SIW with several modes

Hi everybody,

I am designing a SIW which propagates TE10 at 10GHz and TE20 at 14GHz in CST.
The cut-off frequency for TE20 is already 12GHz in my SIW, which is feed by a microstrip tappered transition.
I though that one excitations would be enough and TE20 would be transmitted inmediatly, but at 14GHz the fundamental mode remains.
Do I need 3 excitations? One at the center of SIW excited by 10GHz-signal and two more at each side of the SIW excited by 14GHz-signal and delayed 180?o to each other?

Thank you and regards

I am talking about SIW, but the same methodology can be applied to a (dielectric-filled) waveguide.