Transmission lines Definition and 12 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. 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.
  2. 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...
  3. 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...
  4. 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...
  5. maearnie

    Smith Chart 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...
  6. 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
  7. 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...
  8. 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...
  9. S

    Transmission Lines

    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}##...
  10. 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...
  11. 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...
  12. 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...