A novel frequency-selective rasorber (FSR) is proposed in this paper which has a nearly transparent window between two absorption bands. The FSR consists of a resistive sheet and a bandpass frequency-selective surface (FSS). The impedance conditions of absorption/transmission for both the resistive sheet and the bandpass FSS are theoretically derived based on equivalent circuit analysis. The insertion loss of FSR at the resonant frequency of lossless bandpass FSS is proven to be only related to the equivalent impedance of the resistive sheet. When the resistive sheet is in parallel resonance at the passband, a nearly transparent window can be achieved regardless of lossy properties. An interdigital resonator (IR) is designed to realize parallel resonance in the resistive element by extending one finger of a strip-type interdigital capacitor to connect the two separate parts of the capacitor. The IR is equivalent to a parallel LC circuit. Lumped resistors are loaded around the IR to absorb the incident wave at lower and upper absorption bands. With the bandpass FSS as the ground plane, the absorption performances at both the lower and upper bands around the resonant frequency are improved compared to a metal-plane-backed absorber structure. The FSR passband is designed at 10 GHz with an insertion loss of 0.2 dB. The band with a reflection coefficient below −10 dB extends from 4.8 to 15.5 GHz. A further extension to dual-polarized FSR is designed, fabricated, and measured to validate the proposed design.

Frequency-Selective Rasorber With Interabsorption Band Transparent Window and Interdigital Resonator

A simple, fast and efficient method for designing wideband radar absorbers is proposed. The idea is to modify the circuit analog absorber method without perturbing the bandwidth. This is done by utilizing the asymptotic behavior of such an absorber at low frequency and replacing the band-stop resonating frequency selective surfaces with low-pass capacitive ones, which can be synthesized by square patches. It is shown that higher frequencies are not influenced by these modifications. A thin wideband capacitive circuit absorber (CCA) is presented with 28% reduction of thickness and 57% increase of bandwidth in comparison to the Salisbury screen. It is also explained why some optimized metamaterial designs fail to compete with the CCA method. For high permittivity layers, it is shown that the CCA is a better solution than the Jaumann absorber and improvements both in thickness and bandwidth are possible. A three layered ultra wideband (4-24 GHz) CCA is presented with total thickness of 15.1 mm. Finally, a design capable of handling oblique angles of incidence for both polarizations and fulfilling different mechanical, thermal and fabrication constraints is given. The absorption band covers the entire C, X and Ku radar bands (4-18 GHz), showing significant improvement compared to the published circuit analog absorbers. (Less)

/pdf/capacitive-circuit-method-for-fast-and-efficient-design-of-2slqpdlohd.pdf

Capacitive Circuit Method for Fast and Efficient Design of Wideband Radar Absorbers

Switchable low-profile broadband frequency rasorber/ absorber based on slot arrays is presented and investigated. First, an equivalent circuit model is proposed to synthesize a rasorber with the transmission window within the absorption band. Based on the slot arrays, simple design guidelines for rasorber design are further developed. Low-profile broadband rasorber and high-selectivity rasorber are, respectively, designed and analyzed to verify the design strategies. Moreover, by loading the switching diodes, a switchable rasorber/absorber is proposed and analyzed, which exhibits small insertion loss at the transmission window and a bandwidth around 100% while the thickness is less than 10% of the free-space wavelength at the lowest operating frequency. Moreover, one extension of dual-polarized rasorber is also designed and analyzed to further validate the design strategy. For demonstration, the rasorber prototypes are fabricated and measured, good agreement between the simulation and measured results is observed.

Switchable Low-Profile Broadband Frequency-Selective Rasorber/Absorber Based on Slot Arrays

A novel design of an absorptive frequency-selective transmission structure (AFST) is proposed. This structure is based on the design of a frequency-dependent lossy layer with square-loop hybrid resonator (SLHR). The parallel resonance provided by the hybrid resonator is utilized to bypass the lossy path and improve the insertion loss. Meanwhile, the series resonance of the hybrid resonator is used for expanding the upper absorption bandwidth. Furthermore, the absorption for out-of-band frequencies is achieved by using four metallic strips with lumped resistors, which are connected with the SLHR. The quantity of lumped elements required in a unit cell can be reduced by at least 50% compared to previous structures. The design guidelines are explained with the aid of an equivalent circuit model. Both simulation and experiment results are presented to demonstrate the performance of our AFST. It is shown that an insertion loss of 0.29 dB at 6.1 GHz and a 112.4% 10 dB reflection reduction bandwidth are obtained under the normal incidence.

Absorptive Frequency-Selective Transmission Structure With Square-Loop Hybrid Resonator

This paper presents a miniaturized frequency-selective rasorber (FSR) that has a wide transmission band with low insertion loss and a wide absorption band below the transmission band. The FSR is composed of a resistive sheet and a bandpass frequency-selective surface (FSS). The unit cell of the resistive sheet is a resistor-loaded hexagonal metallic loop, each side of which is inserted with a circular spiral resonator (CSR) in the center. The CSR is equivalent to a parallel LC circuit that has a high inductance and low parasitic capacitance. At a high frequency of 10 GHz, the CSR resonates to be infinite impedance, around which a 0.5 dB transmission band of 8.68–11.34 GHz is produced. The bandpass FSS is a triple-layer FSS in which two layers of identical hexagonal patches are coupled through a layer of hexagonal aperture; it has a fast rolloff 0.5 dB transmission band of 8.2–11.33 GHz, which almost coincides with that of the resistive sheet. The 1 dB transmission band of the FSR by placing the resistive sheet on the bandpass FSS is 8.3–11.07 GHz. At low frequency, the FSR performs as an absorber, and the 10 dB absorption band is 2.4–7.1 GHz. In addition, due to its tiny physical sizes of only $1.2\,\,\text {mm}\times 1.6$ mm, the CSR can be viewed as a lumped element circuit with a high inductance, in which the parallel resonance is almost independent to the incident angle. The transmission performance of the FSR has a good independence of polarizations and incident angles. A prototype of the proposed FSR is fabricated and measured to validate the design.

Miniaturized Frequency-Selective Rasorber With a Wide Transmission Band Using Circular Spiral Resonator

Two novel dual-polarized frequency-selective absorbers (FSA) are proposed based on two-layered structures under multimode resonances Each FSA consists of a two-layered metal structure with an air gap in the middle At the top layer, a hybrid resonator is applied to generate a wide absorption band and a stop/passband At the bottom layer, two different types of structures are separately applied to perform the opposite senses of bandpass and band-notched filtering for the final FSAs Moreover, the design strategies were examined using the corresponding equivalent circuit models The resultant absorbers have the thickness of only approximately $008~\lambda _{L}$ at the lowest operating frequencies and exhibit wide operating bands of over 100% To demonstrate this approach, two prototypes of the proposed absorbers were fabricated and tested, and reasonable agreement between the simulation results and experimental data was obtained

Dual-Polarized Bandpass and Band-Notched Frequency-Selective Absorbers Under Multimode Resonance

The low-profile dual-polarization frequency-selective rasorbers with very simple structures are investigated. Based on an equivalent circuit model, the operating principle and design method are first studied. One rasorber using lossy cross-frame elements and double-square loops is then proposed, which exhibits two absorption bands at the two sides of one passband. Moreover, the rasorbers with reduced cell size are further developed to stabilize the angular response. Under normal incidence, an insertion loss of about 0.26 dB is obtained at 4.25 GHz, the fractional bandwidth for reflection less than −10 dB is over 100%, and the thickness is around ${\text{0.1}}\,\lambda _{L}$ . For demonstration, the rasorber prototypes are fabricated and measured, while reasonable agreements are observed accordingly.

Low-Profile Dual-Polarization Frequency-Selective Rasorbers Based on Simple-Structure Lossy Cross-Frame Elements

In this paper, a novel thin, broadband, single polarization capacitive surface-based absorber is proposed and investigated. First, the impedance of a substrate and matching surface are analyzed and used for the determination of the structure parameters of the microwave absorber. Second, by utilizing infinite strip gratings with lumped resistors, a capacitive surface-based absorber is presented with 48.6% thickness reduction and 11.7% bandwidth enhancement in comparison to the Salisbury screen. Finally, a prototype of the proposed absorber is fabricated and measured for validation. The proposed method provides an efficient way for the design of thin broadband microwave absorbers.

Investigation of Thin and Broadband Capacitive Surface-Based Absorber by the Impedance Analysis Method

Low-profile and broadband absorbers based on capacitive surfaces are proposed and implemented. First, the impedance of a dielectric substrate and capacitive surface is analyzed separately to characterize the surface impedance. The capacitive surfaces are therefore achieved with the combination of two ring resonators and used to design a three-fold and four-fold symmetric absorber. The resulting absorbers, with the thickness of about 0.076 $\;{\lambda _L}$ at the lowest operating frequency, exhibit good absorption at −10 dB return loss over the fractional bandwidth of 82.5%–117.4%. For demonstration, one prototype of the proposed absorber was fabricated and measured, and good agreement is observed.

Ye Han

Papers

Switchable Low-Profile Broadband Frequency-Selective Rasorber/Absorber Based on Slot Arrays

Dual-Polarized Bandpass and Band-Notched Frequency-Selective Absorbers Under Multimode Resonance

Low-Profile Dual-Polarization Frequency-Selective Rasorbers Based on Simple-Structure Lossy Cross-Frame Elements

Investigation of Thin and Broadband Capacitive Surface-Based Absorber by the Impedance Analysis Method

Low-Profile Broadband Absorbers Based on Capacitive Surfaces