Zhixin Yao

In the analysis of most circuit analog (CA) absorbers, it is only the absorption performance under normal incidence that is taken seriously, leading to a much worse absorption with the increase of incident angle, especially when it is larger than 30°. In this communication, a novel absorber, which consists of a conductive square-loop (SL) array embedded with lumped resistors and a well-designed wide-angle impedance matching (WAIM) layer, is proposed. Equivalent circuit (EC) and strict formula derivations are introduced to present a great insight into the deterioration of absorption performance under oblique incidence and the effect of WAIM layer. An absorber sample is further fabricated and measured for verification. It is shown through measurement that the structure has a fractional bandwidth of 137.1% for at least 10 dB reflection reduction under normal incidence. When the incidence angle is increased to 45°, the overlapped bandwidth for both transverse electrical (TE) and transverse magnetic (TM) polarizations is still measured to be 110.5%.

In this letter, a novel and effective method for non-Foster active absorber with ultralow profile and wide absorption band is proposed. The three-layer absorber consists of a square loop frequency selective surface array printed on the FR-4 dielectric substrate, a non-Foster matching layer, and a backed metal plane.Equivalent circuit model for the active absorber is proposed and analyzed, providing great insight into the effect of non-Foster matching layer on breaking the Rozanov limit effectively. A 1×10 absorber absorbing structure sample is fabricated.It is shown by measurement that the absorber has a fractional bandwidth of 36.0%, with a total thickness of only 0.008 λ_L (wavelength at the lowest absorption frequency). The simulation and experimental results are in good agreement in the operating band, which verifies the accuracy of the design method.

In this letter, a novel and effective design method for multilayer and multiresonant absorbers is proposed. The two-layer absorber is composed of two resistor-loaded square loop frequency selective surface (FSS) arrays printed on the Rogers 4003 dielectric substrate and a backed metal plane. Equivalent circuits for two-layer absorbers with resistive sheets and resonant FSSs are proposed, analyzed, and further improved to provide great insight into the existence of the four resonances and ultrawide bandwidth. In the equivalent circuit model, the substrate is transformed to a parallel admittance and a positive susceptance is introduced in the whole operation band, which contributes to an extra resonance. After the measurement of a fabricated sample, a fractional bandwidth of 171.2% is obtained for at least 10 dB reflection reduction, with a total thickness of only 0.087λ _L (wavelength at the lowest absorption frequency). The good agreement between the calculated, simulated, and measured results validates the designed absorber.

An accurate and efficient design method for resonant frequency selective surfaces (FSSs) based on the macro-model of units in the basic strip-gap FSS is proposed. Several variables are introduced in the formulas to improve the accuracy of computation. Without dielectric substrate, two effective widths are introduced in the calculation of equivalent L and C due to the current inhomogeneity caused by the gap. Novel equivalent circuit (EC) formulas are proposed after full-wave simulation and parameter fitting. Furthermore, with substrate, another two variables, the effective permittivity for inductance and capacitance, are introduced with detailed calculation equations. The results obtained by simulation and EC calculation show that not only the resonance frequencies but also the frequency responses over a wide frequency band are found to be in reasonable agreement, when the wavelength of the operating frequency is smaller than the period. For verification of complex structures and actual application, two absorbers based on cross dipole and square loop FSSs, respectively, are proposed. Good agreement is gained between the measurements of a fabricated sample working from 2.03 to 9.36 GHz, and the results obtained by EC calculation and simulation. The results verify that it is feasible to use this method in the study of more complex and novel structures which are based on resonant FSSs.

This paper presents a fully planar ultrawideband wide-angle scanning tightly coupled array. A tightly coupled element, which has a low input resistance, is used as the basic array element. The element can be fed by a simple microstrip-to-coaxial transition, obviating the need for an additional impedance transform structure. To improve the scan performance of the array in both E-and H-planes, a polarization-sensitive frequency-selective surface (FSS) structure, whose effective dielectric constant for the TM and TE modes incidence wave can be changed respectively by different structural parameters, is designed to serve as the wide-angle impedance matching (WAIM) layer. The FSS structure is printed on the superstrate dielectric directly without an additional dielectric layer, resulting in a low-cost array. The proposed array can work in 3.7:1 bandwidth with voltage standing wave ratio (VSWR) < 3 for E-plane scanning up to 75° and H-plane scanning up to 60°. Based on the designed unit cell, an 11 × 11 prototype is fabricated and measured. The measured results of the fabricated array show good agreement with simulations, proving the feasibility of the design.

A two-layer design which has a wideband transmission window within an ultra-wide absorption band, named as frequency-selective rasorber (FSR), is presented. This structure enables small insertion loss in the wide transmission window, with two absorption bands below and above the passband. The equivalent circuit (EC) is employed to make the design clearer and straightforward. Compared with other FSRs, it has a much wider transmission band with an ultra-wide absorption band and relative low profile. The simulated results exhibit that the insertion loss is less than 3 dB from 9.59 to 12.16 GHz (23.6%), while the frequency for -10dB reflection coefficient ranges from 5.4 to 17GHz (103.6%) with a thickness of 0.106λ _L (λ _L is wavelength at the lowest working frequency).