On the Reflection Characteristics of a Reflectarray Element with Low-Loss and High-Loss Substrates

This paper revisits the loss phenomenon (particularly, the dielectric loss) for a microstrip patch in reflectarray mode, and discusses the reflection characteristics (magnitude and phase) for a reflectarray element with low- and high-loss substrates. First, the dielectric losses that occur in a lossy slab backed by a perfect electric conductor are both analytically and numerically investigated. Using similar numerical analysis, the reflectarray element (a patch on top of a slab backed by a conductor) is characterized, based on dielectric losses and reflection behavior. It is observed that for low-loss substrates, the dielectric loss decreases with increasing substrate thickness (as previously suggested in the literature). More importantly, for high-loss substrates, the dielectric loss no longer follows the expected trend (decreasing loss with increasing substrate thickness). The dielectric loss becomes a complex phenomenon, involving the dielectric loss tangent and substrate thickness. It is therefore noted that it is important to recognize the well-behaved and misbehaved phase-swing region for high-loss substrates for a reflectarray element. A simple circuit-model representation is provided for the reflectarray element. The anomalous phase behavior observed for high-loss substrates is explained using pole-zero analysis. Waveguide measurements are performed to quantify these reflectarray losses for low- and high-loss substrates. Finally, the loss mechanisms in a patch reflectarray (scattering mode) are compared to a patch antenna (radiation mode), using parameters such as reflection power and radiation efficiency, and similar loss mechanisms for both structures are apparent.