hfss differential pair

Hi, everyone. I have a question about the differential pair definition in HFSS Driven Terminal simulation.

I am wondering is the Differential Terminal Port Impedance equal to the odd impedance and the common terminal port impedance equal to the even mode impedance in the couple line.

Actually, from the manual of the HFSS chap. 8: signal integrity example, it defines two edge coupled stripline and assign two terminal line in one waveport. And then the differential pair is assigned.

However, the simulation results show that the differential impedance is 116.76 Ohm but the common one is 32.61 Ohm. It contradicts to my knowledge that the even mode impedance should always larger than the odd one.

Thanks.

Hi, winglj
Based on my knowledge, the differential terminal port impedance should be twice the odd impedance while the common terminal port impedance half the even impedance. Because the even and odd impedance is defined on single microstrip line while the differntial and common impedance is defined on coupled lines.

impedance
odd = sqrt(L/(Cgnd/2+2*Cgap))
even = sqrt(L/C/2)

differential impedance should be much smaller than the common mode impedance
the smaller the gap the larger difference

www.hfss.com

Terminal mode Excitation is iner of Antenna but Driven Terminal is outer of Antenna.
At transmision line Tow port required and but in Antenna is one Exitation port.