In radio-frequency applications up to a few gigahertz, the wave propagates in the transverse electric and magnetic mode TEM only, which means that the electric and magnetic fields are both perpendicular to the direction of propagation the electric field is radial, and the magnetic field is circumferential. However, at frequencies for which the wavelength in the dielectric is significantly shorter than the circumference of the cable other transverse modes can propagate. These modes are classified into two groups, transverse electric TE and transverse magnetic TM waveguide modes.
When more than one mode can exist, bends and other irregularities in the cable geometry can cause power to be transferred from one mode to another. The most common use for coaxial cables is for television and other signals with bandwidth of multiple megahertz. A microstrip circuit uses a thin flat conductor which is parallel to a ground plane.
Microstrip can be made by having a strip of copper on one side of a printed circuit board PCB or ceramic substrate while the other side is a continuous ground plane. The width of the strip, the thickness of the insulating layer PCB or ceramic and the dielectric constant of the insulating layer determine the characteristic impedance.
Microstrip is an open structure whereas coaxial cable is a closed structure. A stripline circuit uses a flat strip of metal which is sandwiched between two parallel ground planes. The insulating material of the substrate forms a dielectric. The width of the strip, the thickness of the substrate and the relative permittivity of the substrate determine the characteristic impedance of the strip which is a transmission line. A coplanar waveguide consists of a center strip and two adjacent outer conductors, all three of them flat structures that are deposited onto the same insulating substrate and thus are located in the same plane "coplanar".
The width of the center conductor, the distance between inner and outer conductors, and the relative permittivity of the substrate determine the characteristic impedance of the coplanar transmission line. A balanced line is a transmission line consisting of two conductors of the same type, and equal impedance to ground and other circuits.
There are many formats of balanced lines, amongst the most common are twisted pair, star quad and twin-lead. Twisted pairs are commonly used for terrestrial telephone communications. In such cables, many pairs are grouped together in a single cable, from two to several thousand. Star quad is a four-conductor cable in which all four conductors are twisted together around the cable axis. It is sometimes used for two circuits, such as 4-wire telephony and other telecommunications applications. In this configuration each pair uses two non-adjacent conductors.
Other times it is used for a single, balanced line , such as audio applications and 2-wire telephony. In this configuration two non-adjacent conductors are terminated together at both ends of the cable, and the other two conductors are also terminated together. When used for two circuits, crosstalk is reduced relative to cables with two separate twisted pairs. When used for a single, balanced line , magnetic interference picked up by the cable arrives as a virtually perfect common mode signal, which is easily removed by coupling transformers.
The combined benefits of twisting, balanced signalling, and quadrupole pattern give outstanding noise immunity, especially advantageous for low signal level applications such as microphone cables, even when installed very close to a power cable. High capacitance causes increasing distortion and greater loss of high frequencies as distance increases. Twin-lead consists of a pair of conductors held apart by a continuous insulator. By holding the conductors a known distance apart, the geometry is fixed and the line characteristics are reliably consistent.
It is lower loss than coaxial cable because the characteristic impedance of twin-lead is generally higher than coaxial cable, leading to lower resistive losses due to the reduced current. However, it is more susceptible to interference. Lecher lines are a form of parallel conductor that can be used at UHF for creating resonant circuits.
Unbalanced lines were formerly much used for telegraph transmission, but this form of communication has now fallen into disuse. Cables are similar to twisted pair in that many cores are bundled into the same cable but only one conductor is provided per circuit and there is no twisting. All the circuits on the same route use a common path for the return current earth return. There is a power transmission version of single-wire earth return in use in many locations. Electrical transmission lines are very widely used to transmit high frequency signals over long or short distances with minimum power loss.
One familiar example is the down lead from a TV or radio aerial to the receiver. Transmission lines are also used as pulse generators. By charging the transmission line and then discharging it into a resistive load, a rectangular pulse equal in length to twice the electrical length of the line can be obtained, although with half the voltage. A Blumlein transmission line is a related pulse forming device that overcomes this limitation.
These are sometimes used as the pulsed power sources for radar transmitters and other devices. If a short-circuited or open-circuited transmission line is wired in parallel with a line used to transfer signals from point A to point B, then it will function as a filter. The method for making stubs is similar to the method for using Lecher lines for crude frequency measurement, but it is 'working backwards'. One method recommended in the RSGB 's radiocommunication handbook is to take an open-circuited length of transmission line wired in parallel with the feeder delivering signals from an aerial.
By cutting the free end of the transmission line, a minimum in the strength of the signal observed at a receiver can be found. At this stage the stub filter will reject this frequency and the odd harmonics, but if the free end of the stub is shorted then the stub will become a filter rejecting the even harmonics.
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Part of this article was derived from Federal Standard C. From Wikipedia, the free encyclopedia. This article is about the radio-frequency component. For the movement of electrical energy, see Electric power transmission. For the usage in acoustics, see Acoustic transmission line. Main article: Telegrapher's equations. See also: Reflections on copper lines. Planar transmission line. Coplanar waveguide. Balanced line. Twisted pair.
Star quad cable. Lecher lines. Head Exam PW Krishnamurthy Electromagnetic Field Theory Fundamentals, 2nd Ed. Cambridge Univ. Electromagnetics Explained: Artech House. Journal of Magnetic Resonance. Ahamed, Victor B. Lawrence, Design and engineering of intelligent communication systems , pp. Eargle's The Microphone Book: Focal Press. Coaxial cable Fiber-optic communication Optical fiber Free-space optical communication Molecular communication Radio waves Transmission line. Space-division Frequency-division Time-division Polarization-division Orbital angular-momentum Code-division. The anti-sidetone circuit accomplishes this reduction by interposing a transformer between the transmitter circuit and the receiver circuit and by splitting the transmitter signals along two paths.
When the divided signals, having opposite polarities, meet at the transformer, they almost entirely cancel each other in crossing to the receiver circuit. The speech signal coming from the other end of the line, on the other hand, arrives at the transformer along a single, undivided path and crosses the transformer unimpeded. Impedance matching allows a more efficient flow of current through the system. Beginning in the early 19th century, several inventors made a number of attempts to transmit sound by electric means. The first inventor to suggest that sound could be transmitted electrically was a Frenchman, Charles Bourseul, who indicated that a diaphragm making and breaking contact with an electrode might be used for this purpose.
In the s Italian American inventor Antonio Meucci had electrical devices in his home called telettrofoni that he used to communicate between rooms, though he did not patent his inventions. By Johann Philipp Reis of Germany had designed several instruments for the transmission of sound. The transmitter Reis employed consisted of a membrane with a metallic strip that would intermittently contact a metallic point connected to an electrical circuit.
As sound waves impinged on the membrane, making the membrane vibrate, the circuit would be connected and interrupted at the same rate as the frequency of the sound. The fluctuating electric current thus generated would be transmitted by wire to a receiver, which consisted of an iron needle that was surrounded by the coil of an electromagnet and connected to a sounding box. The fluctuating electric current would generate varying magnetic fields in the coil, and these in turn would force the iron needle to produce vibrations in the sounding box.
In the s two American inventors, Elisha Gray and Alexander Graham Bell , each independently, designed devices that could transmit speech electrically. An electromagnetic coil was located near each of the reeds. When a reed in the transmitter was vibrated by sound waves of its resonant frequency—for example, hertz—it induced an electric current of corresponding frequency in its matching coil. Thus, simple tones could be transmitted. In the spring of Gray realized that a receiver consisting of a single steel diaphragm in front of an electromagnet could reproduce any of the transmitted tones.
Gray, however, was initially unable to conceive of a transmitter that would transmit complex speech vibrations and instead chose to demonstrate the transmission of tones via his telegraphic device in the summer of However, since Bell too had no transmitter, the membrane device was never constructed. Following some earlier experiments, Bell postulated that, if two membrane receivers were connected electrically, a sound wave that caused one membrane to vibrate would induce a voltage in the electromagnetic coil that would in turn cause the other membrane to vibrate.
Working with a young machinist, Thomas Augustus Watson , Bell had two such instruments constructed in June An application for a U. Several hours later that same day, Gray filed a caveat on the concept of a telephone transmitter and receiver.
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A caveat was a confidential , formal declaration by an inventor to the U. Patent Office of an intent to file a patent on an idea yet to be perfected; it was intended to prevent the idea from being used by other inventors. At this point neither Gray nor Bell had yet constructed a working telephone that could convey speech.
On March 7, , Bell was awarded U. This patent is often referred to as the most valuable ever issued by the U. Patent Office, as it described not only the telephone instrument but also the concept of a telephone system. Gray had earlier come up with an idea for a transmitter in which a moving membrane was attached to an electrically conductive rod immersed in an acidic solution. Another conductive rod was immersed in the solution, and, as sound waves impinged on the membrane, the two rods would move with respect to each other.
Variations in the distance between the two rods would produce variations in electric resistance and, hence, variations in the electric current. In contrast to the magnetic coil type of transmitter, the variable-resistance transmitter could actually amplify the transmitted sound, permitting use of longer cables between the transmitter and the receiver. Watson—come here—I want to see you. One of the earliest demonstrations occurred in June at the Centennial Exposition in Philadelphia.
Further tests and refinement of equipment followed shortly afterward. On October 9, , Bell conducted a two-way test of his telephone over a 5-km 2-mile distance between Boston and Cambridgeport, Massachusetts. In May the first commercial application of the telephone took place with the installation of telephones in offices of customers of the E. Holmes burglar alarm company.
The telephone instrument
The poor performance of early telephone transmitters prompted a number of inventors to pursue further work in this area. Among them was Thomas Alva Edison , whose design for a voice transmitter consisted of a cavity filled with granules of carbonized anthracite coal. The carbon granules were confined between two electrodes through which a constant electric current was passed. One of the electrodes was attached to a thin iron diaphragm, and, as sound waves forced the diaphragm to vibrate, the carbon granules were alternately compressed and released.
As the distance across the granules fluctuated, resistance to the electric current also fluctuated, and the resulting variations in current were transmitted to the receiver. The telephone instrument continued to evolve over time, as can be illustrated by the succession of American instruments described below. The concept of mounting both the transmitter and the receiver in the same handle appeared in in instruments designed for use by telephone operators in a New York City exchange.
The earliest telephone instrument to see common use was introduced by Charles Williams, Jr. Designed for wall mounting, this instrument consisted of a ringer, a hand-cranked magneto for generating a ringing voltage in a distant instrument , a hand receiver, a switch hook, and a transmitter. Various versions of this telephone instrument remained in use throughout the United States as late as the s.
As is noted in the section Switching , the telephone dial originated with automatic telephone switching systems in Desk instruments were first constructed in Patterned after the wall-mounted telephone, they usually consisted of a separate receiver and transmitter. The ringer and much of the telephone electronics remained in a separate box, on which the transmitter-receiver handle was cradled when not in use. The first telephone to incorporate all the components of the station apparatus into one instrument was the so-called combined set of Some 25 million of these instruments were produced until they were superseded by a new design in The telephone was totally new, incorporating significant improvements in audio quality, mechanical design, and physical construction.
Push-button versions of this set became available in Modern telephone instruments are largely electronic. Wire coils that performed multiple functions in older sets have been replaced by integrated circuits that are powered by the line voltage. Mechanical bell ringers have given way to electronic ringers. Finally, a number of other features have become available on the telephone instrument, including last-number recall and speed-dialing of multiple telephone numbers. Cordless telephones are devices that take the place of a telephone instrument within a home or office and permit very limited mobility—up to metres feet.
Because they communicate with a base unit that is plugged directly into an existing telephone jack, they essentially serve as a wireless extension to existing home or office wiring. The first cordless phones employed analog modulation methods and operated over a pair of frequencies, 1. Beginning in the s, cordless phones operated over a pair of frequencies in the and megahertz bands, and in the late s phones operating in the —megahertz band began to appear.
These phones employed either analog modulation, digital modulation, or spread-spectrum modulation. Some digital cordless telephones now operate in the gigahertz region—for example, 5. Generally speaking, each successive generation of cordless phones has offered improved quality and range to the consumer.
In a number of countries throughout the world, a wireless service called the personal communication system PCS is available. In the broadest sense, PCS includes all forms of wireless communication that are interconnected with the public switched telephone network, including mobile telephone and aeronautical public correspondence systems, but the basic concept includes the following attributes: The first PCS to be implemented was the second-generation cordless telephony CT-2 system, which entered service in the United Kingdom in The CT-2 system was designed at the outset to serve as a telepoint system.
In telepoint systems, a user of a portable unit might originate telephone calls but not receive them by dialing a base station located within several hundred metres.
The base unit was connected to the PSTN and operated as a public pay telephone, charging calls to the subscriber. Because of its limited coverage, the CT-2 system went out of service, giving way to the popular GSM digital cellular system see mobile telephone. The DECT system was designed initially to provide cordless telephone service for office environments , but its scope soon broadened to include campus-wide communications and telepoint services. The PHS became popular throughout urban areas as an alternative to cellular systems. Supporting data traffic at 32 and 64 kilobits per second, it could perform as a high-speed wireless modem for access to the Internet.
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Written By: David E. Last Updated: Jan 15, See Article History. Alternative Title: Page 1 of 2. Next page The telephone network.
Learn More in these related Britannica articles: The modern push-button telephone handset provides a good example of a relatively simple device that has required a great deal of human-factors engineering. The layout of the keys in the four rows of three buttons, for example, was selected only after extensive tests on….