Miniaturized-element Frequency Selective Surface Research Articles

Low-Profile Higher-Order Narrowband Bandpass Miniaturized-Element Frequency-Selective Surface

We present a low-profile miniaturized-element frequency-selective surface (MEFSS) with narrowband, second-order bandpass response. The miniaturized-element unit cell comprises a capacitively loaded parallel LC unit cell resulting in a narrowband bandpass response. A higher-order filtering response is obtained by implementing five metallic layers FSS formed by two layers of parallel LC resonators with an inductive impedance inverter layer in between them. The proposed design offers a very thin profile and an insensitive frequency response to the polarization of the incident electromagnetic wave. A design procedure is developed for the proposed FSS structure based on an equivalent circuit model analysis. Using this model, a prototype of MEFSS with the second-order bandpass response at 3.34 GHz and a fractional bandwidth of 3.7% is designed, realized, and characterized experimentally. The measurement results of the FSS verify the effectiveness of the FSS design procedure and the stability of its frequency response up to 60° oblique incidence angles.

Metamaterial-inspired optically transparent active dual-band frequency selective surface with independent wideband tunability.

This paper presents an optically transparent active bandstop frequency selective surface (FSS) with wideband tunability of two resonance frequencies using the concept of miniaturized element FSS (MEFSS). The proposed design consists of metallic square loop arrays on a new optically transparent substrate as the top layer, a glass interlayer, and periodic patterns of cross dipoles on the substrate as the bottom layer. Two kinds of resonant elements loaded with varactors and the designed bias networks achieve two independent tunable stopbands. The proposed FSS has two large tuning ranges, one is from 1.20 GHz to 2.63 GHz and another is from 2.0 GHz to 5.9 GHz (75% and 99% with respect to the center frequency, respectively). The wideband dual-tuning mechanism is theoretically analyzed and demonstrated by deriving its equivalent circuit (EC) model. The experiment results exhibit reasonable agreement with the numerical simulation responses. This proposed design, with low profile, angular stability, polarization insensitivity, optical transparency, and wideband dual-tunability can play an important role in manipulating electromagnetic wave propagation for manifold applications.

Wideband Transmitarrays Based on Polarization- Rotating Miniaturized-Element Frequency Selective Surfaces

We present a new low-profile, wideband, transmitarray design based on polarization-rotating (PR), miniaturized-element, frequency-selective surfaces (MEFSSs). These MEFSS unit cells rotate the polarization of the transmitted wave by 90° either in the counterclockwise or in the clockwise direction. Each PR element consists of three metal layers separated by two dielectric substrates. The constituent metallic layers comprise metallic strips with specific alignments to twist the polarization of the transmitted electromagnetic wave by ±90°. We used the two rotation directions to achieve a 0°/180° phase difference in the transmission mode to achieve a 1 bit phase shifter. We also show that variations in the unit cell dimensions can be combined with polarization rotation to create an additional 90° phase shifts, which can be combined with the PR phase shift mechanism to achieve a 2 bit phase shifter. We designed, fabricated, and experimentally characterized two illustrative transmitarray prototypes with 1 and 2 bit phase-correction patterns. The measured results agree well with the simulation predictions and demonstrate wideband operation for both arrays. The measured 3 dB gain bandwidth and maximum gain are 33.4% and 24.5 dBi, respectively, for the 1 bit prototype, and 24.1% and 27.2 dBi, respectively, for the 2 bit prototype.

Miniaturized-Element Frequency Selective Surface Metamaterials: A Solution to Enhance Radiation of RFICs

In this article, a technique to improve the radiation characteristics of an active CMOS circuit is presented. A miniaturized-element frequency selective surface (MEFSS) cover is utilized on top of the chip to provide gain enhancement, which is essential for many applications such as radar and communication systems. This is important due to the restrictions on the thickness of metallic layers, losses in substrate, and limitations in real estate. The MEFSS cover consists of partially reflecting surface (PRS) and high-impedance surface (HIS) creating a Fabry–Perot-type cavity (FPC). When the MEFSS structure is used, the height of FPC can be reduced without causing any deterioration to the antenna radiation. A proximity patch antenna is embedded in the HIS layer, which is fed by a small metallic trace on the last layer of chip using the coupling mechanism. By utilizing the transverse-equivalent network (TEN) of the structure and imposing the resonance condition, the geometrical parameters of the MEFSS unit cell is calculated. For demonstration purposes, a scaled MEFSS cover for gain enhancement at the center frequency of 10 GHz is designed, fabricated, and tested. The measured results, which are in agreement with simulations, show that the proposed technique is reliable in practice and gain enhancement of 9 dB can be achieved for a single-patch antenna.

Generation of High-Efficiency Vortex Beam Carrying OAM Mode Based on Miniaturized Element Frequency Selective Surfaces

In this paper, a new method for generating high-efficiency vortex beam carrying orbital angular momentum (OAM) mode is proposed based on the miniaturized element frequency selective surfaces (MEFSS) in the microwave region. The unit cell is a class of sub-wavelength periodic structure, which is composed of non-resonant elements with a low profile. A flat metalens is designed to control the wavefront of the propagating electromagnetic waves based on different phase responses of eight kinds of MEFSS unit cells. Under linearly-polarized normal incidence, a vortex beam carrying OAM mode with a topological charge of 1 is excited. The measurement results are in good agreement with the theoretical prediction and full-wave simulation. The proposed method shows great potential applications in generating OAM mode with high efficiency at microwave frequencies.

A novel miniaturized-element frequency selective surface with a second-order bandpass response

A novel miniaturized-element frequency selective surface with a second-order bandpass response

An angular-stable miniaturized frequency selective surface with controllable frequency band ratio using lumped elements

A novel dual-band miniaturized-element frequency selective surface (MEFSS) using lumped elements is proposed to achieve angular-stable response and controllable frequency band ratio. The MEFSS consists of two metal layers of the same period for the unit cell. The operating principle and the synthesis procedure of the MEFSS are analyzed based on the equivalent circuit model. The prototype of the MEFSS is simulated, fabricated, and tested, and the simulation results have a good agreement with the measurement results. To further reduce the element size and achieve a controllable band ratio, inductors and capacitors are, respectively, loaded on the top layer and the bottom layer of interlayer substrate to decrease the resonant frequency. Choosing the values of loaded lumped components properly, the resonance frequencies and the band ratio can be controlled. The unit size of this modified MEFSS is consumedly miniaturized to , where represents the resonance wavelength in free space. Compared to the previous works, the proposed MEFSS has a better miniaturization performance. The results show that the modified dual-band MEFSS has a large band ratio and high stability in the scope of incidence angles of 80° for TE and TM polarization.

A Hybrid Miniaturized-Element Frequency Selective Surface With a Third-Order Bandpass Response

A new architecture for a miniaturized-element frequency selective surface (MEFSS) capable of providing a third-order bandpass response is presented and experimentally verified. The proposed MEFSS consists of three metal layers separated from one another by two dielectric substrates. The exterior metal layers are nonresonant inductive wire grids, while the center layer is a hybrid resonator composed of an inductive wire and an I-shaped resonator. The exterior metal layers and the dielectric substrates act as two inductively coupled series resonators sandwiching the hybrid resonator layer in the middle. This results in a filter with a third-order bandpass response that can also provide an out-of-band transmission null at a frequency higher than that of the main passband. An equivalent-circuit-based synthesis procedure for this MEFSS is also presented in this letter that allows for designing the FSS from its desired system-level performance indicators (e.g., center frequency of operation, bandwidth, etc.). The validity of this design procedure is verified by presenting a design example and conducting full-wave electromagnetic simulations. Finally, an experimental proof-of-concept demonstration is performed by characterizing the unit cell of this MEFSS designed to operate in a WR-112 waveguide environment.

A High Selectivity, Miniaturized, Low Profile Dual-Band Bandpass FSS with a Controllable Transmission Zero

A novel, highly selective, low profile dual-band, and bandpass miniaturized-element frequency selective surface is proposed to realize stable angular responses and a controllable transmission zero. This FSS is a three-layer structure consisting of three metal layers that are separated from each other by two dielectric substrates. The equivalent circuit model of the FSS and its operating principle are presented and analyzed based on the microwave filter theory. The prototype of this FSS is simulated, fabricated, and measured, and its theoretical analysis, simulation, and measurement results show a good agreement. This FSS has achieved an excellent angular stability and wide out-of-band rejection performance in the scope of incidence angle of 80 degrees. Compared with the other multilayered FSSs and 3D FSSs proposed in previous works, it possesses a lower profile as well as a smaller size. A transmission zero is produced by etching slots on the edges of the middle metal layer to achieve superior frequency selectivity. By properly choosing the size and direction of the slots, the transmission zero and the polarization selectivity are able to be changed, respectively.

Broadband True-Time-Delay Circularly Polarized Reflectarray With Linearly Polarized Feed

We introduce a new technique for designing low-profile circularly polarized reflectarray antennas with ultrawideband, true-time-delay responses. The proposed reflectarray uses the unit cells of ground-plane-backed, anisotropic miniaturized-element frequency selective surfaces as its spatial time-delay units (TDUs). Each TDU is composed of a stack of nonresonant rectangular-shaped capacitive patches featuring asymmetric gap spacings and separated from one another by thin dielectric substrates. The TDUs are designed to provide a reflection phase difference of 90° between the horizontal and vertical components of the incident wave over a wide bandwidth. This way, a linearly polarized incident field is converted to a circularly polarized radiated wave. A device prototype that operates at $X$ -band is designed, fabricated, and measured. Measurement results demonstrated that the reflectarray antenna provides a gain of 23.7 dB with variations less than 3 dB within the 8–12 GHz operating frequency range or equivalently 40% bandwidth. Time-domain measurement results demonstrate the suitability of this device for operation with wideband pulses. The reflectarray is also used in a multibeam antenna and it is shown that the structure provides a wide-angle scanning performance with a field of view of ±45°.

A Dual-Band, Inductively Coupled Miniaturized-Element Frequency Selective Surface With Higher Order Bandpass Response

We present a new approach for designing dual-band miniaturized-element frequency selective surfaces (MEFSSs) with independent control of the frequencies of operation of each band. The proposed device is composed of 2-D periodic arrays of subwavelength inductive wire grids and capacitive patches separated by dielectric substrates. The structure is built on an inductively coupled MEFSS that uses capacitively loaded dielectric spacers as its main resonators. The two operating bands of this MEFSS correspond to the first and second resonant frequencies of its constituting resonators. By judiciously choosing the locations where the resonators are loaded and the load values, the frequencies of the first and second resonances can be controlled individually. In this way, a dual-band MEFSS with independent frequency bands of operation can be obtained. Using the equivalent circuit model of this MEFSS, a synthesis procedure is developed that can be used to synthesize the dual-band MEFSS from its system-level performance indicators. A prototype of the proposed dual-band MEFSS with second-order bandpass response at each band is designed, fabricated, and experimentally characterized. The measurement results of this device show a stable frequency response with respect to the angle of incidence up to ±45° for both the TE and TM polarizations of incidence.

A broadband, circular-polarization selective surface

We introduce a new technique for designing wideband circular-polarization selective surfaces (CPSSs) based on anisotropic miniaturized element frequency selective surfaces. The proposed structure is a combination of two linear-to-circular polarization converters sandwiching a linear polarizer. This CPSS consists of a number of metallic layers separated from each other by thin dielectric substrates. The metallic layers are in the form of two-dimensional arrays of subwavelength capacitive patches and inductive wire grids with asymmetric dimensions and a wire grid polarizer with sub-wavelength period. The proposed device is designed to offer a wideband circular-polarization selection capability allowing waves with left-hand circular polarization to pass through while rejecting those having right-hand circular polarization. A synthesis procedure is developed that can be used to design the proposed CPSS based on its desired band of operation. Using this procedure, a prototype of the proposed CPSS operating in the 12–18 GHz is designed. Full-wave electromagnetic simulations are used to predict the response of this structure. These simulation results confirm the validity of the proposed design concept and synthesis procedure and show that proposed CPSS operates within a fractional bandwidth of 40% with a co-polarization transmission discrimination of more than 15 dB. Furthermore, the proposed design is shown to be capable of providing an extremely wide field of view of ±60°.

Wideband Linear-to-Circular Polarization Converters Based on Miniaturized-Element Frequency Selective Surfaces

We introduce a new technique for designing wideband polarization converters based on miniaturized-element frequency selective surfaces (MEFSSs). The proposed structure is a two-dimensionally anisotropic periodic structure composed of arrays of subwavelength capacitive patches and inductive wire grids separated by thin dielectric substrates. The structure is designed to behave differently for field components of the two orthogonal polarizations and transmits a circularly polarized wave once illuminated by a linearly polarized plane wave. Using equivalent circuit models for MEFSSs, a synthesis procedure is developed that can be used to design the polarization converter from its required bandwidth and center frequency of operation. Using this procedure, a prototype of the proposed polarization converter operating within the X-band is designed, fabricated, and experimentally characterized using a free-space measurement system. The measurement results confirm the theoretical predictions and the design procedure of the structure and demonstrate that the proposed MEFSS-based polarization converter operates in a wide field of view of $\pm45^{\circ}$ with a fractional bandwidth of 40%.

Miniaturized-Element Frequency-Selective Surfaces Based on the Transparent Element to a Specific Polarization

In this study, a novel technique has been introduced for the first time, which is based on the transparent element to a specific polarization in order to realize bandpass frequency selective surfaces. It is shown that the proposed technique is more successful than preceding miniaturized structures presented in a variety of literature with respect to reducing the unit cell size. The element sizes of the structures are about 0.017λ 0 to 0.053λ 0 . Furthermore, frequency response of the structures demonstrates perfect stability against the variations of the incident angle of the electromagnetic (EM) wave.

Inductively-Coupled Miniaturized-Element Frequency Selective Surfaces With Narrowband, High-Order Bandpass Responses

We introduce a new technique for designing miniaturized-element frequency selective surfaces (MEFSSs) with narrowband, bandpass responses of order $N\ge2$ . The proposed structure is composed of two-dimensional periodic arrays of subwavelength inductive wire grids separated by dielectric substrates. A simple equivalent circuit model, composed of transmission line resonators coupled together with shunt inductors, is presented for this structure. Using this equivalent circuit model, an analytical synthesis procedure is developed that can be used to synthesize the MEFSS from its desired system-level performance indicators such as the center frequency of operation and bandwidth. Using this synthesis procedure, a prototype of the proposed MEFSS with a second-order bandpass response, center frequency of 21 GHz, and fractional bandwidth of 5% is designed, fabricated, and experimentally characterized. The measurement results confirm the theoretical predictions and the design procedure of the structure and demonstrate that the proposed MEFSS has a stable frequency response with respect to the angle of incidence of the EM wave in the $\pm40^\circ$ range for both TE and TM polarizations of incidence.

An Effective Via-Based Frequency Adjustment and Minimization Methodology for Single-Layered Frequency-Selective Surfaces

This paper proposes a new methodology for minimizing the size of the periodic element and tuning the resonant frequency of a single-layered frequency-selective surface (FSS). Vertical vias are introduced to the FSS design in the methodology. This via-based methodology can be applied to nearly all of the single-layered FSSs presented in the literature, which exhibit much more convenience in practice than the multilayered FSSs. Through the proposed methodology, users can easily vary the resonant frequency of the considered via-inserted FSS (VI-FSS), adding only some additional vias and without having to modify the original element pattern. In addition, the proposed methodology can be applied to minimize the element size or lower its resonant frequency. To illustrate the methodology, the well-known Jerusalem Cross (JC) FSS was first considered. A high-order equivalent circuit was suggested to model the physical mechanism of the VI-FSS with the applied methodology. A set of closed-form equations was also provided to calculate efficiently the circuit inductances and capacitances, especially those caused by the added vias. The VI-FSS prototype was fabricated and examined in a fully anechoic chamber. Favorable agreement among the proposed circuit, full-wave simulation, and measurement was achieved. For demonstrating the capability of the via-based methodology further, it was applied to two advanced applications: 1) two miniaturized-element FSSs and 2) an FSS absorber. Thus, a significant performance improvement in these applications reconfirmed the capability of the proposed methodology.

Ultra-Wideband, True-Time-Delay Reflectarray Antennas Using Ground-Plane-Backed, Miniaturized-Element Frequency Selective Surfaces

We introduce a new method for designing low-profile reflectarray antennas with broadband, true-time-delay (TTD) responses. Such structures are composed of numerous reflective spatial time-delay units distributed over a planar surface. Each spatial time-delay unit is a unit cell of a ground-plane-backed miniaturized-element frequency selective surface (MEFSS) composed of nonresonant elements. Each element is a lowpass type MEFSS composed of a stack of nonresonant patches separated from one another by thin dielectric substrates, and the whole structure is backed with a ground plane. A prototype of the proposed MEFSS-based TTD reflectarray with the focal length to aperture diameter ratio $(f/D)$ of 0.87 operating at the center frequency of 10 GHz is designed, fabricated, and experimentally characterized both in time and frequency domains. It is demonstrated that the fabricated TTD reflectarray operates over a bandwidth of 40% without any significant chromatic aberrations. The proposed antenna provides a realized gain of 23 dB when fed with an X-band horn antenna and shows a gain variation of about 4 dB in the 8–12 GHz range. The antenna also shows consistent radiation characteristics and relatively low side-lobe levels across its entire band of operation.

Design of Wideband, FSS-Based MultiBeam Antennas Using the Effective Medium Approach

We present the design, simulation, and measurement results of a broadband, low-profile, multibeam antenna. The antenna uses multiple feed elements placed on the focal plane of a planar microwave lens to achieve high-gain, multibeam operation with a wide field of view. The lens is based on a recently reported design employing the constituting unit cells of appropriately designed miniaturized-element frequency selective surfaces (MEFSSs) as its spatial time-delay units. A new technique for modeling such lenses is also presented that greatly simplifies the full-wave electromagnetic simulation of MEFSS-based lenses. This technique is based on treating the pixels of the lens as effective media with the same effective permittivity and permeability and significantly reduces the difficulty of modeling and optimizing the proposed multibeam antenna with its relatively large aperture size in a full-wave electromagnetic simulation tool. Using this procedure, a prototype multibeam antenna operating in the 8-10 GHz range is designed. The prototype is fabricated and characterized using a multiprobe, spherical near field system. The measurement results are in good agreement with the simulation results obtained using the proposed simplified modeling technique. Measurements demonstrate consistent radiation characteristics over the antenna's entire operational band with multiple beams in a field of view of ±45°.

A Novel Wideband Miniaturized-Element Frequency Selective Surface

This letter presents a novel wideband miniaturized-element frequency selective surface (MEFSS). The simulation and measurement results show that the bandwidth of the proposed MEFSS is remarkably enhanced compared to that of an original second-order MEFSS while its size and total thickness are still small. A parametric study is also conducted to understand the operating mechanism of the proposed structure. The phenomenon observed in the parametric study is explained with an equivalent circuit model.

Reflectarray antenna based on grounded loop‐wire miniaturised‐element frequency selective surfaces

A reflectarray antenna based on a metal-backed co-planar loop-wire Miniaturized Element Frequency Selective Surface (MEFSS) is presented in this paper. Since the unit cells are small compared to the wavelength, an equivalent lumped-element model can describe the spectral behaviour of the proposed MEFSS structure. It is shown that the proposed unit cell provides a wider and gentler phase variation as a function of the loop size compared to a metallic patch or loop element and that the phase response of the loop-wire element has very low sensitivity to the angle of incidence. In addition, the response of the unit cell, to a great extent, depends on the geometric and electrical parameters of the cells themselves with minimum dependency on the neighbouring elements and therefore the reflected signal phase can be controlled locally. Three samples of the proposed MEFSS structure, each giving different reflection phase values, are fabricated and measured to validate the circuit model and the design procedure. A prototype MEFSS-based reflectarray antenna at X-band is fabricated and measured. The prototype (260 mm × 260 mm and F/D = 1.02) shows a maximum gain of 26.3 dBi with a 1 dB-gain bandwidth of 17% and 3-dB beamwidth of 6° in agreement with the design parameters.