Svarun,
a few more references on this subject
regards
S. A. Bokhari, J.R. Mosig, F.E. Gardiol (1992). Radiation Pattern computation of Microstrip Antennas on Finite Size Ground Planes. IEE Proceedings - H, 139(3), 278-286.
M. Ando, T. Murasaki, Y. Inasawa, M. Sato, K. Natsuhara (0). GTD Pattern Analysis of Antennas on a Finite Ground Plane by Using Modified Edge Representation.
S. Cui, D. Fang (1997). Analysis of the Diffraction Coefficients of Corrugated Surfaces with the Use of a Numerical GTD Method. Microwave and Optical Technology Letters, 15(5), 297-303.
R. Marg, I. Schön, A.F. Jacob (2000). Finite Ground Plane Effects on the Radiation Pattern of Small Microstrip Arrays. IEE Proceedings Microwaves Antennas and Propagation, 147(2), 139-143.
S. Noghanian, L. Shafai (1998). Control of Microstrip Antenna Radiation Characteristics by Ground Plane Size and Shape. IEE Proceedings Microwaves Antennas and Propagation, 145(3), 207-212.
Z. Ying, P. S. Kildal (1996). Improvements of Dipole, Helix, Spiral, Microstrip Patch and Aperture Antennas with Ground Planes by Using Corrugated Soft Surfaces. IEE Proceedings Microwaves Antennas and Propagation, 143(3), 244-248.