A detailed study of a single-layer circuit analog absorber using double-square-loop array reveals that three resonances can be obtained within its operating frequency band. An equivalent circuit model is proposed to explain how these three resonances can be produced and its absorption bandwidth can be widened. Simple guidelines for the design of this double-square-loop absorber are then formulated. It is shown through measurements that the fractional bandwidth of 126.8% is realized for at least 10 dB reflectivity reduction under the normal incidence. Furthermore, the total thickness of the proposed design is only 0.088λL at the lowest operating frequency. A good agreement between simulated and measured results demonstrates the validity of our design.

On the Design of Single-Layer Circuit Analog Absorber Using Double-Square-Loop Array

The intent of this paper is to provide an overview of basic concepts, types, techniques, and experimental studies of the current state-of-the-art Frequency Selective Surfaces (FSSs). FSS is a periodic surface with identical two-dimensional arrays of elements arranged on a dielectric substrate. An incoming plane wave will either be transmitted (passband) or reflected back (stopband), completely or partially, depending on the nature of array element. This occurs when the frequency of electromagnetic (EM) wave matches with the resonant frequency of the FSS elements. Therefore, an FSS is capable of passing or blocking the EM waves of certain range of frequencies in the free space; consequently, identified as spatial filters. Nowadays, FSSs have been studied comprehensively and huge growth is perceived in the field of its designing and implementation for different practical applications at frequency ranges of microwave to optical. In this review article, we illustrate the recent researches on different categories of FSSs based on structure design, array element used, and applications. We also focus on theoretical breakthroughs with fabrication techniques, experimental verifications of design examples as well as prospects and challenges, especially in the microwave regime. We emphasize their significant performance parameters, particularly focusing on how advancement in this field could facilitate innovation in advanced electromagnetics.

Frequency Selective Surfaces: A Review

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)

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

In this letter, a single-layer wideband absorber with compact thickness is proposed, and its ability to reduce the radar cross section (RCS) is investigated. The absorber consists of two-dimensional array of conductive crossed dipoles with lumped resistor elements on the top of a single-layer FR4 substrate, backed by continuous metallic layer. Simple methodologies for design and analysis of the absorber are presented. The thickness of the fabricated sample is less than 0.077λ
 L 
. (wavelength at the lowest cutoff frequency). It offers the 10-dB reflection reduction bandwidth of 70.7% (5.3-11.2 GHz) and full width at half-maximum (FWHM) absorption bandwidth of 104.76% (3.75-12 GHz). The experimentally demonstrated performance of the absorber shows good agreement with the numerical simulations.

Single-Layer Wideband Microwave Absorber Using Array of Crossed Dipoles

This communication proposes miniaturized implantable antenna systems for biomedical applications, specifically for scalp implantation. The proposed designs exhibit dual-band characteristics on the industrial, scientific, and medical bands (i.e., 915 and 2450 MHz) with small volumes: 344 mm3 (System A) and 406 mm3 (System B). Each system is integrated with the microelectronic components and a battery. The key feature of the proposed implantable antenna is its small volume ( $8 \times 6 \times 0.5$ = 24 mm3) with a slotless and a vialess ground plane, thereby reducing design complexity. Moreover, the structure exhibits satisfactory peak gain values of −28.5 and −22.8 dBi at a lower and higher resonant band, respectively. Simulations using the finite-element method and finite-difference time domain are performed to evaluate the implantable systems. For validation, measurements are carried out in a saline solution. The antenna offers a good agreement between the measured and simulation results. In addition, a link budget is calculated to analyze the data transmission range.

Scalp-Implantable Antenna Systems for Intracranial Pressure Monitoring

A wideband circuit analog absorber in X-band is proposed. The design is discussed by the unit cell and equivalent circuit model. A unit cell of the periodic structure for the absorber is designed and simulated. The absorber consists of two RO 4003 layers separated by air distance. With the thickness of 3 mm, the absorber offers a -20-dB bandwidth of 40%. A novel method for estimating the optimality ratio of planar absorbers is also proposed. The optimality ratio of the designed X-band absorber is calculated with the proposed method, and the result is about 80% optimality in frequency response and total thickness.

A Circuit Analog Absorber With Optimum Thickness and Response in X-Band

The effect of liquid on the performance of a UHF RFID tag is investigated in this letter. It is found that the presence of the liquid increases the loss resistance in the equivalent circuit of the antenna. This leads to impedance mismatching with a narrower bandwidth and degraded return loss. The proposed solution for compensating this effect is to add a resonant $RLC$ circuit with smaller resistance in parallel to the antenna body. This resistance corrects the increased loss resistance when the resonant frequency of the added part is close to the working frequency of the tag. The simulation and measurement results confirm an improvement in impedance matching that leads to the increase of the reading range.

A UHF RFID Tag With Improved Performance on Liquid Bottles

In most applications, a frequency-selective surface (FSS) needs to be attached to a wide variety of dielectric materials. The performance of a traditional FSS is greatly influenced by the dielectric material to which it is attached. In this paper, a novel multilayer structure is proposed to construct an FSS. The performance of the proposed structure is shown to be very stable when it is attached directly to a wide variety of dielectric materials of arbitrary thickness. Both single- and dual-polarized structures are designed. The shape of the FSS element is designed by using stepped-impedance transmission lines. A new methodology is proposed to design the FSS by maximizing the value of the capacitance between adjacent layers. The proposed structure offers three distinctive advantages. First, the strong cross-layer capacitance makes the FSS element very compact. The dimensions of the miniaturized element are as small as $0.012 \lambda \times 0.012 \lambda $ . Second, for the proposed structure, the lower the profile, the stronger the cross-layer capacitance, and the lower the resonant frequency. This is unique to the proposed structure since the resonant frequency is usually higher for a lower profile for traditional structures. Third and most importantly, any external dielectric material attached to the FSS will not significantly affect the performance of the FSS due to this strong cross-layer capacitance. Through examples of a single-polarized bandpass FSS at 1 GHz and a dual-polarized bandpass FSS at 1.96 GHz, it is demonstrated that a stable resonant frequency under various incident angles up to 75° can be achieved.

A Miniaturized Low-Profile Multilayer Frequency-Selective Surface Insensitive to Surrounding Dielectric Materials

Traditionally, the element of a frequency selective surface (FSS) is rotationally symmetrical and the element arrays in a multilayer FSS are aligned with each other. A new approach to miniaturize the size of the FSS array element is proposed in this paper by interconnecting the array elements only in one direction in a two-layer FSS structure. One layer acts as an enhanced inductor while the other layer provides capacitance. The interconnection between the adjacent array elements changes the equivalent circuit and produces a strong cross-layer capacitance, which lowers the resonant frequency significantly. The dimensions of the miniaturized FSS element are much smaller than the wavelength at the resonant frequency (periodicity $\ll \lambda )$ . The element can also have a low profile since the cross-layer capacitance is stronger with a thinner substrate. The sensitivity to the incident angle of the proposed structure is comparable with traditional ones. A theoretical equivalent circuit model is developed to characterize the structure, based on the analysis of the geometrical configuration of the FSS structure and the electric field distribution on it. The theory was verified by the experimental results.

Frequency Selective Surface Structure Miniaturization Using Interconnected Array Elements on Orthogonal Layers

A novel hybrid ultra-high frequency radio frequency identification tag is proposed in this paper. The tag offers the advantage of both dipole type and patch antenna depending on the object it is placed on. The label-type dipoles are not functional on metallic objects and patch antennas are not efficient on non-metallic objects, whereas the proposed tag is a dual port design thus it is capable of working in two different modes in off-metal and on-metal scenarios. In off-metal mode, the tag works as a dipole antenna designed on an FR-4 substrate with 3.2 mm thickness. The wide capacitive loaded dipole arms and optimum tuned impedance matching between the chip and the antenna increase the robustness and limit the detuning effect in off-metal mode. The metallic surface of the host material acts as the ground plane in on-metal mode and the tag transforms into a short-ended quarter wavelength patch antenna. The measurement result confirms a long read range between 8 and 12 m in off-metal mode depending on the host material and 5.6 m in on-metal mode.

Abed Pour Sohrab

Papers

A Circuit Analog Absorber With Optimum Thickness and Response in X-Band

A UHF RFID Tag With Improved Performance on Liquid Bottles

A Miniaturized Low-Profile Multilayer Frequency-Selective Surface Insensitive to Surrounding Dielectric Materials

Frequency Selective Surface Structure Miniaturization Using Interconnected Array Elements on Orthogonal Layers

A Hybrid UHF RFID Tag Robust to Host Material