An orthomode transducer is a waveguide component. It is referred to as a polarisation duplexer. Orthomode transducers serve either to combine or to separate two orthogonally polarized microwave signal paths. One of the paths forms the uplink, transmitted over the same waveguide as the received signal path, or downlink path; such a device may be part of a terrestrial microwave radio feed. For VSAT modems the transmission and reception paths are at 90° to each other, or in other words, the signals are orthogonally polarized with respect to each other; this orthogonal shift between the two signal paths provides an isolation of 40 dB in the Ku band and Ka band radio frequency bands. Hence this device serves in an essential role as the junction element of the outdoor unit of a VSAT modem, it protects the receiver front-end element from burn-out by the power of the output signal generated by the block up converter. The BUC is connected to the feed horn through a wave guide port of the OMT junction device.
Orthomode transducers are used in dual-polarized Very small aperture terminals, in sparsely populated areas, radar antennas and communications links. They are connected to the antenna's down converter or LNB and to the High Power Amplifier attached to a transmitting antenna. Wherever there are two polarizations of radio signals, the transmitted and received radio signal to and from the antenna are said to be “orthogonal”; this means that the modulation planes of the two radio signal waves are at 90 degrees angles to each other. The OMT device is used to separate two equal frequency signals, of low signal power. Protective separation is essential as the transmitter unit would damage the sensitive low micro-voltage, front-end receiver amplifier unit at the antenna; the transmission signal of the up-link, of high power originating from BUC, the low power received signal power coming from the antenna to the LNB receiver unit, in this case are at an angle of 90° relative to each other, are both coupled together at the feed-horn focal-point of the Parabolic antenna.
The device that unites both up-link and down-link paths, which are at 90° to each other, is known as an Orthogonal Mode Transducer OMT. In the VSAT Ku band of operation case, a typical OMT Orthomode Transducer provides a 40 dB isolation between each of the connected radio ports to the feed horn that faces the parabolic dish reflector; the port facing the parabolic reflector of the antenna is a circular polarizing port so that horizontal and vertical polarity coupling of inbound and outbound radio signal is achieved. The 40 dB isolation provides essential protection to the sensitive receiver amplifier against burn out from the high-power signal of the transmitter unit. Further isolation may be obtained by means of selective radio frequency filtering to achieve an isolation of 100 dB; the second image demonstrates two types of outdoor units, a 1-watt Hughes unit and a composite configuration of a 2-watt BUC/OMT/LNB Andrew, Swedish Microwave units. The following images show a Portenseigne & Hirschmann Ku band configuration, that highlights the horizontal the vertical, circular polarized wave-guide ports that join to the Feed-horn, the LNB or BUC elements of an outdoor unit.
An ortho-mode transducer is a component found on high capacity terrestrial microwave radio links. In this arrangement, two parabolic reflector dishes operate in a point to point microwave radio path with four radios, two mounted on each end. On each dish a T-shaped ortho-mode transducer is mounted at the rear of the feed, separating the signal from the feed into two separate radios, one operating in the horizontal polarity, the other in the vertical polarity; this arrangement is used to increase the aggregate data throughput between two dishes on a point to point microwave path, or for fault-tolerance redundancy. Certain types of outdoor microwave radios have integrated orthomode transducers and operate in both polarities from a single radio unit, performing cross-polarization interference cancellation within the radio unit itself. Alternatively, the orthomode transducer may be built into the antenna, allow connection of separate radios, or separate ports of the same radio, to the antenna. An ortho-mode transducer can be modelled as a 4-port device, 2 of these representing the single-polarization ports and the remaining embodied by the degenerate modes in the dual-polarized port.
The scattering parameters can be gathered in a 4×4 scattering matrix S, symmetrical for a reciprocal OMT, thus leaving 10 independent terms for a general lossy device: S = [ S H H S H V S H h S H v S H V S
Coaxial cable, or coax is a type of electrical cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Many coaxial cables have an insulating outer sheath or jacket; the term coaxial comes from the outer shield sharing a geometric axis. Coaxial cable was invented by English engineer and mathematician Oliver Heaviside, who patented the design in 1880. Coaxial cable is a type of transmission line, used to carry high frequency electrical signals with low losses, it is used in such applications as telephone trunklines, broadband internet networking cables, high speed computer data busses, carrying cable television signals, connecting radio transmitters and receivers to their antennas. It differs from other shielded cables because the dimensions of the cable and connectors are controlled to give a precise, constant conductor spacing, needed for it to function efficiently as a transmission line. Coaxial cable is used as a transmission line for radio frequency signals.
Its applications include feedlines connecting radio transmitters and receivers to their antennas, computer network connections, digital audio, distribution of cable television signals. One advantage of coaxial over other types of radio transmission line is that in an ideal coaxial cable the electromagnetic field carrying the signal exists only in the space between the inner and outer conductors; 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 provides protection of the signal from external electromagnetic interference. Coaxial cable conducts electrical signal using an inner conductor surrounded by an insulating layer and all enclosed by a shield one to four layers of woven metallic braid and metallic tape; the cable is protected by an outer insulating jacket. 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 and magnetic fields outside the cable are kept from interfering with signals inside the cable. Larger diameter cables and cables with multiple shields have less leakage; this property makes coaxial cable a good choice for carrying weak signals that cannot tolerate interference from the environment or for stronger electrical signals that must not be allowed to radiate or couple into adjacent structures or circuits. Common applications of coaxial cable include video and CATV distribution, RF and microwave transmission, computer and instrumentation data connections; 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. In radio frequency systems, where the cable length is comparable to the wavelength of the signals transmitted, a uniform cable characteristic impedance is important to minimize loss; the source and load impedances are chosen to match the impedance of the cable to ensure maximum power transfer and minimum standing wave ratio.
Other important properties of coaxial cable include attenuation as a function of frequency, voltage handling capability, shield quality. Coaxial cable design choices affect physical size, frequency performance, power handling capabilities, flexibility and cost; the inner conductor might be stranded. To get better high-frequency 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 inner conductor may be solid plastic, a foam plastic, or air with spacers supporting the inner wire. The properties of the dielectric insulator determine some of the electrical properties of the cable. A common choice is a solid polyethylene insulator, used in lower-loss cables. Solid Teflon is used as an insulator; some coaxial lines have spacers to keep the inner conductor from touching the shield. Many conventional coaxial cables use braided copper wire forming the shield; this allows the cable to be flexible, but it means there are gaps in the shield layer, the inner dimension of the shield varies because the braid cannot be flat.
Sometimes the braid is silver-plated. For better shield performance, some cables have a double-layer shield; the shield might be just two braids, but it is more common now to have a thin foil shield covered by a wire braid. Some cables may invest in more than two shield layers, such as "quad-shield", which uses four alternating layers of foil and braid. Other shield designs sacrifice flexibility for better performance; those cables cannot be bent as the shield will kink, causing losses in the cable. When a foil shield is used a small wire conductor incorporated into the foil makes soldering the shield termination easier. For high-power radio-frequency transmission up to about 1 GHz, coaxial cable with a solid copper outer conductor is available in sizes of 0.25 inch upward. The outer conductor is corrugated like a bellows to permit flexibility and the inner conductor is held in position by a plastic spiral to approximate an air dielectric. One brand name for such cable is Heliax. Coaxial cables require an internal structure of an insulating material to maintain the spacing between the center conductor and shield.
In telecommunications and electronics, an antenna feed refers to several different parts of an antenna system: The antenna feed is the wire or cabling that connects between the antenna and the radio called the feed line. The antenna feed is the location on the antenna where the feedline from the receiver or transmitter connects or attaches; the antenna feed is the matching system at the attachment point that converts the feedline impedance to the antenna’s intrinsic impedance, makes any balanced-to-unbalanced conversion. In a transmitting antenna system the term can refer to any one or all of the components involved conveying the RF electrical current into the radiating part of the antenna, where the current is converted to radiation; because of the several meanings, “antenna feed system” is used to refer to all of the parts of the antenna feed between the radio and the radiator. Simple antennas, such as monopole or whip antennas, dipole antennas, large loop antennas are directly connected to a feedline cable that matches the impedance of the antenna and the radio.
Compound antennas are made of multiple simple antennas, similar to the way that compound lenses are made of several simple lenses. Examples of compound antennas are array antennas, Yagi-Uda antennas, log periodic antennas, quad antennas, small loop antennas. One of the simple sub-antennas, part of a compound antenna is a feeder antenna called the “driven element”: The driven element converts the RF electrical currents to free space radio waves, or vice versa, it radiates the signal into the space nearby the other elements of the compound antenna, which in turn absorb and re-radiate the signal. Those elements are called “passive” or “parasitic” elements and they re-radiate the radio waves they absorb in the form of a beam in the desired direction; the passive elements function as reflecting and directing structures in the same way that mirrors and focusing lenses function in compound lenses. For example, in a rooftop Yagi-Uda television antenna, the feed consists of a dipole driven element, which converts the radio waves to an electric current, a coaxial cable or twin lead transmission line which conducts the received signal from the driven element into the house to the television receiver.
The rest of the antenna consists of rods called parasitic elements, which strengthen reception from a given direction. In more complex antenna systems the feed can be more complicated; the term “antenna feed system” refers to all of the components between the beam-shaping part of the antenna and the receiver's first amplifier. For a transmitting antenna, the feed system consists of everything after the last power amplifier, might include an antenna tuner unit near the amplifier, it certainly includes any impedance matching sections adjacent to, or incorporated into the structure of the antenna. In a radar or satellite communications antenna the feed might consist of a feed horn, orthomode transducer, frequency diplexer, waveguide switches, rotary joint, etc. With a transmitting antenna, the antenna feed is a critical component that must be adjusted to function compatibly with the antenna and transmitter; each type of transmission line and each type of antenna has a specific characteristic impedance, the ratio of voltage to current, the “favorite” of the antenna, or line, or radio.
The impedances of radios and feedlines are constant. Antenna impedances, swing by factors well over 1 000: 1 , with changing frequency, as the antenna passes through an evenly-spaced sequence of resonances and “anti-”resonances at different frequencies; the line impedance must be matched to the impedance of the antenna at one end and the transmitter at the other to efficiently transfer power between the transmitter and its antenna. If the impedances at either end of the line do not match, it will cause a condition called “standing waves" on the feed line, in which the RF energy is reflected back toward the transmitter, wasting energy and overheating the transmitter; the impedance is matched through a device called an antenna tuner or matching network, which can be in the transmitter, next to the transmitter, near the antenna, on the antenna, or any combination, including none. The degree of mismatch between the feedline and the antenna is measured by an instrument called an SWR meter, which measures the standing wave ratio on the line: The ratio of the adjacent maximum and minimum voltage amplitudes, or adjacent maximum and minimum current amplitudes.
Standing wave ratio Antenna tuner Silver, H. Ward, ed.. The ARRL Antenna Book. Newington, CT: American Radio Relay League. ISBN 978-0-87259-680-1
Spatial multiplexing is a transmission technique in MIMO wireless communication, Fibre-optic communication and other communications technologies to transmit independent and separately encoded data signals, known as "streams". Therefore, the space dimension is multiplexed, more than one time. If the transmitter is equipped with N t antennas and the receiver has N r antennas, the maximum spatial multiplexing order is, N s = min if a linear receiver is used; this means that N s streams can be transmitted in parallel, ideally leading to an N s increase of the spectral efficiency. The practical multiplexing gain can be limited by spatial correlation, which means that some of the parallel streams may have weak channel gains. In an open-loop MIMO system with N t transmitter antennas and N r receiver antennas, the input-output relationship can be described as y = H x + n where x = T is the N t × 1 vector of transmitted symbols, y, n are the N r × 1 vectors of received symbols and noise and H is the N r × N t matrix of channel coefficients.
An encountered problem in open loop spatial multiplexing is to guard against instance of high channel correlation and strong power imbalances between the multiple streams. One such extension, being considered for DVB-NGH systems is the so-called enhanced Spatial Multiplexing scheme. A closed-loop MIMO system utilizes Channel State Information at the transmitter. In most cases, only partial CSI is available at the transmitter because of the limitations of the feedback channel. In a closed-loop MIMO system the input-output relationship with a closed-loop approach can be described as y = H W s + n where s = T is the N s × 1 vector of transmitted symbols, y, n are the N r × 1 vectors of received symbols and noise H is the N r × N t matrix of channel coefficients and W is the N t × N s linear precoding matrix. A precoding matrix W is used to precode the symbols in the vector to enhance the performance; the column dimension N s of W can be selected smaller than N t, useful if the system requires N s streams because of several reasons.
Examples of the reasons are as follows: either the rank of the MIMO channel or the number of receiver antennas is smaller than the number of transmit antennas. Space–time code Space–time trellis code WiMAX MIMO 3G MIMO Fibre-optic communications
XPIC, or cross-polarization interference cancelling technology, is an algorithm to suppress mutual interference between two received streams in a Polarization-division multiplexing communication system. The cross-polarization interference canceller is a signal processing technique implemented on the demodulated received signals at the baseband level, it is necessary in Polarization Division Multiplexing systems: the data sources to be transmitted are coded and mapped into QAM modulating symbols at the system's symbol rate and upconverted to a carrier frequency, generating two radio streams radiated by a single dual-polarized antenna. A corresponding dual-polarized antenna is located at the remote site and connected to two receivers, which downconvert the radio streams into baseband signals; this multiplexing/demultiplexing technique is based on the expected discrimination between the two orthogonal polarizations: an ideal, infinite XPD of the whole system guarantees that each signal at the receivers contains only the signal generated by the corresponding transmitter.
Some of the factors causing such cross-polarization interference are listed in Polarization-Division Multiplexing. As a practical consequence, at the receiving site the two streams are received with a residual mutual interference. In many practical cases for high-level M-QAM modulations, the communication system cannot tolerate the experienced levels of cross-polarization interference and an improved suppression is necessary; the two received polarizations at the antenna outputs linear horizontal H and vertical V, are routed each to a receiver whose baseband output is further processed by an ad-hoc cross-polarization cancelling scheme implemented as a digital stage. The XPIC algorithm attains the correct reconstruction of H by summing V to H to cancel any residual interference, vice versa; the cancelling process is implemented using two blocks: a baseband equalizer and the baseband XPIC. The output from the latter is subtracted from the former and sent to the decision stage, responsible for yielding the estimation of the data stream.
The equalization and XPIC blocks are adaptive for a correct tracking of the time-variant channel transfer function: XPIC must provide a shaping of the received cross signal equal to the portion of the cross interference affecting the main one. The feedback control to drive the adapting criteria comes from the measure of the residual error across the decision block. In the example, both blocks are based on the typical structure of the Finite Impulse Response digital filter and whose the coefficients are not fixed, but adapted to minimize a suitable functional J while multiple delays D act on the input signal. Given: ϵ k: residual complex error at time instant k, s k: baseband main received signal complex sample at time instant k, s k: baseband cross received signal complex sample at time instant k, C j, k: complex coefficient of baseband equalizer on the tap j and time instant k, C j, k: XPIC complex coefficient on tap j and time instant k, j = − n... N: index of tap y k: result of cancelling action feeding the decision device at time instant k, d k: estimated transmitted data at time instant k, so ϵ k = y k - d k γ: step-size or compression factor for adaptativity,if the function to minimize is for example the mean power the residual error, J = E, the adapting gradient algorithm prescribes that the coefficients are updated after every time step k as: C j, k + 1 = C j, k − γ ϵ k ∗.
Polarization is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. A simple example of a polarized transverse wave is vibrations traveling along a taut string. Depending on how the string is plucked, the vibrations can be in a vertical direction, horizontal direction, or at any angle perpendicular to the string. In contrast, in longitudinal waves, such as sound waves in a liquid or gas, the displacement of the particles in the oscillation is always in the direction of propagation, so these waves do not exhibit polarization. Transverse waves that exhibit polarization include electromagnetic waves such as light and radio waves, gravitational waves, transverse sound waves in solids. In some types of transverse waves, the wave displacement is limited to a single direction, so these do not exhibit polarization. An electromagnetic wave such as light consists of a coupled oscillating electric field and magnetic field which are always perpendicular.
In linear polarization, the fields oscillate in a single direction. In circular or elliptical polarization, the fields rotate at a constant rate in a plane as the wave travels; the rotation can have two possible directions. Light or other electromagnetic radiation from many sources, such as the sun and incandescent lamps, consists of short wave trains with an equal mixture of polarizations. Polarized light can be produced by passing unpolarized light through a polarizer, which allows waves of only one polarization to pass through; the most common optical materials are isotropic and do not affect the polarization of light passing through them. Some of these are used to make polarizing filters. Light is partially polarized when it reflects from a surface. According to quantum mechanics, electromagnetic waves can be viewed as streams of particles called photons; when viewed in this way, the polarization of an electromagnetic wave is determined by a quantum mechanical property of photons called their spin.
A photon has one of two possible spins: it can either spin in a right hand sense or a left hand sense about its direction of travel. Circularly polarized electromagnetic waves are composed of photons with only one type of spin, either right- or left-hand. Linearly polarized waves consist of photons that are in a superposition of right and left circularly polarized states, with equal amplitude and phases synchronized to give oscillation in a plane. Polarization is an important parameter in areas of science dealing with transverse waves, such as optics, seismology and microwaves. Impacted are technologies such as lasers and optical fiber telecommunications, radar. Most sources of light are classified as incoherent and unpolarized because they consist of a random mixture of waves having different spatial characteristics, frequencies and polarization states. However, for understanding electromagnetic waves and polarization in particular, it is easiest to just consider coherent plane waves. Characterizing an optical system in relation to a plane wave with those given parameters can be used to predict its response to a more general case, since a wave with any specified spatial structure can be decomposed into a combination of plane waves.
And incoherent states can be modeled stochastically as a weighted combination of such uncorrelated waves with some distribution of frequencies and polarizations. Electromagnetic waves, traveling in free space or another homogeneous isotropic non-attenuating medium, are properly described as transverse waves, meaning that a plane wave's electric field vector E and magnetic field H are in directions perpendicular to the direction of wave propagation. By convention, the "polarization" direction of an electromagnetic wave is given by its electric field vector. Considering a monochromatic plane wave of optical frequency f, let us take the direction of propagation as the z axis. Being a transverse wave the E and H fields must contain components only in the x and y directions whereas Ez = Hz = 0. Using complex notation, the instantaneous physical electric and magnetic fields are given by the real parts of the complex quantities occurring in the following equations; as a function of time t and spatial position z these complex fields can be written as: E → =
An optical fiber is a flexible, transparent fiber made by drawing glass or plastic to a diameter thicker than that of a human hair. Optical fibers are used most as a means to transmit light between the two ends of the fiber and find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths than electrical cables. Fibers are used instead of metal wires. Fibers are used for illumination and imaging, are wrapped in bundles so they may be used to carry light into, or images out of confined spaces, as in the case of a fiberscope. Specially designed fibers are used for a variety of other applications, some of them being fiber optic sensors and fiber lasers. Optical fibers include a core surrounded by a transparent cladding material with a lower index of refraction. Light is kept in the core by the phenomenon of total internal reflection which causes the fiber to act as a waveguide. Fibers that support many propagation paths or transverse modes are called multi-mode fibers, while those that support a single mode are called single-mode fibers.
Multi-mode fibers have a wider core diameter and are used for short-distance communication links and for applications where high power must be transmitted. Single-mode fibers are used for most communication links longer than 1,000 meters. Being able to join optical fibers with low loss is important in fiber optic communication; this is more complex than joining electrical wire or cable and involves careful cleaving of the fibers, precise alignment of the fiber cores, the coupling of these aligned cores. For applications that demand a permanent connection a fusion splice is common. In this technique, an electric arc is used to melt the ends of the fibers together. Another common technique is a mechanical splice, where the ends of the fibers are held in contact by mechanical force. Temporary or semi-permanent connections are made by means of specialized optical fiber connectors; the field of applied science and engineering concerned with the design and application of optical fibers is known as fiber optics.
The term was coined by Indian physicist Narinder Singh Kapany, acknowledged as the father of fiber optics. Guiding of light by refraction, the principle that makes fiber optics possible, was first demonstrated by Daniel Colladon and Jacques Babinet in Paris in the early 1840s. John Tyndall included a demonstration of it in his public lectures in London, 12 years later. Tyndall wrote about the property of total internal reflection in an introductory book about the nature of light in 1870:When the light passes from air into water, the refracted ray is bent towards the perpendicular... When the ray passes from water to air it is bent from the perpendicular... If the angle which the ray in water encloses with the perpendicular to the surface be greater than 48 degrees, the ray will not quit the water at all: it will be reflected at the surface.... The angle which marks the limit where total reflection begins is called the limiting angle of the medium. For water this angle is 48°27′, for flint glass it is 38°41′, while for diamond it is 23°42′.
In the late 19th and early 20th centuries, light was guided through bent glass rods to illuminate body cavities. Practical applications such as close internal illumination during dentistry appeared early in the twentieth century. Image transmission through tubes was demonstrated independently by the radio experimenter Clarence Hansell and the television pioneer John Logie Baird in the 1920s. In the 1930s, Heinrich Lamm showed that one could transmit images through a bundle of unclad optical fibers and used it for internal medical examinations, but his work was forgotten. In 1953, Dutch scientist Bram van Heel first demonstrated image transmission through bundles of optical fibers with a transparent cladding; that same year, Harold Hopkins and Narinder Singh Kapany at Imperial College in London succeeded in making image-transmitting bundles with over 10,000 fibers, subsequently achieved image transmission through a 75 cm long bundle which combined several thousand fibers. Their article titled "A flexible fibrescope, using static scanning" was published in the journal Nature in 1954.
The first practical fiber optic semi-flexible gastroscope was patented by Basil Hirschowitz, C. Wilbur Peters, Lawrence E. Curtiss, researchers at the University of Michigan, in 1956. In the process of developing the gastroscope, Curtiss produced the first glass-clad fibers. A variety of other image transmission applications soon followed. Kapany coined the term fiber optics, wrote a 1960 article in Scientific American that introduced the topic to a wide audience, wrote the first book about the new field; the first working fiber-optical data transmission system was demonstrated by German physicist Manfred Börner at Telefunken Research Labs in Ulm in 1965, followed by the first patent application for this technology in 1966. NASA used fiber optics in the television cameras. At the time, the use in the cameras was classified confidential, employees handling the cameras had to be supervised by someone with an appropriate security clearance. Charles K. Kao and George A. Hockham of the British company Standard Telephones and Cables were the first, in 1965, to promote the idea that the attenuation in optical fibers could be reduced below 20 decibels per kilometer, making fibers a practical communication medium.
They proposed th