substrate modes/surface waves in HFSS

Hi Guys,

I am working on cpw and microstrip lines with HFSS.

I am tryimg to visualize the losses I can have along my transmission line in the form of surface waves.

I better explain, I know that along my transmission line, because of a thick substrate, I lose some power in the substrate (GaAs).

Defining multi-mode ports does not help since it gives me just the cut-off frequencies of the possible propagation modes from the ports. I already checked my propagation constant, and they are out of band.

But I still lose power along the transmission line, this has been double checked from a proper measurement on a real chip.

I am sure that there are some modes excited along the structure as evanescent....and some surface mode generation.

Have you got any idea opn how to identify and visualize them?

I hope there is someone out there who can enlight me.

best regards

oby