Transmission properties of frequency selective structures with air gaps

The transmission properties of compound frequency selective structures with dielectric slab and air gaps were investigated by computation and experimentation. Mechanism analyses were also carried out. Results show that the air gaps have a distinct influence on the transmission properties. Resonant frequency of the structure would increase rapidly when the air gap appears. After the gap gets larger to a specific value, generally 1/5 wavelength corresponding to the resonant frequency, the transmission properties would change periodically with the gap thickness. The change of transmission properties in one period has a close relationship with the dielectric thickness. These results provide a new method for designing a bandpass radome of large incidence angle and low loss with the concept of stealth shield radome.

A band-pass frequency selective surface with polarization rotation

A band-pass frequency selective surface(FSS) with polarization rotation property is proposed. The proposed polarization rotating FSS(PR-FSS) is a two-dimensional periodic structure, its unit cell is an antenna-filter-antenna(AFA) module,and the polarization directions of the upper and lower antennas in each AFA module are orthogonal to each other, so the PR-FSS can achieve frequency selection and 90 degrees polarization rotation at the same time. The numerical simulation demonstrate that the anticipated frequency selection and polarization rotation are realized by the PR-FSS in the frequency band from 8.84 GHz to 10.30 GHz with a relative bandwidth of 15.26%, and the maximum insertion loss in the pass band is only 0.17 d B. Finally, one effective experiment validation is carried out, a reasonable agreement is observed between the experimental and simulated results except for a slight deviation caused by fabrication error and measurement tolerance.

Frequency selective surface structure optimized by genetic algorithm

Frequency selective surface （FSS） is a two-dimensional periodic structure which has promiaent characteristics of bandpass or bandbloek when interacting with electromagnetic waves. In this paper, the thickness, the dielectric constant, the element graph and the arrangement periodicity of an FSS medium are investigated by Genetic Algorithm （GA） when an electromagnetic wave is incident on the FSS at a wide angle, and an optimized FSS structure and transmission characteristics are obtained. The results show that the optimized structure has better stability in relation to incident angle of electromagnetic wave and preserves the stability of centre frequency even at an incident angle as large as 80°, thereby laying the foundation for the application of FSS to curved surfaces at wide angles.

Improvement on the wave absorbing property of a lossy frequency selective surface absorber using a magnetic substrate

An equivalent-circuit model is used to analyse the improvement of the wave absorbing performance of the lossy frequency selective surface（FSS） absorber by using a magnetic substrate,showing that it is possible to widen the wave absorbing bandwidth.Three pieces of magnetic substrates are prepared.According to the complex permittivity and permeability,the reflectivity of the corresponding absorber is calculated by the finite difference time-domain（FDTD） method,and the bandwidth of the reflectivity below 10 dB is optimized by genetic algorithm.The calculated results indicate that the wave absorbing performance is significantly improved by increasing the complex permeability of the substrate;the reflectivity bandwidth below 10 dB of the single layer FSS absorber can reach 3.6-18 GHz with a thickness of 5 mm,which is wider than that with a dielectric substrate.The density of the FSS absorber is only 0.92 g/cm 3.Additionally,the absorption band can be further widened by inserting a second lossy FSS.Finally,a double layer lossy FSS absorber with a magnetic substrate is fabricated based on the design result.The experimental result is consistent with the design one.

Frequency selective surface with better polarization independency for arbitrary incident angle

This paper investigates the frequency-selective property of a planar layer consisting of period arrays both theoretically and experimentally for different polarizations at arbitrary incident angle. The novel element is designed by loading the rectangular microstrip element with L-shaped conducting patch at its two ends. Based on the spectral-domain method, the frequency response including angle effect and polarization effect of the frequency selective surface （FSS） structure are analysed and the plots of the frequency versus transmission coefficient are obtained. As a result of the numerical analysis, it is shown that if the source polarization is changed, polarization-independence of previous FSS design can be achieved only for normal incidence, which limits most FSS applications. But in our proposed structure, the better polarization-independency for arbitrary incident angle can be achieved. It is observed that the simulated result comes very close to the experimental result.

Design of a tunable frequency selective surface absorber as a loaded receiving antenna array

An effective approach to designing a tunable electromagnetic absorber is presented and experimentally verified; it is based on an idea that an existing frequency selective surface （FSS） absorber is regarded as a loaded receiving antenna array. The existing absorber is effectively simplified by withdrawing half of the loaded resistors; a more compact one is obtained when lumped capacitors are introduced. Building on this, a varactor-tunable absorber with a proper bias network is proposed. Numerical simulations of one tunable absorber with 1.6 mm in thickness show that a wide tuning range from 3.05 GHz to 1.96 GHz is achieved by changing the capacitance of the loaded varactor from 0.5 pF to 5.0 pF. An experiment is carried out using a rectangular waveguide measurement setup and excellent agreement between the simulated and measured results is demonstrated.

Frequency-selective microwave polarization rotator using substrate-integrated waveguide cavities

A frequency selective polarization rotator that can rotate the polarization angle of an incident electromagnetic wave at the microwave frequency by 45 is presented. The polarization rotator is based on a two-dimensional periodic array of substrate integrated waveguide cavities, realizing the polarization rotation by coupling the input signal to the output wave through three metallic slots. Two layers of frequency selective surfaces are cascaded by substrate and form the polarization rotator. A vertical slot on the top layer is used to select the horizontal polarization from the incident wave, the vertical and the horizontal slots on the bottom layer are, respectively, used to obtain horizontally and vertically polarized outgoing waves. The two orthogonal outgoing waves are combined to result in the 45~ polarized wave. Both full wave simulation and experimental measurement are carried out, together validating the proposed method.

A new layered space-time detection algorithm for frequency selective fading multiple-input multiple-output channels based on particle filter

In this paper, a new observation equation of non-Gaussian frequency selective fading Bell Labs layered space time （BLAST） architecture system is proposed, which is used for frequency selective fading channels and non-Gaussian noise in an application environment of BLAST system. With othogonal matrix triangularization （QR decomposition） of the channel matrix, the static observation equation of frequency selective fading BLAST system is transformed into a dynamic state space model, and then the particle filter is used for space-time layered detection. Making the full use of the finite alphabet of the digital modulation communication signal, the optimal proposal distribution can be chosen to produce particle and update the weight. Incorporated with current method of reducing error propagation, a new space-time layered detection algorithm is proposed. Simulation result shows the validity of the proposed algorithm.

All-dielectric frequency selective surface design based on dielectric resonator

In this work,we propose an all-dielectric frequency selective surface（FSS） composed of periodically placed highpermittivity dielectric resonators and a three-dimensional（3D） printed supporter.Mie resonances in the dielectric resonators offer strong electric and magnetic dipoles,quadrupoles,and higher order terms.The re-radiated electric and magnetic fields by these multipoles interact with the incident fields,which leads to total reflection or total transmission in some special frequency bands.The measured results of the fabricated FSS demonstrate a stopband fractional bandwidth（FBW）of 22.2%,which is consistent with the simulated result.

A novel four-legged loaded element thick-screen frequency selective surface with a stable performance

A thick-screen frequency selective surface （FSS） has not only a broad bandwidth but also the advantages of overcoming the multilayer FSS shortcoming of complex structure and low transmittance of centre frequency due to the cascade of FSSs, and this means it could potentially be applied in a stealth curved streamlined radome. However, there is an unsteadiness of centre frequency in a wide range of incident angles and another unsteadiness of polarization in a big incident angle. In order to solve these problems, in this paper we provide a novel four-legged loaded element thick-screen FSS. The structure is analysed and simulated using the mode matching method and moment method. The centre frequency, the transmittance of centre frequency, and bandwidth of the structure are investigated when some parameters including the polarization at a big incident angle and the incident angles of TE ＆： TM waves are changed. The novel four-legged loaded element thick-screen FSS has better transmission properties with a better steadiness of polarization and incident angle independence. The novel structure of the four-legged loaded element thick-screen FSS provides a valuable reference for their application in a stealth curved streamlined radome.

A dual-band flexible frequency selective surface with miniaturized elements and maximally flat (Butterworth) response

A dual-band flexible frequency selective surface （FSS） with miniaturized elements and maximally flat （Butterworth） response is presented in this paper. It is composed of three metallic layers, which are fabricated on thin flexible polyimide substrates and bonded together using thin bonding films. The overall thickness of the proposed structure is only about 0.3 mm, making it an attractive choice for conformal FSS applications. All the three layers can constitute a miniaturized- element FSS （MEFSS） and produce the first pass-band with miniaturization property, while the up and bottom layers can constitute a symmetric biplanar FSS and produce the second pass-band with maximally fiat （Butterworth） response. The two pass-bands are independent and there is a wide band spacing up to 30 GHz between them. The principles of operation, the simulated results by using the vector modal matching method, and the experimental values of the fabricated prototype are also presented and discussed.

A STEALTHY ANTENNA WITH FREQUENCY SELECTIVE REFLECTOR

A Cassegrainian antenna with frequency selective reflector is introduced, and the measured radiation properties and radar cross-section (RCS) of the antenna are given and discussed. In comparison with ordinary metallic reflector, this antenna can obtain similar radiation pattern and about 15dB of RCS reduction.

Single-layer angularly-stable bandpass frequency-selective surface with interdigital resonator

A miniaturized periodic element for constructing bandpass frequency selective surface(FSS)independent of incident angles and polarizations is presented.An interdigital resonator(IR)with one extending finger to connect the two separate parts of the interdigital capacitor is explored to achieve parallel resonance.The equivalent circuit model(ECM)and electric field distributions are introduced to explain frequency performance of FSS.The whole structure has only one layer and possesses a low profile(a thickness of 0.0015λ,where λ represents the resonant wavelength in free space)as well as a small size(0.03λ×0.03λ).This FSS performs as a spatial bandpass filter which exhibits a great angular stability with incident angles ranging from 0° to 80° for both transverse electric(TE)and transverse magnetic(TM)polarizations.As an example,a prototype of one proposed FSS is fabricated and tested.The measured results show a good angular stability.

Design of double-layer active frequency-selective surface with PIN diodes for stealth radome

An experimental double-layer active frequency-selective surface（AFSS） for stealth radome is proposed. The AFSS is a planar structure which is composed of a fixed frequency-selective surface（FSS）, a PIN diodes array, and a DC bias network. The AFSS elements incorporating switchable PIN diodes are discussed. By means of controlling the DC bias network, it is possible to switch the frequency response for reflecting and transmitting. Measured and simulated data validate that when the incidence angle varies from 0°to 30° the AFSS produces more than-11.5 dB isolation across6–18 GHz when forward biased. The insertion loss（IL） is less than 0.5 dB across 10–11 GHz when reverse biased.

Performance Improvement of Multiband Triangular Microstrip Patch Antenna Using Frequency Selective Surface

Today's antennas have to operate in multiple resonant frequencies to satisfy the need of recent advances in communication technologies.This paper presents split ring resonator based triangular multiband antenna whose antenna performance is enhanced with the help of frequency selective surfaces(FSSs).The antenna has multiple resonances at S,C,and X bands.An array of 4×3 crisscross-shaped unit cells are arranged to form the FSS layer.The antenna is fed with a microstrip line feeding technique.The proposed antenna operates at 3.5 GHz,4.1 GHz,5.5GHz,9.4GHz,and 9.8 GHz with a better return loss and gain.Simulated and measured results yield a good match.

Study on high-order frequency selective surface with interdigital capacitance loading

The application of frequency selection surfaces(FSSs)is limited by large area,narrow bandwidth,low stopband inhibition and large ripple in the passband.A method for designing high-order wide band miniaturized-element frequency selective surface(MEFSS)with capacitance loading is introduced.The proposed structure is composed of multiply sub-wavelength interdigital capacitance layer,sub-wavelength inductive wire grids separated by dielectric substrates.A simple equivalent circuit model,composed of short transmission lines coupled together with shunt inductors and capacitors,is presented for this structure.Using the equivalent circuit model and electromagnetic(EM)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,bandwidth and stopband inhibition.Using this synthesis procedure,a prototype of the proposed MEFSS with a third-order bandpass response,center frequency of 2.75 GHz,fractional bandwidth of 8%is designed,fabricated,and measured.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±30°range incidence,and the in-band return loss is greater than 18 dB,and the rejection in the stopband is greater than 25 dB at the frequency of 3.2 GHz.

基于频率选择表面(FSS)技术的微小卫星隐身天线罩设计

天线是卫星的强散射源,降低天线的RCS是卫星隐身设计的重要研究课题之一,本文提出了一种基于频率选择表面技术的微小卫星天线罩设计方案,可有效降低卫星天线的RCS。首先针对隐身天线罩设计要求,设计天线罩壁,包括各层介质的介电常数、厚度等。然后设计附着在罩壁上的FSS屏。进一步制作天线罩实物,通过在微波暗室进行天线罩扫频测试,实际测量该天线罩的频率响应特性,并测试星上天线方向图。结果表明,本文提出的天线罩设计可满足微小卫星天线在给定频段的阻带特性,并保持在给定正常工作频段的通带特性,说明该微小卫星隐身天线罩设计是有效的。

即贴型单层带通FSS的传输特性

以狭缝阵列为例,利用模匹配法并结合实验对即贴型频率选择表面(FSS)与介质板的复合结构进行了研究,分析了不同厚度的聚酰亚胺基底膜和粘合胶层对谐振频率的影响以及不同复合方式下的频率响应。结果表明,即贴型FSS结构中,基底膜与粘合胶层等细节的影响不可忽略,影响程度与复合方式和介质厚度均密切相关。复合结构中应当含有粘合胶层,避免空气间隙的影响。仅当基底膜和粘合胶层的介电常数小于介质板或与之相近,且介质板厚度为基底膜和粘合胶层厚度的10倍以上时,采用合适的复合方式,才能使基底膜和粘合胶层对复合结构传输特性的影响降至最低。一系列厚度的介质单侧加载时的FSS结构频率响应测试结果与计算结果的对比表明,粘合工艺的缺陷会导致即贴型FSS结构的传输性能发生变化,FSS的设计应当全面考虑上述因素的影响。

带有多层介质衬底FSS的损耗和带宽特性分析

本文简要介绍应用模匹配方法分析带有多层介质衬底的FSS频率响应技术，在此基础上研究带有多层介质衬底的平面Y形缝隙周期阵列对平面波的频率传输特性。重点讨论介质层厚度变化对FSS传输损耗及带宽的影响。