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I was reading an Electromagnetism Lecture and about how they are transmitted in Transmission Lines and I got a little confused.

The question is:

• EM waves propagate between the transmission lines however do they propagate as EM waves or as Voltage SIGNALS? IF they propagate as voltage signals do they lose their wave properties/identity?

Or do they actually propagate as waves passing in the electromagnetic FIELD generated by the coaxial cable for example or what have you.

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    $\begingroup$ Is there a difference? Voltage and electromagnetic field are intimately connected. $\endgroup$ – probably_someone Feb 12 '18 at 18:02
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They are two different models for the same underlying physical phenomenon. The telegrapher's equations (voltage and current model for propagation on a transmission line) is a simplification of the full EM model.

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  • $\begingroup$ So physically which occurs? $\endgroup$ – Hasan Hammoud Feb 12 '18 at 18:45
  • $\begingroup$ @HasanHammoud, How do you want me to separate what physically occurs from our models of what physically occurs? Practically, you can count on Maxwell's equations describing the physical reality to a very high accuracy. Every other classical model is a simplification of that. $\endgroup$ – The Photon Feb 12 '18 at 19:16
  • $\begingroup$ Sorry for my questions but I'm a bit confused so basically can we say that for a coaxial cable for example an electromagnetic field is generated and the wave propagate as it is in this field ? $\endgroup$ – Hasan Hammoud Feb 12 '18 at 19:48
  • $\begingroup$ Yes, an EM wave propagates between the outer and inner conductors. We can model this accurately enough for many purposes by talking about the voltages and currents on the conductors. $\endgroup$ – The Photon Feb 12 '18 at 21:24
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The usual electromagnetic wave propagating on transmission lines are TEM waves that can be modeled by solving Maxwell's equations with the proper boundary conditions on the conductors or with a distributed circuit model leading to telegrapher's equations. Both approaches are equivalent as long as you are considering the TEM wave propagating along the transmission line and as long you are not considering too high frequencies. Thus the usual EM waves on the coaxial cable correspond to voltage (and current) waves that can propagate as signals on the transmission line.

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  • $\begingroup$ TEM modes only occur in certain kinds of waveguide, such as coaxial cable. They don't occur in many other important types of waveguide such as hollow rectangular waveguide and microstrip lines. $\endgroup$ – The Photon Feb 13 '18 at 3:06
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    $\begingroup$ @ThePhoton - You are right, but hollow rectangular waveguides are not transmission lines and the open microstrip lines have fields that are still designated as quasi-TEM modes. All open wire transmission lines, e.g, parallel wires (Lecher-lines), support TEM modes at least in good approximation and are also well described by telegrapher's equations, although they usually have higher (also radiative) damping. The advantage of transmission lines compared to wave guides is that they have no lower cut-off frequency. $\endgroup$ – freecharly Feb 13 '18 at 3:33

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