I am modelling a microwave structure using circuit theory, but I have a small confusion regarding characteristic admittances (I am assuming rotational symmetry and TM modes so I only have E_z, E_r and H_phi).
Let's say that I have a cylinder and a wave that propagates in the z-direction (i.e. perpendicular to a planar face of the cylinder). Moreover, the cylinder has two ports: an axial one (located at a planar face) and a radial one (located at the outer radius). I need to calculate the characteristic admittance (both, incident and reflected) at both ports, so I have to make an analysis port by port (short-circuiting the remaining ones). In the axial case (i.e. port 1) it is easy because each EM mode inside the structure depends only on one component of the incident fields (i.e. of the wave) and because the fields tangential to the port are also tangential to the propagating wave (i.e. the tangential fields are E_r and H_phi). However, in the radial case (i.e. port 2), each EM mode is coupled to several field components, and the fields tangential to the wave are E_r and H_phi, but the ones tangential to the port are E_z and H_phi.
Thus, I am not sure about which components I should be using for calculating the characteristic admittance. Any help, pointers or info is appreciated.
So the wave at Port 2 does not pentrate the port plane or surface?
Well, now that you mention it, I suppose it does not (since the wave travels tangential to the port). However, there should be an effect at the surface (i.e. the port) considering that there is an interaction between the port and the wave. Moreover, the analysis of the structure leads to an admittance matrix that indicates some interaction between those elements...
Right now, I am considering it is made of air, since I need to validate my equations and I am thinking of doing so by comparing the resonant frequencies of the cylinder against those calculated analiytically (I already did it for the axial port and it worked).
Hi! Excuse my bad English.
If I correctly understood Your question, it is about definition and calculation of characteristic admittance or characteristic impedance for evaluation parameters of two transmission lines junction. Both of these quantity aren't rigorous physical quantities because can't be measured generally. It follows from the circuit theory. Therefore the circuit theory is insufficient for calculation of a junction parameters. It is also needs to use results of experiment, or electrodynamic calculation. In both cases diffraction will be considered in a junction place.
Yours faithfully, Jakob Reyzenkind.
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