Slot Frequency Selective Surface Research Articles

A configuration to double the gain of pyramidal horn antenna using single layer frequency selective surface

AbstractA gain enhancement technique in pyramidal horn antenna using a slot type band pass frequency selective surface (FSS) is presented. The horn antenna without FSS has achieved a measured gain of 10.5 dBi at 10 GHz, while the antenna with the single layer slotted FSS yields an enhanced measured gain of 14.2 dBi (an improvement of 3.7 dB or 134%). Conventionally, such a gain enhancement would 5 times increase in its dimensions (flaring). However, the authors achieve the same with only 2 times increase in dimensions. Another interesting value addition of the proposed idea is that an FSS sheet can be added to the existing horn antenna to increase its gain without altering the antenna itself. Such a scheme is highly beneficial in strategic applications. The proposed gain enhanced horn antenna can be recommended to be utilised in several X‐band applications, such as radar systems (weather, surveillance, traffic control and military), satellite communications and point–point microwave links that require high gain and small form factor.

Design of compact dual‐band shared‐aperture antenna system based on frequency selective surface

AbstractA miniaturized dual‐band dual‐polarization base station antenna array operating at 2.4–2.8 and 4.6–5.0 GHz is proposed in this paper. The upper‐band unit is embedded in the lower‐band unit to miniature the array size. The size of the proposed shared‐aperture antenna element is 0.28 × 0.28 . To ensure the normal radiation of dual‐band under the small frequency ratio, a meander slot frequency selective surface structure is introduced, which is electromagnetically transparent to the lower‐band unit and function as metal ground for the upper‐band unit to ensure the normal radiation of the upper and lower‐band units, respectively. Simulation and measured results show that the antenna array meets the following design specifications: dual‐band operating (voltage standing wave ratio [VSWR] < 2), mutual coupling less than −20 dB basically, satisfactory gain and radiation characteristics.

A Low-RCS Folded Reflectarray Combining Dual-Metasurface and Rasorber

A strategy for folded reflectarray with low detectability and enhanced gain is presented. For this purpose, the folded reflectarray is engineered by combining a polarization-conversion focusing metasur face, a frequency selective rasorber (FSR), and a diffusion metasurface. The FSR is composed of a lossy layer and a single-polarized I-shaped slotted frequency selective surface (FSS) between which the diffusive metasurface is inserted for in-band and out-of-band cross-polarized low radar cross-sectional (RCS) reduction. Therein, the antenna exhibits a transmission window and two out-of-band absorptions due to the FSR for copolarized radiation. In contrast to detection, the system operated for both wave incidence. For cross-polarized wave incidence, a wideband RCS reduction is enabled due to the wave-diffusive function of the inserted metasurface. The half profile is guaranteed since the wave bounces two times between the FSS and focusing metasurface. The numerical and experimental results coincide well, both indicating that the low-RCS folded reflectarray achieves a peak gain of 23.5 dB at 10.2 GHz, corresponding to an aperture efficiency of 37.8%. Besides, the antenna achieves 10 dB monostatic RCS reduction within 4.6–8.8 GHz and 9.6–13.8 GHz under copolarization detection and a 10 dB monostatic RCS reduction from 8.4 to 19 GHz under cross-polarization detection. Moreover, good invisibility is also achieved for bistatic RCS reduction.

Analysis and design of a single layer double square slot frequency selective surface with single stopband between double passbands

AbstractSquare loop and square slot are the simplest and commonly used elements of frequency selective surfaces (FSSs) providing bandstop and bandpass responses, respectively. Despite it is already known that employment of a double square slot structure presents a tunable stopband between two passbands, it is not clear which parameters of the structure are crucial for tuning the inbetween stopband. We propose an analysis of a double square slot FSS considering its element as a parallel connection of square slot and square loop predicting two geometrical parameters of the constituent square loop are crucial for tuning the inbetween stopband. The stopband is tunable by changing geometrical parameters of the constituent square loop and does not depend on geometries of the constituent square slot. The validity of this analytical prediction is verified by full wave EM simulation results.

Approximate analytical method for hexagonal slot frequency selective surface analysis

In this article, an efficient approximate analytical method for hexagonal slot frequency selective surface (FSS) analysis is proposed. The main contribution of this article is to derive the tangential incident electric field along each oblique strip complementary to the hexagonal slot, so that Kirchhoff type circuit can be established according to the transmission line parameters and distributed sources. The current distribution is solved using Kirchhoff current law and continuity condition for potential. The transmission characteristic can then be calculated following Munk's scheme. Meanwhile, the physical nature of Munk's scheme is studied. The periodic circuit equations are derived from the corresponding electromagnetic field boundary conditions. The approximate closed-form expressions for aperture field and patch surface current distribution of common FSS elements can be calculated by the method of moments. From the analysis, it is known that the receiving current and transmitting current corresponds to the basis function and test function, respectively. Measured and simulated results of a symmetric hybrid radome composed of hexagonal slot are carried out to verify the accuracy and efficiency of the presented method. The CPU time is about 600 and 0.4 second for CST and the proposed method, respectively.

Wideband transverse electromagnetic cell design and its application in frequency selective surface measurement

An open transverse electromagnetic cell (TEM cell) that covers L-band, S-band, and C-band is proposed in this paper. According to impedance matching theory analysis, the characteristic impedance of coaxial connector and microstrip with air as substrate are considered, which are combined with a taper gradient section to realize the function of TEM cell. The proposed TEM cell was modeled and simulated in CST Microwave Studio. Through full-wave simulation, the proposed TEM cell demonstrates a wideband transmission characteristic, S11 less than −10 dB from 1 to 8 GHz, which is relatively rare among the designs that have been reported. The test flow and data post-processing method are given. To verify the effectiveness of the test method, an example of measuring a classical square slot frequency selective surface that their resonance frequency is 3.7 GHz is designed and simulated in the TEM cell. To verify the simulated results, a prototype is fabricated and measured with the proposed TEM cell. The experimental result demonstrates a good agreement with the simulated result, which indicates that the designed TEM cell has good test performance.

Thermally Tunable Frequency‐Selective Surface Based on VO2 Thin Film

An approach to a design of tunable frequency‐selective surfaces (FSSs) for radio frequency (RF) domain based on vanadium dioxide (VO2) thin films is proposed. As an example, a thermally tunable annular slot FSS with integrated VO2 switchable elements for C‐band of the RF domain is described. Switching is based on abrupt change of conductivity of the VO2 film, initiated by heating from room temperature to a temperature of the metal–insulator phase transition of 44 °C. A tuning mechanism of the FSS allows continuous change of frequency response from room temperature state, where the FSS acts as a band‐pass filter, to high‐temperature state, where the FSS works as a reflector. In the process of tuning, a reflection coefficient at a central frequency of the filter can be increased from −26.4 dB up to −2.2 dB. The proposed approach can be used for the design of tunable FSS for applications that allow thermal control of the frequency response.

A novel low‐radar cross section microstrip antenna based on slotted frequency selective surface

AbstractA novel low‐radar cross section (RCS) microstrip antenna with radiation performance and antenna volume maintained is proposed in this paper. Different from other antenna RCS reduction methods, the proposed antenna applies shaping method on frequency selective surface (FSS) ground plate to achieve further RCS reduction. In order to obtain the effect of subtracting square slots, an antenna with normal FSS ground is designed as reference antenna. The modified antenna with slotted FSS ground shows more significant RCS reduction compared with reference antenna in the frequency range 4 to 7.5 GHz, the maximum RCS reduction is 14.62 dBsm at 4.22 GHz. And the gain of the modified antenna is increased about 0.2 dB in contrast with the reference antenna. Besides, the modified and reference antenna are fabricated and measured. It is shown that the simulation and measured results are in good agreement.

Design of a wideband square slot bandpass frequency‐selective surface using phase range analysis

This article presents the design of a wideband bandpass frequency‐selective surface (FSS). The structure is designed using the analysis of the generalized scattering matrix (GSM)‐based transmission phase range. A relationship between the transmission phase range and the bandpass fractional bandwidth (FBW) is developed. The analysis reveals that the FBW of the FSS unit‐cell is proportional to the phase range. Based on the interpretation of results, a design curve has been drawn, and using this curve, a simple square slot FSS on a low‐cost FR‐4 substrate has been fabricated and measured. Using the design curve, a 53% FBW has been predicted. The simulated results show a 52% FBW with the 3.52‐GHz center frequency (fo). The response for the transverse electric (TE) and transverse magnetic (TM) modes has been measured. The structure has a bandpass FBW equal to 50% and 53.3% in TE and TM modes, respectively. The method presented in this article may be used to increase the bandpass response of different FSS topologies.

Design of Modified Single-Layer Double U-Type Linear to Circular Polarization Convertor

This paper proposes single-layer periodic arrays of freestanding slot frequency-selective surface (FSS) as an approach to convert linearly polarized wave to circularly polarized wave. The double U-type structure is etched on a metal plate rather than a traditional print circuit board (PCB) material. It has the characteristics of broadband and could be applied to the situation of large incident angles. Numerical simulations have been carried out, and the prototype structures have been fabricated and experimentally validated. The whole thickness is only 1 mm, and the results show that 3 dB axial ratio (AR) bandwidth is 36.6% at normal incident (16.5% at ±60°). An insertion loss of under 3 dB can be achieved within the whole AR bandwidth.

Transmitting-absorbing material based on resistive metasurface

We theoretically and experimentally studied a transmitting-absorbing material that had transmission window within the absorption band. The material had a sandwich structure comprised of a resistive metasurface, a dielectric substrate and a metallic slot frequency-selective surface (FSS). The material showed lower reflectivity in a wide frequency range; the -10 dB absorbing bandwidth could be over 76% (64% in experiments). Meanwhile in the middle of the range the transmittance up to -1.7 dB was observed at about 10 GHz where slot FSS had good transparency. We showed the lower reflectivity of our material was mainly due to the absorption of incident radiation energy except for the transparent window where the radiation energy passed through the materials. The transmitting window could be adjusted by changing the geometry parameters of slot FSS. Our material had the advantages of thin thickness, small size of unit cell, convenient fabrication and easy to extend to other frequency bands. This kind of transmitting-absorbing material provided a new way for the applications, such as light harvesting, sensing and radome materials.

Transmission performance analysis of circular polarisation frequency selective surface and fast accurate optimisation based on equivalent circuit

In this study, a promising application of frequency selective surface (FSS) operating in circular polarisation is analysed and designed. Both magnitude and phase consistencies between transverse electric (TE) and transverse magnetic (TM) waves are emphasised, especially transmission phase agreement. Equivalent circuit model (ECM) provides a deep physical insight into phase discrepancy between TE and TM waves. The effects of geometrical parameters of FSS on magnitude and phase consistencies are also investigated by ECM. Additionally, an optimisation process is utilised to obtain desired frequency responses for multi-layer FSS design. This process involves iterative correction and update of coarse circuit model on the basis of a few accurate full wave simulations. Moreover, a dual-layer square slot FSS is efficiently designed using this optimisation method. Finally, a prototype of the optimised dual-layer square slot FSS is fabricated and tested to validate the simulated results.

Tunable square slot FSS EC modelling and optimisation

An equivalent circuit model for tunable square slot frequency selective surfaces (FSS) is proposed. Tuning is achieved with the inclusion of a discrete capacitance element, at key points within the FSS unit cell. The equivalent circuit model takes into account the FSS physical attributes. The performance of the proposed equivalent circuit model was evaluated against results obtained from appropriate electromagnetic simulations. Results demonstrate the validity of the proposed equivalent circuit model, in which good estimations of the frequency response of FSS structures were obtained. A RMSE value of 190 MHz was extracted from the complete parametric simulation set. The model was further validated against measurements performed on a tunable active FSS prototype inside an anechoic chamber. Tuning of the FSS was achieved by means of bias voltage to modify the varicaps capacitance value. A relatively good agreement was verified between equivalent circuit model, electromagnetic simulations and measurements. Finally, this work is sought to be a useful contribution to the literature on this topic, by improving on the elementary models of such FSS structures and be useful as a good and fast first approach to determine approximate FSS unit cell dimensions.

3-D Mechanically Tunable Square Slot FSS

We introduce an innovative 3-D mechanically tunable frequency selective surface (FSS), which is inspired by the classical flat square slot FSS. The proposal improves the performance of classical 2-D FSS designs, and it also represents a novel method of achieving mechanical frequency tuning, despite other 3-D designs that consist of a collection of stacked 3-D layers exist. In our proposal, the rotation of an inner element provides tuning capability to the squared cell structure, consisting of metallic grids with a movable inner element. An aluminum prototype was built, which can be tuned from 2.4 to 4 GHz, and also compared its measured performance and numerical simulations. Some characteristics of the proposed structure are the rejection level at main polarization, up to 20 dB, and the maximum frequency sweep of approximately 50% of the fundamental frequency. The prototype showed a stable frequency response for angles of incidence up to 45°. Since results are in good agreement with simulations, we provide parametric equations to design 3-D structures at desired frequencies.

Square Loop and Slot Frequency Selective Surfaces Study for Equivalent Circuit Model Optimization

This paper presents a parametric study of square loop and square slot frequency selective surfaces (FSSs) aimed at their equivalent circuit (EC) model optimization. Consideration was given to their physical attributes, i.e., the unit cell dimensions and spacing, substrate thickness and dielectric properties, for several frequencies and plane wave incident angles. Correlation analysis and evaluation of the influence of physical related input parameters on the FSS performance are presented. Subsequent optimization factor for the square loop classical EC model is analyzed, and a novel EC model formulation for the square slot FSS is proposed. The performance of the proposed EC model was assessed against results obtained from appropriate electromagnetic (EM) simulations, based on a root-mean-square error (RMSE) criteria. Results demonstrate the validity of the optimized EC model, in which good estimations of the frequency response of FSS structures were obtained. Significant reduction of the resonant frequency offsets, in the order of 650 (from 910 to 260) and 460 (770 to 310) MHz, was obtained for square loops and square slots, respectively. The models were further validated against measurements performed on two physical FSS prototypes inside an anechoic chamber at 2.4 GHz. Relatively good agreement was obtained between measurements of real FSS prototypes and results obtained with the EC model. Finally, this work is sought to provide the necessary refinement of elementary models for further studies with more complex and novel FSS structures.

Cylindrical Slot FSS Configuration for Beam-Switching Applications

A novel design for a beam-switching antenna using active cylindrical slot frequency selective surface (ACSFSS) is presented. The antenna system is composed of an omnidirectional monopole antenna and the ACSFSS, which employs a new technique of switching slot arrays. The ACSFSS is made up of 12 columns with 8 slots each, dividing the cylinder by 30 $^{\circ}$ . To steer the beam of the antenna the diodes are set off and on, so that the radiation pattern of the antenna is determined by the number of off state columns. To estimate the general dimension of the cylindrical FSS, an equivalent metallic reflector is introduced and optimized, and then parametric studies for the unit cell dimensions are discussed. The fabricated prototype works within the WLAN band, centered around 2.45 GHz, and can agilely select either a narrow-beam or wide-beam operating mode. Simulation and measurements confirm the operation of the ACSFSS antenna, with good matching and gain observed. In particular, the narrow-beam mode $-$ 3 dB beamwidth is 47 $^{\circ}$ which offers enhanced angular resolution compared with other reported beam-sweeping work.

基于十字单元的可调谐互补屏频率选择表面

To tune the operating frequency of a Frequency Selective Surface (FSS), the load impedance of the loop slot FSS forms was separated to an inductive surface and a capacitive surface, and the coupling mechanism between the two surfaces was used to develop a Complementary FSS(CFSS). An equivalent circuit model of the CFSS was established and its tunable resonance mechanism was analyzed quantitatively. The influences of the rotating angle of the load patch, coupled dielectric thickness and the relative dielectric constant on the transmission characteristics of the CFSS were calculated accurately by using the coupled integral equation method, then the inductive surface and the capacitive surface were fabricated on both sides of the coupled dielectric by the coating and lithography methods. The transmission response of a 250 mm 250 mm CFSS prototype was measured by using the free space method. The calculated and measured results show that the CFSS resonance will be tuned from 18.2 GHz to 14.8 GHz by rotating the cross patch from 0 degree to 10 degree. The coupling effect between the capacitive surface and the inductive surface will disappear by increasing the physical thickness of the coupling dielectric from 0.1 mm to 1 mm. With increasing the relative dielectric constant of the coupling medium, the coupling between the CFSSs will be strengthened and the operating frequency will shift to lower frequencies. It concludes that the CFSS can realize the active resonance conversion function as the rotating angle changes, which provides references for the design and manufacture of active FSSs.

Analysis of FSS Radomes Based on Physical Optics Method and Ray Tracing Technique

In this letter, we analyze the electromagnetic characteristic of a frequency selective surface (FSS) radome using the physical optics (PO) method and ray tracing technique. We consider the cross-loop slot FSS and the tangent-ogive radome. Radiation pattern of the FSS radome is computed to illustrate the electromagnetic transmission characteristic.

Varactor-tunable frequency selective surface with an embedded bias network

A new technique for designing a varactor-tunable frequency selective surface (FSS) with an embedded bias network is proposed and experimentally verified. The proposed FSS is based on a square-ring slot FSS. The frequency tuning is achieved by inserting varactor diodes between the square mesh and each unattached square patch. The square mesh is divided into two parts for biasing the varactor diodes. Full-wave numerical simulations show that a wide tuning range can be achieved by changing the capacitances of these loaded varactors. Two homo-type samples using fixed lumped capacitors are fabricated and measured using a standard waveguide measurement setup. Excellent agreement between the measured and simulated results is demonstrated.

Double split ring slot FSS reflectarray for difference pattern generation

A report is presented on a split ring slot frequency selective surface (FSS) reflector whose element design and distribution allows generation of far-field difference patterns. The reflector operates by converting linearly polarised plane wave fronts into two orthogonal polarisations each with a deep null in the centre of the radiation pattern. The far-field measurement presented is in good agreement with the simulation and demonstrates a null depth of -20dB in the centre of the radiation pattern.