Quad‐band linear to circular reflective polarisation transformer and its application in dual‐sense circularly polarised antenna design

This study presents the design and application of a novel quad‐band reflective polarisation transformer (RPT) with linear to dual‐sense circular polarisation (LP to DS‐CP) conversion. The proposed reflector is a highly miniaturised ( 0.1 λ 0 × 0.025 λ 0 ) two‐layered structure built on a thin dielectric substrate (0.005 λ 0 , where λ 0 is the free‐space wavelength of the lowest frequency with zero reflection phase) and backed by a ground plane. The design consists of anisotropic capacitive patches due to which the reflection phase of the orthogonal components of the LP wave differs by +90° (yielding right‐hand circularly polarised (LHCP)) at 2 GHz and by −90° (yielding right‐hand circularly polarised (RHCP)) at 2.4, 3.5 and 5.8 GHz. Further, it is proposed that the CP sense can be controlled by introducing narrow rectangular slots on both sides of the two‐layered RPT. These perturbations delay (or advance) the reflections by 180°, thereby converting RHCP component to LHCP (or vice‐versa). This diversity principle is verified using full‐wave and circuit simulations. A prototype of the RPT is fabricated and measured to validate these results. For practical verification, the RPT is placed behind a quad‐band LP antenna operating exactly at the ±90° reflection phase frequencies. The polariser converts LP to RHCP at 2.4–2.55/3.38–3.6 GHz and LP to LHCP at 2, 5.8–5.92 GHz.