For TEM modes, H_z = 0 and E_z = 0, so we can easily find from Maxwell's equations that TEM modes in, say, coax have no low-frequency cut-off. The TEM modes are dispersionless and exist from the upper frequency cut-off all the way down to dc. This in every emag text book.
However, the question is what is the actual physical picture here? What is a good mental visualization to physically explain this propagation down to dc?
TEM modes only exist in waveguides with two or more conductors. Usually they are called transmission lines. If they have more than two conductors, they can be used to carry a DC current. Consider the electric fields around the conductors in the DC limit. They will "point" straight from one conductor to the other and be perpendicular to the direction the current is propagating. The magnetic field will do loops, but also be perpendicular. Thus, we call them TEM modes.
There actually is no upper frequency cutoff for the TEM mode. The upper frequency cutoff refers to when there is suddenly a second order mode that can also propagate. Usually we want waveguides (or transmission lines) to support only a single mode. Thus, it is common to talk about transmission lines operating from DC up to a cutoff, but really this cutoff is just when there is a second order mode now in addition to the TEM mode.
Hope this helps!!
TEM modes only exist in waveguides with two or more conductors. Usually they are called transmission lines. If they have more than two conductors, they can be used to carry a DC current. Consider the electric fields around the conductors in the DC limit. They will "point" straight from one conductor to the other and be perpendicular to the direction the current is propagating. The magnetic field will do loops, but also be perpendicular. Thus, we call them TEM modes.
There actually is no upper frequency cutoff for the TEM mode. The upper frequency cutoff refers to when there is suddenly a second order mode that can also propagate. Usually we want waveguides (or transmission lines) to support only a single mode. Thus, it is common to talk about transmission lines operating from DC up to a cutoff, but really this cutoff is just when there is a second order mode now in addition to the TEM mode.
Hope this helps!!
The shape of the TEM field distribution in the waveguide cross section is constant, independent of longitudinal distance, two dimensional, and independent of frequency. This distribution is simply multiplied by the longitudinal wave function (exp[-j k z], with k=2 pi f /c), with frequency varying 0 < f < infinity.
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