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This paper presents, a novel second order E-plane planar insert based dual band bandstop waveguide filter. The circular shape with two T-shape resonators are printed on Roger RO4350 dielectric substrate to form planar insert. The insert are placed in such a way that positioned in E-plane of the standard WR-90 rectangular waveguide. The independent resonant frequency and bandwidth of the stopband can be adjusted by tuning the resonators. To achieve second order dual band bandstop filter response four asymmetrical circular shape with two T-shape resonators are designed on dielectric substrate to form different stopbands and separated by some specific distance to eliminate the unwanted coupling between them. The numerical analysis of the proposed filter is carried out using CST microwave studio. To validate the numerical analysis of the proposed filter, resonator, WR-90 rectangular waveguide is fabricated and measured. The measured results shows stopband characteristics having centre frequencies f01g = 8.56 GHz and f02 = 11.21 GHz and exhibits the bandwidth of 0.4146 GHz and 0.3824 GHz. The simulated and measured result are in good agreement. [ABSTRACT FROM AUTHOR]

In this paper, a dual-bandstop frequency-selective surface (FSS) implemented using a four-legged Minkowski square and s hexagonal ring is presented. The proposed FSS shows dual stopbands at 4.9 GHz and 7.1 GHz in the C band. The size of the unit cell is 0.36λ0 × 0.36λ0, which is designed on an FR4 substrate with a thickness of 1.6 mm and a loss tangent of 0.02. The frequency response of the bandstop FSS is verified using circuit simulation, and its optimised dimensions are determined at the desired frequencies of operation. The results reveal that the ratio of the two stopband frequencies can be adjusted over a wide range by properly selecting the geometrical parameters. The impedance bandwidths for S11< –10 dB of 0.9 GHz and 1 GHz have been realised at the lower and upper frequency bands, respectively. A prototype is fabricated and measurements show a reasonable agreement with the simulated results, thus validating the design concept. It has been shown that the proposed bandstop FSS exhibits excellent stability upto 60° at oblique incident angles in both TE and TM polarisations. This FSS is suitable for C-band maritime applications due to wide stopbands and highly stable angular operation. [ABSTRACT FROM AUTHOR]

In this paper, a hybrid model chipless radio frequency identification (RFID) tag is introduced. It is constructed by using multi-resonator circuit with a bandstop filter simultaneously. Bandstop filter section is conventionally designed by quarter-wave open-circuited stubs, while the multi-resonator circuit is developed by coupling dual-band codirectional split ring resonators (CDSRRs) to a feeding line. Each stub and CDSRR can provide two and four frequency codes, respectively. The open-circuited stubs can form the resonant frequencies within wide frequency range, while the CDSRRs have notch band characteristics. The proposed hybrid model approach has the ability of combining notch and wide band resonant frequencies on a single circuit. A 12-bit chipless RFID tag is designed by locating two wideband monopole antennas vertically and horizontally at the end of the multi-resonator circuit. The monopole antennas are formed by using two orthogonal ellipses. 000000000000 and 100100100100 frequency coded two tags have been experimentally investigated. [ABSTRACT FROM AUTHOR]

This paper reports on the design and experimental validation of a new class of frequency-tunable filters with switchable bandpass-to-bandstop transfer function characteristics. Single- and dual-band filter topologies are developed by coupling dual-mode loop resonators (DMLRs) to the feedline between the RF input and output ports of the filter. The bandpass-to-bandstop reconfigurability is realized by adding/removing an in-parallel cascaded thru line using single pole double throw (SPDT) switches. Center frequency tunability is achieved by incorporating three varactor diodes in the DMLRs. For experimental validations purposes, two filters with single- and dual-band responses were implemented and tested and appear to be in a good agreement with the EM simulated performance. For the single-band bandpass-to-bandstop filter, the RF-measured tuning range for the bandpass mode of operation is 1.65-1.96 GHz, and for the bandstop mode or operation it is 1.68-1.93 GHz. The dual-band bandpass-to-bandstop filter exhibits the RF-measured tuning ranges of 1.38-1.53 GHz for the first passband and 1.82-2.08 GHz for the second passband in the bandpass mode operation. The minimum insertion loss varies between 3.3 and 6.8 dB for the first passband, whereas it is between 2.2 and 4.7 dB in the second passband. The RF-measured tuning ranges in its bandstop mode of operation were measured between 1.35-1.47 GHz and 1.75-2.01 GHz.

A bandstop filter with flat negative group delay (NGD) is proposed. The flatter NGD and reduced insertion loss have been achieved in the passband of the bandstop filter. This paper provides the group delay and insertion loss equations for each component of bandstop filter, then analyzes the effects of different unit values on bandwidth, flatness, and minimum NGD. The experimental results show that the flatness is 5.06% and the NGD is −0.498 ns. The insertion loss is 6.72 dB, and the relative NGD bandwidth is 85.2%. The simulation and measurement results are basically consistent with the theoretical results. The feasibility of the design is proved.

This study presents a comprehensive numerical investigation of silicon Bragg grating (BG) waveguide structures with cladding modulation. The device design features a uniform silicon ridge waveguide with corrugated cladding on both sides. Two distinct architectures are explored: one where silicon serves as the cladding and another where gold (Au) is employed. Our detailed analysis uncovers compelling results for both configurations. The silicon corrugated cladding BG waveguide demonstrates a bandstop bandwidth of ~50 nm, accompanied by an extinction ratio (ER) of 7.98 dB. The device footprint is compact, measuring approximately 16.4 × 3 µm2. In contrast, the Au corrugated cladding BG waveguide exhibits exceptional performance, boasting a wideband bandstop bandwidth of ~143 nm and an impressive ER of 19.96 dB. Despite this enhanced functionality, the device maintains a reasonably small footprint at around 16.9 × 3 µm2. This investigation underscores the potential of Au corrugated cladding BG waveguides as ideal candidates for achieving high-spectral-characteristic bandstop filters. The significant improvement in bandstop bandwidth and ER makes them promising for advanced optical filtering applications. [ABSTRACT FROM AUTHOR]

A filtering antenna consists of a reflective filtering antenna (RFA) and a bandstop filter (BSF) is presented. Due to the L‐shaped feeding structure, slots etching on the radiator and the ground plane, the RFA obtains good impendence matching, stable in‐band gain, as well as four reflective radiation nulls, and that contribute an operating band of 19% and good frequency selectivity to the proposed filtering antenna. The RFA and BSF are cascaded directly, and the compact BSF is terminated by a resistor to absorb the stopband energy of the RFA, bringing good reflectionless characteristics in a wide frequency range to the proposed filtering antenna. The proposed filtering antenna has a 2‐dB gain bandwidth of 22% (1.78–2.22 GHz), in‐band gain of about 6.81 dBi, radiation suppression levels of higher than 23.4 and 21.4 dB for the lower and upper stopbands, respectively. Within 1.0–3.0 GHz, |S11| ≤ −12 dB, implying slight effect of reflected wave on the devices connected to the proposed filtering antenna. [ABSTRACT FROM AUTHOR]

This work proposes a spiral filter-based diplexer antenna for dual-band full-duplex 5G application in C-band. The shared radiator is formed by a cross-shape Yagi-Uda antenna. The dual-band fullduplex characteristic is obtained by applying a diplexer with two different band-stop filters (BSFs) based on a high-order rectangular spiral-shaped open-stub filter. The proposed diplexer antenna is suitable for modern 5G full-duplex communication system applications with a small frequency ratio and high isolation between two ports by applying a Wilkinson power divider. The diplexer antenna is designed, fabricated, and measured, showing good performance of channel isolations of 27 dB/23 dB and the antenna gain of 4.7 dBi/4.2 dBi at two operation bands from 3.56 GHz to 3.68 GHz and from 3.72 GHz to 3.83 GHz covering the required 100 MHz maximum bandwidth in C-band while its frequency ratio is only 1.04. [ABSTRACT FROM AUTHOR]

Nowadays, global system for mobile communication (GSM) systems are widely used. Therefore, the impact of mobile GSM signals on human health is one of the current research topics. Furthermore, electromagnetic interference caused by GSM signals can be a significant threat to electronic devices today. To overcome these problems, the surfaces of electronic equipment and exterior or interior walls of buildings can be covered with frequency selective surface (FSS) that stops unwanted GSM signals and passing the other signal. This work presents a novel dual-band FSS that attenuates the 900 MHz and 1800–2170 MHz GSM bands by at least 10 dB in the transmission direction. At the design stage, nested geometries are used to achieve multi-stop band frequency response. The proposed design is effectively optimized by utilizing the equivalent circuit geometries and current and electric field distribution diagrams. So, minimum of 10 dB attenuation is achieved in targeted GSM bands for all polarizations and up to 60 degrees (except TM 60 degrees) of incidence angles. [ABSTRACT FROM AUTHOR]

Two wideband bandstop filters (BSFs) for single and dual-band are proposed and then extended to reflectionless BSFs based on the analysis from input impedance/admittance perspective. Also, topologies of higher-number-stopband input-reflectionless BSF are provided to broaden the design scope. Open/shorted coupled lines are adopted to obtain multi transmission zeros and desired stopband bandwidth by adjusting the even-/odd-mode impedance of coupled lines. Resistor-loaded coupled lines are connected with Port 1 to absorb unwanted signals and obtain input-reflectionless behavior. For validation of the proposed theory analysis, BSFs with corresponding absorptive prototypes are constructed and measured. [ABSTRACT FROM AUTHOR]

In this paper first a dual band bandstop filter is proposed using a unique spiral defect at the signal plane for Wi-Fi communication and remote sensing applications. The spiral defects is studied, simulated and measured. Next proposed spiral defect is combined with the hexagonal head defected resonators at ground plane to improve the rejection band. In addition a study is also done to observe the effects of the proposed defects at both signal plane and ground plane. A comparison is done among the response of individual defects filters and combined defect filter. It is observed that later shows improved overall bandwidth with compactness. The proposed combined defect filter with improved stopband displays 92.6% FBW and 112.5 dB/GHz sharpness factor with very compact in size as 0.26 λ0 × 0.25 λ0. All measurement results are in good agreement with the simulated results.

The resonant interaction of a dielectric-coated conductive rod with the X-band microwave field is investigated. The magnetic field distribution of the Goubau standing radial surface waves was experimentally visualized by using a thermo-elastic optical indicator microscope, and the corresponding electric field distribution was determined via numerical simulations. These field distributions are characterized by a certain pattern of antinodes distinctive for standing waves. An analysis of these field distributions allows one to couple a coated rod with a cut Goubau line. A rod placed in the gap region perpendicular to the Goubau line results in a sharp rejection band in the transmission spectrum which is extremely sensitive to the changes in the surrounding media. The shifting rate of the resonance as a function of the dielectric shell thickness is approximately 1.4 GHz/mm. The Q-factor of copper rods depends on their size and dielectric shell thickness. Longer rods with more energy localization areas have higher Q-factors, typically 1.7 times higher (12.7 vs. 7.5). Moreover, incorporating a dielectric shell enhances energy confinement and can elevate the Q-factor by as much as 22 %. When a 25 mm Cu rod is situated inside a cut Goubau line system, the Q-factor values are significantly higher, with a ratio of 275 to 13. With the addition of a dielectric shell, the Q-factor can be elevated by 58 %. The versatility of the proposed controllable system makes it possible to tune the operating spectrum towards higher GHz and THz frequencies.

In this paper, we present the design of a new wide dual-band bandstop filter (DBBSF) using nonuniform transmission lines. The method used to design this filter is to replace conventional uniform transmission lines with nonuniform lines governed by a truncated Fourier series. Based on how impedances are profiled in the proposed DBBSF structure, the fractional bandwidths of the two 10 dB-down rejection bands are widened to 39.72% and 52.63%, respectively, and the physical size has been reduced compared to that of the filter with the uniform transmission lines. The results of the electromagnetic (EM) simulation support the obtained analytical response and show an improved frequency behavior. [ABSTRACT FROM AUTHOR]

This study presents a comprehensive numerical investigation of silicon Bragg grating (BG) waveguide structures with cladding modulation. The device design features a uniform silicon ridge waveguide with corrugated cladding on both sides. Two distinct architectures are explored: one where silicon serves as the cladding and another where gold (Au) is employed. Our detailed analysis uncovers compelling results for both configurations. The silicon corrugated cladding BG waveguide demonstrates a bandstop bandwidth of ~50 nm, accompanied by an extinction ratio (ER) of 7.98 dB. The device footprint is compact, measuring approximately 16.4 × 3 µm2. In contrast, the Au corrugated cladding BG waveguide exhibits exceptional performance, boasting a wideband bandstop bandwidth of ~143 nm and an impressive ER of 19.96 dB. Despite this enhanced functionality, the device maintains a reasonably small footprint at around 16.9 × 3 µm2. This investigation underscores the potential of Au corrugated cladding BG waveguides as ideal candidates for achieving high-spectral-characteristic bandstop filters. The significant improvement in bandstop bandwidth and ER makes them promising for advanced optical filtering applications.

With the increased requirements of dynamic wireless charging scenarios, the industry puts forward higher technical specifications for system efficiency. However, the traditional millisecond level response time cannot meet the requirements of dynamic maximum efficiency tracking (MET) system for real-time control. Therefore, this article proposes a wireless power transfer system based on harmonic-informatization, which simplifies the circuit through the harmonic and bandstop filter, so that the mutual inductance can be extracted by harmonic, and then MET can be implemented in real time. Compared with the traditional fundamental system, the harmonic-informatization system has the advantages of no iteration, no communication, and simple calculation. Because the operation of MET under harmonic is simple, the experimental system in this article successfully achieves the microsecond level response time. At the same time, the experiment shows that the system efficiency and power change little after adding the filter, which is due to the low equivalent reactance of the filter under the fundamental. [ABSTRACT FROM AUTHOR]

This article presents a simple and effective method for designing a dual-band power amplifier (PA). A microstrip coupled-line bandstop filter (BSF) is cascaded with a broadband PA, thus a notched band can be obtained in the wide operating band, realizing a dual-band characteristic of the PA. The simplified real frequency technique (SRFT) is employed to derive a broadband input matching networks covering the requisite frequency band. Appropriate fundamental impedance and second harmonic impedance matching are achieved at the ideal region of the Smith chart. For validation, an experimental prototype is fabricated and tested using a commercial GaN transistor Cree's CGH40010F. The proposed dual-band PA is operated at 1.95–2.4 GHz and 2.65–3.1 GHz, with 61%–71% measured power-added efficiency (PAE) and 40.5–41.5 dBm of measured output power. [ABSTRACT FROM AUTHOR]

The ever-growing field of wireless communication requires efficient radio frequency transceivers with tunable absorptive bandstop filters that can prevent the interference of out-of-band and reflected signals. This paper presents 4-pole tunable absorptive bandstop filters using folded coupled-lines with an inductor. A 4-pole filter transfer function is achieved to obtain an absorptive bandstop filter coupling diagram with 45° electrical-length transmission-line source/load coupling. The folded-loaded coupled-lines with an inductor are proposed to obtain the control for the required coupling coefficients based on the filter transfer functions. The required quality factor extracted through the transfer function is accomplished using a distributed resonator loaded with asymmetrical varactors without auxiliary circuits, such as an impedance-matching network. Two types of filters were designed and built on a substrate with $\varepsilon _{\mathrm {r}} = 3.42$ and $h = 30$ mil. The center frequency tuning was performed using silicon varactors. The first filter shows a return loss of 16.2–38 dB and a rejection level of 41–48 dB in the tuning range 0.83–1.09 GHz. The second filter results in a measured return loss of 14–34 dB and a rejection level of 40.1–51 dB at 1.16–1.47 GHz. 4-pole tunable absorptive bandstop filters with high attenuation and reflectionless responses were achieved which have applications in cognitive radios and carrier aggregation systems.

In this work, a switchable bandstop filter is used as a decoupling structure for developing a miniaturized reconfigurable multiple input multiple output (MIMO) antenna. Initially, a dual band ((2.43-2.60 GHz and 3.51-3.79 GHz)) single monopole antenna structure is developed on FR4 substrate. Then the single monopole antenna and its replica are accommodated in a small space with an edge to edge separation distance of 11 mm to form a 2 port MIMO antenna. Now, a switchable bandstop filter is used as a decoupling network between two closely spaced monopole antenna elements to prevent mutual coupling and reconfigure the antenna characteristics. The dual pole switchable bandstop filter is configured in such a way that one of its poles lies at 2.5 GHz in one state (Mode 1) and at 3.68 GHz in another state (Mode 2) under the switching action of two PIN diodes. Controlling the ON/OFF states of the PIN diodes in the bandstop filter, high isolation is achieved alternately in lower (2.43-2.60 GHz) and upper (3.51-3.79 GHz) frequency bands of the MIMO antenna. Also, stub network is used to improve impedance matching in the upper frequency band. The proposed isolation technique helps the antenna to yield high isolation (>30 dB), fair gain (>2.97 dBi), reasonable radiation efficiency (>86.8 %), low envelope correlation coefficient (< 0.16), high diversity gain (DG >9.88 dB), low Mean effective gain ratio (MEG 1/MEG 2 < 0.05 dB) and low channel capacity loss (CCL < 0.06 bits/s/Hz) for both the operating frequency bands. The overall dimension of the antenna is restricted to 44mm $\times $ 22mm ( $0.36\lambda _{o}\,\,\times \,\,0.18\lambda _{o}$ ) for its easy integration in compact wireless devices. This type of reconfigurable MIMO antenna is best suited for cognitive radio communication, which promotes efficient spectrum utilization.

Abstract Design procedure of a novel transmission line low‐pass filter is presented. The filter has a flexible rejection response for a prescribed cut‐off frequency and an in‐band ripple factor. It is compact by using a Z‐shaped coupled‐line section, which is loaded with open‐circuited stubs. All of the transmission line elements are identical in electrical length, and the impedances can be calculated by solving certain specific equations according to prescribed in‐band specifications. The simulated results show that the circuit can generate low‐pass responses with third‐, fifth‐, and seventh‐order filtering functions. The filter order depends on the number and position of the loading stubs. For verification, two sample filters with cut‐off frequency of 0.5/0.3 GHz are fabricated using a three‐/five‐layer PCB process, which generate seventh‐order low‐pass filtering responses with good selectivity. The experimental results agree well with the predictions.

This paper presents two novel on-chip bandstop to bandpass reconfigurable filters in C and X bands. Designed on a silicon substrate, filter reconfigurability is achieved using semiconductor-distributed doped areas (ScDDAs), such as an N+PP+ junction integrated into the substrate. The active element is therefore co-designed with the passive parts, allowing flexibility in ScDDA size and position. This flexibility offers advantages in terms of integration, ease of manufacture, and performance. The synthesis was developed in the OFF-state in order to match with the well-known one in the ON-state. As proof of concept, 5 GHz and 10 GHz filters were built. The simulated and measured results showed good agreement in both bandpass and bandstop configurations. [ABSTRACT FROM AUTHOR]

In this letter, a compact electromagnetic (EM) structure for miniaturized notch filter design in standard silicon technology is presented. Unlike prior-art on-chip bandstop filters, this filter configuration achieves sharp selectivity and large notch attenuation depth by producing a strong coupling between the top layer (i.e., resonating stub-loaded input-to-output direct path) and the bottom layer (i.e., two spiral-line grounded resonators). Moreover, as these vertically-stacked layers are broadside coupled, the physical footprint of the EM structure is significantly reduced. To demonstrate the proposed principle of notch filter, a simplified lossless behavioral model using ideal lumped elements is developed and verified. In addition, EM simulations are performed for parametric studies. As proof of concept, a prototype is fabricated in 0.13- $\mu \text{m}$ bulk CMOS technology. The measurement results show that the designed filter exhibits a notch located at 19 GHz with an attenuation depth higher than 37 dB. The footprint, excluding the pads, is only 0.028 mm2 (${0.096}\times {0.296}$ mm $^{{2}}$). [ABSTRACT FROM AUTHOR]

This paper presents a compact multifunctional reconfigurable filter based on double coupled‐line structures with three different filtering modes. Three different working modes of the wideband bandpass filter (BPF), bandstop filter (BSF) and wideband filter with single notch can be switched. One prototype of the reconfigurable filter is constructed and measured. For the wideband BPF mode, the 3‐dB bandwidth is 84.5%with in‐band return loss (RL) higher than 21.5 dB and minimum insertion losses (IL) of 0.9 dB, and rejections for the upper stopband is higher than 22.5 dB. For the wideband filter with single notch mode, the 3‐dB bandwidth is 39.7% and 24.9%, with the minimum in‐band IL of 1.13 dB and 1.5 dB, respectively. The maximum rejections between the two bands are 32 dB, and the upper stop band rejections are higher than 19 dB. For the BSF mode, the 3‐dB bandwidth is 29.6%. [ABSTRACT FROM AUTHOR]

In this article, novel single- and dual-band reconfigurable microwave component (RMC) designs with very compact circuit sizes are proposed, which can be reconfigured into multiple functions, including a bandpass filter, a bandstop filter, and a quadrature coupler. The proposed RMCs with filter and coupler functions are based on lumped elements and transistor-based switches such that they are very suitable for on-chip implementation. Specifically, the proposed 10-GHz RMC in GaAs can achieve the functions of a bandpass filter, a bandstop filter, and a quadrature coupler with a circuit size of only 1.8 mm $\times1.35$ mm. On the other hand, the proposed 9.5/24-GHz RMC in GaAs can be reconfigured as either a dual-band bandpass filter or a dual-band quadrature coupler, while the circuit size is only 1.36 mm $\times1.15$ mm. In addition, to demonstrate the possible application of the proposed RMC designs, a proof-of-concept 2.45-GHz radar system prototype based on a proposed 2.45-GHz RMC is implemented in PCB, which successfully integrates the functions of target speed detection and transmission in a single hardware. [ABSTRACT FROM AUTHOR]

Defected Ground Structure (DGS) based on-chip bandstop filter (BSF) design is proposed for millimeter-wave applications. In the proposed structure, a capacitively loaded T-shape resonator is embedded in the original DGS resonator, which forms two high quality (${Q}$ -) factor small loop resonators. Moreover, this structure, in combination with the feedline and series capacitor, independently realizes the position of two transmission poles: one at each side of the parallel resonance without increasing the layout size. The proposed BSF presents a sharp scattering (S-) parameter response due to the appearance of two transmission poles. As a result, the loaded ${Q}$ -factor and negative group delay of the BSF are improved. The prototype of the proposed BSF is fabricated in 0.18- $\mu \text{m}$ Complementary Metal-Oxide-Semiconductor (CMOS) process and measured. The measurement result shows a return loss of 1.78 dB at 53.2 GHz center stopband frequency with the negative group delay of 161 pS. The measurement results also agree well with the electromagnetic simulation results. Without pads, the active area of the prototyped BSF is only 0.024 mm2. [ABSTRACT FROM AUTHOR]

The novel microstrip bandstop filters with extra-high attenuation and wide stopband have been proposed. In order to realize the bandstop filter with broad stopband and great attenuation within the stopband, the coupled lines with single-ended ground have been adopted at both input and output ports. Moreover, for a greater fractional bandwidth, a higher impedance ratio $Z_{ei}/Z_{oi}$ of two adopted coupled lines is required. Furthermore, by differentiating the even- and odd-mode impedances of coupled lines at the input and output ports, the attenuation within the stopband can be increased and the stopband’s bandwidth would be slightly narrower. Two bandstop filters, with the attenuations of 46 dB and 55 dB and a fractional bandwidths of 58.1% and 50.3%, respectively, have been developed to verify the proposed method. [ABSTRACT FROM AUTHOR]

A highly selective tri‐band bandpass (BP)‐to‐bandstop (BS) filter with the third tunable band and the other two fixed ones is proposed in this article. This filter is based on a novel crossed resonator that is composed of a two‐end‐open UIR with an open stub and a short stub connected in parallel on both sides of the resonator center. A PIN diode is utilized to achieve BP‐to‐BS switchable characteristics. Then, varactor diodes loaded at the open stub are employed to obtain the independently tunable third band. In addition, due to the symmetry of the resonator structure, the even‐ and odd‐ mode analysis method is used to derive the resonant frequencies. For validation, a tri‐band BP‐to‐BS filter operating at 2.4 GHz (WLAN), 3.3 GHz (WiMAX), and 4.9 GHz (LAA/LTE‐U) is designed, manufactured, and measured that achieves the continuous frequency tuning range (FTR) of the third band from 4.52 to 4.90 GHz with return loss (RL) better than 10.05 dB under the BP characteristic, from 4.55 to 4.90 GHz with the rejection level (RL) higher than 15.53 dB under the BS characteristic. The measured results agree well with the simulated ones. [ABSTRACT FROM AUTHOR]

This paper presents the synthesis and design of the multi-mode dual-band bandstop filter (MM-DBBSF). A highly selective multi-mode dual-band bandstop response is obtained using a quarter wavelength coupled line structure. It has been shown that by increasing the coupled line’s order, the selectivity and the transmission zeros are increased in the desired stopband. Moreover, a step impedance resonator (SIR) is used between two coupled lines to achieve more transmission poles for better out of band selectivity. The paper show a detailed theoretical synthesis of the coupled line and SIR structure. In order to validate the theoretical model, ideal and microstrip topologies are designed and simulated. Furthermore, a high-frequency substrate is used to fabricate four prototypes. The simulated and measured results show good concurrence.

This paper presents an absorptive microstrip-fed slot antenna with a wide reflectionless band and high radiation efficiency. The proposed antenna consists of a microstrip-fed slot antenna, a quarter-wavelength coupled-line bandstop filter (BSF), and absorptive resistors attached to a grounding pad. The BSF and absorptive resistors are connected to the end of the microstrip feedline. The slot antenna and the BSF are designed to work at the same frequency. In the working band, the high input impedance of the BSF exactly matches the slot antenna for effective radiation. While out of the working band, since the input impedance looking towards the end of the slot antenna matches well with that of the BSF, the energy will flow through the BSF and then be absorbed by the absorptive resistors. In this way, the energy both in and out of the working band will not be reflected back to the input port, which presents a wide reflectionless band. To demonstrate this idea, a prototype operating at 3 GHz is designed, fabricated, and measured. The measurements show that the proposed antenna is well matched ( $|S_{11}|< -10$ dB) in a wide frequency band from DC to 5.3 GHz, the 3-dB gain bandwidth covers from 2.85 GHz to 3.24 GHz with a peak gain of 4.54 dBi, the peak radiation efficiency can reach 93%, and a bidirectional radiation pattern is realized with more than 20 dB cross-polarization suppression in main radiation directions.

Abstract A highly selective tri‐band bandpass (BP)‐to‐bandstop (BS) filter with the third tunable band and the other two fixed ones is proposed in this article. This filter is based on a novel crossed resonator that is composed of a two‐end‐open UIR with an open stub and a short stub connected in parallel on both sides of the resonator center. A PIN diode is utilized to achieve BP‐to‐BS switchable characteristics. Then, varactor diodes loaded at the open stub are employed to obtain the independently tunable third band. In addition, due to the symmetry of the resonator structure, the even‐ and odd‐ mode analysis method is used to derive the resonant frequencies. For validation, a tri‐band BP‐to‐BS filter operating at 2.4 GHz (WLAN), 3.3 GHz (WiMAX), and 4.9 GHz (LAA/LTE‐U) is designed, manufactured, and measured that achieves the continuous frequency tuning range (FTR) of the third band from 4.52 to 4.90 GHz with return loss (RL) better than 10.05 dB under the BP characteristic, from 4.55 to 4.90 GHz with the rejection level (RL) higher than 15.53 dB under the BS characteristic. The measured results agree well with the simulated ones.

In this paper, a new kind of absorptive bandstop filter composing three series‐coupled lines is designed, investigated, and fabricated. The proposed circuit is indeed a combination of a dual‐band conventional notch filter and a single‐band absorptive filter generating two conventional and an absorptive notch. More precisely, the circuit introduces a fixed absorptive notch at 4.22 GHz and two electronically tunable (approximately 20%) conventional notches at 1.62–2 and 4.71–5.75 GHz, respectively. Besides, the FBWs of the first conventional notch, the second one, and the absorptive notch are 5.5 ± 1, 3.5 ± 1, and 3 ± 1%, respectively. The instantaneous surface current density of the circuit is depicted to investigate the generation of notches. The circuit has a compact size of 44.7 × 11.85 mm2 equaling to 0.489 × 0.129 λg2; moreover, it can successfully absorb more than 98% of the input power at its absorptive notch. [ABSTRACT FROM AUTHOR]

Design procedure of a novel transmission line low‐pass filter is presented. The filter has a flexible rejection response for a prescribed cut‐off frequency and an in‐band ripple factor. It is compact by using a Z‐shaped coupled‐line section, which is loaded with open‐circuited stubs. All of the transmission line elements are identical in electrical length, and the impedances can be calculated by solving certain specific equations according to prescribed in‐band specifications. The simulated results show that the circuit can generate low‐pass responses with third‐, fifth‐, and seventh‐order filtering functions. The filter order depends on the number and position of the loading stubs. For verification, two sample filters with cut‐off frequency of 0.5/0.3 GHz are fabricated using a three‐/five‐layer PCB process, which generate seventh‐order low‐pass filtering responses with good selectivity. The experimental results agree well with the predictions. [ABSTRACT FROM AUTHOR]

A self-complementary metasurface is presented for application in multiband terahertz filters. The unit cell structures of the self-complementary metasurface consist of a combination of an ordinary Jerusalem cross and its complementary counterpart that resonates in the THz regime. The columnar repetition of ordinary and complementary resonator structures enables complementary spectral responses for incident waves with mutually orthogonal linear polarizations. The operating principles of the self-complementary metamaterial with the interaction between the juxtaposed ordinary and complementary Jerusalem crosses are explained using an equivalent circuit method and are confirmed with a full-wave electromagnetic simulation. The designed self-complementary metasurface functions as a selective bandstop filter (BSF) or bandpass filter (BPF) depending on the polarization states of the incident wave. The fabricated metasurface exhibits high polarization purity, exemplified by a polarization extinction ratio as high as 24 dB. The transmittance phases of the two orthogonally polarized waves have phase differences between -73° and 83° within a broad frequency range.

A 1-D finite electromagnetic bandgap (EBG) periodic structure is studied. In the structure, EBG behaviour arises from a unit cell comprised of a metallic patch sandwiched between microstrip line and ground plane. Reactive loading offered by patch size determines the bandgap position. A detailed parametric study of various physical structure parameters is presented as a basis to develop a interrelation between physical parameters of the structure and cutoff frequencies. Closed-form synthesis equations are then formulated using curve fitting techniques. Subsequently, a step-by-step design methodology is presented to get a close first pass approximation of structure dimensions for a given specification. This design method reduces the effort required for a designer to perform extensive electromagnetic simulations at early stages of the design. The proposed synthesis method is tested for a variety of commercially available substrates and different frequency ranges for validation. Comparison with electromagnetic (EM) simulations and measurement show that the proposed synthesis method provides first pass approximation of the physical structure dimensions with 94% accuracy.

This brief presents the ultra-miniaturized wideband input-absorptive bandstop filters with one and three transmission zeros using the thin film integrated passive devices (TFIPD) technology. It is based on a novel T-shape structure with lumped-element resonators and one resistor chained on the input port. The basic bandstop block composed of lumped-element resonators can be synthesized by the Chebyshev reflective prototype values (${g}$ -parameters). In order to further expand the stopband bandwidth and improve the frequency selectivity, the series inductance and capacitance resonator (SICR) is loaded on the basic bandstop block, thus resulting in two extra transmission zeros. The chained resistor on the input port is used to absorb the input-signal power. For demonstration, the one-zero and three-zero design cases operating at 5 GHz are simulated, manufactured and measured, respectively. [ABSTRACT FROM AUTHOR]

This paper proposes a novel design of a triple band frequency selective surface (FSS) that acts as a bandstop filter at 1.92 GHz, 3.5 GHz and 5.64 GHz. The bandstop frequencies resemble to GSM, WLAN and WiMAX application bands respectively. The structure is polarization insensitive since its frequency response remains unaltered for both TE and TM modes of electromagnetic wave propagation. The main attractive feature of the structure is its highly stable response under oblique incident angle upto ± 800 at both TE and TM polarizations. Surface current distribution and equivalent circuit modelling are presented to demonstrate the resonance characteristic of the FSS. The structure is compact with an overall unit cell area of 0.09λ x 0.09λ, where λ is the wavelength corresponding to the lowest resonant frequency. The proposed structure is compared with other multi-bandstop FSS structures in literature to highlight its superior functionality in terms of stability under oblique incidence and compactness. A prototype comprising of 6 x 6 array of the proposed unit cell is fabricated and the measured results are in good agreement with the simulated results.

In this paper, a novel compact wide-frequency tunable filter with switchable bandpass and bandstop frequency response is presented. By employing two PIN diodes between the source and load port, the bandpass and bandstop filtering response can be switched, respectively. Three varactor diodes have been used to tuning the operating frequency of the tunable filter at bandpass and bandstop frequency response. The even- and odd-mode method is applied to analyze the presented tunable filter. When PIN diode is forward-biased, the filter has the bandstop response, and the filter has bandpass response while it is reversed. To verify the proposed tunable filter circuits, the tunable bandpass filter and bandstop filter are designed, fabricated, and measured. It is demonstrated that the center frequency of the fabricated tunable filter is tuned from 0.95 to 1.35 GHz. The measured insertion loss in the band is greater than 13 and 11 dB for bandstop mode and bandpass mode, respectively. Moreover, the fabricated tunable filter has a compact size of 0.24λg × 0.24λg (λg is the wavelength of the center frequency during the tunable frequency range).

A high-selectivity low-profile filtering end-fire antenna is proposed using a bandstop filtering (BSF) element integrated into the feedline of the conventional quasi-Yagi antenna. Three transmission zeros (TZs) and six transmission poles (TPs) of the BSF element can be obtained through the odd–even mode equivalent circuit analysis and calculation. With the help of TP and TZ of the BSF element, an additional resonant mode within the operating band and additional radiation null at the upper band edge of the antenna can be obtained, respectively. Consequently, the impedance bandwidth and frequency selectivity of the proposed antenna can be improved compared with the conventional quasi-Yagi antenna. For demonstration, an antenna is fabricated whose measured results exhibit a stable end-fire gain response with two radiation nulls close to two sides of the operation band.

A new type of miniaturized tunable bandstop filter (BSF) based on multilayer technology is presented in this letter. One of the transmission zero could be individually tuned by replacing an outer replaceable element, which can be considered as part of the inner resonant cell of the bandstop filter. Thus, a tradeoff between the stopband rejection depth and bandwidth can be conveniently realized according to different applications. A bandstop filter works from 3.1 to 4.2 GHz has been designed and fabricated in a low temperature cofired ceramic (LTCC) substrate to verify the proposed concept. By virtue of the LTCC multilayer technology, the filter occupies a very compact volume of 2.0 mm × 1.2 mm × 0.8 mm. [ABSTRACT FROM AUTHOR]

In this work, amorphous InSe thin films coated with 30–160-nm-thick SiO2 are used as an active material to fabricate multifunctional devices. The n-InSe/p-SiO2 layers that are deposited onto ytterbium substrates are optically and electrically characterized. It was observed that the coating of SiO2 nanosheets onto the surface of InSe enhances the light absorbability in the near-infrared range without remarkable altering of the bandgap. Significant increase in the steady-state photocurrent values accompanied by faster photocurrent responses resulted from the coating of SiO2 nanosheets. Electrically, while the Yb/InSe/Au channels display tunneling Schottky barrier characteristics, the Yb/InSe/SiO2/Au channels show pn junction features. Both channels displayed metal–oxide–semiconductors (MOS) capacitance–voltage characteristics. In addition, the analyses of the current–voltage characteristics have shown that the currents in the Yb/InSe/Au and Yb/InSe/SiO2(30 nm)/Au channel are dominated by electric field-assisted thermionic emission (tunneling) of charge carriers through barriers of widths of 18/14 and 30/16 nm under reverse-/forward-biasing conditions, respectively. Further increase in the oxide layer thickness lowered the barrier height of the devices. On the other hand, when an ac signal of low amplitude is imposed through the device channels, the conductance, capacitance, and reflection coefficient spectra displayed bandstop filter characteristics near 1.6 GHz. The microwave cutoff frequency spectra show a remarkable increase in the cutoff frequency values as a result of the coating of InSe with SiO2 nanosheets. The features of the device assure its applicability as rectifying diodes, fast photosensors, MOS capacitors, and microwave bandstop filters. [ABSTRACT FROM AUTHOR]

Herein, we report the design and experimental characterization of a broken gap heterojunction devices fabricated by vacuum evaporation of Yb/InSe/Sb2Te3/Au stacked layers. The structural characterizations of the stacked layers revealed an amorphous/polycrystalline heterojunction type. The measurements of capacitance-voltage characteristics in the frequency domain of 1.0-9.0 MHz displayed tunable metal-oxidesemiconductor (MOS) characteristics. The frequency dependent built-in voltage, depletion width, and free carrier density is also investigated. In addition, the analyses of currentvoltage characteristics have shown that the device displays highly stable current rectification ratios of ~103 above 0.20 V. Moreover, the ac signal analyses in the frequency domain of 10-1800 MHz have shown the possible tunability of the conductance and capacitance over a wide range of frequency. Furthermore, the microwave cutoff frequency spectra indicated increasing cutoff frequency limits with increasing incident signal frequency. The microwave cutoff frequency reached 7.1 GHz for a propagating signal of frequency of 1800 MHz. [ABSTRACT FROM AUTHOR]

In the present work, a U-shaped CPW resonator (CPWR) with, generally, unequal arms is proposed to produce high Q-factor bandstop filter (BSF) based on broadside-coupling between the CPWR and a CPW through-line (CPWTL), which are printed on opposite faces of a thin substrate. The unequal arms of the U-shape and the finite width of the ground strips of the CPWR are shown to produce much higher Q-factor than that of equal arms and infinitely extending side ground planes. The dimensions of the CPWTL are optimized for impedance matching while the dimensions of the CPWR are optimized to obtain the highest Qfactor. The effect of the loss tangent of the dielectric substrate material on the Q-factor is investigated. It is shown that the difference between the lengths of the unequal arms of the U-shaped resonator can be used to control the Q-factor. Thanks to the computational efficiency of the employed electromagnetic simulator, enough number of trials has been successfully performed in reasonable time to arrive at the final design of the BSF. A prototype of the proposed BSF is fabricated for experimental investigation of its performance. The experimental measurements show good agreement when compared with the corresponding simulation results. [ABSTRACT FROM AUTHOR]

Abstract The design and analysis of two defected ground structures (DGS)‐based filters have been proposed in this article. The first is a multifrequency bandstop filter (BSF) which utilizes a semi‐H defect in the ground plane. This structure is then prototyped on a Rogers 4350B substrate of overall size 45 mm × 15 mm with an external SMD capacitors employed to control the resonance of the circuit for the stopband frequencies of 433, 700, and 915 MHz. An equivalent π‐circuit is modified to validate this multiband filter. The second filter is a combination of a BSF and bandpass filter in one structure. The filter operating with a controllable passband and stopband frequency is fabricated on Rogers 4350B lossy substrate. Two SMD capacitors are loaded in the filter structure to control the passband and stopband frequencies of the filter with a structure size of 20 mm x 20 mm. Finally, the equivalent circuit from the BSF is modified further to encompass the bandpass and bandstop frequency response of the proposed work.

An ultra-wideband (UWB) filtering-antenna with controllable band notch is reported in this paper. The filtering-antenna consists of a modified monopole antenna and defected microstrip structure (DMS). The monopole antenna is modified using microstrip transition in the feedline and block with a triangular-shape slot on each side of the circular patch to produce wider impedance bandwidth with better return loss. The DMS is constructed using U-shaped slot etched on the feedline to provide band notch and remove WLAN band (5.1-5.8 GHz). A switch is employed in the DMS to control the created band notch. The measured results show that the proposed design exhibits a wide impedance bandwidth with controllable WLAN band rejection, realized peak gain of 4.85 dB and omnidirectional radiation pattern. Therefore, the proposed design is suitable for UWB applications.

This paper reports on multi-band bandpass and bandstop RF filtering four-port couplers with programmable bands in terms of center frequency and number of passbands/stopbands. They are based on four multi-resonant bandpass/bandstop-type modules that are inter-coupled by means of 90°/270°-long transmission lines. Each multi-resonant module is shaped by K in-parallel-cascaded resonators through impedance inverters. The center frequency of each band is controlled by the resonant frequency of each resonator and its bandwidth by the interconnecting impedance inverter. The proposed multi-band coupler with co-integrated spectrally adaptive filtering capability can be designed for symmetrically and asymmetrically located bands, passbands/stopbands with equal or unequal bandwidths, output signals with 0° or 180° phase difference, and equal or unequal power-division ratios at its output ports. For practical-validation purposes, microstrip prototypes of reconfigurable dual-band bandpass and bandstop filtering couplers have been designed, manufactured, and measured. They exhibit two bands that can be independently tuned in frequency and additionally merged to create a wider band within the frequency range between 1.3 and 1.8 GHz.

In this paper, miniature absorptive bandstop filters (ABSFs) with one to three stopbands are demonstrated. First, the proposed single-band ABSF is realized by the introduction of one lossy resonator to a conventional stub bandstop filter, while the circuit size is largely reduced by employing bridged-T coils to replace the transmission line sections. Next, by employing the dual-band design of bridged-T coil, a miniature dual-band ABSF can be achieved. The proposed tri-band ABSF is then implemented through the cascade of the proposed single- and dual-band ABSFs. As a demonstration, three proposed single-, dual-, and triple-band ABSFs are implemented in a commercial GaAs pHEMT process. Much smaller circuit size than the conventional multi-band ABSFs and good measured performance are achieved.

A tunable dual-band bandstop filter (BSF) in the far-infrared region is proposed based on a graphene surface plasmon. The BSF consists of a graphene waveguide and two graphene-ribbon resonators on the SiO2/Si substrate, supporting waveguide mode and edge mode, respectively. The influence of graphene structure parameters on transmission spectra is numerically investigated, which shows that the resonant properties are sensitive to the lengths of the graphene-ribbon resonators, and each stopband of dual-band BSF can be individually designed and tuned without affecting the other stopband. Moreover, the center frequencies of the two stopbands can be actively tuned independently by respective chemical potential of graphene-ribbon resonators. The proposed filter structure is promising to be utilized in future photonic integrated circuit applications as an ultracompact dual-band BSF.

In this brief, a class of microstrip bandpass filters (BPFs) with quasi-reflectionless behavior on both input and output ports is presented. A single-port reflectionless BPF is proposed by shunt connecting one bandstop filter cell namely one-cell. This one-cell single-port reflectionless BPF is made up of a high impedance transmission line as a series band-pass filter section and a shunt-connected band-stop section with resistively terminated. In the operation passband, the bandpass filter section is used to transmit spectral energy, while the bandstop section is regarded as an open circuit. Besides, the out-of-band energy is absorbed by resistively component in the bandstop filter section. In order to achieve better out-of-band reflectionless characteristic, a two-cell two-port reflectionless BPF is constructed by shunt connecting two cells at input/output ports and one series bandpass filter section in the center. In addition, a three-cell reflectionless BPF is designed to further verify the improvement of out-of-band performance. Finally, two-cell and three-cell two-port reflectionless BPFs are fabricated and tested to verify the design concept. The measured results of these two reflectionless BPFs are in good agreement with the simulation ones. [ABSTRACT FROM AUTHOR]

This article presents a novel dielectric support for RF circuits of helix traveling wave tubes (TWTs) for the backward wave cancellation. RF circuit comprised two sections: the first section is a conventional dielectric material and the second section is a ferrite rod which replaces the dielectric support. Considering nonreciprocal electromagnetic property of ferrites, the proposed solution acts as a one-way tunable bandstop filter which only affects the propagating waves in one direction. By tuning this filter in the backward wave frequency, the backward wave can be suppressed without any effect on the forward wave. The electromagnetic characteristics and hot test behavior of ferrite-added slow wave structure (SWS) are observed through the numerical simulations. The proposed structure successfully damps the backward wave oscillation and higher gain is achieved in comparison with conventional TWTs. [ABSTRACT FROM AUTHOR]

Acoustic-wave lumped-element resonator (AWLR)-based filters with reconfigurable all-pass-to-bandstop transfer function are reported. They are based on in-series cascaded resonant stages that comprise two parallel RF signal paths, namely: 1) a thru-line and 2) an AWLR-based resonant section. Reconfigurability is achieved by altering the phase of the thru-line through varactors. In this manner, the filter response can be reconfigured from an all-pass to an enhanced fractional bandwidth (FBW)—i.e., wider than the electromechanical coupling coefficient $k_{t}^{2}$ —bandstop one. For practical validation, two prototypes were designed and tested. They include 1) a single-stage prototype with insertion loss (IL) < 0.38 dB in the all-pass mode and a bandstop mode with a center frequency of 917.5 MHz, an FBW of 0.29% ($2.4~k_{t}^{2}$), and a stopband isolation of 21.2 dB and 2) a two-stage prototype that exhibits an all-pass mode with IL < 0.67 dB and a bandstop mode centered at 916.9 MHz, with an FBW of 0.47% ($3.8~k_{t}^{2}$) and a tunable isolation between 20 and 43 dB. [ABSTRACT FROM AUTHOR]

In this letter, a general approach to synthesize arbitrary order dual-band narrow-band bandstop filter (BSF) is proposed. Simple formulas are derived to calculate the design parameters for the filter. To validate the method, a third-order dual-band BSF using substrate integrated waveguide (SIW) technology is fabricated. Taking the advantage of the high Q SIW resonators, the fabricated filter shows good stopband suppression. The measured center frequencies of the filter are 6.81 and 7.213 GHz, and the corresponding relative 3-dB bandwidths are 4.6% and 4.1%. [ABSTRACT FROM AUTHOR]