: A signal into the transmission line only when the source signal's frequency is close to that of the design frequency of the antenna, or one of the resonant multiples. This makes resonant antenna designs inherently narrow-band: Only useful for a small range of frequencies centered around the resonance(s). It is possible to use simple impedance matching techniques to allow the use of monopole or dipole antennas substantially shorter than

3125-485: A single antenna, to transmit or receive radio waves . The individual antennas (called elements ) are usually connected to a single receiver or transmitter by feedlines that feed the power to the elements in a specific phase relationship. The radio waves radiated by each individual antenna combine and superpose , adding together ( interfering constructively ) to enhance the power radiated in desired directions, and cancelling ( interfering destructively ) to reduce

3250-440: A smaller physical size is usually not required. The quarter-wave elements imitate a series-resonant electrical element due to the standing wave present along the conductor. At the resonant frequency, the standing wave has a current peak and voltage node (minimum) at the feed. In electrical terms, this means that at that position, the element has minimum impedance magnitude , generating the maximum current for minimum voltage. This

3375-405: A source of disadvantages in both transmission and reception. In fact, in transmission, they can lead to radiation in unwanted directions, while, in reception, they can be a source of ambiguity since the desired signal entering the mainlobe region could be strongly disturbed by other signals (unwanted interfering signals) entering the regions of the various grating lobes. Therefore, in periodic arrays,

3500-498: A standard resistive impedance needed for its optimum operation. The feed point location(s) is selected, and antenna elements electrically similar to tuner components may be incorporated in the antenna structure itself, to improve the match . It is a fundamental property of antennas that most of the electrical characteristics of an antenna, such as those described in the next section (e.g. gain , radiation pattern , impedance , bandwidth , resonant frequency and polarization ), are

3625-585: A thin plating to mitigate skin effect losses at high frequencies. Famously, 14,700 short tons (13,300 t) of silver on loan from the United States Treasury were used in the making of the calutron magnets during World War II due to wartime shortages of copper. Aluminum wire is the most common metal in electric power transmission and distribution . Although only 61% of the conductivity of copper by cross-sectional area, its lower density makes it twice as conductive by mass. As aluminum

3750-476: A total 360 degree phase change, returning it to the original signal. The current in the element thus adds to the current being created from the source at that instant. This process creates a standing wave in the conductor, with the maximum current at the feed. The ordinary half-wave dipole is probably the most widely used antenna design. This consists of two ⁠ 1  / 4 ⁠  wavelength elements arranged end-to-end, and lying along essentially

3875-402: A wide angle. To create a directional antenna ( high gain antenna ), which radiates radio waves in a narrow beam, two general techniques can be used: One technique is to use reflection by large metal surfaces such as parabolic reflectors or horns , or refraction by dielectric lenses to change the direction of the radio waves, to focus the radio waves from a single low gain antenna into

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4000-473: A wide range of bandwidths . The most familiar example is the log-periodic dipole array which can be seen as a number (typically 10 to 20) of connected dipole elements with progressive lengths in an endfire array making it rather directional; it finds use especially as a rooftop antenna for television reception. On the other hand, a Yagi–Uda antenna (or simply "Yagi"), with a somewhat similar appearance, has only one dipole element with an electrical connection;

4125-465: Is a monopole antenna, not balanced with respect to ground. The ground (or any large conductive surface) plays the role of the second conductor of a monopole. Since monopole antennas rely on a conductive surface, they may be mounted with a ground plane to approximate the effect of being mounted on the Earth's surface. More complex antennas increase the directivity of the antenna. Additional elements in

4250-434: Is a change in electrical impedance where the feedline joins the antenna. This leads to the concept of impedance matching , the design of the overall system of antenna and transmission line so the impedance is as close as possible, thereby reducing these losses. Impedance matching is accomplished by a circuit called an antenna tuner or impedance matching network between the transmitter and antenna. The impedance match between

4375-417: Is a component which due to its shape and position functions to selectively delay or advance portions of the electromagnetic wavefront passing through it. The refractor alters the spatial characteristics of the wave on one side relative to the other side. It can, for instance, bring the wave to a focus or alter the wave front in other ways, generally in order to maximize the directivity of the antenna system. This

4500-489: Is a consequence of the reciprocity theorem of electromagnetics. Therefore, in discussions of antenna properties no distinction is usually made between receiving and transmitting terminology, and the antenna can be viewed as either transmitting or receiving, whichever is more convenient. A necessary condition for the aforementioned reciprocity property is that the materials in the antenna and transmission medium are linear and reciprocal. Reciprocal (or bilateral ) means that

4625-410: Is adjusted according to the receiver tuning. On the other hand, log-periodic antennas are not resonant at any single frequency but can (in principle) be built to attain similar characteristics (including feedpoint impedance) over any frequency range. These are therefore commonly used (in the form of directional log-periodic dipole arrays ) as television antennas. Gain is a parameter which measures

4750-426: Is assumed that radiators have the same orientation and the same polarization of the electric field. Based on this, the array factor can be written as follows F ( u ) = ∑ n = 1 N I n e j k x n u {\displaystyle F(u)=\sum _{n=1}^{N}I_{n}\,e^{jk\,x_{n}u}} where N {\displaystyle N}

4875-400: Is called an isotropic radiator ; however, these cannot exist in practice nor would they be particularly desired. For most terrestrial communications, rather, there is an advantage in reducing radiation toward the sky or ground in favor of horizontal direction(s). A dipole antenna oriented horizontally sends no energy in the direction of the conductor – this is called the antenna null – but

5000-406: Is connected to a transmission line . The conductor, or element , is aligned with the electrical field of the desired signal, normally meaning it is perpendicular to the line from the antenna to the source (or receiver in the case of a broadcast antenna). The radio signal's electrical component induces a voltage in the conductor. This causes an electrical current to begin flowing in the direction of

5125-438: Is equal to 1. Therefore, the effective area A eff in terms of the gain G in a given direction is given by: For an antenna with an efficiency of less than 100%, both the effective area and gain are reduced by that same amount. Therefore, the above relationship between gain and effective area still holds. These are thus two different ways of expressing the same quantity. A eff is especially convenient when computing

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5250-456: Is intended to receive independent excitations during emission, and to deliver more or less independent signals during reception. Here also, the subject matters of matching and efficiency are involved, especially in the case of an antenna array of a mobile device (see chapter 10 of ), since, in this case, the surroundings of the antenna array influence its behavior, and vary over time. Suitable matching metrics and efficiency metrics take into account

5375-465: Is its radiation pattern . The frequency range or bandwidth over which an antenna functions well can be very wide (as in a log-periodic antenna) or narrow (as in a small loop antenna); outside this range the antenna impedance becomes a poor match to the transmission line and transmitter (or receiver). Use of the antenna well away from its design frequency affects its radiation pattern , reducing its directive gain. Generally an antenna will not have

5500-404: Is not exact: It assumes the current density is totally uniform in the conductor, which is not always true in practical situation. However, this formula still provides a good approximation for long thin conductors such as wires. Another situation this formula is not exact for is with alternating current (AC), because the skin effect inhibits current flow near the center of the conductor. Then,

5625-404: Is passed through it. Liquids made of compounds with only covalent bonds cannot conduct electricity. Certain organic ionic liquids , by contrast, can conduct an electric current. While pure water is not an electrical conductor, even a small portion of ionic impurities, such as salt , can rapidly transform it into a conductor. Wires are measured by their cross sectional area. In many countries,

5750-434: Is redirected toward the desired direction, increasing the antenna's gain by a factor of at least 2. Likewise, a corner reflector can insure that all of the antenna's power is concentrated in only one quadrant of space (or less) with a consequent increase in gain. Practically speaking, the reflector need not be a solid metal sheet, but can consist of a curtain of rods aligned with the antenna's polarization; this greatly reduces

5875-650: Is roughly one-third the cost of copper by weight, the economic advantages are considerable when large conductors are required. The disadvantages of aluminum wiring lie in its mechanical and chemical properties. It readily forms an insulating oxide, making connections heat up. Its larger coefficient of thermal expansion than the brass materials used for connectors causes connections to loosen. Aluminum can also "creep", slowly deforming under load, which also loosens connections. These effects can be mitigated with suitably designed connectors and extra care in installation, but they have made aluminum building wiring unpopular past

6000-418: Is the transmission line , or feed line , which connects the antenna to a transmitter or receiver. The " antenna feed " may refer to all components connecting the antenna to the transmitter or receiver, such as an impedance matching network in addition to the transmission line. In a so-called "aperture antenna", such as a horn or parabolic dish, the "feed" may also refer to a basic radiating antenna embedded in

6125-410: Is the ideal situation, because it produces the maximum output for the minimum input, producing the highest possible efficiency. Contrary to an ideal (lossless) series-resonant circuit, a finite resistance remains (corresponding to the relatively small voltage at the feed-point) due to the antenna's resistance to radiating , as well as any conventional electrical losses from producing heat. Recall that

6250-589: Is the international standard to which all other electrical conductors are compared; the International Annealed Copper Standard conductivity is 58 MS/m , although ultra-pure copper can slightly exceed 101% IACS. The main grade of copper used for electrical applications, such as building wire, motor windings, cables and busbars , is electrolytic-tough pitch (ETP) copper (CW004A or ASTM designation C100140). If high conductivity copper must be welded or brazed or used in

6375-607: Is the number of antenna elements, k {\displaystyle k} is the wavenumber, I n {\displaystyle I_{n}} and x n {\displaystyle x_{n}} (in meters) are the complex excitation coefficient and the position of the n-th radiator, respectively, u = sin ⁡ θ cos ⁡ ϕ {\displaystyle u=\sin \theta \cos \phi } , with θ {\displaystyle \theta } and ϕ {\displaystyle \phi } being

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6500-416: Is the radio equivalent of an optical lens . An antenna coupling network is a passive network (generally a combination of inductive and capacitive circuit elements) used for impedance matching in between the antenna and the transmitter or receiver. This may be used to minimize losses on the feed line, by reducing transmission line's standing wave ratio , and to present the transmitter or receiver with

6625-440: Is unidirectional, designed for maximum response in the direction of the other station, whereas many other antennas are intended to accommodate stations in various directions but are not truly omnidirectional. Since antennas obey reciprocity the same radiation pattern applies to transmission as well as reception of radio waves. A hypothetical antenna that radiates equally in all directions (vertical as well as all horizontal angles)

6750-449: Is usable in most other directions. A number of such dipole elements can be combined into an antenna array such as the Yagi–Uda in order to favor a single horizontal direction, thus termed a beam antenna. The dipole antenna, which is the basis for most antenna designs, is a balanced component, with equal but opposite voltages and currents applied at its two terminals. The vertical antenna

6875-419: Is usually another name for a Yagi–Uda antenna . A phased array usually means an electronically scanned array ; a driven array antenna in which each individual element is connected to the transmitter or receiver through a phase shifter controlled by a computer. The beam of radio waves can be steered electronically to point instantly in any direction over a wide angle, without moving the antennas. However

7000-404: Is usually insulated with PVC insulation that is only rated to operate to about 60 °C, therefore, the current in such wires must be limited so that it never heats the copper conductor above 60 °C, causing a risk of fire . Other, more expensive insulation such as Teflon or fiberglass may allow operation at much higher temperatures. If an electric field is applied to a material, and

7125-448: The ⁠ 1  / 4 ⁠ or ⁠ 1  / 2 ⁠   wave , respectively, at which they are resonant. As these antennas are made shorter (for a given frequency) their impedance becomes dominated by a series capacitive (negative) reactance; by adding an appropriate size " loading coil " – a series inductance with equal and opposite (positive) reactance – the antenna's capacitive reactance may be cancelled leaving only

7250-484: The geometrical cross-section is different from the effective cross-section in which current actually flows, so the resistance is higher than expected. Similarly, if two conductors are near each other carrying AC current, their resistances increase due to the proximity effect . At commercial power frequency , these effects are significant for large conductors carrying large currents, such as busbars in an electrical substation , or large power cables carrying more than

7375-405: The lens antenna . The antenna's power gain (or simply "gain") also takes into account the antenna's efficiency, and is often the primary figure of merit. Antennas are characterized by a number of performance measures which a user would be concerned with in selecting or designing an antenna for a particular application. A plot of the directional characteristics in the space surrounding the antenna

7500-403: The resonance principle. This relies on the behaviour of moving electrons, which reflect off surfaces where the dielectric constant changes, in a fashion similar to the way light reflects when optical properties change. In these designs, the reflective surface is created by the end of a conductor, normally a thin metal wire or rod, which in the simplest case has a feed point at one end where it

7625-440: The service drop . Organic compounds such as octane, which has 8 carbon atoms and 18 hydrogen atoms, cannot conduct electricity. Oils are hydrocarbons, since carbon has the property of tetracovalency and forms covalent bonds with other elements such as hydrogen, since it does not lose or gain electrons, thus does not form ions. Covalent bonds are simply the sharing of electrons. Hence, there is no separation of ions when electricity

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7750-467: The addition of a loading coil. Then it may be said that the coil has lengthened the antenna to achieve an electrical length of 2.5 meters. However, the resulting resistive impedance achieved will be quite a bit lower than that of a true ⁠ 1  / 4 ⁠  wave (resonant) monopole, often requiring further impedance matching (a transformer) to the desired transmission line. For ever shorter antennas (requiring greater "electrical lengthening")

7875-427: The antenna array has multiple ports, so that the subject matters of matching and efficiency are more involved than in the single-port case. Moreover, matching and efficiency depend on the excitation, except when the interactions between the antennas can be ignored. An antenna array used for spatial diversity and/or spatial multiplexing (which are different types of MIMO radio communication) always has multiple ports. It

8000-402: The antenna consisting of a thin conductor. Antennas for use over much broader frequency ranges are achieved using further techniques. Adjustment of a matching network can, in principle, allow for any antenna to be matched at any frequency. Thus the small loop antenna built into most AM broadcast (medium wave) receivers has a very narrow bandwidth, but is tuned using a parallel capacitance which

8125-400: The antenna structure, which need not be directly connected to the receiver or transmitter, increase its directionality. Antenna "gain" describes the concentration of radiated power into a particular solid angle of space. "Gain" is perhaps an unfortunately chosen term, by comparison with amplifier "gain" which implies a net increase in power. In contrast, for antenna "gain", the power increased in

8250-474: The antenna to the power radiated by a half-wave dipole antenna I dipole {\displaystyle I_{\text{dipole}}} ; these units are called decibels-dipole (dBd) Since the gain of a half-wave dipole is 2.15 dBi and the logarithm of a product is additive, the gain in dBi is just 2.15 decibels greater than the gain in dBd High-gain antennas have the advantage of longer range and better signal quality, but must be aimed carefully at

8375-427: The broadside direction. If higher gain is needed one cannot simply make the antenna larger. Due to the constraint on the effective area of a receiving antenna detailed below , one sees that for an already-efficient antenna design, the only way to increase gain (effective area) is by reducing the antenna's gain in another direction. If a half-wave dipole is not connected to an external circuit but rather shorted out at

8500-454: The component antennas' axis relates to the radiation direction. There are also arrays (such as phased arrays ) which don't belong to either of these categories, in which the direction of radiation is at some other angle to the antenna axis. Array antennas can also be categorized by how the element antennas are arranged: Let us consider a linear array whose elements are arranged along the x-axis of an orthogonal Cartesian reference system. It

8625-462: The conductor is made from (as described above) and the conductor's size. For a given material, conductors with a larger cross-sectional area have less resistance than conductors with a smaller cross-sectional area. For bare conductors, the ultimate limit is the point at which power lost to resistance causes the conductor to melt. Aside from fuses , most conductors in the real world are operated far below this limit, however. For example, household wiring

8750-549: The current (the current source ) to those consuming it (the loads ). Instead, the charged particle simply needs to nudge its neighbor a finite amount, who will nudge its neighbor, and on and on until a particle is nudged into the consumer, thus powering it. Essentially what is occurring is a long chain of momentum transfer between mobile charge carriers ; the Drude model of conduction describes this process more rigorously. This momentum transfer model makes metal an ideal choice for

8875-420: The currents are fed to the antennas with the proper phase , due to the phenomenon of interference the spherical waves from the individual antennas combine (superpose) in front of the array to create plane waves , a beam of radio waves traveling in a specific direction. In directions in which the waves from the individual antennas arrive in phase , the waves add together ( constructive interference ) to enhance

9000-470: The degree of directivity of the antenna's radiation pattern . A high-gain antenna will radiate most of its power in a particular direction, while a low-gain antenna will radiate over a wide angle. The antenna gain , or power gain of an antenna is defined as the ratio of the intensity (power per unit surface area) I {\displaystyle I} radiated by the antenna in the direction of its maximum output, at an arbitrary distance, divided by

9125-557: The design operating frequency, f o , and antennas are normally designed to be this size. However, feeding that element with 3  f o (whose wavelength is ⁠ 1  / 3 ⁠ that of f o ) will also lead to a standing wave pattern. Thus, an antenna element is also resonant when its length is ⁠ 3  / 4 ⁠ of a wavelength. This is true for all odd multiples of ⁠ 1  / 4 ⁠  wavelength. This allows some flexibility of design in terms of antenna lengths and feed points. Antennas used in such

9250-463: The desired direction is at the expense of power reduced in undesired directions. Unlike amplifiers, antennas are electrically " passive " devices which conserve total power, and there is no increase in total power above that delivered from the power source (the transmitter), only improved distribution of that fixed total. A phased array consists of two or more simple antennas which are connected together through an electrical network. This often involves

9375-404: The electromagnetic field. Radio waves are electromagnetic waves which carry signals through the air (or through space) at the speed of light with almost no transmission loss . Antennas can be classified as omnidirectional , radiating energy approximately equally in all horizontal directions, or directional , where radio waves are concentrated in some direction(s). A so-called beam antenna

9500-427: The emission of energy from the resonant antenna to free space. The Q of a narrow band antenna can be as high as 15. On the other hand, the reactance at the same off-resonant frequency of one using thick elements is much less, consequently resulting in a Q as low as 5. These two antennas may perform equivalently at the resonant frequency, but the second antenna will perform over a bandwidth 3 times as wide as

9625-418: The entire system of reflecting elements (normally at the focus of the parabolic dish or at the throat of a horn) which could be considered the one active element in that antenna system. A microwave antenna may also be fed directly from a waveguide in place of a (conductive) transmission line . An antenna counterpoise , or ground plane , is a structure of conductive material which improves or substitutes for

9750-494: The extent of the visible space also changes accordingly. Now, suppose that the excitation coefficients are positive real variables. In this case, always in the domain of u {\displaystyle u} , the array factor magnitude has a main lobe with maximum value at u = 0 {\displaystyle u=0} , called mainlobe , several secondary lobes lower than the mainlobe, called sidelobes and mainlobe replicas called grating-lobes . Grating lobes are

9875-526: The feedline and antenna is measured by a parameter called the standing wave ratio (SWR) on the feedline. Consider a half-wave dipole designed to work with signals with wavelength 1 m, meaning the antenna would be approximately 50 cm from tip to tip. If the element has a length-to-diameter ratio of 1000, it will have an inherent impedance of about 63 ohms resistive. Using the appropriate transmission wire or balun, we match that resistance to ensure minimum signal reflection. Feeding that antenna with

10000-430: The feedpoint, then it becomes a resonant half-wave element which efficiently produces a standing wave in response to an impinging radio wave. Because there is no load to absorb that power, it retransmits all of that power, possibly with a phase shift which is critically dependent on the element's exact length. Thus such a conductor can be arranged in order to transmit a second copy of a transmitter's signal in order to affect

10125-580: The field of radio astronomy , in which multiple radio telescopes consisting of large parabolic antennas are linked together into an antenna array, to achieve higher resolution. Using the technique called aperture synthesis such an array can have the resolution of an antenna with a diameter equal to the distance between the antennas. In the technique called Very Long Baseline Interferometry (VLBI) dishes on separate continents have been linked, creating "array antennas" thousands of miles in size. Most array antennas can be divided into two classes based on how

10250-417: The flow of charge ( electric current ) in one or more directions. Materials made of metal are common electrical conductors. The flow of negatively charged electrons generates electric current, positively charged holes , and positive or negative ions in some cases. In order for current to flow within a closed electrical circuit , one charged particle does not need to travel from the component producing

10375-436: The focal point of parabolic reflectors for both transmitting and receiving. Starting in 1895, Guglielmo Marconi began development of antennas practical for long-distance, wireless telegraphy, for which he received the 1909 Nobel Prize in physics . The words antenna and aerial are used interchangeably. Occasionally the equivalent term "aerial" is used to specifically mean an elevated horizontal wire antenna. The origin of

10500-445: The gain of an antenna used for transmitting must be proportional to its effective area when used for receiving. Consider an antenna with no loss , that is, one whose electrical efficiency is 100%. It can be shown that its effective area averaged over all directions must be equal to λ/4π , the wavelength squared divided by 4π . Gain is defined such that the average gain over all directions for an antenna with 100% electrical efficiency

10625-438: The geometrical divergence of the transmitted wave. For a given incoming flux, the power acquired by a receiving antenna is proportional to its effective area . This parameter compares the amount of power captured by a receiving antenna in comparison to the flux of an incoming wave (measured in terms of the signal's power density in watts per square metre). A half-wave dipole has an effective area of about 0.13  λ seen from

10750-426: The geometry of the conductor and therefore its characteristic resistance. However, this effect is generally small, on the order of 10 . An increase in temperature will also increase the number of phonons generated within the material. A phonon is essentially a lattice vibration, or rather a small, harmonic kinetic movement of the atoms of the material. Much like the shaking of a pinball machine, phonons serve to disrupt

10875-538: The grating lobes are outside the interval [-1,1]. As seen above, when the spacing is constant between adjacent radiators, the array factor is characterized by the presence of grating lobes. In the literature, it has been amply demonstrated that to destroy the array factor's periodicity, the same array's geometry must also be made aperiodic. It is possible to act on the positions of the radiators so that these positions are not commensurable with each other. Several methods have been developed to synthesize arrays in which also

11000-474: The ground. It may be connected to or insulated from the natural ground. In a monopole antenna, this aids in the function of the natural ground, particularly where variations (or limitations) of the characteristics of the natural ground interfere with its proper function. Such a structure is normally connected to the return connection of an unbalanced transmission line such as the shield of a coaxial cable . An electromagnetic wave refractor in some aperture antennas

11125-413: The increase in signal power due to an amplifying device placed at the front-end of the system, such as a low-noise amplifier . The effective area or effective aperture of a receiving antenna expresses the portion of the power of a passing electromagnetic wave which the antenna delivers to its terminals, expressed in terms of an equivalent area. For instance, if a radio wave passing a given location has

11250-405: The intensity I iso {\displaystyle I_{\text{iso}}} radiated at the same distance by a hypothetical isotropic antenna which radiates equal power in all directions. This dimensionless ratio is usually expressed logarithmically in decibels , these units are called decibels-isotropic (dBi) A second unit used to measure gain is the ratio of the power radiated by

11375-428: The interval [ − 1 , 1 ] {\displaystyle [-1,1]} , which is associated with the values of θ {\displaystyle \theta } and ϕ {\displaystyle \phi } . In this case, the interval [-1,1] is called visible space . As shown further, if the definition of the variable u {\displaystyle u} changes,

11500-415: The loading coil, relative to the decreased radiation resistance, entail a reduced electrical efficiency , which can be of great concern for a transmitting antenna, but bandwidth is the major factor that sets the size of antennas at 1 MHz and lower frequencies. The radiant flux as a function of the distance from the transmitting antenna varies according to the inverse-square law , since that describes

11625-435: The log-periodic principle it obtains the unique property of maintaining its performance characteristics (gain and impedance) over a very large bandwidth. When a radio wave hits a large conducting sheet it is reflected (with the phase of the electric field reversed) just as a mirror reflects light. Placing such a reflector behind an otherwise non-directional antenna will insure that the power that would have gone in its direction

11750-550: The material has the same response to an electric current or magnetic field in one direction, as it has to the field or current in the opposite direction. Most materials used in antennas meet these conditions, but some microwave antennas use high-tech components such as isolators and circulators , made of nonreciprocal materials such as ferrite . These can be used to give the antenna a different behavior on receiving than it has on transmitting, which can be useful in applications like radar . The majority of antenna designs are based on

11875-443: The material, measured in ohm-metres (Ω·m). The resistivity and conductivity are proportionality constants, and therefore depend only on the material the wire is made of, not the geometry of the wire. Resistivity and conductivity are reciprocals : ρ = 1 / σ {\displaystyle \rho =1/\sigma } . Resistivity is a measure of the material's ability to oppose electric current. This formula

12000-652: The other parasitic elements interact with the electromagnetic field in order to realize a highly directional antenna but with a narrow bandwidth. Even greater directionality can be obtained using aperture antennas such as the parabolic reflector or horn antenna . Since high directivity in an antenna depends on it being large compared to the wavelength, highly directional antennas (thus with high antenna gain ) become more practical at higher frequencies ( UHF and above). At low frequencies (such as AM broadcast ), arrays of vertical towers are used to achieve directionality and they will occupy large areas of land. For reception,

12125-409: The other antenna. An example of a high-gain antenna is a parabolic dish such as a satellite television antenna. Low-gain antennas have shorter range, but the orientation of the antenna is relatively unimportant. An example of a low-gain antenna is the whip antenna found on portable radios and cordless phones . Antenna gain should not be confused with amplifier gain , a separate parameter measuring

12250-464: The other side connected to ground or an equivalent ground plane (or counterpoise ). Monopoles, which are one-half the size of a dipole, are common for long-wavelength radio signals where a dipole would be impractically large. Another common design is the folded dipole which consists of two (or more) half-wave dipoles placed side by side and connected at their ends but only one of which is driven. The standing wave forms with this desired pattern at

12375-401: The others present a high impedance. Another solution uses traps , parallel resonant circuits which are strategically placed in breaks created in long antenna elements. When used at the trap's particular resonant frequency the trap presents a very high impedance (parallel resonance) effectively truncating the element at the location of the trap; if positioned correctly, the truncated element makes

12500-427: The path of electrons, causing them to scatter. This electron scattering will decrease the number of electron collisions and therefore will decrease the total amount of current transferred. Conduction materials include metals , electrolytes , superconductors , semiconductors , plasmas and some nonmetallic conductors such as graphite and conductive polymers . Copper has a high conductivity . Annealed copper

12625-465: The phases applied to each element the radiation pattern can be shifted without physically moving the antenna elements. Another common array antenna is the log-periodic dipole array which has an appearance similar to the Yagi (with a number of parallel elements along a boom) but is totally dissimilar in operation as all elements are connected electrically to the adjacent element with a phase reversal; using

12750-572: The positions represent further degrees of freedom (unknowns). There are both deterministic and probabilistic methodologies. Since the probabilistic theory of aperiodic arrays is a sufficiently systematised theory, with a strong general methodological basis, let us first concentrate on describing its peculiarities. Suppose that the radiators positions, { x n } n = 1 N {\displaystyle \{x_{n}\}_{n=1}^{N}} , are independent and identically distributed random variables whose support coincides with

12875-409: The power radiated in other directions. Similarly, when used for receiving, the separate radio frequency currents from the individual antennas combine in the receiver with the correct phase relationship to enhance signals received from the desired directions and cancel signals from undesired directions. More sophisticated array antennas may have multiple transmitter or receiver modules, each connected to

13000-416: The power radiated. In directions in which the individual waves arrive out of phase , with the peak of one wave coinciding with the valley of another, the waves cancel ( destructive interference ) reducing the power radiated in that direction. Similarly, when receiving, the oscillating currents received by the separate antennas from radio waves received from desired directions are in phase and when combined in

13125-488: The power that would be received by an antenna of a specified gain, as illustrated by the above example. The radiation pattern of an antenna is a plot of the relative field strength of the radio waves emitted by the antenna at different angles in the far field. It is typically represented by a three-dimensional graph, or polar plots of the horizontal and vertical cross sections. The pattern of an ideal isotropic antenna , which radiates equally in all directions, would look like

13250-444: The radiation pattern (and feedpoint impedance) of the element electrically connected to the transmitter. Antenna elements used in this way are known as passive radiators . A Yagi–Uda array uses passive elements to greatly increase gain in one direction (at the expense of other directions). A number of parallel approximately half-wave elements (of very specific lengths) are situated parallel to each other, at specific positions, along

13375-420: The radiation resistance plummets (approximately according to the square of the antenna length), so that the mismatch due to a net reactance away from the electrical resonance worsens. Or one could as well say that the equivalent resonant circuit of the antenna system has a higher Q factor and thus a reduced bandwidth, which can even become inadequate for the transmitted signal's spectrum. Resistive losses due to

13500-416: The radio beam electronically to point in different directions, and for radio direction finding (RDF). The term antenna array most commonly means a driven array consisting of multiple identical driven elements all connected to the receiver or transmitter. A parasitic array consists of a single driven element connected to the feedline, and other elements which are not, called parasitic elements . It

13625-480: The radio waves into a beam or other desired radiation pattern . Strong directivity and good efficiency when transmitting are hard to achieve with antennas with dimensions that are much smaller than a half wavelength . The first antennas were built in 1888 by German physicist Heinrich Hertz in his pioneering experiments to prove the existence of electromagnetic waves predicted by the 1867 electromagnetic theory of James Clerk Maxwell . Hertz placed dipole antennas at

13750-419: The receiver reinforce each other, while currents from radio waves received from other directions are out of phase and when combined in the receiver cancel each other. The radiation pattern of such an antenna consists of a strong beam in one direction, the main lobe , plus a series of weaker beams at different angles called sidelobes , usually representing residual radiation in unwanted directions. The larger

13875-441: The reflector's weight and wind load . Specular reflection of radio waves is also employed in a parabolic reflector antenna, in which a curved reflecting surface effects focussing of an incoming wave toward a so-called feed antenna ; this results in an antenna system with an effective area comparable to the size of the reflector itself. Other concepts from geometrical optics are also employed in antenna technology, such as with

14000-422: The resulting induced electric current is in the same direction, the material is said to be an isotropic electrical conductor . If the resulting electric current is in a different direction from the applied electric field, the material is said to be an anisotropic electrical conductor . Antenna array An antenna array (or array antenna ) is a set of multiple connected antennas which work together as

14125-458: The same axis (or collinear ), each feeding one side of a two-conductor transmission wire. The physical arrangement of the two elements places them 180 degrees out of phase, which means that at any given instant one of the elements is driving current into the transmission line while the other is pulling it out. The monopole antenna is essentially one half of the half-wave dipole, a single ⁠ 1  / 4 ⁠  wavelength element with

14250-422: The same whether the antenna is transmitting or receiving . For example, the "receiving pattern" (sensitivity to incoming signals as a function of direction) of an antenna when used for reception is identical to the radiation pattern of the antenna when it is driven and functions as a radiator, even though the current and voltage distributions on the antenna itself are different for receiving and sending. This

14375-432: The same. Electrically this appears to be a very high impedance. The antenna and transmission line no longer have the same impedance, and the signal will be reflected back into the antenna, reducing output. This could be addressed by changing the matching system between the antenna and transmission line, but that solution only works well at the new design frequency. The result is that the resonant antenna will efficiently feed

14500-469: The signal's instantaneous field. When the resulting current reaches the end of the conductor, it reflects, which is equivalent to a 180 degree change in phase. If the conductor is ⁠ 1  / 4 ⁠ of a wavelength long, current from the feed point will undergo 90 degree phase change by the time it reaches the end of the conductor, reflect through 180 degrees, and then another 90 degrees as it travels back. That means it has undergone

14625-408: The size is expressed in square millimetres. In North America, conductors are measured by American wire gauge for smaller ones, and circular mils for larger ones. The ampacity of a conductor, that is, the amount of current it can carry, is related to its electrical resistance: a lower-resistance conductor can carry a larger value of current. The resistance, in turn, is determined by the material

14750-529: The spacing between adjacent radiators must not exceed a specific value to prevent the appearance of grating lobes (in the visible space) in the visible space ), the spacing between adjacent radiators must not exceed a specific value. For example, as seen previously, the first grating lobes for d = λ / 2 {\displaystyle d=\lambda /2} occur in u = ± 2 {\displaystyle u=\pm 2} . So, in this case, there are no problems since, in this way,

14875-554: The term "phased array" is sometimes used to mean an ordinary array antenna. From the Rayleigh criterion , the directivity of an antenna, the angular width of the beam of radio waves it emits, is proportional to the wavelength of the radio waves divided by the width of the antenna. Small antennas around one wavelength in size, such as quarter-wave monopoles and half-wave dipoles , don't have much directivity ( gain ); they are omnidirectional antennas which radiate radio waves over

15000-441: The wavelength, then the magnitude of the array factor has a period, in the domain of u {\displaystyle u} , equal to 2 {\displaystyle 2} . It is worth emphasising that u {\displaystyle u} is an auxiliary variable. In fact, from a physical point of view, the values of u {\displaystyle u} that are of interest for radiative purposes fall in

15125-457: The whole array aperture. Consequently, the array factor is a stochastic process, whose mean is as follows E [ F ( u ) ] = ∫ − L / 2 L / 2 f ( x ) e j k x u d x {\displaystyle E\left[F(u)\right]=\textstyle \int \limits _{-L/2}^{L/2}f(x)\,e^{jkxu}\,dx} In an antenna array providing

15250-408: The width of the antenna and the greater the number of component antenna elements, the narrower the main lobe, and the higher the gain which can be achieved, and the smaller the sidelobes will be. Arrays in which the antenna elements are fed in phase are broadside arrays; the main lobe is emitted perpendicular to the plane of the elements. The largest array antennas are radio interferometers used in

15375-559: The word antenna relative to wireless apparatus is attributed to Italian radio pioneer Guglielmo Marconi . In the summer of 1895, Marconi began testing his wireless system outdoors on his father's estate near Bologna and soon began to experiment with long wire "aerials" suspended from a pole. In Italian a tent pole is known as l'antenna centrale , and the pole with the wire was simply called l'antenna . Until then wireless radiating transmitting and receiving elements were known simply as "terminals". Because of his prominence, Marconi's use of

15500-545: The word antenna spread among wireless researchers and enthusiasts, and later to the general public. Antenna may refer broadly to an entire assembly including support structure, enclosure (if any), etc., in addition to the actual RF current-carrying components. A receiving antenna may include not only the passive metal receiving elements, but also an integrated preamplifier or mixer , especially at and above microwave frequencies. Antennas are required by any radio receiver or transmitter to couple its electrical connection to

15625-589: The zenith angle and azimuth angle, respectively. If the spacing between adjacent elements is constant, then it can be written that x n + 1 − x n = d {\displaystyle x_{n+1}-x_{n}=d} , and the array is said to be periodic. The array is periodic both spatially (physically) and in the variable u {\displaystyle u} . For example, if d = λ / 2 {\displaystyle d=\lambda /2} , with λ {\displaystyle \lambda } being

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