Circular and elliptical loop resonators are proposed as the unit cell of 3-D frequency-selective structure aiming to reduce the structure thickness. The structure exhibits a pseudoelliptic response by utilizing the coupling between the two quasi-TEM modes, which are concentrated in air and substrate regions of the unit cell. Using elliptical loop with stepped-impedance resonator (SIR), the thickness of the structure can be reduced by 80 compared to previous structures. A comparison between the response of the structure employing the microstrip line as the resonator and the structure using the proposed loop resonators is performed. Three prototypes are designed, fabricated, and tested. The first structure is designed to have a dual-band bandstop response using two half-circular loops with end via holes; it has center frequencies at 10 and 14.3 GHz with -3 dB fractional bandwidths of 6.7 and 6.6, respectively. A half-elliptical loop with SIR is employed in the second structure to achieve more thickness reduction. The third structure is designed to exhibit a dual-polarized bandstop response. Stable frequency response under various angles of incidence is obtained.
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