Control TEM field pattern of waveport.

How to get desired TEM field pattern with 3 or more conductors?

I draw modal/terminal line between the 2 conductors.
When I plot the E field pattern of the waveport,
I find, sometimes, HFSS treat the last conductor as floating,
and sometimes, sig/gnd.

How to control that?

When you set up the waveport ask for more than one mode. Five or six would be a good place to start but you may need even more. You can then examine the various modes after letting the code iterate several passes. Then focus your attention on the desired mode. You may need to modify your impedance line to keep things in sync.

Take a look at the manual as well under impedance calculations.

My problem is that the TEM mode that I want is the linear combination of the fundamental modes that HFSS gets.

How can I get my mode directly?

have you discussed this wiht Ansoft technical support?

You can excite multiple modes and control their relative phase and amplitude in your post processing. Since you have a mode that is a linear combination of several others I would think that is the way to go.