Frequency-selective radome absorbers based on interdigital resonators for radar cross section reduction

Abstract

In the midst of increasing advances in the detection capability of modern radar systems, stealth technology aims to reduce the radar cross section (RCS) of a platform, making it difficult to detect. Among the principles on which stealth technology is based, active systems have stood out because of the various advantages they present. Due to their spatial filtering characteristics, frequency-selective surfaces (FSSs) have been widely explored as active systems. Considering the strong out-of-passband reflections in various directions, bandpass FSSs can significantly reduce monostatic RCS, however they can cause large bistatic RCS. In this context, a new structure, called frequency-selective radome absorber or frequency-selective rasorber (FSR), has attracted the attention of researchers in recent years. This type of structure combines the design of a bandpass FSS with a resistive absorber FSS, to obtain two absorption bands (A) located on both sides of a transmission band (T), also known as absorption-transmission-absorption (A-T-A). Therefore, it is capable of reducing out-of-passband reflections in other directions. In this work, two dual-polarization A-T-A FSRs based on interdigital resonators (IR) are proposed, whose lossy FSS designs feature elements with four incorporated resistors and different layouts; split-ring and crossed dipole. The two rasorbers presented provide good operational bandwidth, and have passbands around 9.7 GHz and 4.78 GHz, whose minimum insertion losses (IL), at normal incidence, are 1.0 dB and 0.77 dB, respectively. The equivalent circuit models (ECM) for the proposed FSRs are presented with the aim of explaining the parallel resonance in each case. The rasorber prototypes are fabricated with low-cost FR4 substrate in both layers, and measured for demonstration. The results are very coherent when compared to those obtained in the simulations. Due to their symmetries, the presented FSRs have good angular stability. Therefore, it appears that the proposed rasorbers are capable of reducing the RCS of communication systems operating in different bands, contributing to making them stealthy. Finally, performance comparisons with other FSRs implemented with parallel resonance in previous works are presented.