Hi,
Look at this link (**broken link removed**) and these publications:
C. Mateo-Segura, M. Garcia-Vigueras, G. Goussetis, J.-L. Gomez-Tornero, A. P. Feresidis, "A Simple Technique for the Dispersion Analysis of Fabry-Perot Cavity Leaky-Wave Antennas," IEEE Transactions on Antennas and Propagation, in press
C. Mateo-Segura, G. Goussetis, A. P. Feresidis, "Sub-wavelength Profile 2-D Leaky-Wave Antennas with Two Periodic Layers," IEEE Transactions on Antennas and Propagation, Vol. 59, No. 2, pp. 416-424, February 2011
S. Wang, A.P. Feresidis, G. Goussetis, J.C. Vardaxoglou, "High-Gain Subwavelength Resonant Cavity Antennas Based on Metamaterial Ground Planes," IEE Proc. Antennas and Propagation, Vol. 153, No. 1, pp.1-6, February 2006
Feresidis, G. Goussetis, S. Wang, and J.C. Vardaxoglou, "Artificial Magnetic Conductor Surfaces and their Application to Low Profile High-Gain Planar Antennas," IEEE Transactions Antennas and Propagation, special issue on Metamaterials, Vol. 53, No. 1, pp. 209-215, Jan-05
S. Wang, A. Feresidis, G. Goussetis, and J.C. Vardaxoglou, "Low Profile Resonant Cavity Antenna with Artificial Magnetic Conductor Ground Plane," Electronic Letters, Vol. 40, No. 7, pp. 405-406, Apr-04
You need to have a radiating source (patch antenna for example) between two partially reflective sheets. You can go through the one in red above, it gives a good over-view on the concept. Then you can copy the same design to HFSS in order to validate the results.