[SOLVED] [HFSS] rectangular waveguide characteristic impedance

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Dear all,

I'm puzzled by the result given by HFSS (v12 or v13) in a simple rectangular waveguide case.

When modelling a rectangular waveguide which cross-section dimensions are 72mm x 34 mm (WR284) and let say a 200mm length, at 3GHz, HFSS solution data Z0 are :

- using Zpi (default) integration line : Z0=304.7 Ohm
- using Zpv integration line : Z0=494 Ohm

However, using analytical expression in this simple case (for TE mode), I find :

% waveguide cross-section dimensions
a = 72e-3;
b = 34e-3;

% frequency
f = 3e9;

% mode indexes
m=1; n=0; 

%% -------
c0 = 299792458;
k0 = 2*pi*f/c0;
omega = 2*pi*f;
mu0 = 4*pi*1e-7; eps0 = 1/(mu0*c0^2);
Z0 = sqrt(mu0/eps0);

% guided wavenumber
beta = sqrt(k0^2-(m*pi/a)^2-(n*pi/b)^2);

% characteristic impedance of the TE mode mn
Zc = k0.*Z0./beta

gives Zc = 523.2295 Ohm


What am I misunderstanding here ?


Best regards,

 

[Hash]

Dear all,

I've checked with CST, which gives the same that the analytical results, i.e. 523Ohm.

CST default impedance definition uses the average of the E/H fields in the port surface. If we force CST to calculate the impedance as in HFSS, i.e. using V²/P integration line (Zpv), then one finds 494Ohm, as in HFSS.

So, it appears that HFSS port impedance (either Zpi or Zpv) does not correspond to the modal impedance of the port (the analytic one, labelled as wave impedance in CST).

This is quite important for hollow waveguide users...