Transmission Lines And Network Cables

ASSIGNMENT NO. 01 (TRANSMISSION LINES AND NETWORK CABLES)

Submitted By: NIVEDITA SRIVASTAVA (BA/08/EC/16) TANYA SINGH (BA/08/EC/20) TINGPILHING SITLHOU (BA/08/EC/22) ANINDITA DAS (BA/08/EC/25) RUBINA KONGSIT (BA/08/EC/26) KUMARI HANSA (BA/08/EC/28) SHUPARNA DEB (BA/08/EC/31)

COURSE NAME: Fundamentals of Telecommunication Engineering COURSE CODE: EC-2101 SUBMISSION DATE: 23/10/2009 SESSION: July-December’09

NORTH EASTERN REGIONAL INSTITUTE OF SCIENCE AND TECHNOLOGY (DEEMED UNIVERSITY) Under the Ministry of Human Resource Development, Govt. of India Itanagar, Arunachal Pradesh, India, Nirjuli-791109

HISTORY 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 correctly 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.

TRANSMISSION LINE A transmission line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic waves or acoustic waves, as well as electric power transmission. It is a device designed to guide electrical energy from one point to another. It is used, for example, to transfer the output RF energy of a transmitter to an antenna. This energy will not travel through normal electrical wire without great losses. Although the antenna can be connected directly to the transmitter, the antenna is usually located some distance away from the transmitter. A transmission line is used to connect the transmitter and the antenna.

PRINCIPLES OF TRANSMISSION LINES Transmission lines are basically impedance matching circuits designed to deliver power (RF) from the transmitter to the antenna, and maximum signal from the antenna to the receiver. The properties of a transmission line are constant for any type of transmission lines. All types of transmission lines are arranged in some uniform pattern which simplifies calculations, reduces costs and increases convenience. The transmission line has a single purpose for both the transmitter and the antenna. This purpose is to transfer the energy output of the transmitter to the antenna with the least possible power loss. How well this is done depends on the impedance and resistance of the transmission line. All transmission lines have two ends .The end of a two-wire transmission line connected to a source is ordinarily called the input end or the generator end. Other names given to this end are transmitter end, sending end, and source. The other end of the line is called the output end or receiving end. Other names given to the output end are load end and sink.

Fig: Basic transmission line.

TYPES OF TRANSMISSION LINES: There are several types of transmission lines whose losses are small: (1) the open-wire line consisting of two parallel wires; (2) the twisted-pair line of two insulated wires twisted together; (3) the concentric cable or coaxial line, and (4) waveguides, and (5) optical fibres. 1.

OPEN WIRE LINE OR PARALLEL WIRE:-

Parallel wire is a transmission line consisting of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits. The chief advantage of the balanced line format is good rejection of external noise. Common forms of balanced line are twin-lead, used for radio frequency signals and twisted pair, used for lower frequencies. Parallel wire lines are to be contrasted to unbalanced lines, such as coaxial cable, which is designed to have its return conductor connected to ground, or circuits whose return conductor actually is ground. Balanced and unbalanced circuits can be interconnected using a transformer called a balun. Circuits driving balanced lines must themselves be balanced to maintain the benefits of balance. This may be achieved by differential signaling, transformer coupling or by merely balancing the impedance in each conductor. 2.

TWISTED PAIR LINE:-

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. The format is also used for data network distribution inside buildings, but in this case the cable used is more expensive with much tighter controlled parameters and either two or four pairs per cable. 3.

COAXIAL LINE:-

Where a central wire is mounted along the axis of a metal tube, (a) with insulating spacers every so often, (b) with continuous rubber insulation along the line (usually used with a flexible outer metal mesh); The Coaxial Transmission Line block models the coaxial transmission line described in the block dialog box in terms of its frequency-dependent S-parameters. A coplanar waveguide transmission line is shown in cross-section in the following figure. Its physical characteristics include the radius of the inner conductor a and the radius of the outer conductor b.

The block models the transmission line as a stub or as a stub less line.

4.

WAVE GUIDES:-

A waveguide is a special form of transmission line consisting of a hollow, metal tube. The tube wall provides distributed inductance, while the empty space between the tube walls provides distributed capacitance. Waveguides are rectangular or circular metallic tubes inside which an electromagnetic wave is propagated and is confined by the tube.

Fig: Wave guides conduct microwave energy at lower loss than coaxial cables. Waveguides are practical only for signals of extremely high frequency, where the wavelength approaches the cross-sectional dimensions of the waveguide. Below such frequencies, waveguides are useless as electrical transmission lines. When functioning as transmission lines, though, waveguides are considerably simpler than two-conductor cables—especially coaxial cables—in their manufacture and maintenance. All transmission lines function as conduits of electromagnetic energy when transporting pulses or high-frequency waves, directing the waves as the banks of a river direct a tidal wave. However, because waveguides are single-conductor elements, the propagation of electrical energy down a waveguide is of a very different nature than the propagation of electrical energy down a two-conductor transmission line. All electromagnetic waves consist of electric and magnetic fields propagating in the same direction of travel, but perpendicular to each other. Along the length of a normal transmission line, both electric and magnetic fields are perpendicular (transverse) to the direction of wave travel. This is known as the principal mode, or TEM (Transverse Electric and Magnetic) mode (Figure below)

Fig: Twin lead transmission line propagation: TEM mode. At microwave signal frequencies (between 100 MHz and 300 GHz), two-conductor transmission lines of any substantial length operating in standard TEM mode become impractical. When an electromagnetic wave propagates down a hollow tube, only one of the fields -- either electric or magnetic -- will actually be transverse to the wave's direction of travel. The other field will “loop” longitudinally to the direction of travel, but still be perpendicular to the other field. (Figure below)

Fig: Waveguide (TE) transverse electric and (TM) transverse magnetic modes. Many variations of each mode exist for a given waveguide, and a full discussion of this is subject well beyond the scope of this book. Signals are typically introduced to and extracted from waveguides by means of small antenna-like coupling devices inserted into the waveguide. Sometimes these coupling elements take the form of a dipole, which is nothing more than two open-ended stub wires of appropriate length. Other times, the coupler is a single stub (a half-dipole, similar in principle to a “whip” antenna, 1/4λ in physical length), or a short loop of wire terminated on the inside surface of the waveguide: (Figure below)

Fig: Stub and loop coupling to waveguide. A cavity's resonant frequency may be altered by changing its physical dimensions. To this end, cavities with movable plates, screws, and other mechanical elements for tuning are manufactured to provide coarse resonant frequency adjustment. Waveguides are not capable of transmitting the transverse electromagnetic mode found in copper lines and must use some other mode. Consequently, they cannot be directly connected to cable and a mechanism for launching the waveguide mode must be provided at the interface. 5.

OPTICAL FIBRES:-

An optical fiber (or fibre) is a glass or plastic fiber that carries light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less loss, and they are also immune to electromagnetic interference. Fibers are also used for illumination, and are wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces. Specially designed fibers are used for a variety of other applications, including sensors and fiber lasers.

CIRCUIT DESCRIPTION:

Uncharged Transmission line

(Switch closes)

Begin wave propagation.

CHARACTERISTIC IMPEDANCE We can describe a transmission line in terms of its impedance. The ratio of voltage to current (E in/Iin) at the input end is known as the input impedance (Zin). This is the impedance presented to the transmitter by the transmission line and its load, the antenna. The ratio of voltage to current at the output (E out/Iout) end is known as the output impedance (Zout). This is the impedance presented to the load by the transmission line and its source. If an infinitely long transmission line could be used, the ratio of voltage to current at any point on that transmission line would be some particular value of impedance. This impedance is known as the characteristic impedance.

Fig: Schematic representation of a transmission line, showing the characteristic impedance Z0. The characteristic impedance or surge impedance of a uniform transmission line, usually written Z0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm. The characteristic impedance of a lossless transmission line is purely real, that is, there is no imaginary component (Z0 = | Z0 | + j0). Characteristic impedance appears like a resistance in this case, such that power generated by a source on one end of an infinitely long lossless transmission line is transmitted through the line but is not dissipated in the line itself. A transmission

line of finite length (lossless) that is terminated at one end with a resistor equal to the characteristic impedance (ZL = Z0) appears to the source like an infinitely long transmission line. The ratio of voltage applied to the current is called the input impedance; the input impedance of the infinite line is called the characteristic impedance.

Schematic representation of the elementary components of a transmission line. Applying the transmission line model based on the telegrapher's equations, the general expression for the characteristic impedance of a transmission line is:

Where, R is the resistance per unit length, L is the inductance per unit length, G is the conductance of the dielectric per unit length, C is the capacitance per unit length, j is the imaginary unit, and ω is the angular frequency. The voltage and current phasors on the line are related by the characteristic impedance as:

Where, the superscripts + and − represent forward- and backward-traveling waves, respectively.

LOSSES IN TRANSMISSION LINES RADIATION LOSSES: RADIATION and INDUCTION LOSSES are similar in that both are caused by the fields surrounding the conductors. Induction losses occur when the electromagnetic field about a conductor cuts through any nearby metallic object and a current is induced in that object Radiation losses occur because some magnetic lines of force about a conductor do not return to the conductor when the cycle alternates. CONDUCTOR HEATING: One type of conductor heating is I2R loss. In RF lines the resistance of the conductors is never equal to zero. Whenever current flows through one of these conductors, some energy is dissipated in the form of heat. This heat loss is a POWER LOSS. Another type of this loss is due to skin effect. When dc flows through a conductor, the movement of electrons through the conductor's cross section is uniform. When ac is applied the expanding and collapsing fields about each electron encircle other electrons. This phenomenon, called self induction. Conductor heating can be minimized and conductivity increased in an RF line by plating the line with silver. Since silver is a better conductor than copper, most of the current will flow through the silver layer. The tubing then serves primarily as a mechanical support. DIELECTRIC LOSSES: Dielectric losses result from the heating effect on the dielectric material between the conductors. Power from the source is used in heating the dielectric. The heat produced is dissipated into the surrounding medium. When there is no potential difference between two conductors, the atoms in the dielectric material between them are normal and the orbits of the electrons are circular. When there is a potential difference between two conductors, the orbits of the electrons change. The excessive negative charge on one conductor repels electrons on the dielectric toward the positive conductor and thus distorts the orbits of the electrons. A change in the path of electrons requires more energy, introducing a power loss. Polythene is often used as a dielectric because less power is consumed when its electron orbits are distorted.

STANDING WAVES The purpose of a transmission line is to convey electrical energy from one point to another. Even if the signals are intended for information only, and not to power some significant load device, the ideal situation would be for all of the original signal energy to travel from the source to the load, and then be completely absorbed or dissipated by the load for maximum signal-to-noise ratio. Thus, “loss” along the length of a transmission line is undesirable, as are reflected waves, since reflected energy is energy not delivered to the end device. Reflections may be eliminated from the transmission line if the load's impedance exactly equals the characteristic (“surge”) impedance of the line.

In essence, a terminating resistor matching the natural impedance of the transmission line makes the line “appear” infinitely long from the perspective of the source, because a resistor has the ability to eternally dissipate energy in the same way a transmission line of infinite length is able to eternally absorb energy. Reflected waves will also manifest if the terminating resistance isn’t precisely equal to the characteristic impedance of the transmission line, not just if the line is left unconnected (open) or jumpered (shorted). Though the energy reflection will not be total with a terminating impedance of slight mismatch, it will be partial. This happens whether or not the terminating resistance is greater or less than the line’s characteristic impedance. Reflected waves will also manifest if the terminating resistance isn’t precisely equal to the characteristic impedance of the transmission line, not just if the line is left unconnected (open) or jumpered (shorted). Though the energy reflection will not be total with a terminating impedance of slight mismatch, it will be partial. This happens whether or not the terminating resistance is greater or less than the line’s characteristic impedance.

Re-reflections of a reflected wave may also occur at the source end of a transmission line, if the source's internal impedance (Thevenin equivalent impedance) is not exactly equal to the line's characteristic impedance. A reflected wave returning back to the source will be dissipated entirely if the source impedance matches the line's, but will be reflected back toward the line end like another incident wave, at least partially, if the source impedance does not match the line. This type of reflection may be particularly troublesome, as it makes it appear that the source has transmitted another pulse.

NETWORK CABLES Networking Cables are used to connect one network device to other or to connect two or more computers to share printer, scanner etc. Different types of network cables like Coaxial cable, Optical fiber cable, Twisted Pair cables are used depending on the network's topology, protocol and size. The devices can be separated by a few meters (e.g. via Ethernet) or nearly unlimited distances (e.g. via the interconnections of the Internet).

1. TWISTED PAIR CABLE:Twisted pair cabling is a type of wiring in which two conductors (the forward and return conductors of a single circuit) are twisted together for the purposes of canceling out electromagnetic interference (EMI) from external sources; for instance, electromagnetic radiation from Unshielded Twisted Pair (UTP) cables, and crosstalk between neighboring pairs. BALANCED PAIR CABLE: In balanced pair operation, the two wires carry equal and opposite signals and the destination detects the difference between the two. This is known as differential mode transmission. Noise sources introduce signals into the wires by coupling of electric or magnetic fields and tend to couple to both wires equally. The noise thus produces a common-mode signal which is cancelled at the receiver when the difference signal is taken. This method starts to fail when the noise source is close to the signal wires; the closer wire will couple with the noise more strongly and the common-mode rejection of the receiver will fail to eliminate it. This problem is especially apparent in telecommunication cables where pairs in the same cable lie next to each other for many miles. One pair can induce crosstalk in another and it is additive along the length of the cable. Twisting the pairs counters this

effect as on each half twist the wire nearest to the noise-source is exchanged. Providing the interfering source remains uniform, or nearly so, over the distance of a single twist, the induced noise will remain common-mode. Differential signaling also reduces electromagnetic radiation from the cable, along with the attenuation that it causes. UNSHIELDED TWISTED PAIR (UTP):

Twisted pair cables were first used in telephone systems by Alexander Graham Bell in 1881. By 1900, the entire American telephone line network was either twisted pair or open wire with similar arrangements to guard against interference. Today, most of the millions of kilometers of twisted pairs in the world are outdoor landlines, owned by telephone companies, used for voice service, and only handled or even seen by telephone workers. CABLE SHIELDING: Twisted pair cables are often shielded in attempt to prevent electromagnetic interference. Because the shielding is made of metal, it may also serve as a ground. However, usually a shielded or a screened twisted pair cable has a special grounding wire added called a drain wire. This shielding can be applied to individual pairs, or to the collection of pairs. When shielding is applied to the collection of pairs, this is referred to as screening. The shielding must be grounded for the shielding to work.

DISADVANTAGES: 

Twisted pair’s susceptibility to the electromagnetic interference greatly depends on the pair twisting schemes (usually patented by the manufacturers) staying intact during the installation. As a result, twisted pair cables usually have stringent requirements for maximum pulling tension as well as minimum bend radius.



In video applications that send information across multiple parallel signal wires, twisted pair cabling can introduce signaling delays known as skew which results in subtle color defects and ghosting due to the image components not aligning correctly when recombined in the display device. The skew occurs because twisted pairs within the same cable often use a different number of twists per meter so as to prevent common-mode crosstalk between pairs with identical numbers of twists. The skew can be compensated by varying the length of pairs in the termination box, so as to introduce delay lines that take up the slack between shorter and longer pairs. MINOR TWISTED PAIR VARIANTS:  Loaded twisted pair: A twisted pair that has intentionally added inductance, common practice on

telecommunication lines, except those carrying higher than voice band frequencies. The added inductors are known as load coils and reduce distortion.  Unloaded twisted pair: A twisted pair that has no added load coils.

 Bonded twisted pair: A twisted pair variant in which the pairs are individually bonded to increase robustness of the cable. Pioneered by Belden, it means the electrical specifications of the cable are maintained despite rough handling. 2. COAXIAL CABLE:Coaxial cable, or coax, is an electrical cable with an inner conductor surrounded by a tubular insulating layer typically of a flexible material with a high dielectric constant, all of which are surrounded by a conductive layer (typically of fine woven wire for flexibility, or of a thin metallic foil), and finally covered with a thin insulating layer on the outside. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis. Coaxial cable was invented by English engineer and mathematician Oliver Heaviside, who first patented the design in 1880.[1] Coaxial cable is used as a transmission line for radio frequency signals, in applications such as connecting radio transmitters and receivers with their antennas, computer network (Internet) connections, and distributing cable television signals. One advantage of coax over other types of 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 transmission lines, and provides protection of the signal from external electromagnetic interference. Coaxial cable should not be confused with other shielded cable used for carrying lower frequency signals such as audio signals. Shielded cable is similar in that it consists of a central wire or wires surrounded by a tubular shield conductor, but it is not constructed with the precise conductor spacing needed to function efficiently as a radio frequency transmission line.

Like an electrical power cord, coaxial cable conducts AC electric current between locations. Like these other cables, it has two conductors, the central wire and the tubular shield. At any moment the current is traveling outward from the source in one of the conductors, and returning in the other. However, since it is alternating current, the current reverses direction many times a second. Coaxial cable differs from other cable because it is designed to carry radio frequency current. This has a frequency much higher than the 50 or 60 Hz used in mains (electric power) cables, reversing direction millions to billions of times per second. Like other types of radio transmission line, this requires special construction to prevent power losses:  If an ordinary wire is used to carry high frequency currents, the wire acts as an antenna, and the high frequency

currents radiate off the wire as radio waves, causing power losses. To prevent this, in coaxial cable one of the conductors is formed into a tube and encloses the other conductor. This confines the radio waves from the central conductor to the space inside the tube. To prevent the outer conductor, or shield, from radiating, it is connected to electrical ground, keeping it at a constant potential.

 The dimensions and spacing of the conductors are uniform. Any abrupt change in the spacing of the two

conductors along the cable tends to reflect radio frequency power back toward the source, causing a condition called standing waves. This acts as a bottleneck, reducing the amount of power reaching the destination end of the cable. The connectors used with coax are designed to hold the correct spacing through the body of the connector.  Each type of coaxial cable has a characteristic impedance depending on its dimensions and materials used,

which is the ratio of the voltage to the current in the cable. In order to prevent reflections at the destination end of the cable from causing standing waves, any equipment the cable is attached to must present an impedance equal to the characteristic impedance. Thus the equipment "appears" electrically similar to a continuation of the cable, preventing reflections. Common values of characteristic impedance for coaxial cable are 50 and 75 ohms. USES: Short coaxial cables are commonly used to connect home video equipment, in ham radio setups, and in measurement electronics. They used to be common for implementing computer networks, in particular Ethernet, but twisted pair cables have replaced them in most applications except in the growing consumer cable modem market for broadband Internet access. Long distance coaxial cable is used to connect radio networks and television networks, though this has largely been superseded by other more high-tech methods (fibre optics, T1/E1,satellite). Micro coaxial cables are used in a range of consumer devices, military equipment, and also in ultra-sound scanning equipment. The most common impedances that are widely used are 50 or 52 ohms, and 75 ohms, although other impedances are available for specific applications. The 50 / 52 ohm cables are widely used for industrial and commercial twoway radio frequency applications (including radio, and telecommunications), although 75 ohms is commonly used for broadcast television and radio.