Anomalous Reflection from Phase Gradient Metasurfaces for Arbitrary Incident Angles

Abstract

Over the past few decades radar cross section (RCS) manipulation has become increasingly important. This increase in interest is due to the development and improvement of stealth technology. While many RCS manipulation techniques exist in the literature, most of these display certain shortcomings. The main disadvantages being complex target designs and narrow frequency bandwidth effectiveness. Metasurfaces are used to address these faults effectively for an array of practical applications. Checkerboard metasurfaces consists of an array of artificial magnetic conductor (AMC) elements, specifically two distinct AMC elements with phase differences of 180◦. This causes phase cancellation between the AMC elements and redirects the scattered energy away from the angle of incidence. The other RCS manipulating metasurface is the phase gradient metasurface (PGM). This study will focus on predicting the reflected wave directions from PGMs with various phase gradients for an arbitrary incident wave. The prediction of the reflected wave direction from PGMs are currently restricted to perpendicular incidence or small angles close to the normal vector. The reflected wave directions from PGMs are determined in the literature by utilising the generalised Snell’s law of reflection. This method is restricted by the relationship of the incident angle and phase gradient magnitude. If the critical value is exceeded the scattered wave direction becomes a complex value. Negative reflection was introduced to the adapted Snell’s law to ensure the predicted reflected wave direction values remain real. However, it is shown that additional energy is also observed close to the plane of the PGM which is not predicted by any of the predicted modes. Array theory is used to determine the scan angle of an antenna array. The PGM can also be viewed as an antenna array where each AMC represents an antenna element with a magnitude and phase value. This study shows that the predicted scattered wave direction is accurately estimated by combining array theory concepts with the adapted Snell’s law. The proposed method of prediction is compared to a variety of simulated and measured metasurfaces. The reflected wave directions for a dual gradient metasurface with various incident angles are simulated in a computational electromagnetic (CEM) software package, CST Studio Suite, and compared to the proposed prediction method. A single gradient metasurface is designed at a different frequency and its bistatic and monostatic RCS is measured in the Compact Antenna Test Range (CATR) at the University of Pretoria.