I was reading in a book how to derive the equivalent circuits for TE, TM, TEM modes of a generic ideal (without losses) waveguide. After some computations, it found this equivalent circuit for TE modes:
It is written that for f > fc (cut - off frequency of the mode), this structure allows propagation because L1 dominates on L1, while for f < fc there is not propagation because C1 dominates on L1. It is shown in the following schemes:
First question: why should the circuit at left allow propagation, and that at right not?
Now let's consider the equivalent circuit for TEM mode:
It is written that it allows propagation at any frequency.
Second question: Why? From a circuital point of view, if f goes to infinity, L becomes open and C becomes short, so we should have 0 output voltage.