An antenna (or aerial) is an electrical device which couples radio waves in free space to an electrical current used by a radio receiver or transmitter. In reception, the antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage that the radio receiver can amplify Lenovo S10-2 Battery 5200mAh .
Alternatively, a radio transmitter will produce a large radio frequency current that may be applied to the terminals of the same antenna in order to convert it into an electromagnetic wave (radio wave) radiated into free space. Antennas are thus essential to the operation of all radio equipment, both transmitters and receivers Lenovo Y730 Battery 5200mAh .
They are used in systems such as radio andtelevision broadcasting, two-way radio, wireless LAN, mobile telephony, radar, and satellite communications.
Typically an antenna consists of an arrangement of metallic conductors (or "elements") with an electrical connection (often through a transmission line) to the receiver or transmitter Lenovo Y710 Battery 5200mAh .
A current forced through such a conductor by a radio transmitter will create an alternating magnetic field according to Ampère's law. Or the alternating magnetic field due to a distant radio transmitter will induce a voltage at the antenna terminals, according to Faraday's law, which is connected to the input of a receiver Lenovo Y530 Battery 5200mAh .
In the so-called far field, at a considerable distance away from the antenna, the oscillating magnetic field is coupled with a similarly oscillating electric field; together these define an electromagnetic wave which is capable of propagating great distances Lenovo Y510 Battery 5200mAh .
Light is one example of electromagnetic radiation, along with infrared and x-rays, while radio waves differ only in their much lower frequency (and much longer wavelength). Electronic circuits can operate at these lower frequencies, processing radio signals conducted through wires Lenovo S10-3 Battery 5200mAh .
But it is only through antennas that those radio frequencyelectrical signals are converted to (and from) propagating radio waves. Depending on the design of the antenna, radio waves can be sent toward and received from all directions ("omnidirectional"), whereas a directional or beam antenna is designed to operate in a particular direction Lenovo G450 Battery 5200mAh .
The first antennas were built in 1888 by Heinrich Hertz (1857–1894) in his pioneering experiments to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. Hertz placed dipole antennas at the focal point of parabolic reflectors for both transmitting and receiving Lenovo E30 Battery 5200mAh .
He published his work and installation drawings in Annalen der Physik und Chemie (vol. 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 Lenovo E40 Battery 5200mAh .
In the United Kingdom and other British English speaking areas the term aerial is more common, even for rigid types. The noun aerial is occasionally written with a diaeresis mark—aërial—in recognition of the original spelling of the adjective aërial from which the noun is derived Lenovo E50 Battery 5200mAh .
The origin of the word antenna relative to wireless apparatus is attributed to Guglielmo Marconi. In 1895, while testing early radio apparatuses in the Swiss Alps at Salvan, Switzerland in the Mont Blancregion, Marconi experimented with early wireless equipment Lenovo G460 Battery 5200mAh .
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 Lenovo Y460 Battery 5200mAh .
Marconi's use of the word antenna (Italian for pole) would become a popular term for what today is uniformly known as the antenna.
In common usage, the word antenna may refer broadly to an entire assembly including support structure, enclosure (if any), etc. in addition to the actual functional components Lenovo Y450 Battery 5200mAh .
Especially at microwave frequencies, a receiving antenna may include not only the actual electrical antenna but an integrated preamplifier and/or mixer.
Antennas are required by any radio receiver or transmitter in order to couple its electrical connection to the electromagnetic field Lenovo N200 Battery 5200mAh .
Radio waves are electromagnetic waves which carry signals through the air (or through space) at almost the speed of light with almost no transmission loss. Radio transmitters and receivers are used to convey signals (information) in systems including broadcast (audio) radio Lenovo T410 Battery 5200mAh ,
television, mobile telephones, wi-fi (WLAN) data networks, trunk lines and point-to-point communications links (telephone, data networks), satellite links, many remote controlled devices such asgarage door openers, and wireless remote sensors, among many others Lenovo T510 Battery 5200mAh .
Radio waves are also used directly for measurements in technologies including RADAR, GPS, and radio astronomy. In each and every case, the transmitters and receivers involved require antennas, although these are sometimes hidden (such as the antenna inside an AM radio or inside a laptop computer equipped with wi-fi) Lenovo SL410 Battery 5200mAh .
According to their applications and technology available, antennas generally fall in one of two catagories:
- Omnidirectional or only weakly directional antennas which receive or radiate more or less in all directions Lenovo SL510 Battery 5200mAh .
- These are employed when the relative position of the other station is unknown or arbitrary. They are also used at lower frequencies where a directional antenna would be too large, or simply to cut costs in applications where a directional antenna isn't required Lenovo C100 Battery 5200mAh .
- Directional or beam antennas which are intended to preferentially radiate or receive in a particular direction or directional pattern.
In common usage "omnidirectional" usually refers to all horizontal directions, typically with reduced performance in the direction of the sky or the ground (a truly isotropic radiator is not even possible) Lenovo T400S Battery 5200mAh .
A "directional" antenna usually is intended to maximize its coupling to the electromagnetic field in the direction of the other station, or sometimes to cover a particular sector such as a 120° horizontal fan pattern in the case of a panel antenna at a cell site Lenovo Y430 Battery 5200mAh .
One example of omnidirectional antennas is the very common vertical antenna or whip antenna consisting of a metal rod (often, but not always, a quarter of a wavelength long). A dipole antenna is similar but consists of two such conductors extending in opposite directions, with a total length that is often, but not always, a half of a wavelength long Ibm ThinkPad G40 Battery 5200mAh .
Dipoles are typically oriented horizontally in which case they are weakly directional: signals are reasonably well radiated toward or received from all directions with the exception of the direction along the conductor itself; this region is called the antenna blind cone or null Ibm ThinkPad X200T Battery 5200mAh .
Both the vertical and dipole antennas are simple in construction and relatively inexpensive. 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 through a balanced transmission line (or to a coaxial transmission line through a so-called balun) Apple A1280 battery 5200mAh .
The vertical antenna, on the other hand, is amonopole antenna. It is typically connected to the inner conductor of a coaxial transmission line (or a matching network); the shield of the transmission line is connected to ground Apple A1281 battery 5200mAh.
In this way, the ground (or any large conductive surface) plays the role of the second conductor of a dipole, thereby forming a complete circuit Samsung R522 battery 5200mAh .
Since monopole antennas rely on a conductive ground, a so-calledgrounding structure may be employed in order to provide a better ground contact to the earth or which itself acts as a ground plane to perform that function regardless of (or in absence of) an actual contact with the earth Samsung P467 battery 5200mAh .
Antennas fancier than the dipole or vertical designs are usually intended to increase the directivity and consequently the gain of the antenna. This can be accomplished in many different ways leading to a plethora of antenna designs. Hp Pavilion dm3 Battery 5400mAh
The vast majority of designs are fed with a balanced line (unlike a monopole antenna) and are based on the dipole antenna with additional components (or elements) which increase its directionality.
For instance, a phased array consists of two or more simple antennas which are connected together through an electrical network Hp Pavilion dm4 Battery 5200mAh .
This often involves 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) Hp Pavilion cq62 Battery 5200mAh .
Antenna arrays may employ any basic (omnidirectional or weakly directional) antenna type, such as dipole, loop or slot antennas. These elements are often identical Hp Pavilion cq42 Battery 5200mAh .
However a log-periodic dipole array consists of a number of dipole elements of different lengths in order to obtain a somewhat directional antenna having an extremely wide bandwidth: these are frequently used for television reception in fringe areas Hp Pavilion cq72 Battery 5200mAh .
The dipole antennas composing it are all considered "active elements" since they are all electrically connected together (and to the transmission line). On the other hand, a superficially similar dipole array, the Yagi-Uda Antenna (or simply "Yagi"), has only one dipole element with an electrical connection Hp cq20 Battery 2600mAh ;
the other so-called parasitic elementsinteract with the electromagnetic field in order to realize a fairly directional antenna but one which is limited to a rather narrow bandwidth. The Yagi antenna has similar looking parasitic dipole elements but which act differently due to their somewhat different lengths Hp 2230 Battery 2600mAh .
There may be a number of so-called "directors" in front of the active element in the direction of propagation, and usually a single (but possibly more) "reflector" on the opposite side of the active element.
Greater directionality can be obtained using beam-forming techniques such as a parabolic reflector or a horn Dell Inspiron Mini 12 battery 7800mAh .
Since the size of a directional antenna depends on it being large compared to the wavelength, very directional antennas of this sort are mainly feasible at UHF and microwave frequencies. On the other hand, at low frequencies (such as AM broadcast) where a practical antenna must be much smaller than a wavelength, significant directionality isn't even possible Dell XPS M1730 battery 7800mAh .
A vertical antenna or loop antenna small compared to the wavelength is typically used, 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 Dell XPS M2010 battery 7800mAh.
An antenna lead-in is the transmission line (or feed line) which connects the antenna to a transmitter or receiver. The antenna feedmay refer to all components connecting the antenna to the transmitter or receiver, such as an impedance matching network in addition to the transmission line Dell Studio XPS 1340 battery 5200mAh/7800mAh .
In a so-called aperture antenna, such as a horn or parabolic dish, the "feed" may also refer to a basic antenna inside the entire system (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 Dell Inspiron 9100 battery 7800mAh .
A microwave antenna may also be fed directly from a waveguide in lieu of a (conductive) transmission line.
An antenna counterpoise or ground plane is a structure of conductive material which improves or substitutes for the ground. It may be connected to or insulated from the natural ground Dell Inspiron N4010 battery 5200mAh .
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 Dell Inspiron N3610 battery 5200mAh .
An electromagnetic wave refractor in some aperture antennas 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 Dell Inspiron N5010 battery 5200mAh .
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 is the radio equivalent of an optical lens Dell Inspiron N7010 battery 5200mAh .
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 improve the standing wave ratio in order to minimize losses in the transmission line Dell Inspiron 1464 battery 5200mAh
(especially at higher frequencies and/or over longer distances) and to present the transmitter or receiver with a standard resistive impedance (such as 75 ohms) that it expects to see for optimum operation Dell Inspiron 1564 battery 5200mAh .
It is a fundamental property of antennas that the characteristics of an antenna described in the next section, such as gain, radiation pattern, impedance, bandwidth, resonant frequency and polarization, are the same whether the antenna is transmitting or receivingDell Inspiron 1764 battery 5200mAh .
For example, the "receiving pattern" (sensitivity as a function of direction) of an antenna when used for reception is identical to theradiation pattern of the antenna when it is driven and functions as a radiator. This is a consequence of the reciprocity theorem of electromagnetics Dell Inspiron 1320 battery 5200mAh .
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 above reciprocity property is that the materials in the antenna and transmission medium are linear and reciprocal Dell Inspiron 1320n battery 5200mAh .
Reciprocal (or bilateral) means that the material has the same response to an electric or magnetic field, or a current, 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 Dell Vostro 1710 battery 5200mAh ,
made of nonreciprocal materials such as ferrite or garnet. 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.
There are several critical parameters affecting an antenna's performance that can be adjusted during the design process Dell Vostro 1720 battery 5200mAh .
These are resonant frequency, impedance, gain, aperture or radiation pattern,polarization, efficiency and bandwidth. Transmit antennas may also have a maximum power rating, and receive antennas differ in their noise rejection properties. All of these parameters can bemeasured through various means Dell Studio XPS 1640 battery 5200mAh .
Many types of antenna are tuned to work at one particular frequency, and are effective only over a range of frequencies centered on this frequency, called the resonant frequency. These are calledresonant antennas Dell Vostro A840 battery 5200mAh .
The antenna acts as an electrical resonator. When driven at its resonant frequency, large standing waves of voltage and current are excited in the antenna elements. These large currents and voltages radiate intense electromagnetic waves, so the power radiated by the antenna is maximum at the resonant frequency Dell Vostro A860 battery 5200mAh .
In antennas made of thin linear conductive elements, the length of the driven element(s) determines the resonant frequency. To be resonant, the length of a driven element should typically be either half or a quarter of the wavelength at that frequency; these are called half-wave and quarter-wave antennas Dell Studio 1535 battery 5200mAh/7800mAh .
The length referred to is not the physical length, but the electrical length of the element, which is the physical length divided by the velocity factor (the ratio of the speed of wave propagation in the wire to c0, the speed of light in a vacuum) Dell Studio 1735 battery 5200mAh/7800mAh .
Antennas are usually also resonant at multiples (harmonics) of the lowest resonant frequency.
Some antenna designs have multiple resonant frequencies, and some are relatively effective over a very broad range of frequencies. or bandwidth Dell Vostro 1310 battery 5200mAh .
One commonly known type of wide band antenna is the logarithmic or log-periodic antenna.
The resonant frequency also affects the impedance of the antenna. At resonance, the equivalent circuit of an antenna is a pure resistance, with no reactive component Dell Latitude E6400 battery,10400mAh/7800mAh .
At frequencies other than the resonant frequencies, the antenna has capacitance or inductance as well as resistance. An antenna can be made resonant at other frequencies besides its natural resonant frequency by compensating for these reactances by adding a loading coil or capacitor in series with it Dell Latitude E6400 Battery,10400mAh/7800mAh .
Other properties of an antenna change with frequency, in particular the radiation pattern, so the antenna's operating frequency may be considerably different from the resonant frequency to optimize other important parameters Dell Latitude E6400 Battery,10400mAh/7800mAh .
Gain is a parameter which measures the degree of directivity of the antenna's radiation pattern. 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 directionsDell Latitude E6500 battery,10400mAh/7800mAh .
Specifically, the antenna 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 the direction of its maximum output, at an arbitrary distance, divided by the intensity radiated at the same distance by a hypothetical isotropic antenna Dell Latitude E6500 battery,10400mAh/7800mAh .
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. An antenna designer must take into account the application for the antenna when determining the gain Dell Latitude E6500 battery,10400mAh/7800mAh .
High-gain antennas have the advantage of longer range and better signal quality, but must be aimed carefully in a particular direction. Low-gain antennas have shorter range, but the orientation of the antenna is relatively inconsequential Dell Latitude E6500 battery,10400mAh/7800mAh .
For example, a dish antenna on a spacecraft is a high-gain device that must be pointed at the planet to be effective, whereas a typical Wi-Fi antenna in a laptop computer is low-gain, and as long as the base station is within range, the antenna can be in any orientation in space HP Pavilion DV6-1210SA battery - 5200mAh/8800mAh .
It makes sense to improve horizontal range at the expense of reception above or below the antenna. Thus most antennas labelled "omnidirectional" really have some gain.
In practice, the half-wave dipole is taken as a reference instead of the isotropic radiator HP Pavilion DV7 battery - 7200mAh .
The gain is then given in dBd (decibels over dipole):
- NOTE: 0 dBd = 2.15 dBi. It is vital in expressing gain values that the reference point be included. Failure to do so can lead to confusion and error Ibm THINKPAD X200 Battery 8800mAh .
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. It is typically represented by a three dimensional graph, or polar plots of the horizontal and vertical cross sections Ibm THINKPAD X200 Battery 8800mAh .
The pattern of an ideal isotropic antenna, which radiates equally in all directions, would look like 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 Ibm THINKPAD X200 Battery 8800mAh ;
this is called an omnidirectional pattern and when plotted looks like a torus or donut.
The radiation of many antennas shows a pattern of maxima or "lobes" at various angles, separated by "nulls", angles where the radiation falls to zero SONY VAIO VGN-FW21E Battery .
This is because the radio waves emitted by different parts of the antenna typically interfere, causing maxima at angles where the radio waves arrive at distant pointsin phase, and zero radiation at other angles where the radio waves arrive out of phase Asus M52N Battery .
In a directional antenna designed to project radio waves in a particular direction, the lobe in that direction is designed larger than the others and is called the "main lobe". The other lobes usually represent unwanted radiation and are called "sidelobes". The axis through the main lobe is called the "principle axis" or "boresight axis" Dell Latitude E6400 ATG Battery .
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.) SONY VAIO VGN-FW21J Battery .
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) SONY VAIO VGN-FW21L Battery .
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 SONY VAIO VGN-FW21M Battery .
Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line and radio by adjusting the impedance of the feed line, using the feed line as an impedance transformer SONY VAIO VGN-FW21Z Battery .
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.
Efficiency is the ratio of power actually radiated to the power put into the antenna terminals SONY VAIO VGN-FW25T/B Battery .
A dummy load may have an SWR of 1:1 but an efficiency of 0, as it absorbs all power and radiates heat but very little RF energy, showing that SWR alone is not an effective measure of an antenna's efficiency. Radiation in an antenna is caused by radiation resistance which can only be measured as part of total resistance including loss resistance SONY VAIO VGN-FW26T/B Battery .
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 SONY VAIO VGN-FW27/B Battery .
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 (array) and allowing the natural impedance of suitable inductive RF filter traps to select the correct antenna SONY VAIO VGN-FW27/W Battery .
All these attempts to increase bandwidth by adding capacitance to the surface area have a detrimental effect on efficiency by reducing the Q factor. They also have an adverse effect on the rejection of unwanted harmonics, on both received and transmitted signal frequencies SONY VAIO VGN-FW31ZJ Battery .
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 SONY VAIO VGN-FW27T/H Battery .
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 SONY VAIO VGN-FW29/B Battery .
"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 SONY VAIO VGN-FW290JRB Battery .
For radio waves the most important reflector is the ionosphere - signals which reflect from it will have their polarization changed unpredictably. For signals which are reflected by the ionosphere, polarization cannot be relied upon. For line-of-sight communications for which polarization can be relied upon SONY VAIO VGN-FW290JTB Battery ,
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 SONY VAIO VGN-FW290JTH Battery .
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 SONY VAIO VGN-FW33G/E1 Battery .
A vertical omnidirectional WiFi antenna will have vertical polarization (the most common type). An exception is a class of elongated waveguide antennas in which vertically placed antennas are horizontally polarized. Many commercial antennas are marked as to the polarization of their emitted signals SONY VAIO VGN-FW33G/W Battery .
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 iselliptical, meaning that the polarization of the radio waves varies over time SONY VAIO VGN-FW33GW Battery .
Two special cases are linear polarization (the ellipse collapses into a line) and circular polarization (in which the two axes of the ellipse are equal). In linear polarization the antenna compels the electric field of the emitted radio wave to a particular orientationDell XPS M1210 Battery) http://www.hdd-shop.co.uk .
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 Dell KM958 battery .
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 Dell Studio 1555 battery .
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 Sony VGP-BPS13 battery .
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 .
Transmission and reception
All of the antenna parameters are expressed in terms of a transmission antenna Sony VGP-BPS13/B battery ,
but are identically applicable to a receiving antenna, due to reciprocity. Impedance, however, is not applied in an obvious way; for impedance, the impedance at the load (where the power is consumed) is most critical. For a transmitting antenna, this is the antenna itself Sony VGP-BPS13/S battery .
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 Sony VGP-BPS13A/B battery .
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 thereactance which combines with the inherent reactance of the active antenna to establish a resonance in a circuit including the active antenna Sony VGP-BPS13B/B battery .
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 Sony VGP-BPL9 battery ,
beyond which heating, arcing or sparking may occur in the components, which may cause them to be damaged or destroyed. Raising this maximum power rating usually requires larger and heavier components, which may require larger and heavier supporting structures Sony VGP-BPS13B/B battery .
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 Sony VGP-BPL15 battery .
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 Dell Inspiron E1505 battery ,
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 HP Pavilion dv6000 Battery ,
or by selecting a polarization different from the noise polarization, or by selecting an antenna that favors either the electric or magnetic field.
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 HP Pavilion dv8000 battery ,
and from natural sources such as lightning. Successfully rejecting these forms of noise is an important antenna feature. A small coil of wire with many turns is more able to reject such noise than a vertical antenna. However, the vertical will radiate much more effectively on transmit, where extraneous signals are not a concern HP PAVILION DV6000 Battery, 8800mAh .
Basic antenna models
There are many variations of antennas. Below are a few basic models. More can be found in Category:Radio frequency antenna types.
- The isotropic radiator is a purely theoretical antenna that radiates equally in all directions. It is considered to be a point in space with no dimensions and no mass HP PAVILION DV8000 Battery, 8800mAh .
- This antenna cannot physically exist, but is useful as a theoretical model for comparison with all other antennas. Most antennas' gains are measured with reference to an isotropic radiator, and are rated in dBi (decibels with respect to an isotropic radiator) SONY VAIO VGN-FZ Battery .
- The dipole antenna is simply two wires pointed in opposite directions arranged either horizontally or vertically, with one end of each wire connected to the radio and the other end hanging free in space. Since this is the simplest practical antenna, it is also used as a reference model for other antennas HP Pavilion DV6000 battery - 8800mAh ;
- ain with respect to a dipole is labeled as dBd. Generally, the dipole is considered to be omnidirectional in the plane perpendicular to the axis of the antenna, but it has deep nulls in the directions of the axis. Variations of the dipole include the folded dipole, the half wave antenna, the ground plane antenna, thewhip, and the J-pole HP Pavilion DV8000 battery - 7800mAh .
- The Yagi-Uda antenna is a directional variation of the dipole with parasitic elements added which are functionality similar to adding a reflector and lenses (directors) to focus a filament light bulb.
- The random wire antenna is simply a very long (at least one quarter wavelength ) wire with one end connected to the radio and the other in free space, arranged in any way most convenient for the space available HP Pavilion DV6000 battery - 8800mAh .
- Folding will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more than the folding typically hurts.) Typically, a random wire antenna will also require an antenna tuner, as it might have a random impedance that varies non-linearly with frequency HP Pavilion DV8000 battery - 6600mAh .
- The horn is used where high gain is needed, the wavelength is short (microwave) and space is not an issue. Horns can be narrow band or wide band, depending on their shape. A horn can be built for any frequency, but horns for lower frequencies are typically impractical. Horns are also frequently used as reference antennas SONY VAIO VGN-FZ18 Battery .
- The parabolic antenna consists of an active element at the focus of a parabolic reflector to reflect the waves into a plane wave. Like the horn it is used for high gain, microwave applications, such assatellite dishes.
- The patch antenna consists mainly of a square conductor mounted over a groundplane SONY VAIO VGN-FZ21E Battery .
- Another example of a planar antenna is the tapered slot antenna (TSA), as the Vivaldi-antenna.
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 SONY VAIO VGN-FW31J Battery 10400mAh .
An example of an inefficient antenna is the simple Hertzian dipole antenna, which radiates over wide range of frequencies and is useful for its small size. A more efficient variation of this is the half-wave dipole, which radiates with high efficiency when the signal wavelength is twice the electrical length of the antenna SONY VAIO VGN-FW31J Battery10400mAh .
One of the goals of antenna design is to minimize the reactance of the device so that it appears as a resistive load. An "antenna inherent reactance" includes not only the distributed reactance of the active antenna but also the natural reactance due to its location and surroundings SONY VAIO VGN-FW21E Battery
(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 SONY VAIO VGN-NR11S/S Battery .
Reactance can be eliminated by operating the antenna at its resonant frequency, when its capacitive and inductive reactances are equal and opposite, resulting in a net zero reactive current. If this is not possible, compensating inductors or capacitors can instead be added to the antenna to cancel its reactance as far as the source is concerned SONY VAIO VGN-NR11M/S Battery .
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 theradiation resistance. Power absorbed by the ohmic resistance becomes waste heat, and that absorbed by the radiation resistance becomes radiated electromagnetic energy SONY VAIO VGN-NR11Z/S Battery .
The greater the ratio of radiation resistance to ohmic resistance, the more efficient the antenna.
Effect of ground
Antennas are typically used in an environment where other objects are present that may have an effect on their performance. Height above ground has a very significant effect on the radiation pattern of some antenna types SONY VAIO VGN-NR11Z/T Battery .
At frequencies used in antennas, the ground behaves mainly as a dielectric . The conductivity of ground at these frequencies is negligible. When an electromagnetic wave arrives at the surface of an object, two waves are created: one enters the dielectric and the other is reflected Sony VAIO VGN-FZ21E Battery .
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 Sony VAIO VGN-FW21E Battery .
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 Sony VAIO VGN-NR11S/S Battery .
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 Sony VAIO VGN-NR11Z/S Battery .
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 Sony VAIO VGN-NR11M/S Battery .
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 Sony VAIO VGN-NR11Z/T Battery .
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 waveSONY VAIO VGN-FZ180E Battery .
As the refractive index of the ground (average value ) is bigger than the refractive index of the air (), the direction of the component of the electric field parallel to the ground inverses at the reflection. This is equivalent to a phase shift of radians or 180°SONY VAIO VGN-FZ220E Battery .
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 SONY VAIO VGN-FZ340E Battery .
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 SONY VAIO VGN-FZ430E Battery :
In these two equations:
- is the electrical field radiated by the antenna if there were no ground.
- is the wave number.
- is the wave length SONY VAIO VGN-FZ460E Battery.
- is the distance between antenna and its image (twice the height of the center of the antenna).
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 SONY VAIO VGN-FZ480E Battery .
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 SONY VAIO VGN-FZ4000 Battery .
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 SONY VAIO VGN-FZ31E Battery .
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 SONY VAIO VGN-FZ31B Battery .
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 SONY VAIO VGN-FZ31J Battery ,
as in mobile phones, the emitter and receivers in base stations use circular polarized electromagnetic waves.
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 appearSONY VAIO VGN-FZ31M Battery .
However, these same buildings reflect the electromagnetic waves and can create ghost images. Using horizontal polarization, reflections are attenuated because of the low reflection of electromagnetic waves whose magnetic field is parallel to the dielectric surface near the Brewster's angle SONY VAIO VGN-FZ31Z Battery .
Vertically polarized analog television has been used in some rural areas. In digital terrestrial television reflections are less obtrusive, due to the inherent robustness of digital signalling and built-in error correction SONY VAIO VGN-FZ38M Battery .
Mutual impedance and interaction between antennas
Current circulating in any antenna induces currents in all others. One can postulate a mutual impedance between two antennas that has the same significance as the in ordinary coupled inductors. The mutual impedance between two antennas is defined as SONY VGP-BPS8 Battery :
- _1}" src="http://upload.wikimedia.org/math/8/1/b/81b7bf076dab670f2a4237ec32202fbc.png" />
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 SONY VGP-BPS13/S Battery :
- is the voltage applied to the antenna i SONY VGP-BPS13A/B Battery
- is the impedance of antenna i
- is the mutual impedance between antennas i and j
Note that, as is the case for mutual inductances, SONY VGP-BPS13B/B Battery
This is a consequence of Lorentz reciprocity. If some of the elements are not fed (there is a short circuit instead a feeder cable), as is the case in television antennas (Yagi-Uda antennas), the corresponding are zero. Those elements are called parasitic elements. Parasitic elements are unpowered elements that either reflect or absorb and reradiate RF energy SONY VGP-BPS13A/S Battery .
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 mast radiator (or 'radiating tower') is a radio mast or tower in which the whole structure itself functions as an antenna SONY VGP-BPS13AS Battery .
This design is commonly used for transmitting antennas operating at low frequencies, in the VLF, LF and MF ranges, in particular those used for AMbroadcasting. The metal mast is electrically connected to the transmitter. Its base is usually mounted on a nonconductive support to insulate it from the ground Dell Inspiron 1320n Battery .
A mast radiator is a form of monopole antenna.
Most mast radiators are built as guyed masts insulated from the ground at the base. Steel lattice masts, of triangular cross-section, are the most common type Dell Studio 1450 Battery .
Square lattice masts and tubular masts are also sometimes used. To ensure that the tower is a continuous conductor, the tower's structural sections are electrically bonded at the joints by short copper jumpers which are soldered to each side Dell Studio 1457 Battery .
At its base, the mast is usually mounted on a thick ceramic insulator, which has the compressive strength to support the tower's weight and the dielectric strength to withstand the high RF voltage applied by the transmitter. The RF power to drive the antenna is supplied by an antenna tuner unit Dell Latitude D610 Battery ,
usually housed in a small building called a helix building next to the mast, and the cable supplying the current is simply bolted to the tower. The actual transmitter is usually located in a separate building, which supplies RF power to the helix building via a transmission line Toshiba NB100 Battery .
Free-standing towers are also used as radiating structures. These towers can have a triangular or a square cross section. One of the best-known radiating towers is the Blosenbergturm in Beromünster, Switzerland. Masts of glass fibre are sometimes used for some small constructions Toshiba Satellite M65 battery .
The ideal height of a mast radiator depends on transmission frequency, demographics for the location, and terrain. For radio systems in thelongwave and mediumwave range the value of the height should be in the range between one sixth and five eighths of the wavelength, with favourite values at the quarter or the half of the radiated wavelength Toshiba Satellite M60 battery .
When this is not always possible, masts with a loading coil, 'capacity hat' or similar loading arrangement are used. The quarter-wave mast is a form of monopole antenna.
The height of the mast determines the radiation properties Dell Latitude D830 Battery .
For high power transmitters in the MW range, masts with heights around half of the radiated wavelength are preferred because they focus the radiated power better to the ground than structures with heights of quarter wavelengths, which are preferred for economical reasons for low power medium wave transmitters Dell Latitude D620 Battery .
A focus of radiated power towards the ground is much desired on frequencies below 3 megahertz, because groundwave propagation is very stable. Masts longer than five eighths of the wavelength are normally not used, because they show bad vertical radiation patterns, so masts for mediumwave transmitters do not normally exceed 300 metres Dell Inspiron Mini 10 Battery .
For longwave transmitters, however, the construction of masts with heights of half-wave wavelength is, because of economical reasons (and perhaps problems with the flight safety), in most cases impossible. The only so far realized radio mast for longwavewith a height of the half length of the radiated wavelength was the Warszawa Radio Mast at Konstantynów, Poland Sony VGN-FW11S Battery .
At the time of its collapse in 1991 it was the tallest manmade structure in the world, at 646.38 metres (2,120.67 ft) tall. For frequencies below longwave masts are electrically enlarged by loading coils or capacity hats on the top, because masts of even quarter wave length would be too high Sony VGN-FW11M Battery .
There are three ways of feeding a mast radiator from a transmitter:
- Base feed: the mast is supported on an insulator, and the transmitter is connected to the mast just above it;
- Shunt feed: the mast is grounded and the transmitter feeds it via a wire connected to the mast part way up Sony VGN-FW139E/H battery .
- (This is a similar approach, on a larger scale, to the 'gamma match' popular among amateur radio operators for VHF and UHF amateur radio antennas.)
- Multiple feed: the structure is divided into two (or more) sections with insulators between, each independently fed Dell Latitude E5400 Battery .
- This collinear arrangement enhances low-angle (ground wave) radiation and reduces high-angle (sky wave) radiation. This increases the distance to the mush zonewhere the ground wave and sky wave are at similar strength at night. This type of antenna is known as an anti-fading aerial (see below) Dell Latitude E4200 Battery .
There is usually an antenna matching unit to match the impedance of the transmitter or feeder to the antenna. Depending on the power involved, this may be a small box or a hut or building. It will typically contain an L-network to transform the modulus of impedance, and a coil or capacitor in series with the mast connection to 'tune out' any reactive component Dell Inspiron 300M Battery .
Location on transmission facility
At some facilities, especially the older and higher-powered installations, the mast radiator may be located at some distance of the transmitter building, in order to reduce the field strength induced by the mast into the building, and to prevent the building from distorting the mast's radiation pattern Dell Studio 1737 battery .
Between the transmitter building and the antenna matching unit next to the mast radiator, there is a feeder: either an underground coaxial cable or an overhead wire 'cage' feeder.
At facilities with multiple masts, spacings are typically smaller, in order to fit them into the available space Dell Inspiron E1505 battery .
At modern transmitters or at low power transmitters situated in very small transmitter buildings the transmitter, matching unit and mast radiator can be very close together and even in the same building. This measure saves on feeders, land area and increases the efficiency of the transmitter, if only one mast radiator is in use Dell RM791 battery .
At most facilities the mast radiator is on a separate base close to the antenna matching unit, but it can be sometimes set up the roof of it. This is for example the case at the main transmission mast of the Mühlacker radio transmitter and the main transmission mast of the Ismaning radio transmitter Toshiba Satellite P10 Battery .
For a good groundwave propagation, mast radiators should be built always on a large flat area with good ground conductivity and if possible without inclination. The construction of a mast radiator on the top of a building or a tower, whose height is in the same magnitude range as the wavelengths being transmitted gives a bad groundwave propagation Acer Aspire One battery 10400mAh .
So mast radiators are (in contrast to FM broadcasting antennas) normally not installed on the top of buildings or towers. Nevertheless in rare cases mast radiators for low power transmissions are installed on top of buildings. So some lighthouses like Reykjanesviti carry a mast radiator for a longwave radio beacon on its roof SONY VGN-FZ210CE Battery .
Also low-power broadcasting stations uses and used such arrangements. The best-known are WGSO in New Orleans and Delaney Building in Spokane. In Europe the low-power broadcasting station at Campobasso uses a mast radiator on a castle Dell Precision M70 Battery .
Mast radiators, as with all other equipment showing over 42 volts on exposed components within 4 metres of the ground,are required to be fenced in. Sometimes wooden fences are used in order to prevent signal interference, which could occur due to currents induced by radio signals in metallic fences Toshiba Satellite L305 Battery .
If the mast radiator is mounted on the top of the helix building, which must be over four meters in height; or the mast is grounded, with the feed being located not less than four meters above the ground, a fence is not required. Nevertheless it is always recommended to fence in any mast radiator in order to prevent unauthorized climbing Toshiba Satellite T4900 Battery .
A mast radiator may need various electrical connections other than the transmitter feed line. Such connections include static drain chokes for lightning protection, power supplies for aircraft warning lamps, and coaxial feeders for ancillary antennas mounted on the mast Toshiba PA3399U-2BRS battery .
A variety of techniques is used to 'isolate' these connections from the high RF voltage on the mast, such as chokes, parallel tuned circuits and coupling loops, on a base-fed mast. On a shunt-fed mast, where the base is grounded, no such measures are necessarySony VGP-BPS10 battery 10400mAh .
An anti-fading aerial is a transmission aerial for long- and mediumwave with a flat vertical radiation pattern, in order to push the mush zone far away from the transmitter site. An anti-fading aerial must reduce radiation at elevations of more than 50 degrees as much as possible Sony VGP-BPS9 battery 10400mAh .
In its simplest form it is an aerial of half wavelength. Such an aerial should be as thin as possible. In early days therefore a wire was hung up in a wood framework tower. Nowadays a radio mast is therefore used, which is fed at its foot and in a certain height Sony VGP-BPS11 battery 10400mAh .
Therefore the radio mast must be insulated against ground and be divided by a separation insulator electrically in two parts, where for feeding of the upper part either a cable inside the mast construction or the ladder, which must be mounted insulated is used. The medium wave transmitters Mühlacker, Wolfsheim and Hamburg use such aerials Sony VGP-BPS22 battery 10400mAh .
There are also radio masts for fading reducing aerials with two separation insulators as at Ismaning.
Because such separation insulators are more sensitive as the whole other construction of the radio mast, bigger horizontal forces, which could be generated by wind caused oscillations, should be inhibited Sony VGP-BPL8 battery 10400mAh .
Therefore some radio mast with built in separation insulators have oscillation dampers just above the separation insulator. You find such device at the radio masts of the transmittersWolfsheim, Hamburg and Ismaning.
Another possibility for reducing an anti-fading aerial is the usage of dipoles hung up in a certain height above ground Sony VGP-BPS14 battery 10400mAh .
A further possibility of the realization an anti-fading aerial is a circle group aerial. Therefore many mast radiators were arranged on a circle and fed in equal phase. With such facilities very flat radiation patterns could be realized, although they are very expensive, because multiple radio masts are required Sony VGP-BPS21 battery 10400mAh .
Taldom transmitter and Tulagino transmitter in Russia are obviously the only radio stations using such an antenna. In earlier days longwave transmitter Orlunda in Sweden also used such an antenna.
The tallest mast radiator ever built was the Warsaw Radio Mast at Konstantynow, Poland Toshiba Satellite A200 Battery .
It was 646.38 metres high but collapsed in 1991. Currently the masts of the Lualualei VLF transmitter, at 458.11 metres, are the tallest mast radiators insulated against ground. In contrast to the Warsaw Radio Mast, the Lualualei antenna uses extensive electrical lengthening measures Toshiba Satellite 1200 Battery .
The tallest mast radiator not using electrical lengthening may be the Hellissandur longwave radio mast. It is 411.48 metres high.
Radio masts and towers are, typically, tall structures designed to support antennas (also known as aerials) for telecommunications and broadcasting, including television Toshiba Satellite M300 Battery .
They are among the tallest man-made structures. Similar structures include electricity pylons and towers for wind turbines.
Masts are sometimes named after the broadcasting organisations that use them, or after a nearby city or town SONY VGP-BPS13A/Q Battery .
The Warsaw Radio Mast was the world's tallest supported structure on land, but it collapsed in 1991, leaving the KVLY/KTHI-TV mast as the tallest.
In the case of a mast radiator or radiating tower, the whole mast or tower is itself the transmitting antenna SONY VGP-BPS13A/Q Battery .
The terms "mast" and "tower" are often used interchangeably. However, in structural engineering terms, a tower is a self-supporting or cantilevered structure, while a mast is held up by stays or guys. By contrast, in broadcast engineering, a tower is an antenna structure attached to the ground SONY VGP-BPS13B/Q Battery ,
whereas a mast is a vertical antenna support mounted on some other structure (which itself may be a tower, a building, or a vehicle). Masts (to use the civil engineering terminology) tend to be cheaper to build but require an extended area surrounding them to accommodate the guy wires SONY VGP-BPS13B/Q Battery .
Towers are more commonly used in cities where land is in short supply.
There are a few borderline designs which are partly free-standing and partly guyed. For example:
- The Gerbrandy tower consists of a self-supporting tower with a guyed mast on top SONY VGP-BPS13/Q Battery .
- The few remaining Blaw-Knox towers do the opposite: they have a guyed lower section surmounted by a freestanding part.
- Zendstation Smilde a tall tower with a guyed mast on top (guys go to ground)
- Torre de Collserola a guyed tower, with a guyed mast on top. (Tower portion is not free-standing SONY VGP-BPS13/Q Battery .)
The steel lattice is the most widespread form of construction. It provides great strength, low weight and wind resistance, and economy in the use of materials. Lattices of triangular cross-section are most common, and square lattices are also widely usedSONY VGP-BPS21A/B Battery .
When built as a stayed mast, usually the whole mast is parallel-sided. One exception is the Blaw-Knox type.
When built as a tower, the structure may be parallel-sided or taper over part or all of its height SONY VGP-BPS21/S Battery .
When constructed of several sections which taper exponentially with height, in the manner of the Eiffel Tower, the tower is said to be an Eiffelized one. The Crystal Palace tower in London is an example SONY VGP-BPS21B Battery .
Guyed masts are sometimes also constructed out of steel tubes. This construction type has the advantage that cables and other equipment is protected from weather influence and that the structure may look nicer. They are mainly used for FM-/TV-broadcasting, but sometimes also as mast radiator SONY VGP-BPS21A Battery ,
wherefore the big mast of Mühlacker transmitting station is a good example. A disadvantage of this mast type is that it is much more affected by winds than masts with open bodies. In fact several tubular guyed masts collapsed: in the UK, these were masts the Emley Moor and Waltham TV stations SONY VGP-BPS21 Battery ,
which collapsed in the 1960s, in Germany that of Bielstein transmitter, which collapsed in 1985. Not in all countries such masts were built: while in Germany, France, UK, Czech, Slovakia and the former Soviet Union multiple tubular guyed masts were built, there are nearly none in Poland and North America Sony VGP-BPS21A/B Battery .
At several cities in Russia and Ukraine, between 1960 and 1965 several tubular guyed masts with crossbars running from the mast structure to the guys were built. All these masts are exclusively used for FM and TV transmission and are except of the mast in Vinnytsia between 150 and 200 metres tall Sony VGP-BPS21/S Battery .
The crossbars of these masts are equipped with a gangway and are equipped with smaller antennas. Their main purpose is oscillation damping.
Reinforced concrete towers are relatively expensive to build but provide a high degree of mechanical rigidity in strong winds Sony VGP-BPS21A Battery .
This can be important when antennas with narrow beamwidths are used, such as those used for microwave point-to-point links, and when the structure is to be occupied by people.
In the 1950s, AT&T built numerous concrete towers, more resembling silos than towers, for its first transcontinental microwave route. Many are still in use today Sony VGP-BPS21 Battery .
In Germany and the Netherlands most towers constructed for point-to-point microwave links are built of reinforced concrete, while in the UK most are lattice towers.
Concrete towers can form prestigious landmarks, such as the CN Tower in Toronto. As well as accommodating technical staff, these buildings may have public areas such as observation decks or restaurants Sony Vaio PCG-5G2L Battery .
The Stuttgart TV tower was the first tower in the world to be built in reinforced concrete. It was designed in 1956 by the local civil engineer Fritz Leonhardt.
Fibreglass poles are occasionally used for low-power non-directional beacons or medium-wave broadcast transmitters Sony Vaio PCG-5G3L Battery .
There are fewer wooden towers now than in the past. Many were built in the UK during World War II because of a shortage of steel. In Germany before World War II wooden towers were used at nearly all medium-wave transmission sites, but all of these towers have since been demolished, except for the Gliwice Radio Tower Sony Vaio PCG-5J1L Battery .
Ferryside Relay is an example of a TV relay transmitter using a wooden pole.
Shorter masts may consist of a self-supporting or guyed wooden pole, similar to a telegraph pole. Sometimes self-supporting tubular galvanized steel poles are used: these may be termed monopoles Sony Vaio PCG-5K2L Battery .
In some cases, it is possible to install transmitting antennas on the roofs of tall buildings. In North America, for instance, there are transmitting antennas on the Empire State Building, the Willis Tower ,and formerly on the World Trade Center towers Sony Vaio PCG-5J2L Battery .
When the buildings collapsed, several local TV and radio stations were knocked off the air until backup transmitters could be put into service. Such facilities also exist in Europe, particularly for portable radio services and low-power FM radio stations.
Many people view bare cellphone towers as ugly and an intrusion into their neighbourhoods Sony Vaio PCG-5K1L Battery .
Even though people increasingly depend upon cellular communications, they are opposed to the bare towers spoiling otherwise scenic views. Many companies offer to 'hide' cellphone towers in, or as, trees, church towers, flag poles, water tanks and other features Sony Vaio PCG-6S2L Battery .
There are many providers that offer these services as part of the normal tower installation and maintenance service. These are generally called "stealth towers" or "stealth installations".
The level of detail and realism achieved by disguised cellphone towers is remarkably high Sony Vaio PCG-6S3L Battery ;
for example, such towers disguised as trees are nearly indistinguishable from the real thing, even for local wildlife (who additionally benefit from the artificial flora). Such towers can be placed unobtrusively innational parks and other such protected places, such as towers disguised as cacti in Coronado National Forest Sony Vaio PCG-6V1L Battery .
Even when disguised, however, such towers can create controversy; a tower doubling as a flagpole attracted controversy in 2004 in relation to the U.S. Presidential campaign of that year, and highlighted the sentiment that such disguises serve more to allow the installation of such towers in subterfuge away from public scrutiny rather than to serve towards the beautification of the landscapeSony Vaio PCG-6W1L Battery .
A mast radiator is a radio tower or mast in which the whole structure works as an antenna. It is used frequently as a transmitting antenna for long or medium wave broadcasting.
Structurally, the only difference is that a mast radiator may be supported on an insulator at its base Sony Vaio PCG-6W2L Battery .
In the case of a tower, there will be one insulator supporting each leg.
Telescopic, pump-up and tiltover towers
A special form of the radio tower is the telescopic mast. These can be erected very quickly Sony Vaio PCG-6W3L Battery .
Telescopic masts are used predominantly in setting up temporary radio links for reporting on major news events, and for temporary communications in emergencies. They are also used in tactical military networks. They can save money by needing to withstand high winds only when raised, and as such are widely used in amateur radio Sony Vaio PCG-7111L Battery .
Telescopic masts consist of two or more concentric sections and come in two principal types:
- Pump-up masts are often used on vehicles, and are raised to their full height pneumatically or hydraulically. They are usually only strong enough to support fairly small antennas Sony Vaio PCG-7112L Battery .
- Telescopic lattice masts are raised by means of a winch, which may be powered by hand or an electric motor. These tend to cater for greater heights and loads than the pump-up type. When retracted, the whole assembly can sometimes be lowered to a horizontal position by means of a second tiltover winch Sony Vaio PCG-7113L Battery .
- This enables antennas to be fitted and adjusted at ground level before winching the mast up.
Balloons and kites
A tethered balloon or a kite can serve as a temporary support. It can carry an antenna or a wire (for VLF, LW or MW) up to an appropriate height Sony Vaio PCG-7133L Battery .
Such an arrangement is used occasionally by military agencies or radio amateurs. The American broadcasters TV Martí broadcast a television program to Cuba by means of such a balloon.
Other special structures
For two VLF transmitters wire antennas spun across deep valleys are used Sony Vaio PCG-7Z2L Battery .
The wires are supported by small masts or towers or rock anchors. See List of spans: Antenna spans across valleys. The same technique was also used for the Criggion VLF transmitter.
For ELF transmitters ground dipole antennas are used. Such structures require no tall masts Sony Vaio PCG-8Y1L Battery .
They consist of two electrodes buried deep in the ground at least a few dozen kilometres apart. From the transmitter building to the electrodes, overhead feeder lines run. These lines look like power lines of the 10 kV level, and are installed on similar pylons Sony Vaio PCG-8Y2L Battery .
- The cost of a mast or tower is roughly proportional to the square of its height.
- A guyed mast is cheaper to build than a self-supporting tower of equal height Sony Vaio PCG-8Z1L Battery .
- A guyed mast needs additional land to accommodate the guys, and is thus best suited to rural locations where land is relatively cheap. An unguyed tower will fit into a much smaller plot.
- A steel lattice tower is cheaper to build than a concrete tower of equal height Sony Vaio PCG-8Z2L Battery .
- Two small towers may be less intrusive, visually, than one big one, especially if they look identical.
- Towers look less ugly if they and the antennas mounted on them appear symmetrical.
- Concrete towers can be built with aesthetic design - and they are, especially in Continental Europe SONY VAIO VGN-FW31J Battery .
- They are sometimes built in prominent places and include observation decks or restaurants.
Masts for HF/shortwave antennas
For transmissions in the shortwave range, there is little to be gained by raising the antenna more than a few wavelengths above ground level. Shortwave transmitters rarely use masts taller than about 100 metres SONY VAIO VGN-FW31E Battery .
Access for riggers
Because masts, towers and the antennas mounted on them require maintenance, access to the whole of the structure is necessary. Small structures are typically accessed with a ladder. Larger structures, which tend to require more frequent maintenance, may have stairs and sometimes a lift, also called a service elevator SONY VAIO VGN-FW31B Battery .
Aircraft warning features
Tall structures in excess of certain legislated heights are often equipped with aircraft warning lamps, usually red, to warn pilots of the structure's existence. In the past, ruggedized and under-run filament lamps were used to maximize the bulb life SONY VAIO VGN-FW31M Battery .
Alternatively, neon lamps were used. Nowadays such lamps tend to use LED arrays.
Height requirements vary across states and countries, and may include additional rules such as requiring a white flashing strobe in the daytime and pulsating red fixtures at night SONY VAIO VGN-FW31Z Battery .
Structures over a certain height may also be required to be painted with contrasting color schemes such as white and orange or white and red to make them more visible against the sky.
Light pollution and nuisance lighting
In some countries where light pollution is a concern, tower heights may be restricted so as to reduce or eliminate the need for aircraft warning lights SONY VAIO VGN-FW38M Battery .
For example in the United States the 1996 Telecommunications Act allows local jurisdictions to set maximum heights for towers, such as limiting tower height to below 200 feet and therefore not requiring aircraft illumination under U.S. Federal Communications Commission (FCC) rules SONY VAIO VGN-AW41JF Battery .
The limit is more commonly set to 190 or 180 feet to allow for masts extending above the tower.
One problem with radio masts is the danger of wind-induced oscillations. This is particularly a concern with steel tube construction. One can reduce this by building cylindrical shock-mounts into the construction SONY VAIO VGN-AW41JF/H Battery .
One finds such shock-mounts, which look like cylinders thicker than the mast, for example, at the radio masts of DHO38in Saterland. There are also constructions, which consist of a free-standing tower (usually from reinforced concrete), onto which a guyed radio mast is installed SONY VAIO VGN-AW41MF Battery .
The best known such construction is the Gerbrandy Tower in Lopik (the Netherlands). Further towers of this building method can be found near Smilde (the Netherlands) andFernsehturm, Waldenburg, Baden-Württemberg, Germany) SONY VAIO VGN-AW41MF/H Battery .
Hazard to birds
Radio, television and cell towers have been documented to pose a hazard to birds. Reports have been issued documenting known bird fatalities and calling for research to find ways to minimize the hazard that communications towers can pose to birds SONY VAIO VGN-FW30B Battery .