How can I find eigenmodes propagating in given direction?

Hi!

I have a general question about eigenmode problem in RF module. How can i find only eigenmodes propagating in given direction? For example, let's consider an infinite number of parallel cylinders lying on substrate. I can model such a geometry using pereodic boundary conditions. I would like to find eigenmodes of this system propagating parallel to the substrate and perpendicular to the cylinders. How can i find such a solution?

Thank you in advance!