# Need help understanding another answer on TEM coaxial cable modes

Background:

If I make a coaxial waveguide from the gap between two long, perfectly conducting, coaxial cylindrical surfaces, (e.g. perfect "coax" cable), then if I apply charges of $+q$ and $-q$ to the inner and outer conductors, the electric field in the gap will be transverse, lying completely within a perpendicular cross-section, and outside of the outer conductor the electric field would be zero.

If I then ran currents of $+I$ and and $-I$ in the inner and outer conductor, the magnetic field in the gap would also be transverse (in-plane) and outside would also be zero.

Question:

I believe that this answer says that this is sufficient to then conclude that at least the lowest order mode of AC (alternating current) propagation would be TEM (Transverse Electric and Magnetic fields). And I believe this could be shown by adding the appropriate $\exp(i(\omega t-kz))$ longitudinal terms and showing that it is still a solution of Maxwell's equations.

But I'm having difficulty understand if the answer is saying something more generalized than this? Is there something more there which is useful to the behavior waveguides made from generalized coaxial pairs than just "if the fields are each transverse in DC (direct current) then they'll also be transverse for AC"?

You are asking a fairly deep question. But in fact, to say "If the fields are transverse in DC they will also be transverse in AC" is wrong.

It is true that the solutions to Maxwell's equations for E and H are transverse at DC, and transverse fields satisfy the equations in the AC case. But at sufficiently high frequency the equations also allow field configurations where E and H are not both transverse. I won't presume to describe the math, especially not in a short space. Suggest you look around the web for higher order modes in coaxial cable.

• Do you think you could include a link to a reference where this is discussed in some more details? Thanks. Nov 20, 2017 at 23:09
• Double check that you are addressing this question, and not the linked Q&A. Here I've asked "Is there something more there which is useful to the behavior waveguides made from generalized coaxial pairs than just 'if the fields are each transverse in DC (direct current) then they'll also be transverse for AC'?" where "here" refers to the linked Q&A.
– uhoh
Nov 21, 2017 at 0:30