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Chapter 44 Transmission lines At the end of this chapter you should be able to: • appreciate the purpose of a transmission line • define the transmission line primary constants R, L, C and G • calculate phase delay, wavelength and velocity of propagation on a transmission line • appreciate current and voltage relationships on a transmission line • define the transmission line secondary line constants Z0, γ , α and β • calculate characteristic impedance and propagation coefficient in terms of the primary line constants • understand and calculate distortion on transmission lines • understand wave reflection and calculate reflection coefficient • understand standing waves and calculate standing wave ratio are often used to couple f.m. and television receivers to 44.1 Introduction their antennas. At frequencies greater than 1000MHz, transmission A transmission lineis asystem of conductors connecting lines are usually in the form of a waveguide which may one point to another and along which electromagnetic be regarded as coaxial lines without the centre con- energy can be sent. Thus telephone lines and power ductor, the energy being launched into the guide or distribution lines are typical examples of transmission abstracted from it by probes or loops projecting into lines; in electronics, however, the term usually implies the guide. a line used for the transmission of radio-frequency (r.f.) energy such as that from a radio transmitter to the antenna. 44.2 Transmission line primary An important feature of a transmission line is that it constants should guide energy from a source at the sending end to a load at the receiving end without loss by radiation. Let an a.c. generator be connected to the input termi- One form of construction often used consists of two nals of a pair of parallel conductors of infinite length. similar conductors mounted close together at a constant A sinusoidal wave will move along the line and a finite separation. The two conductors form the two sides of current will flow into the line. The variation of voltage a balanced circuit and any radiation from one of them with distance along the line will resemble the variation is neutralized by that from the other. Such twin-wire of applied voltage with time. The moving wave, sinu- lines are used for carrying high r.f. power, for exam- soidal in this case, is called a voltage travelling wave. ple, at transmitters. The coaxial form of construction is As the wave moves along the line the capacitance of the commonly employed for low power use, one conduc- line is charged up and the moving charges cause mag- tor being in the form of a cylinder which surrounds the netic energy to be stored. Thus the propagation of such other at its centre, and thus acts as a screen. Such cables an electromagnetic wave constitutes a flow of energy. DOI: 10.1016/B978-1-85617-770-2.00044-6 662 Electrical Circuit Theory and Technology After sufficient time the magnitude of the wave may the fact that there are two conductors in a partic- be measured at any point along the line. The line does ular length of line. not therefore appear to the generator as an open cir- (iii) Capacitance C exists as a result of the electric cuit but presents a definite load Z . If the sending-end 0 field between conductors of a transmission line. voltage is V and the sending-end current is I then S S The capacitance of an isolated twin line is con- Z = V /I . Thus all of the energy is absorbed by the 0 S S sidered in Section 40.3. From equation (14), page line and the line behaves in a similar manner to the gen- 567, the capacitance between the two conductors erator as would a single ‘lumped’ impedance of value is given by Z0 connected directly across the generator terminals. There are four parameters associated with transmis- πε ε C = 0 r farads/metre sion lines, these being resistance, inductance, capaci- ln(D/a) tance and conductance. In most practical lines εr =1 (i) Resistance R is given by R = ρl/A,whereρ is the resistivity of the conductor material, A is the (iv) Conductance G is due to the insulation of the cross-sectional area of each conductor and l is the line allowing some current to leak from one con- length of the conductor (for a two-wire system, l ductor to the other. Conductance is measured in represents twicethelength of theline).Resistance siemens per metre length of line and represents is stated in ohms per metre length of a line and the imperfection of the insulation. Another name represents the imperfection of the conductor. A for conductance is leakance. resistance stated in ohms per loop metre is a little Each of the four transmission line constants, R, L, C more specific since it takes into consideration the and G, known as the primary constants, are uniformly fact that there are two conductors in a particular distributed along the line. length of line. From Chapter 41, when a symmetrical T-network is (ii) Inductance L is due to the magnetic field sur- terminated in its characteristic impedance Z0, the input rounding the conductors of a transmission line impedance of the network is also equal to Z0. Similarly, when a current flows through them. The induc- if a number of identical T-sections are connected in cas- tance of an isolated twin line is considered in cade, the input impedance of the network will also be Section 40.7. From equation (23), page 574, the equal to Z0. inductance L is given by A transmission line can be considered to consist of a network of a very large number of cascaded T-sections each a very short length (δl ) of transmission line, as μ μ 1 D = 0 r + shown in Figure 44.1. This is an approximation of the L π ln henry/metre 4 a uniformly distributed line; the larger the number of lumped parameter sections, the nearer it approaches the Part 3 where D is the distance between centres of the true distributed nature of the line. When the genera- conductor and a is the radius of each conductor. In tor VS is connected, a current IS flows which divides most practical lines μr =1. An inductance stated between that flowing through the leakage conductance in henrys per loop metre takes into consideration G, which is lost, and that which progressively charges Figure 44.1 Transmission lines 663 each capacitor C and which sets up thevoltagetravelling given by: √ wave moving along the transmission line. The loss or β = ω (LC) radians/metre (1) attenuation in the line is caused by both the conductance where L and C are the inductance and capacitance per G and the series resistance R. metre of the line. 44.3 Phase delay, wavelength and Wavelength velocity of propagation The wavelength λ on a line is the distance between a given point and the next point along the line at which Each section of that shown in Figure 44.1 is simply the voltage is the same phase, the initial point leading a low-pass filter possessing losses R and G. If losses the latter point by 2π radian. Since in one wavelength a are neglected, and R and G are removed, the circuit phase change of 2π radians occurs, the phase change per simplifies and the infinite line reduces to a repetitive metre is 2π/λ. Hence, phase change per metre, β =2π/λ T-section low-pass filter network as shown in π λ = 2 Figure 44.2. Let a generator be connected to the line or wavelength, β metres (2) as shown and let the voltage be rising to a maximum positive value just at the instant when the line is con- Velocity of propagation nected to it. A current IS flows through inductance L1 = λ into capacitor C1. The capacitor charges and a voltage The velocity of propagation, u,isgivenbyu f , develops across it. The voltage sends a current through where f is the frequency and λ the wavelength. Hence inductance L1 and L2 into capacitor C2. The capaci- 2πf ω tor charges and the voltage developed across it sends u = fλ= f (2π/β) = = (3) β β a current through L2 and L3 into C3, and so on. Thus all capacitors will in turn charge up to the maximum The velocity of propagation of free space is the same × 6 input voltage. When the generator voltage falls, each as that of light, i.e. approximately 300 10 m/s. The capacitor is charged in turn in opposite polarity, and as velocity of electrical energy along a line is always less λ before the input charge is progressively passed along to than the velocity in free space. The wavelength of λ= the next capacitor. In this manner voltage and current radiation in free space is given by c/ f where c is waves travel along the line together and depend on each the velocity of light. Since the velocity along a line is other. always less than c, the wavelength corresponding to any The process outlined above takes time; for exam- particular frequency is always shorter on the line than it would be in free space. ple, by the time capacitor C3 has reached its maximum voltage, the generator input may be at zero or mov- ing towards its minimum value. There will therefore be Problem 1. A parallel-wire air-spaced a time, and thus a phase difference between the gen- transmission line operating at 1910Hz has a phase Part 3 erator input voltage and the voltage at any point on shift of 0.05rad/km.

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