Research on Advanced RF Reflection Control Technologies

This dissertation presents a comprehensive investigation into advanced radio frequency (RF) reflection control technologies, with a focus on the design, simulation, and experimental validation of nonreciprocal metasurfaces. These metasurfaces are engineered to achieve unidirectional electromagnetic wave transmission, suppress undesired reflections, and enhance electromagnetic compatibility, stealth performance, and surface wave mitigation. In recent years, nonreciprocity has drawn increasing attention for its ability to break time-reversal symmetry, thereby enabling functionalities that are unattainable using conventional reciprocal or passive devices. In this study, both passive and active mechanisms are explored to realize efficient nonreciprocal behaviour over the C-band frequency range. Initially, equivalent circuit models based on transmission-line theory are developed to describe the response of ferrite-loaded metasurfaces. These models are verified by full-wave electromagnetic simulations, demonstrating high accuracy in the frequency range below 7 GHz. Ferrite-based metasurfaces using planar metallic structures are then fabricated and tested in waveguide measurement setups. Strong nonreciprocity is observed near 6.5 GHz, with isolation levels exceeding 10 dB, and the response is shown to shift depending on the magnetic bias and ferrite permeability. To address limitations such as bulky magnet requirements and thermal instability in ferrite systems, this work further explores active metasurfaces using PIN diode-controlled C-ring resonators. These structures allow low-voltage electronic tuning of the transmission characteristics, achieving direction-dependent behaviour without magnets. Simulations and experiments confirm the effectiveness of this approach. Furthermore, dual-polarization structures and 45-degree rotated configurations are investigated to realize polarization-independent designs. While the isolation is somewhat reduced due to field orientation, the desired nonreciprocal response remains.
Overall, this study demonstrates that well-designed nonreciprocal metasurfaces, integrating both passive and active control methods, can achieve compact, low-loss, and reconfigurable behaviour suitable for integration into modern antenna systems, radomes, and surface wave suppression platforms.