Transmission resonance through small apertures

We deal with the transmission cross section for various resonant small apertures. As examples we consider H-shaped aperture and C-shaped aperture (as a bisected structure of the H-shaped aperture), ridged circular aperture, CSRR (Complementary split ring resonator), and λ/2 narrow slit type of resonant aperture. It is observed that the maximum TCS is given by 2G/4π λ² [m²] irrespectively of the aperture shape and physical size for the four cases (H-shaped, C-shaped, ridged circular apertures, and CSRR type aperture) where the gain G corresponds to that (1.5) of short Hertzian dipole. Also for the λ/2-resonant narrow slot, the maximum TCS is found to be the value given by 2G/4π λ² [m²], where G(1.64) corresponds to that of λ/2 - electric dipole. This result indicates that the transmission cross section can be increased by enhancing the gain(directivity) of the resonant aperture as expected from the fact that the directivity of the aperture increases as the physical area of the aperture increases.