Bandpass Frequency Selective Structure Research Articles

Hybrid Rasorber Based on 3-D Bandpass Frequency-Selective Structures

Hybrid Rasorber Based on 3-D Bandpass Frequency-Selective Structures

Synthesis Design of Inductively-Coupled 3-D Bandpass Frequency-Selective Structures Based on Equivalent SOL-Extracted Circuit Model

In this article, an accurate and unified synthesis method for a class of 3-D bandpass frequency-selective structures (FSSs) is proposed. Each element of the presented 3-D FSSs is composed of stacked planar slotline sections cascaded with inductively coupled structures. Here, each inductively coupled structure is modeled as an equivalent <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${K}$ </tex-math></inline-formula> -inverter, and each stacked slotline section is regarded as an equivalent transmission-line resonator. By defining three ideal calibration standards, i.e., short circuit, open circuit, and matched load in a commercial electromagnetic full-wave simulator, a numerical short-open-load calibration technique is utilized to effectively extract all the equivalent circuit parameters involved in the presented 3-D FSS. Furthermore, a generalized synthesis design procedure is presented by mapping the proposed 3-D FSS topology onto an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${n}$ </tex-math></inline-formula> th-order bandpass filtering network. To verify the feasibility of this design concept, two groups of design examples, including a single-polarized third-order Chebyshev bandpass FSS and two dual-polarized second-order ones, are, respectively, synthesized and designed. Finally, two prototypes of the designed 3-D FSS examples are fabricated, assembled, and then tested by using a free-space measurement method. The measured results exhibit good agreement with the predicted ones. The proposed 3-D FSSs feature simple topologies with high-order bandpass filtering responses and stable filtering responses under a large variation of incident angles of spatial waves.

An Interlaced Grid Dual-Band Dual-Polarized Bandpass FSS With a Large Band Ratio

Based on an interlaced grid arrangement, a dual-band dual-polarized bandpass frequency selective structure (FSS) with a large band ratio (BR) is proposed in this letter. The FSS is assembled from vertical layers arranged along <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> - and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i> -directions, and the vertical layers arranged along the two orthogonal directions are polarization independent. The grating structures printed on the vertical layers form a higher frequency passband, and by having additional wire-grid polarizers with a high-pass response, a lower frequency passband is formed. The proposed interlaced grid FSS is a combination of two square grid FSSs. Since the distance between adjacent vertical layers is smaller than that of the square grid FSS, the proposed FSS has a wider bandstop response to achieve the design of a large BR. The measured results show that for both TE and TM polarizations, the center frequencies of the two passbands are 2.5 GHz and 15.5 GHz, respectively, with a corresponding BR of 6.2.

A Generalized Synthesis Technique for High-Order and Wideband 3-D Frequency-Selective Structures With Chebyshev Functions

A generalized synthesis technique for a new class of high-order and wideband 3-D frequency-selective structures (FSSs) is presented in this article. First, a general circuit model with shunt short-ended stubs and an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$i$ </tex-math></inline-formula> -section nonuniform transmission line is discussed, whose frequency response can be synthesized with Chebyshev functions over a wide frequency range. Following the synthesis procedure, all the circuit parameters can be derived from the specifications of the filtering response, such as filtering order <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N$ </tex-math></inline-formula> (≥4), fractional bandwidth (FBW), and equal-ripple return loss (RL). Then, the general circuit model is applied to design high-order and wideband 3-D bandpass FSS, which is physically realized by using stacked printed circuit board pieces etched with slotlines and microstrip lines. As an example, a fourth-order 3-D FSS with FBW = 80% and equal-ripple RL = 10 dB is designed to show the establishment of good relationship between the circuit model and the physical model. To further improve the frequency selectivity, another two coupling paths independent of the main transmission path are created by simultaneously adjusting the FSS thickness and the separation of slotline sections, thus introducing two transmission zeros at both skirts of the passband. Both design examples are fabricated and measured. Good agreement between measured results and simulated ones indicates that the proposed synthesis method can provide an efficient and accurate solution to design such a class of wideband and high-order 3-D FSSs.

Wideband Bandpass Frequency-Selective Structures on Stacked Slotline Resonators: Proposal and Synthetic Design

This article presents the proposal, synthetic design, and implementation of a class of wideband bandpass frequency-selective structures (FSSs) with multiple in-band transmission poles. Each FSS element is formed by introducing parallel-coupled stepped-impedance slotline resonators (SISRs) on a printed circuit board (PCB). By periodically printing the FSS elements in long PCB pieces and then stacking them together with a certain distance, the incident spatial waves in free space can be converted into guided waves propagating along the slotline resonators of our proposed FSS and can be reconverted from guided waves into spatial waves. Therefore, the slotline resonators are designed to manipulate the incident spatial waves by controlling the filtering response of the guided waves within the FSS. An equivalent transmission-line (TL) model is then established to explain the manipulation mechanism. In order to systematically synthesize the filtering response, the transfer function of the equivalent TL model is mapped to the generalized Chebyshev equal-ripple response. The closed-form equations are derived, where all of the electrical and physical parameters are determined by the design specifications of in-band equal-ripple return loss (RL) and fractional bandwidth (FBW). To verify the theoretically predicted performances, three examples with different RLs (20, 30, and 10 dB) and FBWs (90%, 60%, and 120%) are designed, fabricated, and measured, respectively. The experimental results demonstrate that the proposed FSSs can achieve a desirable and stable wideband bandpass performance under oblique incidences. Our proposed method can also be applied to the design of other high-order wideband FSSs.

Application of Metaheuristic Optimization Algorithm and 3D Printing Technique in 3D Bandpass Frequency Selective Structure

A 3D band pass frequency selective surface structure with desired center frequency and bandwidth, designed by multi-objective optimization algorithms, is introduced in this paper. The ant colony optimization technique is used to achieve optimal design properties with the required characteristics. The performance of the manufactured structure using the 3D printing method is measured to verify the optimized structure for normal incidence. Come up with the results, the convenience of the optimization algorithm accordingly 3D printing can help in designing and constructing complex 3D structures.

Thin 3-D Bandpass Frequency-Selective Structure Based on Folded Substrate for Conformal Radome Applications

A new design method based on a folded substrate is proposed in this paper to reduce the thickness of 3-D bandpass frequency-selective structure (FSS). A 67% and 79% thickness reduction compared to the basic 3-D bandpass FSS is achieved by employing three-layer and five-layer folded substrates, respectively. Single- and dual-polarized designs are presented. By integrating one more substrate with a different dielectric constant, dual-band thin structure is achieved. One structure with a five-layer folded substrate is designed, fabricated, and measured using the parallel-plate waveguide measurement setup. It has a center frequency of 3.57 GHz with 26.9% transmission bandwidth. The structure thickness is only $0.06\lambda _{0}$ , where $\lambda _{0}$ is the free-space wavelength at the center frequency of the passband. Stable frequency response is achieved under oblique incidence. A good agreement is accomplished between simulated and measured results. Moreover, a semicylindrical radome is constructed based on the thin 3-D FSS and integrated with a broadband horn antenna. The radiation characteristics of the entire antenna-radome system are finally investigated and its good filtering feature is demonstrated. A fabricated prototype of this radome is measured in the presence of a broadband horn antenna.

Bandpass Frequency Selective Structure With Improved Out-of-Band Rejection Using Stacked Single-Layer Slotlines

In this paper, a class of bandpass frequency selective structures (FSSs) with improved out-of-band rejection is proposed by using stacked slotlines. The proposed FSS consists of a periodic array of slotlines etched in pieces of single-layer printed circuit boards, which are vertically placed along the periodic plane and then stacked together with a certain distance. Due to this special arrangement, slotline resonators are constructed based on the desired propagation modes, thus leading to a stable bandpass filtering response in a wide frequency range. To better understand the operating principle, design procedure of this class of FSSs with uniform slotline unit cells, inductor-loaded uniform slotline unit cells, and stepped-impedance slotline unit cells is presented step by step. In particular, the equivalent transmission line models are established and the even–odd mode analysis method is introduced to intensively analyze the resonant and filtering mechanisms. As two examples, a single-polarized FSS and a dual-polarized one, both based on stepped-impedance slotline unit cells, are designed, fabricated, and measured. Measured results indicate that the proposed FSSs feature miniaturized unit-cell size, wide upper rejection band, and stable frequency filtering response under a large variation of incident angles.

Dual-Polarized Frequency-Selective Surface With Two Transmission Zeros Based on Cascaded Ground Apertured Annular Ring Resonators

An easily assembled 3-D bandpass frequency-selective structure (FSS) is proposed, which exhibits quasi-elliptical bandpass filtering performance with two transmission poles in the passband and wide out-of-band rejection with two transmission zeros for both transverse electric (TE) and transverse magnetic (TM) polarizations. Each unit cell of the proposed FSS consists of back-to-back cascaded ground apertured annular ring resonators, while the ensemble transparent solid circular interior aperture between the resonators serves as a coupling structure, and then a resonator-coupler-resonator module is achieved. The operating principle for this FSS is also explained with the aid of an equivalent circuit model. Moreover, a parametric study is carried out and design guidelines for the proposed FSS are summarized. A design example is fabricated and the frequency responses of the FSS are tested. Measurements show that the frequency performance of the proposed FSS provides a passband with two transmission poles at around 7.2 and 7.9 GHz and a wide stopband with two transmission zeros at around 9.6 and 12.3 GHz under oblique incident wave at $\theta =0 {^{\circ }}$ and 40° for both TE and TM polarizations. This demonstrates that the proposed structure is an excellent candidate of high-performance dual-polarized FSS with wide stopband rejection.

Analysis and implementation of 3D bandpass frequency selective structure with high frequency selectivity

A novel 3D frequency selective structure (FSS), exhibiting bandpass filtering response with high frequency selectivity, is presented. In each unit cell, multiple resonators are constructed to provide multiple transmission zeros/poles. By establishing an equivalent circuit model, it is obtained that the transmission poles in the passband are produced by a short-circuited resonator and an open-circuited resonator, and the transmission zeros close to the passband are attributed to the fundamental and spurious resonant frequencies of another short-circuited resonator. To implement it in high frequencies, multi-layer printed circuit board technology is introduced, where several replacements are carried out. The measured results show good agreement with simulations. Furthermore, the proposed FSS exhibits stable performance with incident angles increased from 0 to 50°.

Tunable 3-D Bandpass Frequency-Selective Structure With Wide Tuning Range

A new three-dimensional (3-D) tunable bandpass frequency selective structure (FSS) with wide tuning range is presented. The tunable FSS consists of a periodic array of printed circuit boards and a number of inserted metallic plates, where varactor-loaded λ/4 stepped-impedance resonators (SIRs) are constructed based on the propagation mode excited in the structure. By properly choosing the impedance ratio of the SIRs, a wide tuning range can be achieved. Detailed discussion is given to determine the key parameters involved in this tunable FSS, including varactor-loaded λ/4 resonators and coupling coefficient. Finally, a prototype tunable FSS is fabricated and measured. The results exhibit a 2:1 (65%) frequency tuning range from 1.4 to 2.75 GHz with an insertion loss of 4.5-1.0 dB within the passband.

Wideband 3D Frequency Selective Rasorber

This communication presents a 3D frequency selective rasorber (FSR) with bandpass filtering response and wideband absorption characteristics. By loading an array of lumped resistors at one side of a microstrip-line based bandpass frequency selective structure (FSS), multiple resonators, including lossy resonators, are constructed. The bandpass performance with high selectivity is provided by resonators in the substrate region of the microstrip line. The absorption characteristic is obtained by the lossy resonators at the resistor-loaded side of the air region. All reflected waves at the resistor-loaded side can be effectively absorbed by appropriately choosing the resistance value. Physical mechanism of the FSR is analyzed with the aid of an equivalent circuit model and current distributions. As an example, a prototype of the designed FSR is fabricated and tested. Experimental results show that the insertion loss at the center frequency is 2.4 dB and a bandwidth of 114% for the absorption better than 10 dB in the upper rejection band is achieved under the normal incidence.

Dual-Band Bandpass Frequency-Selective Structures With Arbitrary Band Ratios

A new type of dual-band bandpass frequency-selective structure (FSS) with arbitrary frequency band ratio is presented. The proposed FSS features a 3-D composite topology, which consists of an array of three-layer parallel strip lines with inserted metallic rods, and two identical single-layer patch arrays. With a special arrangement of these periodic arrays, two independent propagation paths, including multimode resonators with different resonant frequencies, are constructed under a linearly polarized incident wave, thus providing dual-band performance. Two operating frequency bands can be independently designed because these two paths can be equivalent to two separate inductor-loaded transmission lines. The operating principle is first explained by analyzing the electric field distributions at resonant frequencies. Based on this, an equivalent circuit model is then established to further understand the mechanism. A group of parametric studies are also conducted to investigate the filtering performance. In order to verify our design concept, two practical examples with large and small band ratios are fabricated and measured. The unique feature of the kind of dual-band FSS is that it can exhibit stable dual-band performance with both small and large band ratios under a large variation of incident angle.

Bandpass Frequency Selective Structure With Wideband Spurious Rejection

This letter presents a miniaturized bandpass frequency selective structure (FSS) based on stepped-impedance resonator (SIR) for spurious suppression. The proposed FSS consists of a periodic array of two-layer printed circuit boards and a number of inserted metallic rods and plates, where inductor-loaded SIRs are constructed based on the propagation modes in each unit cell. By properly selecting the impedance ratio of the SIRs and the inductance of the inserted metallic rods, the FSS can provide a bandpass response with two transmission poles in the passband and a very wide stopband capable of suppressing multiple spurious modes in the upper band. An equivalent circuit model is established to explain its operating principle, and it is analyzed by using the even-odd-mode analysis method. As an example, a prototype of the proposed FSS is designed, fabricated, and tested. Good agreement between simulated and measured results verifies the design concept of the bandpass FSS.

Three-Dimensional Dual-Polarized Frequency Selective Structure With Wide Out-of-Band Rejection

A new three-dimensional bandpass frequency-selective structure (FSS) is proposed for dual-polarized applications. Each square unit cell of the proposed FSS consists of vertical and horizontal double-sided parallel-strip lines (DSPSLs) and a thin metallic plate. Furthermore, the parallel-strip lines of every DSPSL are connected together through via holes. Using this arrangement, multimode resonators are established in the proposed structure. At lower frequencies, the substrate mode propagates and two resonators along the DSPSL are constructed, leading to a bandpass response with two transmission poles. At higher frequencies, the air mode is reflected by two resonates along the inserted plate, resulting in a stopband with two transmission zeros. The operating principle for this FSS is explained with the aid of an equivalent circuit model. Moreover, a parametric study of the proposed FSS is also carried out and design guidelines are formulated. As an example, a prototype of the proposed FSS is designed, fabricated, and tested. Measured results demonstrate that the FSS exhibits dual polarizations, high out-of-band rejection, and stable frequency response under both transverse electric (TE) and transverse magnetic (TM) polarizations of an obliquely incident wave.

Angular-stable and polarization-independent frequency selective structure with high selectivity

We present a three-dimensional bandpass frequency selective structure (FSS) with high selectivity and wide out-of-band rejection. Furthermore, the structure is insensitive to the variation of incident angle and polarization of a striking plane electromagnetic wave. By studying the electric fields and currents excited in the proposed structure, it is revealed that four resonators are constructed to produce the desired filtering response. Experiments show that the fabricated FSS has a 3 dB fractional bandwidth of 18.75% for the passband and a bandwidth of 268.25% for the upper out-of-band rejection better than 20 dB under the normal incidence.

Three-Dimensional Bandpass Frequency-Selective Structures With Multiple Transmission Zeros

A new class of three-dimensional (3-D) bandpass frequency-selective structures (FSSs) with multiple transmission zeros is presented to realize wide out-of-band rejection. The proposed FSSs are based on a two-dimensional (2-D) array of shielded microstrip lines with shorting via to ground, where two different resonators in the substrate are constructed based on the excited substrate mode. Furthermore, metallic plates of rectangular shape and “T-type” are inserted in the air region of shielded microstrip lines, which can introduce additional resonators provided by the air mode. Using this arrangement, a passband with two transmission poles can be obtained. Moreover, multiple transmission zeros outside the passband are produced for improving the out-of-band rejection. The operating principles of these FSSs are explained with the aid of equivalent circuit models. Two examples are designed, fabricated, and measured to verify the proposed structures and circuit models. Measured results demonstrate that the FSSs exhibit high out-of-band rejection and stable filtering response under a large variation of the incidence angle.

An Elliptical Bandpass Frequency Selective Structure Based on Microstrip Lines

A high-performance bandpass frequency selective structure (FSS) is presented, which consists of a 2-D periodic array of shielded microstrip lines. It exhibits stable elliptical filtering response under large variation of the angle of incidence. Its operating principle and design guidelines are discussed through equivalent circuit models. An efficient full-wave mode-matching method has been developed to analyze the proposed FSS. The proposed structure provides a simple and practically feasible solution to high-performance FSS since it can be fabricated using readily available microstrip line technology.