Multilayer Fss

Introduction: Periodic structures have contributed to many technological advancements in recent years. Among other characteristics, these materials offer the possibility of creating perfect magnetic conductors (PMC) with in-phase reflection. Insertion of such surfaces enhances matching and hence efficiency of some antennas (e.g. printed planar antennas) when they have to be installed close to a parallel conducting surface [l]. Frequency-selective surfaces (FSS) or partial reflecting surfaces have been demonstrated in printed antenna configurations for enhancing the performance of the antenna over a narrow or a broad band [2]. A multilayer FSS ground plane has been incorporated into a reconfigurable printed dipole array to achieve broad band operation and to control the phase of a reflector array. FSS has also been used as a backing reflector in order to extend the useable frequency range [3]. In this paper an optimized multilayer FSS is investigated. The objective is to achieve controllable phase coherence over an ultra-wide frequency band. The FSS reflector consists of four different layers, each of them having its own periodicity. In particular, the first layer strongly reflects the higher frequencies and the bottom layer contributes to the reflection at lower frequencies. The intermediate layers reflect mid frequencies. When the FSS is designed to deliver a linearly decreasing reflection phase over an ultra-wide frequency band (referred to the level of the top layer), a substantial gain enhancement can be achieved over the considered frequencies. This FSS is not backed by a perfect metal reflector hence a small amount of transmission through FSS is intentionally allowed, which also leads to back radiation of the antenna. Although this slightly reduces the gain in the main beam direction, it helps to achieve good gain stability. To prove the concept we have chosen a slot antenna available in the literature as the main radiator. Without the FSS, it yields an...