We experimentally discovered that the efficient coupling of surface plasmon wave to free-space radiation beyond the interaction zone via random metallic particles. In the experiments of terahertz wave transmission through an ensemble of randomly packed aluminium particles with sub-wavelength sizes, we observed a strong dependence of the transmission on the lateral boundary conditions. Our results showed that the processes of terahertz wave propagating in the disk-shaped samples and reemitting to the free space were resulted from surface modes excitation, which is critically affected by lateral sizes of the particle ensembles.