In this study, a subwavelength aperture antenna with multiple corrugations and a superstrate layer was investigated based on Fourier transform and mode matching, demonstrating a rigorous and fast convergent series solution. The aperture antenna with a corrugated conducting flange and a dielectric superstrate was fabricated in the X-band frequency range to demonstrate the validity of the derived series solution. The measured reflection coefficient and radiation pattern of the proposed antenna are also presented and compared to the computational results to assess the effect of the dielectric superstrate. A maximum gain enhancement of 5.19 dB was achieved by employing the superstrate.