[HFSS] rectangular waveguide (characteristic) port impedance Z

Dear all,

I've previously started (and closed) a thread in this forum (06-04-11) about the characteristic impedance calculation of rectangular waveguide in HFSS.

The conclusion at that time was that the results given by HFSS were not in accordance with the classical electromagnetic books (Pozar, Harrington, Colin, etc.). (CST gave the same results than the analytic formula.)

You will find attached to this post a HFSS project which proves that HFSS can give the same result than the analytic formula for the fundamental TE10 mode only when the ratio between the rectangular waveguide cross section lengths (a/b) is exactly 2 (with the Zpv line integration port setup) !

I personally have no clue why ; and the HFSS support doesn't help neither.

Best regards,

Hello Hash,

the point is that you cannot define an unambigous impedance for waveguides with non-TEM modes. The voltage and current depend on the integration paths you choose. An unambigous definition is the one for the field impedance which is the relation of the fields Ex/Hy. You also get the same value if you use so-called generalized voltages and currents for which the E^2 and H^2 field is normalized to 1 integrating over the cross section (see Marcuvitz, Waveguide Handbook).

The important thing is: Everything works well, as long as you always use the same impedance definition in your circuits. The reason is that only the relation of impedances Z1/Z2 matters and not their absolute value.

Regards, gsassle

Sure, I perfectly agree with you.

However, since the HFSS documentation is quite poor, I'm almost certain that a lot of HFSS users who model waveguides are not aware of that...