3-D Frequency Selective Structure With Flexibly Controllable Response Based on Triangle Slotline

In this work, a three-dimensional frequency selective structure (3-D FSS) based on the triangular slot is proposed. The proposed second-order 3-D FSS has a transmission zero on each side of the passband, thereby improving the selectivity. The equivalent topological circuits of this FSS are investigated. The two utilized degenerate modes are uncoupled from each other and the filtering response can be flexibly tuned. A 3-D FSS operating at 5 GHz is designed and fabricated with a fractional bandwidth of $8 \%(4.8 \mathrm{GHz}-5.2 \mathrm{GHz})$. Within the passband, the insertion loss is kept under 1.4 dB. This 3-D FSS is distinguished by its structural simplicity, flexibility in tuning, and high selectivity.