What is Transmission lines: Definition and 79 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. Daniel Petka

    I Does a Fabry Perot cavity charge in discrete steps?

    I'm trying to understand the Fabry Perot interferometer and came across this amazing video. Basically it all comes down to adding E-fields together with each of them being delayed by the cavity round trip time. In the extreme case, either they all interfere constructively at the second mirror...
  2. E

    Propagation speed and LC model of transmission line

    I am a bit confused on the speed of voltage propagation in the transmission line and I hope someone can enlighten me. I understand the speed of voltage propagation is in the same order as the speed of light and it is much faster than current. But if we can model the transmission line with a...
  3. T

    I Are voltage and current waves in transmission lines an artifice?

    The operation of a transmission line is based on the axial propagation of electromagnetic waves between the two line conductors. However, the study of the transmission lines does not focus on E and B waves but on voltage and current waves. It is considered that there are resistance...
  4. stephen8686

    Questions about Circuit Models of Transmission Lines

    This is for my graduate EM Theory class. My background is physics/optics, so I was able to understand when we solved maxwells eqs. for waves propagating between parallel conducting plates, but that lead into the lumped element circuit model of transmission lines which I don't understand. I've...
  5. A

    How transmission lines deliver (established?) power from generator

    Hello: I'm confused about transmission lines. According to Faraday's Law, what's induced is an emf that depends on how fast the coils spin, or whatever equivalent to a simple model seen on Physics textbooks. Power, however, is then assumed constant when talking about the power loss due to...
  6. ergospherical

    Resonance in transmission lines

    I've calculated the relationships between the input impedance, line impedance and terminal impedance for a mismatched transmission line (parallel-wire type and of length ##l##). I'm now asked to consider the case where "the source [an oscillator] is in resonance with the line". What exactly does...
  7. therealist

    How do I find the frequency, speed and direction of a wave

    Hey I am trying to learn how waves move in time and I am not sure how to solve the following question. Can someone please guide me through it.
  8. F

    EMF below Power Transmission Lines

    Many years ago I was responsible for maintaining the integrity of a buried fiber optic cable that utilized a railroad right-of-way. Kinda like watching submarine races ! Along much of this routing, transmission towers / power lines would be adjacent to the railroad. Talking with railroad...
  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. L

    Ground clearances for transmission lines

    Why are ground clearances so large for 765 kV towers? I did a little research on this and even if the switching surge factor is 3 and you have a factor of safety of 1.5, in air (3 kV/mm), it corresponds to 1.62 m of maximum arc length, which doesn't explain why towers are so large. According...
  11. B

    Electricity in transmission lines (Simple V^2/R =P)

    I understand how to get the answer but i don't understand how the answer 0.2 ohms is equal to the resistance of the transmission lines If V^2/P = R then (132kV)^2/800MW = the resistance of the transmission lines, why is the 1% tansmitted as heat relevant at all as this is just one form of...
  12. 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...
  13. 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...
  14. g98

    To reduce cost and weight, power transmission lines are made

    Homework Statement Determine the resistance of a 30 km long wire. b. Assume that the line transfers 1000 MW of power at a potential difference of 500 kV. What is the current through the wires? c. How much power is lost during transmission? What fraction of the transmitted power is lost...
  15. 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...
  16. Dave Johnson

    Current Flowing through High Voltage AC Transmission Lines

    Homework Statement Q1: A power plant produces energy at a voltage of Vi = 18667 V. Before being sent along long distance power lines this electricity is sent through a transformer with 143 turns in the primary coil and 8971 turns in the secondary coil. Voltage is calculated to be...
  17. V

    Question on: Smith charts, power and transmission lines

    I would simply like to know: Given a lossless transmission line with a certain characteristic impedance Z terminated in a load L. How can I go about computing the fraction of the time average incident power arriving that is dissipated in the load? Is this possible without knowing the voltage or...
  18. A

    Why is del(E_t) zero in TEM mode of transmission lines?

    I was going through MIT opencourseware https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-013-electromagnetics-and-applications-spring-2009/readings/MIT6_013S09_chap07.pdf and I didnt understand exactly why in equation 7.1.37 and 7.1.38 in page 192 the divergence of...
  19. 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...
  20. maearnie

    Smith Chart: Find Input Impedance for Antenna Problem

    Homework Statement An antenna is modeled by a 40ohm resistor in parallel with a capacitor with Xc = -j25. The antenna is fed by a lossless 50ohm signal line 0.15 wavelengths long. Use a Smith Chart to find the input impedance to the transmission line.Homework Equations The Attempt at a Solution...
  21. S

    Transmission lines & reflections

    Homework Statement [/B] Can someone help me solving this problem ? A lossless coaxial cable is used in a data network. it has an er =3 (relative permittivity) It is driven by a pulse source with internal impedance of 10 ohm, 1 V amplitude, square wave with duration of both positive and...
  22. 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
  23. T

    Impedance matching a purely resistive load

    Homework Statement A 25-Ω antenna is connected to a 75-Ω lossless transmission line. Reflections back toward the generator can be eliminated by placing a shunt reactance Z at a distance l from the load (Fig. 1). Determine the values of Z and l. Homework Equations...
  24. 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...
  25. T

    Current of AC transmission lines and electricity in general

    Hi, I realized I didn't understand the physical model of something I know the theory of pretty well. I was considering the real power loss of an AC transmission line I2*R, then I realized that from source to load the electrons in an AC line don't actually move anywhere. (as far as I know) Yet if...
  26. S

    Prove That Relationship Given is True for Transmission Lines Homework

    Homework Statement I am having problems with the second part of the question - proving that the relationship given is true. Homework Equations See question. The Attempt at a Solution Firstly, consider a single pair of transmission lines with characteristic impedances ##Z_{1}## and ##Z_{2}##...
  27. toforfiltum

    Power loss in transmission lines

    Homework Statement My textbook says that high voltages are used for power transmission because according to the equation P=VI and P= I2R, power loss can be reduced. What confuses me is why the equation P=V2/R cannot used here? And if used, would be contradictory? I know that R is the...
  28. Frank Coutinho

    Help understanding Partial Mutual Inductance

    I'm sure you are all familiar with calculating the inductance of a long transmission line. We first calculate the partial self inductance and we add to the partial mutual inductance due to the current in the other conductors. Looking at the image of a single-phase system, where I1 + I2 = 0...
  29. N

    Inverse Fourier Tranform of Transmission Lines Wave Equation

    Homework Statement From the derivation of v(x,t) and i(x,t) I am stuck on how the inverse Fourier transform of e^(-jwx/u) was calculated. I am trying to understand how the PDE was fully solved here: http://fourier.eng.hmc.edu/e84/lectures/transmission_line/node1.htmlHomework Equations Not...
  30. cnh1995

    Power Transmission through Transmission Lines

    In the numericals based on transmission line, the wording is like this-" A transmission line feeds power to a load at 132kV, 0.8 pf lagging and so on." In practice, are pf and current really constant? Doesn't it depend on the load? I mean, every minute someone is switching something on and...
  31. E

    Transverse resonance method

    The Transverse resonance method is used to determine the propagation constant of a wave in several waveguides, like the rectangular waveguide, or also dielectric waveguides. It takes advantage of the fact that a standing wave is present along a certain direction (transverse with respect to the...
  32. H

    Calculating power absorbed in transmission lines

    Homework Statement Calculate the power (Pso) absorbed from the supply by the transmission line on open circuit. Given parameters are: A1= 0.8698 A2= 0.03542 B1= 47.94Ω B2= 180.8Ω C1= 0S C2= 0.001349S D1= 0.8698 D2= 0.03542 I have calculated Vs and Is earlier in the question Vs= 77325 +...
  33. J

    Transmission Line Reflected Waves

    Hi , I'm studying transmission lines and how waves move through the device , so far I haven't delved into the maths behind it as I have been trying to get an overview of it.So far I have seen the effects when there is an open circuit , shorted and matched impedance loads. The example I saw...
  34. can12345

    Static Pressure in Transmission Lines - Understanding the Differences

    Homework Statement Is there any difference if I measure static pressure value from the down side of the pipe or up side of the pipe? I am getting an error just small as 300 pascal when I do zero measurement. Here I draw the system also. The diameter of pipe is 10 cm. What could be the possible...
  35. C

    Transmission Lines: Reflection Coefficient vs Frequency

    Hey guys - I've been using this website to study transmission lines. (great study aid - posting for others to use. Great for smith charts) However, I've noticed this graph they have for load matching: I'm having trouble finding or understanding a relationship for reflection vs frequency...
  36. S

    Stubs in Transmission Lines

    Hello experts! What is stubs in regards with transmission lines? I don't find any precise and point to point definition of it. Every where book tries to make us understand with a concept of 4 to 5 lines and this makes me confused. Please tell me the very precise and easiest definition of...
  37. U

    EM Waves in Transmission lines

    Homework Statement (a) Find the capacitance per unit length and find the inductance per unit length of a transmission wire above a infinite conducting plane. I used the image method (b) Show that EM waves can propagate, and find their speed and the characteristic impedance of the line...
  38. T

    Wavelength on power transmission lines

    At the power-line frequency (50Hz), the wavelength of electromagnetic waves in free space is 6,000 km. Similarly, the phase velocity of voltage/current waves is about 4,000 km at this frequency. The question is, why do we bother about the wavelength at all at the power-line frequency, when the...
  39. S

    Compass deflection by overhead transmission lines

    Problem Description: A hiker is reading a compass under an overhead transmission line that is 5.1m above the ground and carries a current of 807A in a horizontal direction from North to South. Assume the Earth's field is of the order 0.5*10^-4T. A) Determine the magnitude of the field produced...
  40. F

    Transmission lines parallel fault

    I am trying to draw the bounce diagram for the trans line in the picture attached to my thread. Sorry won't let me upload... https://www.dropbox.com/home#!/lightbox/home/Year%202/Semester%202/mbgm9mw6 I am not sure what to do here because I have never seen anything like it before. It...
  41. U

    Electric Grid Question - Elevated Transmission Lines

    Electric Grid Question -- Elevated Transmission Lines Hi, Our teacher gave to us an homework which is if we carry transmission lines at 20000 m (20km) high from Earth face, what is this application's benefits and harms? Can anyone help me? Already thanks for your replies...
  42. C

    High power transmission lines: underground?

    In the Midwest (Iowa, etc.), we get plenty of storms, mostly Winter, and power lines go down frequently. Someone is about to build a new high-power transmission line across my County (and too near my property!), and I can't figure out why it is not mandatory in the 21st Century that all...
  43. Q

    Transmission Lines: Frequency Limitations & Attenuation

    I am writing my senior thesis on communication networks and this question sort of popped into my head: What determines what frequencies you can use in a landline connection? (e.g. say I have a wire connecting point A to B to send a bunch of signals, what prevents me from using the entire...
  44. D

    Why does the capacitance of open transmission lines vary near the end?

    I had an occasion to hook up a piece of open ended coax to a network analyzer. I know the wire has some pF per foot but I expected it to be linear over inches but it wasn't. In other words instead of 5 inches = 5X the pF per foot, the capacitance rises sharply near the end of the coax. Any idea why?
  45. F

    Transmission lines 2 wires ?

    Hello, a beginners question here. Can someone please tell me why transmission lines use two wires instead of just one ?
  46. I_am_learning

    Electric Field Generated by Transmission lines

    I am trying to make a software to calculate electric field near transmission lines. When I checked it I got one awkward looking result. At any point near the Transmission line, I guess the time average (or net) Electric Field should be 0. Why should it have any preference on a value and hence...
  47. P

    Best book for Electromagnetic Theory & Transmission Lines

    best book for "Electromagnetic Theory & Transmission Lines" We've this subject of "electromagnetic theory and transmission lines" this semester..and i wanted to know which books deal the content properly ...mastering this topic may require that one cool book (and ofcourse, regular effort)...and...
  48. E

    Transmission lines and Waveguides

    Ok, it's clear to me that waveguides are for sending EM waves from one place to another (e.g. TEM, TE, TM modes). But what about TLs? I've seen them described as carrying AC signals (V or I) but also EM waves, e.g. TEM waves in coax cable. What gives? Does one imply the existence of the...
  49. A

    Calculation of capacitance of transmission lines

    Capacitance between two plates is defined as the charge accumulated per unit potential difference between those plates. But in the case of transmission lines how can we define it? The potential difference any two conductors in a group of several conductors can be derived in terms of the charges...
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