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While reading the transmission lines in electromagnetic field theory , I observed that while analyzing different transmission lines like co-axial cable, and two wire lines, we are using the electric circuit model where we take the circuit as a distributed network, but while analyzing the wave-guide we are mainly focused on the electric and magnetic field lines.

Why do we analyze the two structures differently? One using circuit theory model and another using electromagnetic theory?

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    \$\begingroup\$ How would you propose to define the voltage at a point along a rectangular waveguide? \$\endgroup\$ – The Photon Feb 21 '18 at 2:41
  • \$\begingroup\$ @ThePhoton inside the waveguide, voltage wave does not exists as it is filled with dielectric, that's why we go for electro magnetic theory ? \$\endgroup\$ – Mrinal Senapati Feb 21 '18 at 2:55
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Generally we use the simplest model we can, that produces useful results.

A two conductor waveguide like co-ax that's supporting a TEM (Tranverse Electric Magnetic) Mode can be analysed in terms of a tranverse voltage across the dielectric (just like capacitors have) and a longitudinal current (just like inductors have) in the wires. When we measure the speed of propagation and impedance, we find they're consistent with ascribing sensible 'per unit length' figures for capacitance and inductance.

In a hollow tube waveguide, the TEM mode does not exist, and the modes that do propagate (TE and TM modes) do not map onto nice two-terminal components.

We can also analyse a two conductor waveguide in terms of fields. It's just that it's harder work than circuit theory when we're dealing with the TEM mode. When we apply too high a frequency to coaxial cable and it 'modes', starts supporting tube-type waveguide modes like TE10, then we need the field approach.

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