This paper deals with resonant transmission through a pair of ridge-loaded circular sub-wavelength apertures in an infinite perfect electric conductor (PEC) plane. The effect of the distance between the two resonant circular sub-wavelength apertures allocated along the ridge direction (``parallel'' case) and perpendicular to the ridge direction (``collinear'' case) on the transmission cross section (TCS) is analyzed numerically by using a method of moments (MoM). It is found that the TCS for the parallel case varies more sensitively to the distance than that for the collinearly located case, and the maximum TCS for the parallel case is tripled compared to the TCS value of a single resonant aperture. For the case of maximum TCS in the parallel configuration, the directivity in the broadside direction is about 8.76 times (=9.43 dB) compared to that for the single resonant aperture. For the purpose of validation, the single resonant aperture and a pair of resonant apertures in the parallel configuration with a distance for maximum TCS are fabricated on a stainless steel plate with 0.3 mm thickness, and their transmission characteristics are measured. Experimental results show that the transmittance, which is a transmitted power density measured at 50 cm away from the aperture plane, for the parallel resonant apertures is about 7 times (=8.43 dB) higher than that for the single aperture, which agrees well with the simulation.
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