Investigations On Absorptive/Transmissive Frequency Selective Surface

Absortive/transmissive Frequency selective surface(ATFSS)is a multi-layer integrated periodical structure with absorptive/transmissive,which is usually composed of a lossy resistive sheet and a lossless bandpass FSS.A well-designed ATFSS is an electromagnetic window with low insertion loss.The key of the ATFSS design is the resistive sheet which should meets both the absorption and the transmission impedance condition.ATFSS absorbs other than reflects the out-of-band incident wave,so that RCS reduction in the whole space can be achieved,which has great value on design of stealthy radome.Firstly,an equivalent circuit(EC)method based on two-port network and transmission line(TL)theory is constructed to analyze and design the ATFSS.The multi-layer ATFSS in three-dimensional space is equivalent to a two-port TL network.The free space and the dielectric spacer are equivalent to TLs with different characteristic impedance.Both the resistive sheet and the bandpass FSS are equivalent to the loaded impedance.A method to obtain the equivalent surface impedance of periodic structure is also proposed.The impedance conditions of the ATFSS to obtain good absorptive/transmissive(A/T)performances are theoretically derived.Effective EC models are constructed for various ATFSS designs,and their A/T performances are verified using ADS software.According to the EC model,a high-frequency-transmissive ATFSS(HT-ATFSS)is designed based on strip-type parallel LC resonance(PLC)structure.The PLC is introduced into the element of the resistive sheet to control the distribution of induced surface current as expected.At the resonance frequency of the PLC(f_h),its resonance impedance becomes infinite and separates the resistive element into several short sections,which has blocked the induced current.If all the short sections are short enough,incident wave can be transmitted with low insertion loss.At lower frequencies,the PLC operates as an inductor;series resonance and absorption can be achieved in the whole resistive element.Besides,the extension of the absorption bandwidth is also studied by cascading serveral PLC structures in the resistive element.The design of HT-ATFSS with wide transmission band has been thoroughly studied.To realize a resistive sheet with wideband transmission property,it is required that the PLC structure should possess large equivalent inductance.A spiral resonator(SR)based on strip-type spiral inductor has been designed which has relative large inductance and little parastatic capacitance.It has been inserted into a resistor-loaded metallic hexagonal loop element to realize a resistive sheet with wideband transparent window.The bandpass FSS is a triple-layer FSS in which two layers of identical hexagonal patches are coupled through a layer of hexagonal aperture.The HT-ATFSS design has a wide transmission band with low insertion loss(IL)and a wide absorption band below the transmission band.A moderm-frequency-transmissive ATFSS(MT-ATFSS)is also designed.The impedance conditions for the resistive sheet to realize a transparent window between two absorption bands are theoretically derived.An interdigital resonator(IR)based on interdigital capacitor is designed and applied in the resistive element to meet the impedance conditions.The influence of the reflection phase difference between the FSS and metallic ground plane on the absorption performances at both sides of the transmission band is analyzed.MT-ATFSS structures under both single and dual polarization(s)have been designed.The A/T performances of all the above ATFSS designs have been verified by numerical simulations and experimental measurements.