Tunable RIS for Mitigating RFI in Radio Telescopes

This paper presents a practical design and simulation framework for a reconfigurable intelligent surface (RIS) that enables high-precision signal cancellation in a three-dimensional space. The system targets airborne ADS-B transmissions at 1090 MHz—now a major source of interference for radio astronomy observatories. Unlike conventional RIS architectures that support only phase or amplitude control, we introduce a novel unit cell that integrates independently biased PIN and varactor diodes to achieve full two-dimensional tunability of the reflection coefficient. The unit cell is modeled using nonlinear circuit simulations in OrCAD PSpice and validated via full-wave electromagnetic analysis in Ansys HFSS. A tailored admittance-matching method translates target reflection coefficients into feasible RIS configurations. Simulations of a 64×64 RIS array show that more than 97% of the 2993 evaluated directions of arrival achieve sub-0 dB residual power at the receiver, demonstrating robust suppression of mobile interference. This work establishes the feasibility of RIS-based spatial RFI cancellation at a fixed frequency, with implications for spectrum coexistence, secure communications, and ultra-sensitive sensing platforms.