Determining the reflection coeficent with the vector Helmholtz solver for a coaxial port

[electroflow]

I want to use Elmer FEM to simulate the reflection coefficent of a high frequency structure that is fed by a coaxial line.
I have successfully adapted the example from the Elmer GUI Tutorial 18 for coaxial feed lines. But I am not sure how to adapt the calculation of the reflection coefficent from the Energy functional. In the tutorial it just states that for a rectangular waveguide the relation between the energy function l(E) and the reflection coeficent rho is given by

l(E) = i beta k_0^2 a b / mu / k_c^2 (1 + rho)

a, b dimensions of the waveguide
k_0 wave number in vacuum
k_c wave number in z direction
beta = sqrt(k_0^2 - k_c^2)
mu permeability

Is it anywhere documented how this expression arises such that I can adapt it for a coaxial mode?

[raback]

Hi

Do you have some nice reference case? We are currently looking at ways to extract S parameters (NxN matrix) and I guess reflection coefficient is closely related to it.

-Peter