We studied coupling phenomena between surface plasmons and electromagnetic waves in the microwave spectrum using circular apertures surrounded by array of grooves. We first present experimental and theoretical results of enhanced microwave transmission though a subwavelength circular aperture with concentric periodic grooves around the surface plasmon resonance frequency. This is followed by transmission studies through circular annular apertures and circular annular apertures surrounded by concentric periodic grooves. We demonstrated that 145 fold enhancement factor could be obtained with a subwavelength circular annular aperture surrounded by concentric periodic grooves. Our results show that, high transmission from a circular annular aperture with grooves is assisted by the guided mode of the coaxial waveguide and coupling to the surface plasmons. (C) 2005 Optical Society of America.