Transmission Lines Review SY2021-2022 2nd Term PDF

Transmission Lines Review Transmission & Antenna Systems 1. Transmission Line Circuits, Losses and parameters. Matching TL, Smith Chart 2. Radio Wave Propagation, Power Density and Field Strength Calculations 3. Antenna Systems 4. Wave guides, Fiber Optics Transmission Media Guided vs. Unguided Medium Guided -this refers to transmission media that requires physical media to facilitate communication Unguided Medium -this refers to transmission media that does not use physical media to facilitate Communication like radio wave propagation, microwave comm., sat comm. etc. Transmission Lines -It is a metallic conductor system used to transfer electrical energy from one point to another. -It is designed to deliver RF power from transmitter to the antenna and maximum signal from the antenna to the receiver. Basic Types Transverse Electromagnetic (TEM) TL - such as coaxial lines and microstrip lines which are designed to support a single electromagnetic wave that propagates along the length of the transmission line with electric and magnetic field vectors perpendicular to the direction of propagation. Higher-order TL - It includes radio-frequency waveguides and multimode optical fiber, are designed to guide waves that have relatively complex structure. Length of Transmission Line Physical Length This refers to the actual length of the transmission line, typically measured in m/km/ft/miles. Electrical Length This refers to the length of transmission line based on the fraction of its wavelength. Question Calculate the minimum length of a pair of conductors to be considered as a transmission line, if its operating frequency is 800 MHz. a. 12.4 mm b. 37.5 mm c. 10.2 mm d. 28.5 mm Types of Transmission Line Balanced or Differential Transmission Line -made up of two parallel conductors(signal or carry line and return line) spaced from one another by a certain distance. -both conductors carry signal currents which are equal in magnitude but opposite in direction(metallic circuit currents). Advantage: It has very good noise rejection (noise immunity). It cancel effect of noise in amplitude due to opposing direction. Disadvantage: Not cost effective due Types of Transmission Line Unbalanced or Single Ended Transmission Line -This is a transmission line, whose conductors have unequal impedances with respect to ground. It usually consist of a conductor that is considered the signal line and another that is grounded, or is ground itself. Advantage: It is cost effective Disadvantage: Noise rejection is not as good as balanced TL Types of Transmission Line Balun This refers to a circuit device used to connect a balanced transmission line to unbalanced load. Types of Transmission Line Parallel-Wire Transmission Line It is composed two conducting wires separated by a dielectric(insulator). Parallel-Wire TL Categories Open-wire It is simply consists of two parallel wires,closely spaced and separated by air. Twin Lead - it is similar to the open wire, except that instead of spacers a solid dielectric is used along the whole length of the wire. Types of Transmission Line Coaxial Cable It is composed of two concentric conductors and is used extensively for high-frequency applications to reduce losses and to isolate transmission paths. This also provides excellent shielding against external interference. Types of Transmission Line Microstrip is a type of transmission line that consists of a conductor fabricated on dielectric substrate with a grounded plane. It is a flat conductor separated from a ground plane by insulating material. 𝒁 𝒐= 𝟖𝟕 ( 𝒍𝒏 𝟓.𝟗𝟖 𝒉 √ 𝜺𝒓 +𝟏.𝟒𝟏 𝟎. 𝟖 𝒘 +𝒕 ) Types of Transmission Line Stripline (Flat Metallic Ground Plane) A microwave transmission line constructed of a center conductor suspended between parallel conductive ground planes. ( ) 𝟔𝟎 𝟒𝒅 𝒅 𝒁 = 𝒍𝒏 ; 𝒉= 𝒐 √ 𝜺𝒓 ( ) 𝒕 𝟐 ( 𝟎. 𝟔𝟕 𝝅 𝒘 ) 𝟎. 𝟖+ 𝒉 Types of Transmission Line Waveguide is a special form of transmission line consisting of a hollow, metal tube. Wave guides conduct microwave energy at lower loss than coaxial cables. Types of Transmission Line Optical Fibers -is used by telecommunications companies to transmit telephone signals, Internet communication and cable television signals. It is used when high bandwidth, long distance, or immunity to electromagnetic interference is required and as light guides, for different applications i.e. imaging tools, lasers, hydrophones for seismic waves, SONAR, and as sensors to measure pressure and temperature. Transmission Line Equivalent Circuit Physical properties like diameter and conductor spacing and electrical properties such as conductivity and relative permittivity (otherwise known as dielectric constant) determine what we call the primary line constants. Primary Line Constants 1. Series Resistance (R) – it is the total resistance of the transmission line per unit length (). 2. Series Inductance (L) - it is the total inductance of the transmission line per unit length (). 3. Shunt Conductance (G) - it is the shunt conductance of the transmission line per unit length (). 4. Shunt Capacitance (C) - it is the shunt capacitance of the transmission line per unit length (). Primary Line Constants (distributed parameters) Where: - absolute permeability const #33 - relative permeability (unitless) unless otherwise stated Primary Line Parallel Wire Coaxial Cable Constant Line Series Resistance Where: (R) - absolute permittivity const #32 (Ω/) - relative permittivity (unitless) Series Inductance (L) (H/m) Dielectric Relative Shunt Capacitanc Permittivity / (C) Dielectric (F/m) Constant Shunt Conductance Vacuum (G) Air Teflon Polyethylene Polyvinyl chloride Lumped VS Distributed Lumped systems -systems in which electrical properties like R, L, C etc are assumed to be located on a small space of the circuit. These systems are applicable for low frequency application. Distributed systems assume that electrical properties R, L, C etc are distributed across the entire circuit. these systems are applicable to for high frequency application. Question A parallel-wire transmission line is constructed #6 AWG copper wire with a 12-inch separation in air. Determine the per-meter values of L, C and R assuming an operating frequency of 1 MHz. Secondary Line Constants It determine the transmission characteristics of the transmission line. These can be calculated using the primary line constants. a) Characteristic Impedance b) Propagation Constant Secondary Line Constants 1. Characteristic Impedance () -it is also known as surge impedance -it is defined as the impedance looking at the input of an infinitely long transmission line. Note: for maximum power transfer from the source to the load, a transmission line terminated in a purely resistive load equal to the characteristic impedance of the line. Note: At maximum power transfer: , all of the power will be absorbed by the load else some of the power Other Formulas for Characteristic Impedance () Characteristic Impedance Equation *for RF/High Frequencies, General (lossless) Lossless Parallel-Wire Line *for Audio Frequencies, Low Coaxial Cable Frequencies, (lossy) At max. power transfer: Coaxia Characteristic Velocity l Cable Impedance RG-8U 0.66c RG- 0.66c 58U RG- 0.79c 59U RG- 0.84c Question Given the following primary line constants: , , and. Determine the characteristic impedance of the line at 60 Hz. a) b) c) d) A lossless transmission line has and. What value of load Question resistance should this line be terminated with to ensure maximum power transfer? a. b. c. d. Calculate the for an air dielectric two-wire parallel transmission Question line. The ratio of the distance between the centers of the two conductors and the radius of the conductors is 12.3. a. b. c. d. Secondary Line Constants 2. Propagation Constant – it is also known as propagation coefficient that is used to express the attenuation and the phase shift per unit length of the transmission line. Where: = attenuation coefficient ( or ); * = phase shift coefficient (rad/m) *for High Frequency Question From the propagation constant, determine the values of the attenuation and phase-shift coefficients. The distributed parameters are: R=0.1 ohm/m, G=0 S/m, L=76.8 nH/m and C=22.2 pF/m. Assume an operating frequency of 750 MHz. a. & b. & c. & d. None of the above Question Attenuation is one of the major concerns in the application of transmission lines in communications engineering. How much attenuation in dB, does a 100 m of air-dielectric parallel-wire transmission line introduce? Assume that and. a. 0.123 dB b. 0.243 dB c. 0.145 dB d. 0.452 dB Question A particular cable has a capacitance of and a characteristic impedance of. What is the inductance per foot of this cable? a. 75 nH b. 75 mH c. d. 75 pH What is the characteristic impedance of an open wire line with Question conductors 4mm in diameter separated by 15mm? a. 241.52 Ω b. 252.41 Ω c. 421.52 Ω d. 452.21 Ω Question What length of standard RG-8/U coaxial cable would be required to obtain a 45 degrees phase shift at 280 MHz? a. 0.088 m b. 0.062 m c. 0.214 m d. 0.412 m Question When dc flows through a conductor, the movement of electrons through the conductor’s cross section is uniform. The situation is somewhat different when ac is applied. The expanding and collapsing fields about each electron encircle other electrons. This retards the movement of the encircled electrons. Which phenomenon is referred to? a. self-charging b. self-discharging c. self-induction d. self-conduction Note: please also review neets Transverse Electromagnetic (TEM) It refers to a form of radiant energy which is essentially an oscillatory disturbance in free space that is composed of two components, electric field (E) and magnetic field (H) which are perpendicular to each other and to the direction of propagation. Velocity in Transmission Lines Velocity of Propagation -it refers to the speed of the TEM wave propagating in a transmission line. This is always less than the speed of the wave in free space. Velocity Factor The ratio of the actual velocity of propagation through a given medium to the velocity of propagation through free space. Velocity in Transmission Lines Derived from Maxwell’s equations: (in free space) In transmission line: for TL Velocity in Transmission Lines Velocity Factor Velocity in Transmission Lines (lossless) Velocity in Transmission Lines Phase-shift coefficient: Phase-shift: Delay Lines -these are transmission lines design to intentionally introduce a time delay in the path of the EM wave. -the amount of time delay is a function of inductance and capacitance. *for 1 meter of delay line: *for 1 foot of delay line: Question A loss-free line transmission line has an inductance of 263.2 nH/m and a capacitance of 46.8 pF/m. If the generator produces a sinusoidal voltage at 3 MHz of 2V peak, determine the phase change if the line is 3m long. a. 10.34 degrees b. 11.34 degrees c. 12.34 degrees d. 13.34 degrees Question Determine the required pulse duration of a pulse so that when the pulse travels on a 20 m line, the trailing edge occurs at the generator end of the line just as the leading edge reaches the load. Assume that the speed of the pulse on the line is the same as its free-space velocity. a. 0.0667 b. 0.0776 c. 0.0833 d. 0.0967 Question Calculate the velocity factor of a coaxial cable, used as a transmission line with a characteristic impedance of 70 ohms, Capacitance is 20 pF/m and an inductance equal to. a. 0.14 b. 0.22 c. 0.44 d. 0.73 Transmission Line Losses Radiation Losses TL acts as an antenna if the separation of the conductors in appreciable fraction of a wavelength. Increased with frequency. Conductor Heating or Losses Proportional to current and inversely proportional to characteristic impedance. Dielectric Heating Proportional to the voltage across the dielectric and inversely proportional to the characteristic impedance for power transmitted. Reflections For maximum power transfer: In the event that the transmission line is not terminated with a load impedance equal to the characteristic impedance, reflections will occur. Incident waves TEM waves that travel along a transmission line from the source towards the load. Reflected waves TEM waves that travel from the load back towards the source. Non-Resonant vs. Resonant Transmission Lines Resonant TL -it is also known as mismatched line, wherein some of the incident power is reflected back to the source. = (0) shorted or () open ; Or entire signal is reflected back to source: Non-Resonant TL -it is also known as flat line (resistive) or matched line, wherein there is no reflected power in the Reflection Coefficient -a vector quantity that represents the ratio of the reflected voltage to the incident voltage or reflected current to the incident current. i.e. if Reflection Coefficient Matched line, : Short circuited line, : Note: all signals will be reflected back but phase inverted Open Circuited line,. Note: all signals will be reflected back but not phase inverted Return Loss Return Loss - it is the loss of power in the signal returned / reflected by a discontinuity in a transmission line. Note: The higher the return loss, the less power is actually lost. Return Loss and Mismatch Loss Mismatch Loss - it is a measure of how much the transmitted power is attenuated due to reflection. Question A communication system uses an RG-58U transmission line terminated at a load impedance of ohms. Determine the reflection coefficient. a. b. c. d. Standing Wave -it is the interference pattern of two traveling waves (incident and reflected) -it is a measure of mismatch between load impedance and Standing Wave Ratio the characteristic impedance of a transmission line Standing Wave Ratio (SWR) For resistive loads: For resistive loads: Relation to : Power Relationships Power Relationships - absorbed power Where: Question A communication system uses an RG-58U transmission line terminated at a load impedance of ohms. Determine the SWR. a. b. c. d. Question A transmitter supplies 50 W through a line with an SWR of. What is the power absorbed by the load? a. b. c. d. Transmission Line Input Impedance () The input impedance of a lossless line seen looking into a transmission line that is terminated in a short or open can be resistive, inductive, capacitive depending on the distance from the termination. Inductor (shorted tx line) Capacitor (open tx line) Parallel resonant Impedance Inversion: Series resonant The impedance at a point in a Capacitor transmission line inverts every Inductor quarter-wave section. Question Calculate the impedance looking into a 50 Ω line 1 m long, terminated with a load impedance of 100 Ω, if the line has a velocity factor of and operates at frequency of 30 MHz. a. b. c. d. Question A 50 Ω short-circuited line operates at 500 MHz. Determine the equivalent reactance and component of this line if it is 0.1 long. a. ; b. ; c. ; d. ; Ω Question In a wire communication system, calculate the return loss in dB, if the load impedance is 600 ohms and the characteristic impedance of the line is 300 ohms. a. 9.54 dB b. 8.54 dB c. 7.54 dB d. 6.54 dB Question What proportion of incident power is reflected back from the load for a 75 ohms line terminated with ohms load? a. 5.84 % b. 6.84 % c. 7.84 % d. 8.84 % Question A transmitter sends 5W of power to a 75 ohms line. Suppose the transmitter and the line is matched but the load is not, determine the power absorbed by the load if the coefficient of reflection is 0.75. a. 2.8125 W b. 2.1875 W c. 4.8125 W d. 4.1875 W Question Voltage and current readings are taken on a transmission line at different points. The maximum voltage reading is 120 Vrms, and the minimum voltage reading is 40 Vrms. If the maximum current reading on the line is 2.5 A, what would be the lowest current reading be? a. 0.338 mA b. 0.338 A c. 0.833 mA d. 0.833 A Question If a 525 MHz transmitter is delivering 50 Watts radio signal via RG-8 coaxial transmission line to a perfectly resistive terminating antenna, both the coax and antenna have the same ohmic value, what will be the measured SWR at the feed line? a. 1:1 b. 1:2 c. 1:3 d. 1:5 Transmission Line Instruments 1. Quarter Wave Transformer -It is a section of a transmission line to match a purely resistive load to a transmission line. It is a transmission line with an adjustable characteristic impedance which is quarter wavelength long - quarter wave transformer characteristic impedance. Question A 50 ohms transmission line is terminated with a resistive load of 120 ohms. Design a quarter-wave transformer that could be installed at the load to match it with the line. Determine its length and characteristic impedance. Operating frequency is 70 MHz. a. 1.07 m and 77.46 Ω b. 20.26 m and 77.46 Ω c. 1.07 m and 85.12 Ω d. 20.26 m and 85.12 Ω Impedance Matching Networks 2. Stub -is essentially a short section of a transmission line open or shorted. - It is used to remove the reactive component of a load with complex impedance and match it to the line. Types of Matching Stub 1. Series Open Stub 2. Series Short Stub 3. Shunt Open Stub 4. Shunt Short Stub Transmission Line Instruments Smith Chart This refers to a special circular graph transmission line calculator named after Philip Smith. Transmission Line Instruments Optical Time Domain Reflectometer This refers to a device used to perform time-domain reflectometry. The TDR is a technique that can be used to locate an impairment in a metallic cable. Question A pulse is transmitted down a cable that has a velocity of propagation of. The reflected signal is received 1 microsec later. How far down is the cable impairment? a. 120 m b. 100 m c. 150 m d. 90 m Transmission Line Instruments Slotted Line It is a short section of air-dielectric coaxial line with a slot in outer conductor through which a capacitively-coupled probe is inserted. It measures the wavelength and SWR of the line. Question Two adjacent minima on a slotted line are 23 cm apart. Find the wavelength and frequency of the TEM wave on the transmission line if a. 23 cm and 230.44 MHz b. 46 cm and 230.44 MHz c. 23 cm and 430.44 MHz d. 46 cm and 430.44 MHz Question A shorted transmission line 6 inches long acts as a ______ resonant circuit at a frequency of 492 MHz. a. Parallel b. series-parallel c. Series d. Capacitive Question What would be the approximate series impedance of a quarter-wave matching line used to match a 600 ohms feed to 70 ohms antenna? a. 105 Ω b. 205 Ω c. 305 Ω d. 405 Ω Question A certain instrument measures the input impedance of a line and gave a reading of 1200 ohms when its far-end terminals is open circuited and 300 ohms when it is short circuited. Calculate the characteristic impedance of the line. a. 400 Ω b. 500 Ω c. 600 Ω d. 700 Ω Question The input impedance of short-circuited line of length L where , is a. resistive b. inductive Inductor c. capacitive Capacitor Parallel d. None of the above resonant Series resonant Capacitor Inductor Question A shorted half-wave line at the operating frequency acts like a(n): a. capacitor b. inductor c. Series resonant circuit d. Parallel resonant circuit Question In signal propagation, the data channel provides the physical path between transmitter and receiver. It could either be guided or unguided. Which of the ff. does not belong to the group? a. wireless b. Optical fiber c. Coaxial cable d. Twisted pair Question Which of the following determines the characteristic impedance of a coaxial antenna feed line? a. The diameter of the braid and the length of the line b. The frequency of the signal and the length of the line c. The diameter of the braid and the frequency of the signal d. The ratio of the diameter of the outer conductor to the diameter of the inner conductor THANK YOU! End

Transmission Lines Review SY2021-2022 2nd Term PDF

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