1.
Dipole antenna
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In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna. A dipole antenna consists of two identical conductive elements such as metal wires or rods, which are usually bilaterally symmetrical. The driving current from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna, each side of the feedline to the transmitter or receiver is connected to one of the conductors. This contrasts with an antenna, which consists of a single rod or conductor with one side of the feedline connected to it. A common example of a dipole is the rabbit ears television antenna found on broadcast television sets and this is the simplest type of antenna from a theoretical point of view. Dipoles are resonant antennas, meaning that the serve as resonators, with standing waves of radio current flowing back. So the length of the elements is determined by the wavelength of the radio waves used. The most common form is the dipole, in which each of the two rod elements is approximately 1/4 wavelength long, so the whole antenna is a half-wavelength long. The radiation pattern of a dipole is omnidirectional, it radiates equal power in all azimuthal directions perpendicular to the axis of the antenna. For a half-wave dipole the radiation is maximum,2.15 dBi perpendicular to the axis, falling monotonically with elevation angle to zero on the axis. Several different variations of the dipole are also used, such as the dipole, short dipole, cage dipole, bow-tie. The dipole was the earliest type of antenna, it was invented by German physicist Heinrich Hertz around 1886 in his investigations of radio waves. The feedpoint impedance of an antenna is very sensitive to its electrical length. Therefore, a dipole will generally only perform optimally over a narrow bandwidth. The real and imaginary components of impedance, as a function of electrical length, are shown in the accompanying graph. The detailed calculation of these numbers are described below, note that the value of the reactance is highly dependent on the diameter of the conductors, this plot is for conductors with a diameter of.001 wavelengths. Dipoles that are smaller than the wavelength of the signal are called short dipoles. These have a low radiation resistance making them inefficient antennas
2.
Loop antenna
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A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor with its ends connected to a balanced transmission line. Small loops have low resistance and thus poor efficiency and are mainly used as receiving antennas at low frequencies. To increase the field in the loop and thus the efficiency, the coil of wire is often wound around a ferrite rod magnetic core. These antennas are used for radio direction finding. In amateur radio, loop antennas are used for low profile operating where larger antennas would be inconvenient, unsightly. Loop antennas are easy to build. Resonant loop antennas are large, governed by the intended wavelength of operation. Thus they are used at higher frequencies, especially VHF and UHF. They can be viewed as a folded dipole deformed into a different shape, although a resonant loop may be in the shape of a circle, distorting it into a somewhat different closed shape does not greatly alter its characteristics. For instance, the quad antenna popular in amateur radio consists of a resonant loop in a rectangular shape, or a large loop can be completely collapsed into a line, in which case it is termed a folded dipole. In the case of large loops, such as the quad or a folded dipole, on the other hand, a small loop antenna is used for wavelengths much bigger than the loop itself, its radiation resistance and efficiency are instead dependent on the area enclosed by the loop. For a given area, the length of the conductor is minimized if the shape is a circle. Although it has a similar appearance, the so-called halo antenna is not technically a loop since it possesses a break in the conductor opposite the feed point. Its characteristics are unlike that of either sort of loop antenna described in this article, also outside the scope of this article is the use of coupling coils for inductive transmission systems including LF and HF RFID tags and readers. The large or resonant loop antenna can be seen as a folded dipole which has been reformed into a circle, in order to be resonant the loop requires a circumference approximately equal to one wavelength. Contrary to the loop antenna, this design radiates in the direction normal to the plane of the loop. Therefore, these loops are installed with the plane of the loop in the vertical direction. Compared to a dipole or folded dipole, it then transmits less toward the sky or ground, further directionality can be obtained by using a loop whose circumference is not one but 3 or 5 wavelengths
3.
Monopole antenna
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A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the end of the monopole and the ground plane. One side of the feedline is attached to the lower end of the monopole, and the other side is attached to the ground plane. This contrasts with an antenna which consists of two identical rod conductors, with the signal from the transmitter applied between the two halves of the antenna. The monopole is a resonant antenna, the rod functions as a resonator for radio waves, oscillating with standing waves of voltage. Therefore, the length of the antenna is determined by the wavelength of the waves it is used with. The most common form is the quarter-wave monopole, in which the antenna is approximately ¼ of the wavelength of the radio waves, the monopole antenna was invented in 1895 by radio pioneer Guglielmo Marconi, for this reason it is sometimes called the Marconi antenna. Common types of antenna are the whip, rubber ducky, helical, random wire, umbrella, inverted-L and T-antenna, inverted-F, mast radiator. The monopole antenna was invented in 1895 and patented 1896 by radio pioneer Guglielmo Marconi during his historic first experiments in radio communication and he began by using dipole antennas invented by Heinrich Hertz consisting of two identical horizontal wires ending in metal plates. For this reason the monopole is also called a Marconi antenna, like a dipole antenna, a monopole has an omnidirectional radiation pattern. It radiates vertically polarized radio waves, a monopole can be visualized as being formed by replacing the bottom half of a vertical dipole antenna with a conducting plane at right-angles to the remaining half. The general effect of small ground planes, as well as imperfectly conducting earth grounds, is to tilt the direction of maximum radiation up to higher elevation angles. This design is used for the mast radiator antennas employed in radio broadcasting at low frequencies, as well as low frequency antennas such as the T-antenna. At VHF and UHF frequencies the size of the ground plane needed is smaller, the most common antenna used in mobile phones is the inverted-F antenna, which is a variant of the inverted-L monopole. Bending over the antenna saves space and keeps the it within the bounds of the mobiles case, to improve the match the antenna is not fed from the end, rather some intermediate point, and the end is grounded instead. The quarter-wave whip and rubber ducky antennas used with handheld radios such as walkie-talkies and these dont use a ground plane, and the ground side of the transmitter is just connected to the ground connection on its circuit board. The hand and body of the person holding them may function as a ground plane. When used for broadcasting, the radio frequency power from the broadcasting transmitter is fed across the base insulator between the tower and a ground system
4.
Satellite dish
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A satellite dish is a dish-shaped type of parabolic antenna designed to receive electromagnetic signals from satellites, which transmit data transmissions or broadcasts, such as satellite television. The parabolic shape of a dish reflects the signal to the focal point. Mounted on brackets at the focal point is a device called a feedhorn. This feedhorn is essentially the front-end of a waveguide that gathers the signals at or near the focal point, the LNB converts the signals from electromagnetic or radio waves to electrical signals and shifts the signals from the downlinked C-band and/or Ku-band to the L-band range. Direct broadcast satellite dishes use an LNBF, which integrates the feedhorn with the LNB, the theoretical gain of a dish increases as the frequency increases. The actual gain depends on factors including surface finish, accuracy of shape. A typical value for a consumer type 60 cm satellite dish at 11.75 GHz is 37.50 dB, with lower frequencies, C-band for example, dish designers have a wider choice of materials. The large size of dish required for lower frequencies led to the dishes being constructed from metal mesh on a metal framework, at higher frequencies, mesh type designs are rarer though some designs have used a solid dish with perforations. A common misconception is that the LNBF, the device at the front of the dish, modern dishes intended for home television use are generally 43 cm to 80 cm in diameter, and are fixed in one position, for Ku-band reception from one orbital position. Prior to the existence of direct broadcast satellite services, home users would generally have a motorised C-band dish of up to 3 m in diameter for reception of channels from different satellites. Overly small dishes can still cause problems, however, including rain fade, in Europe, the frequencies used by DBS services are 10. 7–12.75 GHz on two polarisations H and V. This range is divided into a low band with 10. 7–11.7 GHz, and this results in two frequency bands, each with a bandwidth of about 1 GHz, each with two possible polarizations. In the LNB they become down converted to 950–2150 MHz, which is the range allocated for the satellite service on the coaxial cable between LNBF and receiver. Lower frequencies are allocated to cable and terrestrial TV, FM radio, in a single receiver residential installation there is a single coaxial cable running from the receiver set-top box in the building to the LNB on the dish. The DC electric power for the LNB is provided through the coaxial cable conductors that carry the signal to the receiver. In addition, control signals are transmitted from the receiver to the LNB through the cable. The receiver uses different power supply voltages to select antenna polarization, a satellite finder may aid in aiming. A dish that is mounted on a pole and driven by a motor or a servo can be controlled and rotated to face any satellite position in the sky
5.
Television antenna
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They can also be mounted in a loft or attic, where the dry conditions and increased elevation are advantageous for reception and antenna longevity. Outdoor antennas are more expensive and difficult to install, but are necessary for adequate reception in areas far from television stations. The most common types of antennas are the dipole and loop antennas, and for outdoor antennas the yagi, log periodic. To cover this range, antennas generally consist of multiple conductors of different lengths which correspond to the range the antenna is intended to receive. The length of the elements of a TV antenna are usually half the wavelength of the signal they are intended to receive, the wavelength of a signal equals the speed of light divided by the frequency. The design of a television broadcast receiving antenna is the same for the analog transmissions. Simple half-wave dipole antenna for VHF or UHF loop antennas that are made to be placed indoors are often used for television, these are often called rabbit ears or bunny aerials. The length of the ears can be adjusted by the user. These are not as efficient as a rooftop antenna since they are less directional. Dipole antennas are bi-directional, that is, they receive evenly forward and backwards and this makes them less efficient than antennas designed to maximize the signal from a narrower angle in one direction. These simple antennas are called set-top antennas because they were placed on top of the television set or receiver. The actual length of the ears is optimally about 91% of half the wavelength of the channel in free space. Quarter-wave television antennas are also used and these use a single element, and use the earth as a ground plane, therefore, no ground is required in the feed line. See also, Dipole antenna#Quarter-wave monopole Soon after television broadcasting switched from analog to digital broadcasting, RCA is one manufacturer which has commercially sold a flat antenna. Flat antennas are very lightweight, very thin, and square-shaped like a thin notebook and they connect to televisions, or to digital converter boxes, with a single coaxial cable, and may be sold with an optional signal amplifier. The amplifier must be plugged into a source, but the flat antenna does not require a power source. The flat antenna may need some moving around to achieve an optimum reception, an outdoor TV antenna generally consists of multiple conductive elements that are arranged such that it is a directional antenna. The length of the elements is one half of the signal wavelength
6.
Whip antenna
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A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the receiver or transmitter. The antenna is designed to be flexible so that it does not easily break, whip antennas for portable radios are often made of a series of interlocking telescoping metal tubes, so they can be retracted when not in use. Longer ones, made for mounting on vehicles and structures, are made of a fiberglass rod around a wire core. The most common type is the whip, which is approximately one-quarter of a wavelength long. Whips are the most common type of antenna, and are used in the HF, VHF and UHF radio bands. Larger versions mounted on roofs and radio masts are used as base station antennas for police, fire, ambulance, taxi, whip antennas 1/4 wavelength long or less have a single main lobe, with field strength maximum in horizontal directions, falling monotonically to zero on the axis. This is because they transmit equally well in all horizontal directions, whip antennas are normally designed as resonant antennas, the rod acts as a resonator for radio waves, with standing waves of voltage and current reflected back and forth from its ends. Therefore, the length of the rod is determined by the wavelength of the radio waves used. The most common length is approximately one-quarter of the wavelength, called a quarter-wave whip. If mounted above a ground plane, a quarter-wave whip has a gain twice that of a half wave dipole, or 5.19 dBi. However without a plane the gain is reduced and the radiation resistance increased. Whips mounted on vehicles use the skin of the vehicle as a ground plane. In hand-held devices usually no explicit ground plane is provided, therefore, the radio itself, and possibly the users hand, serves as a rudimentary ground plane. Since these are no larger than the size of the antenna itself and this type of antenna is called a ground plane antenna. To reduce the length of an antenna to make it less cumbersome. This allows the antenna to be much shorter than the normal length of a quarter-wavelength. The coil is added at the base of the whip or occasionally in the middle, in the most widely used form, the rubber ducky antenna, the loading coil is integrated with the antenna itself by making the whip out of a narrow helix of springy wire
7.
Balun
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A balun /ˈbælʌn/ is an electrical device that converts between a balanced signal and an unbalanced signal. A balun can take many forms and may include devices that also transform impedances, transformer baluns can also be used to connect lines of differing impedance. Sometimes, in the case of transformer baluns, they use magnetic coupling, common-mode chokes are also used as baluns and work by eliminating, rather than ignoring, common mode signals. The origin of the word balun is “balanced to unbalanced”, a variation of this device is the UNUN, which transfers signal from one unbalanced line to another. In classical transformers, there are two electrically separate windings of wire coils around the transformer’s core and this type is sometimes called a “voltage balun”. The ‘primary’ winding receives the signal, and the ‘secondary’ winding puts out the converted signal. The electrical signal in the coil is converted into a magnetic field in the transformer’s core. When the electrical current through the primary reverses, it causes the magnetic field to collapse. The collapsing magnetic field induces an electric field in the secondary winding. The ratio of loops in each winding and the efficiency of the magnetic coupling determines the ratio of electrical potential to electrical current. For idealized transformers, although the ratio of Voltage to Amperage will change in proportion to the square of the winding ratio. Generally a balun consists of two wires and a core, it converts the electrical energy of the primary wire into a magnetic field. Depending on how the secondary wire is done, the field is converted back to an electric field. An autotransformer balun has only one coil, or is made of two or more coils that have an electrical connection, the coil is typically wound on a ferrite rod or doughnut-shaped toroid. One can also make an autotransformer from a transformer by cross-wiring the primary and secondary windings. Baluns made with autotransformer windings are also called “Voltage baluns”, since they produce balanced output voltage, in all autotransformers, the single winding must have at least one extra electrical connection – called a tap or tap point – between the two ends of the winding. The current sent into the balun through one pair of connections acts as if it were a primary coil, electrical connections to parts of the coil different from the input connections have higher or lower voltages depending on the length of the coil that the output is tapped from. Unlike transformer-type baluns, autotransformer baluns connect all the connected to them to a single ground voltage level
8.
Coaxial cable
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Coaxial cable, or coax, is a type of cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Many coaxial cables also have an outer sheath or jacket. The term coaxial comes from the conductor and the outer shield sharing a geometric axis. Coaxial cable was invented by English engineer and mathematician Oliver Heaviside, Coaxial cable is used as a transmission line for radio frequency signals. Its applications include feedlines connecting radio transmitters and receivers with their antennas, computer network connections, digital audio and this allows coaxial cable runs to be installed next to metal objects such as gutters without the power losses that occur in other types of transmission lines. Coaxial cable also provides protection of the signal from external electromagnetic interference, the cable is protected by an outer insulating jacket. Normally, the shield is kept at ground potential and a signal carrying voltage is applied to the center conductor, the advantage of coaxial design is that electric and magnetic fields are restricted to the dielectric with little leakage outside the shield. Conversely, electric and magnetic fields outside the cable are largely kept from interfering with signals inside the cable, larger diameter cables and cables with multiple shields have less leakage. Common applications of coaxial cable include video and CATV distribution, RF and microwave transmission, the characteristic impedance of the cable is determined by the dielectric constant of the inner insulator and the radii of the inner and outer conductors. A controlled cable characteristic impedance is important because the source and load impedance should be matched to ensure maximum power transfer, other important properties of coaxial cable include attenuation as a function of frequency, voltage handling capability, and shield quality. Coaxial cable design choices affect physical size, frequency performance, attenuation, power handling capabilities, flexibility, strength, the inner conductor might be solid or stranded, stranded is more flexible. To get better performance, the inner conductor may be silver-plated. Copper-plated steel wire is used as an inner conductor for cable used in the cable TV industry. The insulator surrounding the conductor may be solid plastic, a foam plastic. The properties of control some electrical properties of the cable. A common choice is a solid polyethylene insulator, used in lower-loss cables, solid Teflon is also used as an insulator. Some coaxial lines use air and have spacers to keep the conductor from touching the shield. Many conventional coaxial cables use braided copper wire forming the shield and this allows the cable to be flexible, but it also means there are gaps in the shield layer, and the inner dimension of the shield varies slightly because the braid cannot be flat
9.
Low-noise block downconverter
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A low-noise block downconverter is the receiving device mounted on satellite dishes used for satellite TV reception, which collects the radio waves from the dish. Also called a block, low-noise converter, or even low-noise downconverter. The LNB is a combination of low-noise amplifier, frequency mixer, local oscillator and it receives the microwave signal from the satellite collected by the dish, amplifies it, and downconverts the block of frequencies to a lower block of intermediate frequencies. The LNB is usually a box suspended on one or more short booms, or feed arms, in front of the dish reflector. The microwave signal from the dish is picked up by a feedhorn on the LNB and is fed to a section of waveguide. One or more pins, or probes, protrude into the waveguide at right angles to the axis and act as antennas. The lower frequency IF output signal emerges from a socket on the box to which the cable connects. The LNB gets its power from the receiver or set-top box and this phantom power travels to the LNB, opposite to the signals from the LNB. A corresponding component, called a block upconverter, is used at the earth station dish to convert the band of television channels to the microwave uplink frequency. The signal received by the LNB is extremely weak and it has to be amplified before downconversion, the low noise amplifier section of the LNB amplifies this weak signal while adding the minimum possible amount of noise to the signal. The low-noise quality of an LNB is expressed as the noise figure and this is the signal to noise ratio at the input divided by the signal to noise ratio at the output. It is typically expressed as a decibels value, the ideal LNB, effectively a perfect amplifier, would have a noise figure of 0 dB and would not add any noise to the signal. Every LNB off the line has a different noise figure because of manufacturing tolerances. Satellites use comparatively high radio frequencies to transmit their TV signals, as microwave satellite signals do not easily pass through walls, roofs, or even glass windows, it is preferable for satellite antennas to be mounted outdoors. The purpose of the LNB is to use the principle to take a block of relatively high frequencies. These lower frequencies travel through cables with much less attenuation, so there is much more signal left at the receiver end of the cable. It is also easier and cheaper to design electronic circuits to operate at these lower frequencies. The local oscillator frequency determines what block of incoming frequencies is downconverted to the expected by the receiver
10.
Radio receiver
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In radio communications, a radio receiver is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna, the antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts the desired information. The information produced by the receiver may be in the form of sound, a radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. The most familiar form of radio receiver is a broadcast receiver, often just called a radio, the sound is produced either by a loudspeaker in the radio or an earphone which plugs into a jack on the radio. The radio requires electric power, provided either by batteries inside the radio or a cord which plugs into an electric outlet. All radios have a control to adjust the loudness of the audio. The frequency of radio stations is usually listed prominently in their advertising, in order to select a particular station to receive, the radio is adjusted to the frequency of the desired transmitter. In some radios this is done by the user turning a tuning knob until the station is heard in the radios loudspeaker. In most countries radio broadcasting is permitted using two different methods of modulation, that is, methods of adding the signal to the radio wave, AM and FM. In amplitude modulation the strength of the signal is varied by the audio signal. AM broadcasting is permitted on AM broadcast bands between 148 and 283 kHz in the low range and 526 and 1706 kHz in the medium frequency range of the radio spectrum. In frequency modulation the frequency of the signal is varied slightly by the audio signal. FM broadcasting is permitted in the FM broadcast bands between about 65 and 108 MHz in the high frequency range. The exact frequency ranges vary somewhat in different countries, radios sold in each receive the correct frequency range for that country. Most broadcast radios, called AM/FM radios, can receive both AM and FM bands, and have a switch to select which band to receive. Limited AM broadcasting is permitted in parts of the high frequency band called the shortwave band. Shortwave listening requires a receiver that can receive these bands, called a shortwave receiver, at these frequencies, radio waves transmitted into the sky can reflect from a layer in the atmosphere called the ionosphere, returning to earth at transcontinental distances. So shortwaves are used for long distance international broadcasting, often by governmental public service broadcasting organizations like the BBC, due to their lower frequency, AM radio stations can often be received at longer distances than FM stations, at hundreds of miles
11.
Antenna tuner
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Similar matching networks are used in other equipment to transform impedance. An antennas impedance is different at different frequencies, coupling through an ATU allows the use of one antenna on a broad range of frequencies. However, despite its name, an antenna ‘tuner ’ actually matches the transmitter only to the complex impedance reflected back to the end of the feedline. If both tuner and transmission line were lossless, tuning at the end would produce a match at every point in the transmitter/feedline/antenna system. With practical systems, feed line losses limit the ability of the antenna ‘tuner’ to match the antenna or change its resonant frequency, with lossy feedlines, maximum power transfer only occurs if matching is done at both ends of the line. Transformers, autotransformers, and baluns are sometimes incorporated into the design of narrow band antenna tuners, solid-state power amplifiers operating from 1–30 MHz typically use one or more wideband transformers wound on ferrite cores. MOSFETs and bipolar transistors are designed to operate into a low impedance, so the transformer primary typically has a single turn. This feedline system design has the advantage of reducing the retuning required when the frequency is changed. A similar design can match an antenna to a line, For example, many TV antennas have a 300 Ohm impedance. A small ferrite core transformer makes the broad band impedance transformation and this transformer does not need, nor is it capable of adjustment. For receive-only use in a TV the small SWR variation with frequency is not a major problem and it should be added that many ferrite based transformers perform a balanced to unbalanced transformation along with the impedance change. When the balanced to unbalanced function is present these transformers are called a balun, the most common baluns have either a 1,1 or a 1,4 impedance transformation. There are several designs for impedance matching using an autotransformer, which is a transformer with different connection points or taps spaced along the windings. When autotransformers connect balanced and unbalanced lines they are called baluns, when two differently-grounded cables or circuits must be connected but the grounds kept independent, a full, two-winding transformer with the desired ratio is used instead. The circuit pictured at the right has three identical windings wrapped in the direction around either an air core or ferrite core. The three equal windings shown are wired for a ground shared by two unbalanced lines, and can be used as 1,1,1,4, or 1,9 impedance match. Notice that on the side, the line with more windings measures greater impedance for the same 10 Ohm load on the right. The narrow-band methods described below cover a much smaller span of frequencies
12.
Antenna farm
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The history of the term antenna farm is uncertain, but it dates to at least the 1950s. In telecom circles, any area more than three antennas could be referred to as an antenna farm. In the case of an AM broadcasting station, the multiple mast radiators may all be part of a system for a single station. Alternatively, a tower with many separate antennas is often called a candelabra tower. This is not only for the security of the location. With a clearer line of sight in both cases, more signal can be received, multiple small towers also allow stations to have backup facilities co-located on each others towers for redundancy. Cleveland, Ohio has its antenna farm in the suburb of Parma, in addition, all three network affiliates and one of the 3 major radio groups have their studio facilities located within the Oklahoma City tower farm. In the Appalachian Mountains of the Eastern United States, Poor Mountain serves most of the FM, holston Mountain in upper East Tennessee is home to most of the FM and TV stations in the Tri-Cities DMA. Other examples are Signal Mountain near Chattanooga, Tennessee, Sharps Ridge in Knoxville, Tennessee, probably the most famous broadcast antenna farm of all is the World Trade Center Tower One, on which many of the New York City television and several FM stations had their antennas. All were lost when Twin Towers One and Two collapsed after the September 11 attacks in 2001, most of those stations now broadcast from their previous home,200 feet lower, on the Empire State Building. Antenna farms have often been the source of complaints from local neighborhoods and this has been increasingly so for TV stations, which have been pursuing with alacrity the construction of new digital television antennas. One such situation was in Colorado, in the late 1990s, many of the Denver metropolitan area TV stations already transmitted analog TV from Lookout Mountain, but needed the extra space for more antennas. Residents and the city of Golden filed legal objections, including challenging the authority of the Federal Communications Commission to override denial of zoning permits by local government, the city of Golden also sought to condemn the site, even though it was outside city limits. It was decided that scenic Eldorado Mountain near Boulder might be a better site, despite other alternatives, the new supertower was forced on Lookout Mountain by the U. S. Congress, allowing the existing towers to be removed in 2009 after analog shutdown. The site began operating in spring 2008
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This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).