Unbalanced CRLH Leaky-wave Antenna With Broadside Radiation Based On Spin Photonic Topological Insulator Featured Hexagonal Configuration In Armchair Arrangement

A new X-band leaky-wave antenna has been developed using spin photonic topological insulators. This antenna features a hexagonal unit cell arranged in an armchair configuration. This arrangement provides advantages over the zigzag configuration, particularly by offering a wider operational region and a more suitable pattern. To design the structure, a parametric study on the cell dimensions has been conducted, and another study has designed the transition region to couple the topological structure with the classical line. The proposed antenna has a low profile and illuminates two sides of the structure simultaneously. Additionally, it offers a 53-degree scanning range and a bandwidth of 2.7 GHz, making it a groundbreaking improvement over other leaky-wave antennas that utilize photonic topological insulators. The proposed antenna is an unbalanced CRLH leaky-wave antenna capable of radiating in both backward and forward directions. Notably, as it transitions through the broadside within its scanning range, there is no significant drop in performance, even in the presence of an open stop band. To the best of our knowledge, this characteristic is unique among unbalanced CRLH leaky-wave antennas.

💡 Research Summary

The paper presents a novel X‑band unbalanced composite right/left‑handed (CRLH) leaky‑wave antenna (LWA) that leverages spin photonic topological insulators (PTIs). The core innovation is the use of a hexagonal unit cell arranged in an armchair configuration rather than the conventional zig‑zag layout. This armchair arrangement places the majority of the topological bandgap within the fast‑wave region, enabling edge modes to radiate over the entire bandgap and providing a much wider operational bandwidth than previously reported PTI‑based LWAs.

A comprehensive parametric study examined the influence of unit‑cell period and border width on the bandgap. Increasing the period reduces both the upper and lower band edges, with the upper edge decreasing more rapidly, while widening the border expands the bandgap, especially for shorter periods. Based on these results, a period of 12 mm and a border width of 0.5 mm were selected, yielding a bandgap that fully covers the X‑band (approximately 8–11 GHz).

Because the matching technique used for zig‑zag structures cannot be directly applied to the armchair geometry, a new transition region between the topological metasurface and a conventional microstrip line (ASL) was designed. Nine cut‑planes were evaluated; the electric‑field profile of cut‑plane 1 (or 9) showed the best similarity to the ASL field while minimizing fragmented remnants that would act as local cavities. Optimized dimensions of the ASL were obtained through full‑wave simulations in both Ansys HFSS and CST Studio, confirming low return loss in the bandgap region and modest insertion loss even when two edge modes with opposite phase velocities coexist.

The fabricated antenna has a radiating length of about 97 mm and radiates simultaneously from both sides of the structure. Measured (simulated) results show backward radiation starting near 8 GHz at ±126°, a smooth transition through broadside at 9.5 GHz with a realized gain of 3.47 dB, HPBW of 18°, and sidelobes below –10 dB, and forward radiation up to ±73° at 10.8 GHz. The total scanning range is 53°, covering both backward and forward directions. Above 11 GHz, two additional backward beams appear due to the coexistence of two edge modes with opposite phase velocities, indicating that the practical operating band should be limited to 8–11 GHz to avoid simultaneous forward and backward beams.

A key achievement is the mitigation of the open stop‑band effect that typically causes a sharp gain drop at broadside in unbalanced CRLH LWAs. In the armchair‑based design, the fast‑wave region dominates the bandgap, so the antenna maintains stable gain when the beam passes through broadside, a behavior not reported in prior unbalanced CRLH LWAs. Consequently, the antenna delivers a 2.7 GHz bandwidth, 53° scanning, dual‑side radiation, and robust broadband performance without the usual broadside degradation.

The work demonstrates that spin PTI metasurfaces, when configured in an armchair lattice, can overcome the limitations of earlier PTI‑based leaky‑wave antennas, offering a practical path toward low‑profile, wide‑scan, broadband LWAs for applications such as radar, communications, and sensing. Future work may explore longer apertures for higher gain, multi‑band extensions, and experimental validation of the fabricated prototype.