**4. Study on effect of orientation of CSRR ring on frequency response**

The effect of changing the orientations of the CSRR ring was exhaustively studied by [12], which was verified by simulations and experiments that modify CSRR's orientations, different passband characteristics can be obtained. The orientation was specified with respect to the direction of the outer ring's split, as shown in **Figure 10**. Hence, they are aligned face to face, back to back, and side by side. The side-by-side type has also been divided into two cases with the CSRRs reversely or equally oriented.

**Figure 10.**

*Configurations of various SIW-CSRR unit cells in which the CSRRs are: (a) face to face, (b) back to back, (c) side-by-side reversely oriented, and (d) side-by-side equally oriented.*

After simulation of various orientations, it was found that by altering the configuration of the CSRRs in a particular position (face-to-face orientation), the propagation of TE10 mode can be suppressed, resulting in enhanced selectivity and stopband rejection of the filter. The waveguide width was chosen as w = 12.3 mm to keep the cutoff frequency of the initial SIW at about 8.7 GHz. The Rogers substrate RT/Duroid 5880 with a thickness of 0.508 mm and a relative permittivity of 2.2 is used in the design. The metalized vias have a diameter of 0.8 mm and a center-to-center spacing of 1.48 mm.

After the simulation of various configurations, it was found that the unit cells with face-to-face and back-to-back oriented CSRR exhibit a similar kind of passband with one transmission zero and one pole located above the passband. Nonetheless, for the second case, the transmission zero is close to the pole leading to a steep upper side transition but with large insertion loss due to the weak coupling. For the third case, two rings are arranged side by side in opposite directions, and two transmission poles with two transmission zeros in the upper band are achieved. The propagation is quite weak for the fourth case due to weak magnetic coupling.

Eventually, a two-stage filter using the unit cell aligned face to face is simulated and fabricated using Rogers RT/Duroid 5880. A distance of 8.8 mm separates the two cells. The proposed bandpass filter achieves one transmission zeros at 6.4 GHz in the upper band, resulting in high selectivity and a wide upper stopband. The two-pole filter has a measured center frequency of 5.0 GHz and a 3-dB bandwidth of 0.33 GHz (3.2% FBW).
