What is Transmission line: Definition and 184 Discussions

In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmission must be taken into account. This applies especially to radio-frequency engineering because the short wavelengths mean that wave phenomena arise over very short distances (this can be as short as millimetres depending on frequency). However, the theory of transmission lines was historically developed to explain phenomena on very long telegraph lines, especially submarine telegraph cables.
Transmission lines are used for purposes such as connecting radio transmitters and receivers with their antennas (they are then called feed lines or feeders), distributing cable television signals, trunklines routing calls between telephone switching centres, computer network connections and high speed computer data buses. RF engineers commonly use short pieces of transmission line, usually in the form of printed planar transmission lines, arranged in certain patterns to build circuits such as filters. These circuits, known as distributed-element circuits, are an alternative to traditional circuits using discrete capacitors and inductors.
Ordinary electrical cables suffice to carry low frequency alternating current (AC) and 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. RF currents also tend to reflect from discontinuities in the cable such as connectors and joints, and travel back down the cable toward the source. These reflections act as bottlenecks, preventing the signal power from reaching the destination. Transmission lines use specialized construction, and 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. 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, and waveguides are used instead which function as "pipes" to confine and guide the electromagnetic waves. At even higher frequencies, in the terahertz, infrared and visible ranges, waveguides in turn become lossy, and optical methods, (such as lenses and mirrors), are used to guide electromagnetic waves.

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  1. T

    A Voltage and current waves in a transmission line

    INTRODUCTION From the boundary conditions of the electromagnetic field in perfect conductors, it is deduced that in a transmission line with a time-varying current, the field vectors E and B in the dielectric lie in planes transverse to the conductors and also that the E field is normal to the...
  2. N

    I Resulting magnetic field due to the passage of an AC current

    Hello, Assuming that we have a single-phase electrical transmission line (short distance (< 50 km), stranded, non-isolated, made of aluminum conductor steel reinforced), I would like to know the resulting magnetic field (shape, structure, absolute value, and all other possible details) when...
  3. LarryS

    I Transmission Line EM Wave vs EM Wave in Free Space

    According to Maxwell’s Equations, the speed an EM plane wave in free space, far from its source, is determined by the electric constant, ε0, and the magnetic constant, μ0, such that c = 1/√( ε0 μ0). The units of ε0 are capacitance per unit length and the units of μ0 are inductance per unit...
  4. L

    Transmission line with 2x termination

    Trying to work out the match for a transmission line with 2x termination. Like the one here: https://ibb.co/p4dY7pF V1=1 R1=R2=T1=T2=50, V1 sees R1 and T1. So it is divided to half. Now you have 0.5V source with a 50 Ohms series impedance(T1) looking into a load that is a parallel combo of...
  5. H

    Rate of energy dissipated by a power transmission line

    VR = IR = 400 [ 29*10^-6 * 800 * 10^3 ] = 9280 P = IV = 400*9280 = 3.7 MW I was able to calculate the correct answer from the above, but why can't I use the equation P= V^2 / R?
  6. P

    A Transmission Line in Accelerating Frame: Energy Conservation Puzzle

    While I believe I have an answer to this problem, I think it's an interesting one and counter-intuitive. I think it might spark some interesting discussions. The bow of a Born rigid accelerating spaceship has a proper acceleration of g. There is a transmission line running from the stern to...
  7. icesalmon

    Engineering Lattice Diagram for Transmission Line

    The propagation speed of the wave is C/sqrt(9) = up if the length of the transmission line is 2m then every 10ns it will pass through the middle of the transmission line. But the switch in the circuit is opened after 5ns so after the current wave bounces off of the Load for the first time it...
  8. M

    Engineering Voltage pulse sketches for reflection on a lossless transmission line

    Hi, I was recently attempting a question about transmission lines and I don't seem to really understand how the voltages travel through the line. Question: If we send a voltage pulse of amplitude ## V_0 ## through a lossless transmission line, what does the voltage at the output look like when...
  9. Amartansh

    I Intuition behind fields in Transmission lines

    How can traveling wave exist on transmission line if wires are perfect conductors (how voltages can be different at different positions on one perfect conductor)? I mean electric fiels should be zero on equipotential source. I know if length is too long compared to wavelength, we get phase...
  10. J

    Engineering 50 Hz High-Voltage Transmission Line

    VS=VR(A1+jA2)+IR(B1+jB2) IS=VR(C1+jC2)+IR(D1+jD2) Given the parameter values in TABLE C and an open-circuit received voltage measured as 88.9 kV, calculate the values of VS and IS and hence the power (PSO) absorbed from the supply by the transmission line on open circuit. VR= 88.9×103 Table...
  11. E

    Transmission line resonator

    Hello everyone, I am studying transmission line resonators and in particular I was considering a particular case of resonator where I have an open circuited quarter wavelength transformer of high impedance terminating a regular 50 ##\Omega## line. If I look at the input impedance from the open...
  12. A

    Active power dissipated by a resistor from DC to RF

    Hello all PF members! I was wondering about the power dissipated by resistor at high frequency which is: ##P=Re\left \{ U\cdot I^{*} \right \}=Re\left \{ \frac{\left |V^{+} \right |^{^{2}}}{2\left |Z_{c} \right |}\cdot (1-\left |\Gamma \right |^{2} )\right \}##, (1) where: ##Z_{c}## -...
  13. John FD

    Transmission Line , OPGW Stringing

    Hi, any of you guys have experience paying out conductors leaving the OPGW later on? any idea or procedures needed so that during OPGW paying out stage later on it will not be damaged by the existing conductors. Mentor Note -- Adding link to information about OPGW...
  14. M

    Impedance graph of a transmission line powersystem in MATLAB/simulink

    I'm trying to plot impedance locus of transmission line in Matlab/Simulink. I have readed some document and thay have plot result like that : I'm trying to calculate impedance when system have short - circuit fault, and plot it on R-X diagram, but i can't get their *red - dot* result like those...
  15. Jason-Li

    Transmission Line Coefficients

    Homework Statement Figure shows a 50 Hz, high-voltage, transmission line. The relationships between the sending and receiving end voltages and currents are given by the complex ABCD equations: where 'S' stands for sending-end and 'R' stands for receiving-end (a) Given the parameter values...
  16. Jason-Li

    Transmission Line Coefficients & Definitions

    Homework Statement (a) State what is meant by a ‘distortionless’ and a ‘lossless’ transmission line.(b) A transmission line has the primary coefficients as given below. Determine the line’s secondary coefficients Zo, α and β at a frequency of 1 GHz. R = 2 Ω/m L = 8 nH/m G=0.5 mS/m C=0.23...
  17. Rx7man

    Microhydro project, transmission line loss

    Well, I've been trying to design a microhydro power project, and I contacted a supplier on Alibaba and we're a bit at odds about the recommended transmission voltage, I'd like to go with 480V and a transformer back down to 120/240V on the other end, their 'engineers' say this isn't necessary and...
  18. Cocoleia

    Power loss in a transmission line

    Homework Statement Homework EquationsThe Attempt at a Solution I have posted the whole solution so it is more complete, but I really only need help with part c (I think) My output power is higher than my input power, I'm not sure where I'm going wrong
  19. S

    Electric current flowing in a power transmission line

    A cable of resistance 12 Ohm carries electric power from a generator producing 250 kilowatt at 10,000 volts. Calculate the current in the cable. Solution using P=VI I=25A but using P=I^2R 250000=I^2×12 I =500/root 12 Amps What is the reason for the different...
  20. G

    Transmission line: leakage current differential equation

    Homework Statement I have a coaxial cable with internal conductor of radius r1 and external conductor of radii r2 and r3. The material of the conductors has a conductivity ##\sigma_1##. Between the conductors there is a imperfect dielectric of conductivity ##\sigma_2##. Consider the...
  21. Marcin H

    Transmission Line Transposition - Conceptual Question

    This is more of a conceptual question and not a homework question. I am having a hard time understanding why we have to transpose transmission lines and how physically moving them changes anything. Do we only transpose in 3 phase transmission lines when we have 3 wires? Does the orientation...
  22. A

    Lattice Diagram of a transmission line

    In lattice diagram, if the voltage wave is generated at Junction that there is an overhead transmission line before it, will the wave be in two directions with the same magnitude, or one in neglected ? Picture may illustrate .
  23. R

    Coupling signals to a stripline transmission line

    Hello. I have a question about the place where signal enters or gets out from a line (strip or not) Sorry I don't know the exact word in english, may be tap or tapping ... For different lines 1/4 or 1/8 wavelength these points are sometimes @ a 1/4 from the cold point, sometimes elsewhere. I...
  24. Marcus95

    Input impedance of a "Ladder" transmission line

    Homework Statement My electronics&physics lecture notes contain the following side note: ___ "A ladder transmission line comprises an alternating sequence of segments of two different transmission lines both of length $l$ with characteristic impedance $Z1$ and $Z2$. If the line is constructed...
  25. billyray

    Transmission line coefficients

    Homework Statement A transmission line has the primary coefficients . R=2Ω/m L=8nH/m G=0.5mS/m C=0.23pF/m Determine the lines secondary coefficients Z0, α and β at a frequency of 1GHz Homework Equations included in upload The Attempt at a Solution included. I think its just a case of...
  26. S

    Figure 3a shows a 50 Hz, high-voltage transmission line

    Homework Statement Figure shows a 50 Hz, high-voltage, transmission line. The relationships between the sending and receiving end voltages and currents are given by the complex ABCD equations: where 'S' stands for sending-end and 'R' stands for receiving-end (a) Given the parameter...
  27. J

    Lossless transmission line question

    Homework Statement A 50 Ω lossless transmission line of length 0.4λ is terminated in a load of (40 + j30) Ω. Determine, using the equation given below, the input impedance to the line.Homework Equations The Attempt at a Solution Zo= 50 Zl= 50∠36.87 or 40+j30 βι= 144° (this is known due to...
  28. J

    Capacitance of a Transmission line

    Homework Statement A single phase transmission line has 2 parallel conductors 1.5 metres apart. The diameter of each conductor being 0.5 cm. Calculate the line to neutral capacitance for a line 80 km long[/B]Homework Equations CAN = 2(3.14)(permittivity)/Loge [D/r] This gives capacitance per...
  29. Marcus95

    Power Transmitted in a Coaxial Cable

    Homework Statement A coaxial transmission line consists of an inner cylindrical conductor of radius a = 1 mm and a cylindrical outer conductor chosen to make the characteristic impedance 75 ohm. The space between the conductors is lled with a gas which can stand a maximum eld of 105 V/m without...
  30. Y

    Transmission line voltage delay

    The voltage value on a wire a distance away from the signal-port (where we apply the initial voltage) has a phase delay because the current takes time to travel down the wire. But how do you visualize this? First we have a voltage at the beginning, and that causes current to flow. Then a...
  31. P

    Engineering Single-Phase Double Circuit Transmission Line

    Homework Statement There are 4 wires with diameter d arranged in a way such that they form a square shape, with the wires on the corners. The horizontal distance is kD while the vertical distance is D. Prove that the inductance per meter of each conductor is 1/2 + 2ln { [2kd√(1 + k^2)] / d }...
  32. T

    Question about Transmission Line Losses

    Hi there, If I'm using the equation P_loss = P^2 * R / V^2 to calculation the power lost on a cable, where R = p(resistivity) * L(length) / A(cross sectional area), then how long is this power loss for exactly? Thanks, Sydney
  33. A

    Calculating the reactive power of a transmission line

    I have a single machine connected to an infinite bus with the following parameters: - Sending end power = 3.75MW - RL=5.18 ohms - XL=130 ohms - VS (Generator bus)=161kV - VR (Infinite bus)=161kV - Sending end voltage angle (delta) = 0.01877 (Angle which allows 3.75MW to be transferred)...
  34. G

    Transmission line impedance etc

    All transmission lines have a characteristic impedance which is different based on their length and voltage and frequency (for AC lines) , Also all transformers and their primaries have a different impedance based on the turnsratio, core steel type etc factors. Now I read that for any given line...
  35. B

    Losses on Power Transmission Lines

    Say a power station transmits 1008 MWh of power per year to a facility however they only receive 900 MWh say the transmission line is 15 km long how would you calculate average power loss per km, now I know that theoretically it will just be 7.2 MWh per km but what calculations would you make to...
  36. C

    I What really is bandwidth?

    Hello, I am reading a book about simulation, modeling and automatic control. In a chapter about hydraulic transmission line, a pipe's "bandwidth" is mentioned. Contex: "Long pipes are used in large hydraulic installations where pipes of length up to 10 m are not uncommon. Moreover, in offshore...
  37. Manoj Sahu

    Neutral Conductor Current: How is it Possible?

    I read in a book that during normal working condition (balanced system) the current through a neutral conductor is zero i.e., Ia+Ib+Ic =0 where Ia, Ib, Ic are phase current of three different phases a, b, c. How is that possible? I mean if you look at mathematical result of the phases they are...
  38. louisnach

    Electrical Engineering, power system, transmission line

    Homework Statement A 400 kV transmission line has a length of 500 km and a reactance of 0,4 Ω/km. How much power can you transmit, if the power angle (the angle between the voltages of the beginning and the end) is limited to 25 degrees? What would then be the line’s boundary power limit for...
  39. Rodolfo

    Engineering Book that covers transfer function of transmission line?

    I've been looking for that in various applied electromagnetics books but none of them cover that subject even on the internet the only website I saw it mentioned was in this forum I guess I'd like a book with a level a little above for example cheng or balanis
  40. D

    50Hz HV Transmission line question

    Homework Statement [/B] This question was raised on the 11th January 2015 but that thread appears to be no longer active now. There are 2 parts to the question (a) and (b). Initially I am only trying to work on (a) Figure shows a 50 Hz, high-voltage, transmission line. The relationships...
  41. D

    Transmission line Secondary Coefficients

    Homework Statement A transmission line has the primary coefficients as given below. ##R=2\Omega/m## ##L=8 nH/m## ##G=0.5 mS/m## ##C=0.23 pF/m## Determine the lines secondary coefficients ##Z0##. ##\alpha## and ##\beta## at a frequency of ##1 GHz## Homework Equations [/B] In my notes I am...
  42. D

    A ##50\Omega## lossless transmission line impedance question

    Homework Statement [/B] a ##50\Omega## lossless transmission line of length ##0.4\lambda## is terminated in a load of ##(40+j30)\Omega##. Determine, using ##Zin=Z0\frac{ZL cos \beta l+j Z0 sin \beta l}{Z0 cos \beta l +j ZL sin \beta l}## the input impedance to the line. ##Z0=50\Omega##...
  43. D

    Phase Change in a transmission line

    Homework Statement (a) A transmission line has a length, ##l##, of 0.4λ. Determine the phase change, ##\beta l##, that occurs down the line. Homework Equations ##\beta=\frac{\omega}{f\lambda}## or ##\beta=\frac{2\pi}{\lambda}## The Attempt at a Solution This question was posted a couple of...
  44. A

    Inductance per unit length of coaxial transmission line

    1. Homework Statement An air coaxial transmission line has a solid inner conductor of radius a and a very thin outer conductor of inner radius b. Determine the inductance per unit length of the line. Homework Equations the book states the methodology to find the inductance as follows: 1)...
  45. M

    Optimal location of receiver on a transmission line

    I am measuring a communications bus with an oscilloscope at various points along the line. I understand that there will be reflections at each stub as well as at the ends of the bus if the impedance doesn't match. I also understand that these reflected signals will add/subtract with the...
  46. I

    Backward Impedance Matching

    Homework Statement https://scontent-lax3-1.xx.fbcdn.net/v/t34.0-12/14804959_10155526179417281_80533891_n.png?oh=649ee14155ac558b21c6553f003e418b&oe=5809EAA0 Homework EquationsThe Attempt at a Solution 1) I determined the input impedance of the first transmission line (Zin1). 2) I calculated...
  47. B

    Lossless transmission line characteristic phase constant

    Homework Statement A lossless transmission line has an inductance of 9.0 nH / m and a capacitance of 3.6 pF / m. a) What is the line's characteristic impedance? b) Calculate the phase constant of a frequency of 1 GHz. Homework Equations Equation of characteristic impedance of a transmission...
  48. L

    Transmission line impedance question

    I am a bit confused on source impedance and Tline impedance, when both are equal. Let's say source impedance is 50 ohms and Tline imepdance is also 50 ohms. Why does the incident wave divide between the source impedance and tline impedance. The impedance is same, so reflection co-efficient...
  49. T

    Transmission line and measuring Zo

    Homework Statement Figure 2: Homework EquationsThe Attempt at a Solution 3.2) I made an equation for the input voltage and input current Vi = Vs*(Zi)/(Zi+Rs+Rm) Ii = Vs/(Zi+Rs+Rm) I am a bit confused about what voltages V1 and V2 are measuring, and what do they mean by using the phase ...