GCPW simulation results (HFSS) at: 60-120 GHz

Hi all,

I hope I can get some advices on this topic

I am trying to simulate a transition from GCPW to Rectangular Waveguide at high frequencies (60-120 GHz).
I had given dimensions for the GCPW:

Line width: W=0.31mm

Gap: G= 0.1mm

Dielectric (ROGERS 5880): H= 0.127mm

therefore I tried to simulate a simple GCPW line, to verify its impedance and other characteristics (see the attached model: BCPW.zip)


I set-up the waveport according to the guidelines of the tutorials and HFSS help but it was impossible to exclude the propagation of high order modes (I am verifying the structure up to 120 GHz), therefore I had to reduce the port width to 1mm.
It seems that the port walls do not interfere too much with the excitation of the port, at least from the attached fundamental mode (Wport 1mm).png file! (am I wrong?)

Unfortunately, the results I am obtaining from the simulation are quite bad.
In spite of being simple BCPW line, the return loss is no better than 10 dB (see RL - IL.png file) and the IL is extremely high.
Anyone could suggest me what is the cause if this high mismatch between the two (identical) ports?

The characteristic impedance has a strong drop-off at high frequency (see Zo.png): should I consider that as normal?

Note: Radiation Boundaries are added to all external faces of the model.