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# HFSS capacitance/impedance between two bodies

##### Newbie
Hi, I am trying to obtain the impedance (capacitance) between two metallic objects. I have one body inside another separated by 1 mm vacuum gap. The brown body is supposed to be powered and the green one grounded. I used a cable and a lumped port to introduce the excitation.

I simulated the model for a sweep of frequencies and then I calculated the impedance of the mirror to the housing with the S-parameters: Im(𝑍𝐵) = Im(50(1 + S11/(1 − S11))), where S11 is the EM voltage wave reflection coefficient and 50 is the characteristic impedance. The result is ~-1j Ω.
However, if I calculate the reactance theoretically, it results in ~-2.6 Ω. The theoretical value was calculated considering 𝐶=(ε·𝐴)/𝑠 and 𝑍𝐵=𝑋𝐶=1𝑗2𝜋𝑓𝐶Z_B=X_C=1/j2πfC, where A=area=86834.78 mm2 and S=separation=1 mm. εr=1 since the bodies are separated with vacuum.

Is this method correct to obtain the reactance/capacitance between two bodies?
I also attach the model here

#### Attachments

• Mirrror_capacitance.zip
113.5 KB · Views: 53

The discrepancy suggests that there might be a setup or calculation issue. Check all the formula and equations that you're using.

In the analystical formula I am not considering parasitic inductances. If were to consider it, the parasitic inductance should be ~3nH in order to obtain Z=-1j. Is this value usual? I thought it was a too big difference and I have a setup problem in the analysis. Honestly, I think it is more probable to have a setup issue, I am a newbie in HFSS...

C is 769 pF for A=86834.78 mm2, s=1 mm and ε=8.85E-12 F/m.