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I am trying to obtain the input impedance (reactance) between a mirror and its housing. The mirror is insulated from the housing (which is grounded) by some dielectrics. To simplify, the dielectrics were removed from the model. So I have a body (mirror) and a gap of 1 mm to its housing. Both bodies are  enclosed in a parallelepiped with ???????? = 1, which performs the function of the environment. The “perfect electric conductor” (PEC) boundary conditions was assigned to the exterior walls of this structure. I introduced a port between the housing and body by means of a coaxial.  Electromagnetic RF excitation is applied to the coaxial port located on the reverse side of the mirror. I simulated the model in HFSS for f=80 MHz using a lumped port. Then, I calculated the impedance of the mirror to the housing with the S-parameters: Im(????????) = Im(50(1 + S11/(1 − S11))), where S11 is the EM voltage wave reflection coefficient and 50 is the characteristic impedance. The result is ~j1 Ω. However, if I calculate the reactance theoretically, it results in ~2.6 Ω. The theoretical value was calculated considering ????=(ε·????)/???? and 𝑍𝐵=𝑋𝐶=1𝑗2𝜋𝑓𝐶Z_B=X_C=1/j2πfC, where A=area=86834.78 mm2 and S=separation=1 mm. ε=1 since the bodies are separated with vacuum. Is this method correct to obtain the reactance/capacitance between two bodies?