In physics, attenuation or, in some contexts, extinction is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, water and air attenuate both light and sound at variable attenuation rates. Hearing protectors help reduce acoustic flux from flowing into the ears; this phenomenon is measured in decibels. In electrical engineering and telecommunications, attenuation affects the propagation of waves and signals in electrical circuits, in optical fibers, in air. Electrical attenuators and optical attenuators are manufactured components in this field. In many cases, attenuation is an exponential function of the path length through the medium. In chemical spectroscopy, this is known as the Beer–Lambert law. In engineering, attenuation is measured in units of decibels per unit length of medium and is represented by the attenuation coefficient of the medium in question. Attenuation occurs in earthquakes. One area of research in which attenuation plays a prominent role, is in ultrasound physics.
Attenuation in ultrasound is the reduction in amplitude of the ultrasound beam as a function of distance through the imaging medium. Accounting for attenuation effects in ultrasound is important because a reduced signal amplitude can affect the quality of the image produced. By knowing the attenuation that an ultrasound beam experiences traveling through a medium, one can adjust the input signal amplitude to compensate for any loss of energy at the desired imaging depth. Ultrasound attenuation measurement in heterogeneous systems, like emulsions or colloids, yields information on particle size distribution. There is an ISO standard on this technique. Ultrasound attenuation can be used for extensional rheology measurement. There are acoustic rheometers that employ Stokes' law for measuring extensional viscosity and volume viscosity. Wave equations which take acoustic attenuation into account can be written on a fractional derivative form, see the article on acoustic attenuation or e.g. the survey paper.
Attenuation coefficients are used to quantify different media according to how the transmitted ultrasound amplitude decreases as a function of frequency. The attenuation coefficient can be used to determine total attenuation in dB in the medium using the following formula: Attenuation = α ⋅ ℓ ⋅ f Attenuation is linearly dependent on the medium length and attenuation coefficient, –approximately– on the frequency of the incident ultrasound beam for biological tissue. Attenuation coefficients vary for different media. In biomedical ultrasound imaging however, biological materials and water are the most used media; the attenuation coefficients of common biological materials at a frequency of 1 MHz are listed below: There are two general ways of acoustic energy losses: absorption and scattering, for instance light scattering. Ultrasound propagation through homogeneous media is associated only with absorption and can be characterized with absorption coefficient only. Propagation through heterogeneous media requires taking into account scattering.
Fractional derivative wave equations can be applied for modeling of lossy acoustical wave propagation, see acoustic attenuation and Ref. Main article: Electromagnetic absorption by waterShortwave radiation emitted from the sun have wavelengths in the visible spectrum of light that range from 360 nm to 750 nm; when the sun's radiation reaches the sea-surface, the shortwave radiation is attenuated by the water, the intensity of light decreases exponentially with water depth. The intensity of light at depth can be calculated using the Beer-Lambert Law. In clear open waters, visible light is absorbed at the longest wavelengths first. Thus, red and yellow wavelengths are absorbed at higher water depths, blue and violet wavelengths reach the deepest in the water column; because the blue and violet wavelengths are absorbed last compared to the other wavelengths, open ocean waters appear deep-blue to the eye. In near-shore waters, sea water contains more phytoplankton than the clear central ocean waters.
Chlorophyll-a pigments in the phytoplankton absorb light, the plants themselves scatter light, making coastal waters less clear than open waters. Chlorophyll-a absorbs light most in the shortest wavelengths of the visible spectrum. In near-shore waters where there are high concentrations of phytoplankton, the green wavelength reaches the deepest in the water column and the color of water to an observer appears green-blue or green; the energy with which an earthquake affects a location depends on the running distance. The attenuation in the signal of ground motion intensity plays an important role in the assessment of possible strong groundshaking. A seismic wave loses energy; this phenomenon is tied into the dispersion of the seismic energy with the distance. There are two types of dissipated energy: geometric dispersion caused by distribution of the seismic energy to greater volumes dispersion as heat called intrinsic attenuation or anelastic attenuat
In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction; some types of repeaters broadcast an identical signal, but alter its method of transmission, for example, on another frequency or baud rate. There are several different types of repeaters. A broadcast relay station is a repeater used in broadcast television; when an information-bearing signal passes through a communication channel, it is progressively degraded due to loss of power. For example, when a telephone call passes through a wire telephone line, some of the power in the electric current which represents the audio signal is dissipated as heat in the resistance of the copper wire; the longer the wire is, the more power is lost, the smaller the amplitude of the signal at the far end. So with a long enough wire the call will not be audible at the other end.
The farther from a radio station a receiver is, the weaker the radio signal, the poorer the reception. A repeater is an electronic device in a communication channel that increases the power of a signal and retransmits it, allowing it to travel further. Since it amplifies the signal, it requires a source of electric power; the term "repeater" originated with telegraphy in the 19th century, referred to an electromechanical device used to regenerate telegraph signals. Use of the term has continued in data communications. In computer networking, because repeaters work with the actual physical signal, do not attempt to interpret the data being transmitted, they operate on the physical layer, the first layer of the OSI model; this is used to increase the range of telephone signals in a telephone line. Land line repeaterThey are most used in trunklines that carry long distance calls. In an analog telephone line consisting of a pair of wires, it consists of an amplifier circuit made of transistors which use power from a DC current source to increase the power of the alternating current audio signal on the line.
Since the telephone is a duplex communication system, the wire pair carries two audio signals, one going in each direction. So telephone repeaters have to be bilateral, amplifying the signal in both directions without causing feedback, which complicates their design considerably. Telephone repeaters were the first type of repeater and were some of the first applications of amplification; the development of telephone repeaters between 1900 and 1915 made long distance phone service possible. Now, most telecommunications cables are fiber optic cables. Before the invention of electronic amplifiers, mechanically coupled carbon microphones were used as amplifiers in telephone repeaters. After the turn of the 20th century it was found that negative resistance mercury lamps could amplify, they were used; the invention of audion tube repeaters around 1916 made transcontinental telephony practical. In the 1930s vacuum tube repeaters using hybrid coils became commonplace, allowing the use of thinner wires.
In the 1950s negative impedance gain devices were more popular, a transistorized version called the E6 repeater was the final major type used in the Bell System before the low cost of digital transmission made all voiceband repeaters obsolete. Frequency frogging repeaters were commonplace in frequency-division multiplexing systems from the middle to late 20th century. Submarine cable repeaterThis is a type of telephone repeater used in underwater submarine telecommunications cables; this is used to increase the range of signals in a fiber optic cable. Digital information travels through a fiber optic cable in the form of short pulses of light; the light is made up of particles called photons, which can be scattered in the fiber. An optical communications repeater consists of a phototransistor which converts the light pulses to an electrical signal, an amplifier to increase the power of the signal, an electronic filter which reshapes the pulses, a laser which converts the electrical signal to light again and sends it out the other fiber.
However, optical amplifiers are being developed for repeaters to amplify the light itself without the need of converting it to an electric signal first. This is used to extend the range of coverage of a radio signal; the history of radio relay repeaters began in 1898 from the publication by Johann Mattausch in Austrian Journal Zeitschrift für Electrotechnik. But his proposal "Translator" was not suitable for use; the first relay system with radio repeaters, which functioned, was that invented in 1899 by Emile Guarini-Foresio. A radio repeater consists of a radio receiver connected to a radio transmitter; the received signal is amplified and retransmitted on another frequency, to provide coverage beyond the obstruction. Usage of a duplexer can allow the repeater to use one antenna for both receive and transmit at the same time. Broadcast relay station, rebroadcastor or translator: This is a repeater used to extend the coverage of a radio or television broadcasting station, it consists of a secondary television transmitter.
The signal from the main transmitter comes over leased telephone lines or by microwave relay. Microwave relay: This is a specialized point-to-point telecommunications link, consisting of a microwave receiver that receives information over a beam of microwaves from an
In radio-frequency engineering, a transmission line is a specialized cable or other structure designed to conduct alternating current of radio frequency, that is, currents with a frequency high enough that their wave nature must be taken into account. Transmission lines are used for purposes such as connecting radio transmitters and receivers with their antennas, distributing cable television signals, trunklines routing calls between telephone switching centres, computer network connections and high speed computer data buses; this article covers two-conductor transmission line such as parallel line, coaxial cable and microstrip. Some sources refer to waveguide, dielectric waveguide, optical fibre as transmission line, however these lines require different analytical techniques and so are not covered by this article. Ordinary electrical cables suffice to carry low frequency alternating current, such as mains power, which reverses direction 100 to 120 times per second, audio signals. However, they cannot be used to carry currents in the radio frequency range, above about 30 kHz, because the energy tends to radiate off the cable as radio waves, causing power losses.
Radio frequency currents tend to reflect from discontinuities in the cable such as connectors and joints, travel back down the cable toward the source. These reflections act as bottlenecks. Transmission lines use specialized construction, impedance matching, to carry electromagnetic signals with minimal reflections and power losses; the distinguishing feature of most transmission lines is that they have uniform cross sectional dimensions along their length, giving them a uniform impedance, called the characteristic impedance, to prevent reflections. Types of transmission line include parallel line, coaxial cable, planar transmission lines such as stripline and microstrip; the higher the frequency of electromagnetic waves moving through a given cable or medium, the shorter the wavelength of the waves. Transmission lines become necessary when the transmitted frequency's wavelength is sufficiently short that the length of the cable becomes a significant part of a wavelength. At microwave frequencies and above, power losses in transmission lines become excessive, waveguides are used instead, which function as "pipes" to confine and guide the electromagnetic waves.
Some sources define waveguides as a type of transmission line. At higher frequencies, in the terahertz and visible ranges, waveguides in turn become lossy, optical methods, are used to guide electromagnetic waves; the theory of sound wave propagation is similar mathematically to that of electromagnetic waves, so techniques from transmission line theory are used to build structures to conduct acoustic waves. Mathematical analysis of the behaviour of electrical transmission lines grew out of the work of James Clerk Maxwell, Lord Kelvin and Oliver Heaviside. In 1855 Lord Kelvin formulated a diffusion model of the current in a submarine cable; the model predicted the poor performance of the 1858 trans-Atlantic submarine telegraph cable. In 1885 Heaviside published the first papers that described his analysis of propagation in cables and the modern form of the telegrapher's equations. In many electric circuits, the length of the wires connecting the components can for the most part be ignored; that is, the voltage on the wire at a given time can be assumed to be the same at all points.
However, when the voltage changes in a time interval comparable to the time it takes for the signal to travel down the wire, the length becomes important and the wire must be treated as a transmission line. Stated another way, the length of the wire is important when the signal includes frequency components with corresponding wavelengths comparable to or less than the length of the wire. A common rule of thumb is that the cable or wire should be treated as a transmission line if the length is greater than 1/10 of the wavelength. At this length the phase delay and the interference of any reflections on the line become important and can lead to unpredictable behaviour in systems which have not been designed using transmission line theory. For the purposes of analysis, an electrical transmission line can be modelled as a two-port network, as follows: In the simplest case, the network is assumed to be linear, the two ports are assumed to be interchangeable. If the transmission line is uniform along its length its behaviour is described by a single parameter called the characteristic impedance, symbol Z0.
This is the ratio of the complex voltage of a given wave to the complex current of the same wave at any point on the line. Typical values of Z0 are 50 or 75 ohms for a coaxial cable, about 100 ohms for a twisted pair of wires, about 300 ohms for a common type of untwisted pair used in radio transmission; when sending power down a transmission line, it is desirable that as much power as possible will be absorbed by the load and as little as possible will be reflected back to the source. This can be ensured by making the load impedance equal to Z0, in which case the transmission line is said to be matched; some of the power, fed into a transmission line is lost because of its resistance. This effect is called resistive loss. At high frequencies, another effect cal
Frequency is the number of occurrences of a repeating event per unit of time. It is referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency; the period is the duration of time of one cycle in a repeating event, so the period is the reciprocal of the frequency. For example: if a newborn baby's heart beats at a frequency of 120 times a minute, its period—the time interval between beats—is half a second. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio signals, radio waves, light. For cyclical processes, such as rotation, oscillations, or waves, frequency is defined as a number of cycles per unit time. In physics and engineering disciplines, such as optics and radio, frequency is denoted by a Latin letter f or by the Greek letter ν or ν; the relation between the frequency and the period T of a repeating event or oscillation is given by f = 1 T.
The SI derived unit of frequency is the hertz, named after the German physicist Heinrich Hertz. One hertz means. If a TV has a refresh rate of 1 hertz the TV's screen will change its picture once a second. A previous name for this unit was cycles per second; the SI unit for period is the second. A traditional unit of measure used with rotating mechanical devices is revolutions per minute, abbreviated r/min or rpm. 60 rpm equals one hertz. As a matter of convenience and slower waves, such as ocean surface waves, tend to be described by wave period rather than frequency. Short and fast waves, like audio and radio, are described by their frequency instead of period; these used conversions are listed below: Angular frequency denoted by the Greek letter ω, is defined as the rate of change of angular displacement, θ, or the rate of change of the phase of a sinusoidal waveform, or as the rate of change of the argument to the sine function: y = sin = sin = sin d θ d t = ω = 2 π f Angular frequency is measured in radians per second but, for discrete-time signals, can be expressed as radians per sampling interval, a dimensionless quantity.
Angular frequency is larger than regular frequency by a factor of 2π. Spatial frequency is analogous to temporal frequency, but the time axis is replaced by one or more spatial displacement axes. E.g.: y = sin = sin d θ d x = k Wavenumber, k, is the spatial frequency analogue of angular temporal frequency and is measured in radians per meter. In the case of more than one spatial dimension, wavenumber is a vector quantity. For periodic waves in nondispersive media, frequency has an inverse relationship to the wavelength, λ. In dispersive media, the frequency f of a sinusoidal wave is equal to the phase velocity v of the wave divided by the wavelength λ of the wave: f = v λ. In the special case of electromagnetic waves moving through a vacuum v = c, where c is the speed of light in a vacuum, this expression becomes: f = c λ; when waves from a monochrome source travel from one medium to another, their frequency remains the same—only their wavelength and speed change. Measurement of frequency can done in the following ways, Calculating the frequency of a repeating event is accomplished by counting the number of times that event occurs within a specific time period dividing the count by the length of the time period.
For example, if 71 events occur within 15 seconds the frequency is: f = 71 15 s ≈ 4.73 Hz If the number of counts is not large, it is more accurate to measure the time interval for a predetermined number of occurrences, rather than the number of occurrences within a specified time. The latter method introduces a random error into the count of between zero and one count, so on average half a count; this is called gating error and causes an average error in the calculated frequency of Δ f = 1 2 T
Telecommunication is the transmission of signs, messages, writings and sounds or information of any nature by wire, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology, it is transmitted either electrically over physical media, such as cables, or via electromagnetic radiation. Such transmission paths are divided into communication channels which afford the advantages of multiplexing. Since the Latin term communicatio is considered the social process of information exchange, the term telecommunications is used in its plural form because it involves many different technologies. Early means of communicating over a distance included visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, optical heliographs. Other examples of pre-modern long-distance communication included audio messages such as coded drumbeats, lung-blown horns, loud whistles. 20th- and 21st-century technologies for long-distance communication involve electrical and electromagnetic technologies, such as telegraph and teleprinter, radio, microwave transmission, fiber optics, communications satellites.
A revolution in wireless communication began in the first decade of the 20th century with the pioneering developments in radio communications by Guglielmo Marconi, who won the Nobel Prize in Physics in 1909, other notable pioneering inventors and developers in the field of electrical and electronic telecommunications. These included Charles Wheatstone and Samuel Morse, Alexander Graham Bell, Edwin Armstrong and Lee de Forest, as well as Vladimir K. Zworykin, John Logie Baird and Philo Farnsworth; the word telecommunication is a compound of the Greek prefix tele, meaning distant, far off, or afar, the Latin communicare, meaning to share. Its modern use is adapted from the French, because its written use was recorded in 1904 by the French engineer and novelist Édouard Estaunié. Communication was first used as an English word in the late 14th century, it comes from Old French comunicacion, from Latin communicationem, noun of action from past participle stem of communicare "to share, divide out.
Homing pigeons have been used throughout history by different cultures. Pigeon post had Persian roots, was used by the Romans to aid their military. Frontinus said; the Greeks conveyed the names of the victors at the Olympic Games to various cities using homing pigeons. In the early 19th century, the Dutch government used the system in Sumatra, and in 1849, Paul Julius Reuter started a pigeon service to fly stock prices between Aachen and Brussels, a service that operated for a year until the gap in the telegraph link was closed. In the Middle Ages, chains of beacons were used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could only pass a single bit of information, so the meaning of the message such as "the enemy has been sighted" had to be agreed upon in advance. One notable instance of their use was during the Spanish Armada, when a beacon chain relayed a signal from Plymouth to London. In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system between Lille and Paris.
However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres. As a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880. On 25 July 1837 the first commercial electrical telegraph was demonstrated by English inventor Sir William Fothergill Cooke, English scientist Sir Charles Wheatstone. Both inventors viewed their device as "an improvement to the electromagnetic telegraph" not as a new device. Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837, his code was an important advance over Wheatstone's signaling method. The first transatlantic telegraph cable was completed on 27 July 1866, allowing transatlantic telecommunication for the first time; the conventional telephone was invented independently by Alexander Bell and Elisha Gray in 1876. Antonio Meucci invented the first device that allowed the electrical transmission of voice over a line in 1849.
However Meucci's device was of little practical value because it relied upon the electrophonic effect and thus required users to place the receiver in their mouth to "hear" what was being said. The first commercial telephone services were set-up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven and London. Starting in 1894, Italian inventor Guglielmo Marconi began developing a wireless communication using the newly discovered phenomenon of radio waves, showing by 1901 that they could be transmitted across the Atlantic Ocean; this was the start of wireless telegraphy by radio. Voice and music had little early success. World War I accelerated the development of radio for military communications. After the war, commercial radio AM broadcasting began in the 1920s and became an important mass medium for entertainment and news. World War II again accelerated development of radio for the wartime purposes of aircraft and land communication, radio navigation and radar. Development of stereo FM broadcasting of radio
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal, with a modulating signal that contains information to be transmitted. Most radio systems in the 20th century used frequency modulation or amplitude modulation for radio broadcast. A modulator is a device. A demodulator is a device that performs the inverse of modulation. A modem can perform both operations; the aim of analog modulation is to transfer an analog baseband signal, for example an audio signal or TV signal, over an analog bandpass channel at a different frequency, for example over a limited radio frequency band or a cable TV network channel. The aim of digital modulation is to transfer a digital bit stream over an analog communication channel, for example over the public switched telephone network or over a limited radio frequency band. Analog and digital modulation facilitate frequency division multiplexing, where several low pass information signals are transferred over the same shared physical medium, using separate passband channels.
The aim of digital baseband modulation methods known as line coding, is to transfer a digital bit stream over a baseband channel a non-filtered copper wire such as a serial bus or a wired local area network. The aim of pulse modulation methods is to transfer a narrowband analog signal, for example, a phone call over a wideband baseband channel or, in some of the schemes, as a bit stream over another digital transmission system. In music synthesizers, modulation may be used to synthesize waveforms with an extensive overtone spectrum using a small number of oscillators. In this case, the carrier frequency is in the same order or much lower than the modulating waveform. In analog modulation, the modulation is applied continuously in response to the analog information signal. Common analog modulation techniques include: Amplitude modulation Double-sideband modulation Double-sideband modulation with carrier Double-sideband suppressed-carrier transmission Double-sideband reduced carrier transmission Single-sideband modulation Single-sideband modulation with carrier Single-sideband modulation suppressed carrier modulation Vestigial sideband modulation Quadrature amplitude modulation Angle modulation, constant envelope Frequency modulation Phase modulation Transpositional Modulation, in which the waveform inflection is modified resulting in a signal where each quarter cycle is transposed in the modulation process.
TM is a pseudo-analog modulation. Where an AM carrier carries a phase variable phase f. TM is f. Digital modulation methods can be considered as digital-to-analog conversion and the corresponding demodulation or detection as analog-to-digital conversion; the changes in the carrier signal are chosen from a finite number of M alternative symbols. A simple example: A telephone line is designed for transferring audible sounds, for example and not digital bits. Computers may, communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols. If there are four alternative symbols, the first symbol may represent the bit sequence 00, the second 01, the third 10 and the fourth 11. If the modem plays a melody consisting of 1000 tones per second, the symbol rate is 1000 symbols/second, or 1000 baud. Since each tone represents a message consisting of two digital bits in this example, the bit rate is twice the symbol rate, i.e. 2000 bits per second. This is similar to the technique used by dial-up modems as opposed to DSL modems.
According to one definition of digital signal, the modulated signal is a digital signal. According to another definition, the modulation is a form of digital-to-analog conversion. Most textbooks would consider digital modulation schemes as a form of digital transmission, synonymous to data transmission; the most fundamental digital modulation techniques are based on keying: PSK: a finite number of phases are used. FSK: a finite number of frequencies are used. ASK: a finite number of amplitudes are used. QAM: a finite number of at least two phases and at least two amplitudes are used. In QAM, an in-phase signal and a quadrature phase signal are amplitude modulated with a finite number of amplitudes and summed, it can be seen as a two-channel system, each channel using ASK. The resulting signal is equivalent to a combination of PSK and ASK. In all of the above methods, each of these phases, frequencies or amplitudes are assigned a u
Microwave transmission is the transmission of information by microwave radio waves. Although an experimental 40-mile microwave telecommunication link across the English Channel was demonstrated in 1931, the development of radar in World War II provided the technology for practical exploitation of microwave communication. In the 1950s, large transcontinental microwave relay networks, consisting of chains of repeater stations linked by line-of-sight beams of microwaves were built in Europe and America to relay long distance telephone traffic and television programs between cities. Communication satellites which transferred data between ground stations by microwaves took over much long distance traffic in the 1960s. In recent years, there has been an explosive increase in use of the microwave spectrum by new telecommunication technologies such as wireless networks, direct-broadcast satellites which broadcast television and radio directly into consumers' homes. Microwaves are used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna.
This allows nearby microwave equipment to use the same frequencies without interfering with each other, as lower frequency radio waves do. Another advantage is that the high frequency of microwaves gives the microwave band a large information-carrying capacity. A disadvantage is. Microwave radio transmission is used in point-to-point communication systems on the surface of the Earth, in satellite communications, in deep space radio communications. Other parts of the microwave radio band are used for radars, radio navigation systems, sensor systems, radio astronomy; the next higher part of the radio electromagnetic spectrum, where the frequencies are above 30 GHz and below 100 GHz, are called "millimeter waves" because their wavelengths are conveniently measured in millimeters, their wavelengths range from 10 mm down to 3.0 mm. Radio waves in this band are strongly attenuated by the Earthly atmosphere and particles contained in it during wet weather. In a wide band of frequencies around 60 GHz, the radio waves are attenuated by molecular oxygen in the atmosphere.
The electronic technologies needed in the millimeter wave band are much more difficult to utilize than those of the microwave band. Wireless transmission of informationOne-way and two-way telecommunication using communications satellite Terrestrial microwave relay links in telecommunications networks including backbone or backhaul carriers in cellular networksWireless transmission of powerProposed systems e.g. for connecting solar power collecting satellites to terrestrial power grids Microwave radio relay is a technology used in the 1950s and 1960s for transmitting signals, such as long-distance telephone calls and television programs between two terrestrial points on a narrow beam of microwaves. In microwave radio relay, microwaves are transmitted on a line of sight path between relay stations using directional antennas, forming a fixed radio connection between the two points; the requirement of a line of sight limits the separation between stations to the visual horizon, about 30 to 50 miles.
Before the widespread use of communications satellites, chains of microwave relay stations were used to transmit telecommunication signals over transcontinental distances. Beginning in the 1950s, networks of microwave relay links, such as the AT&T Long Lines system in the U. S. carried long distance telephone calls and television programs between cities. The first system, dubbed TD-2 and built by AT&T, connected New York and Boston in 1947 with a series of eight radio relay stations; these included long daisy-chained series of such links that traversed mountain ranges and spanned continents. Much of the transcontinental traffic is now carried by cheaper optical fibers and communication satellites, but microwave relay remains important for shorter distances; because the radio waves travel in narrow beams confined to a line-of-sight path from one antenna to the other, they don't interfere with other microwave equipment, so nearby microwave links can use the same frequencies. Antennas must be directional.
Typical types of antenna used in radio relay link installations are parabolic antennas, dielectric lens, horn-reflector antennas, which have a diameter of up to 4 meters. Directive antennas permit an economical use of the available frequency spectrum, despite long transmission distances; because of the high frequencies used, a line-of-sight path between the stations is required. Additionally, in order to avoid attenuation of the beam, an area around the beam called the first Fresnel zone must be free from obstacles. Obstacles in the signal field cause unwanted attenuation. High mountain peak or ridge positions are ideal. Obstacles, the curvature of the Earth, the geography of the area and reception issues arising from the use of nearby land are important issues to consider when planning radio links. In the planning process, it is essential that "path profiles" are produced, which provide information about the terrain and Fresnel zones affecting the transmission path; the presence of a water surface, such as a lake or river, along the path must be ta