ansylsis of open guided-wave structure

A commerical simulator is used to analyse a new guided-wave structure with open boundaries. Some questions need to be solved:

(1) how to determine the fundamental mode, the first-order modes, the second-order mode,... because there are many parastic modes existing in this structure?

(2) how to determine the cutoff frequencies of the modes?

Hi ! What kind if simulator do you use ?

Most RF simulators (HFSS, CST) allow you to define a port with open boundaries and do 2D calculations to find port modes.
The modes are sorted according their freqs; the first one will be a fundamental mode usually.
If yuo need to look to a particular high order mode, just define more number of port modes in the initial solver setup.

Thank you, navuho.

You are right, but there are many spurious modes when HFSS or CST is used. The existing of these spurious modes have an effect on the determination of fundamental mode or higher-order mode (cut-off frequency and dispersive characteristics). How should I discard these spurious modes?

I really do not clearly inderstand your problem. If you need look to 10-th mode just do it.
Ok, if you need to calculate 100-th mode it could be a problem, do you ?
Please upload the port geometry if possible.