Slotline Structure Research Articles

Balanced Filtering Power Divider based on Odd-mode Hybrid Microstrip and Slotline Spoof Surface Plasmon Polaritons

In this paper, an ultra-wideband balanced filtering power divider (B-FPD) with high common mode (CM) suppression level is proposed. The B-FPD is designed based on the odd mode spoof surface plasmon polaritons (SSPPs), which are constructed by the hybrid folded microstrip and slotline (HFMS) structure. Firstly, the dispersion characteristic and electric field distribution of the HFMS SSPP unit cell are investigated, which can decrease the upper cut-off frequency for size reduction of nearly 50%. Next, an equal balanced power divider with low cut-off frequency is proposed based on the microstrip-to-slotline transition structure and slotline Y-junction. Then, the balanced power divider is used to excite the odd mode of HFMS SSPP structure. An equal B-FPD with center frequency of 4.0 GHz is implemented, and the 3-dB bandwidth covers 0.9 to 7.0 GHz. Finally, the B-FPD with advantages of high CM suppression level and compact size is fabricated and measured.

Compact wideband filter with wide stopband using transition-free spoof surface plasmon polaritons

In this article, a newly spoof surface plasmon polaritons (SSPPs) is proposed and used to design a compact wideband bandpass filter (BPF). Different from the traditional SSPP unit cell with one-shaped structure, the unit cell of the proposed SSPPs consists of an H-shaped slotline and a bilateral T-shaped slotline. Benefiting from the hybrid structure, no transition portion is needed for impedance matching in the design of the microwave filter, which highly reduces the horizontal size of the SSPPs-based filter. The transition structure of microstrip to slotline provides a capacitance with a highpass frequency response, which enables the realization of a passband by cooperating with the natural lowpass property of SSPPs. Also, the upper and lower cutoff frequencies of the proposed filter can be adjusted independently by the key parameters. Besides, a wide upper stopband can be obtained by properly designing the SSPPs unit cell and the microstrip to slotline structure. Finally, a well-designed BPF operated at 1.3–6.2 GHz and its upper stopband rejection reaching 20 GHz is fabricated and measured. The test results are in good agreement with the simulations, which verifies the feasibility of the proposed structure and design method.

Microstrip to Slot-Line-Fed Microstrip Patch Antenna with Radiation Pattern Diversity for X-Band Application

This paper proposes a planar wideband microstrip feedline to a slot-line-based patch antenna for high-frequency pattern diversity applications. The antenna consists of two adjacent rectangular patches separated by the ground slots, with a directive patch slot along the edge of the substrate. A compact common feedline-to-slot configuration is used to miniaturize the antenna. The antenna is designed in stages, starting with an in-phase feedline, followed by a slot line structure, with two radiating patches on top and a director on the side. This creates radiation diversity with directive radiation patterns. The antenna was fabricated and analyzed inside a far-field anechoic chamber. The experimental results validate the return loss, gain, and radiation performance. The measured results of the antenna within a frequency range of 8.5 to 11 GHz show good agreement with the simulation. The antenna has a maximum gain of 9.2 dBi and has the potential to be beneficial for beam steering and X-band applications due to its low profile, broad bandwidth, high gain, and good directivity.

Dual-Band Power Divider With Wide Passbands and Wide Harmonic Suppression Utilizing Multi-Mode T-Stub Loaded Slotline Resonators

This brief proposes a novel dual-wideband slotline power divider (DWSPD) with ultrawide stopband, which is suitable for the multi-band wireless communication system application. The proposed DWSPD is mainly constructed with two multi-mode T-stub loaded slotline resonators (MTSRs), four folded slotline structures (FSSs), four microstrip-slotline transitions and two isolation resistors. The MTSR can generate four transmission poles (TPs), which forms two wide passbands. Furthermore, the FSS contributes to four additional transmission zeros (TZs), which enhances the harmonic suppression. The fabricated DWSPD demonstrates the 3-dB fractional bandwidths of 45.3% @2.25 GHz and 47.1% @4.37 GHz and the stopband with a suppression more than 18 dB @ up to 40 GHz ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$17.8~f_{01}$ </tex-math></inline-formula> ). To the best of the author’s knowledge, the 3-dB FBWs and stopband achieve the best among the existing dual-band filtering power dividers (DBFPDs).

Novel Unbalanced-to-Balanced Quad-Channel Diplexer With Controllable Bandwidth and High Isolation

A novel unbalanced-to-balanced (UBTB) quad- channel diplexer consisting of two balun filters sharing a common single-ended microstrip feeder is proposed. By loading a stepped-impedance quad-mode resonator (SIQMR) into the balun filter, not only each mode can be flexibly adjusted, but also the size of the circuit can be reduced. In addition, motivated by the internal hybrid coupling, two adjacent modes are utilized to form a passband filter with a transmission zero. On the basis of which, two channels with flexibly adjustable bandwidth and good frequency selectivity are implemented on Rx or Tx path. Finally, a U-shaped microstrip slot-line structure is utilized as the balanced port to achieve good common-mode (CM) suppression. To verify the proposed diplexer, a quad-channel prototype is designed, fabricated and tested. Measured results of the fabricated circuit agree well with the simulated ones.

Synthesis Design of Bandpass Frequency Selective Surface With Multiple Transmission Zeros Using Slotline Structures

This article presents a synthesis method to design a new type of bandpass frequency selective surfaces (FSSs) with multiple transmission zeros (TZs). To briefly show the concept of our synthesis method, a generalized topology of the bandpass FSS based on cascaded slotline sections paralleled with bandpass resonators is first presented to map with an alternative <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> th-order bandpass filtering network with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$J$ </tex-math></inline-formula> -inverters and typical LC parallel resonators. Then, two kinds of composite LC parallel resonators, named as LC – LC and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${L}$ </tex-math></inline-formula> – LC parallel resonators, are introduced and discussed to replace the typical LC parallel resonators, which can produce TZs at skirts of the passband and increase the out-of-band selectivity. The derivations of equivalent relationships between the composite and typical LC resonators are given. Furthermore, for the exploration of bandpass FSS with multiple out-of-band TZs, the realization of these two discussed composite LC parallel resonators is exhibited by inductive and capacitive strip lines loading on slotline structures. The design curves and guidelines are given and discussed correspondingly. To validate our design concept, two FSS examples, a second-order bandpass FSS with four out-of-band TZs and a third-order one with five out-of-band TZs, are synthesized and designed. In final, these two designed bandpass FSS prototypes are fabricated and then measured by the free-space measurement method. The measured results show that the desired stable bandpass filtering performance and independently multiple out-of-band TZs are successfully realized.

Direct Synthesis and Design of Wideband Linear-to-Circular Polarizers on 3-D Frequency Selective Structures

A novel direct synthesis method for wideband linear-to-circular polarizers on 3-D frequency selective structures is presented. The proposed linear-to-circular polarizer consists of two orthogonal linearly polarized (LP) transmission paths, both of which are comprised by three cascaded sections of slotline structures. Under the incidence of an LP wave with its electric-field ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E$ </tex-math></inline-formula> -field) vector of a titled angle 45°, each path can achieve a transmission passband for one of the two orthogonal components decomposed by the incident <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E$ </tex-math></inline-formula> -field vector. To obtain a transmitted wave with circularly polarized (CP) property, the well-known conditions on magnitude and phase difference of these two paths are presented. Then, to develop a unified synthesis method, two new conditions of phase difference and phase slope at the center frequency are introduced to replace the typical phase condition. The closed-form equations are derived, and a synthesis procedure is given. It can be obtained that all the involved circuit parameters can be derived from the specifications of these two paths, such as in-band insertion loss (IL) ripples and fractional bandwidths. Moreover, the phase difference condition of these two paths is further modified to obtain an arbitrary axial ratio (AR) at the center frequency and enhance the AR bandwidth of the polarizer. A group of design examples are theoretically investigated to demonstrate and verify the design concept. An alternative design example is finally selected and fabricated. Good agreement between simulated and measured results features its advantages of wide AR bandwidth and low IL. Therefore, the proposed synthesis method can provide a promising solution to design many other types of wideband linear-to-circular polarizers.

3-D Pattern Modulation Surfaces Using Short-Circuited Slotline Structure: Proposal, Design, and Implementation

This article presents the proposal, design, and implementation of 3-D pattern modulation surfaces (PMSs) using short-circuited slotline structure. First, the reflection coefficient of short-circuited slotline with a certain length is studied using the equivalent transmission line model. Since the magnitude of its reflection coefficient nearly equals 1, the TE-polarized incident plane wave can be almost fully reflected at the input terminal of each short-circuited slotline. Most importantly, the phase of its reflection coefficient can be freely changed and adjusted by only varying the length of short-circuited slotline. Then, the short-circuited slotline structure is used as the key element of the proposed PMS. In this way, the functionality of pattern modulation is obtained by unequally selecting the lengths of a row of short-circuited slotlines. Finally, three prototypes with modulated angles of 15°, 30°, and 45° are fabricated and tested to validate our design concept. The measured results agree well with the calculations, showing that the predicted reflection angles are realized using short-circuited slotlines. Therefore, the proposed 3-D PMS is evidently verified to be a promising candidate for future wireless networks to achieve pattern modulation in a smart radio environment.

Slotline‐based frequency‐reconfigurable quadrature coupler with wide tunable range and simple tuning mechanism

AbstractFrequency‐reconfigurable couplers presented before show some shortages such as limited tunable range and complex tuning mechanism. To address these problems, a frequency‐reconfigurable coupler with wide tuning range is proposed based on a slotline structure. A T‐shaped shorted feeding line with certain impedance transform is connected to each port of the coupler. Beneath the shorted end, the slotline structure is utilized to transfer the signal coupled from the feeding line. The 3‐dB quadrature coupler performance is achieved by setting different widths of the slotlines. To tune the frequency, a varactor with tuning capacitance is loaded at the center of each shorted feeding line. Equivalent circuit is given by transmission line model to verify the design concept. For demonstration, the coupler is fabricated and measured. Good agreements between the simulation and measurement and a wide frequency tunable range from 1.2 to 2.6 GHz are achieved based on a single tuning mechanism.

A Balanced Filtering Directional Coupler with Wide Common-Mode Suppression Based on Slotline Structure

In this paper, a balanced-to-balanced filtering directional coupler (FDC) that can realize a 3 dB coupling degree directional coupler with high isolation and directivity is proposed. The design of the proposed FDC is primarily based on microstrip/slotline transition structures, resonance structures, and odd–even mode phase velocity compensation structures. A U-type microstrip feed line integrated with a stepped-impedance slotline resonator is adopted at the input and output ports, which makes the differential-mode (DM) responses independent of the common-mode (CM) ones, and brings superior DM transmission and CM suppression. In addition, by loading the microstrip stub-loaded resonators (SLRs), a DM passband with sharp filtering performance is realized, and transmission zeros (TZs) can be added into the design, which makes it more selective. Moreover, phase compensating slotlines are added into the coupling structure to enhance the isolation. In order to verify the feasibility of the proposed design method, an FDC prototype circuit was made and tested. The simulation results are in good agreement with the measured results. The designed coupler’s DM operating band covers 2.65 GHz to 3 GHz (FBW = 12.4%), and the insertion and return losses are 4.6 dB and 20 dB, respectively. The isolation degree is better than 15 dB, and the CM suppression is more than 55 dB. The total coupler size is about 67.7 mm × 63.8 mm. The designed balanced-to-balanced FDC can be widely used in S-band wireless communication systems.

High-confinement ultra-wideband bandpass filter using compact folded slotline spoof surface plasmon polaritons

A novel bandpass filter (BPF) based on spoof surface plasmon polaritons (SSPPs) using a compact folded slotline structure is proposed and experimentally demonstrated. The proposed novel SSPPs structure compared with a conventional plasmonic waveguide with slot line SSPPs unit structure at the same size, the considerable advantages in much lower asymptotic frequency with tight field confinement, which enable the proposed filter to be more miniaturization. A high-efficient mode conversion structure is designed to transition from TE-mode to SSPPs-mode by gradient slotline lengths. The low-frequency stop-band can be committed with microstrip to slotline evolution on both sides of the dielectric, while the high-frequency cutoff band is realized by the proposed SSPPs structure. The influence of dispersion relation, electric field distribution, surface current, and structural parameters on the transmission characteristics of the proposed BPF are analyzed by finite difference time domain (FDTD). To validate the design concept, the prototype of the miniaturized SSPPs BPF has been manufactured and measured. The experimental results show high performance of the fabricated sample, in which the working in a range of 0.9 GHz–5.2 GHz with the relative bandwidth is 142%, the insertion loss less than 0.5 dB, the reflection coefficient less than −10 dB, and the group delay is less than one ns. This works provides a mirror for realizing the miniaturization of waveguides, and the application and development of high-confinement SSPPs functional devices in the microwave and THz regimes.

A balanced ultra‐wideband bandpass filter based on H‐type sandwich slotline

In this paper, a balanced ultra-wideband (UWB) band-pass filter (BPF) with compact size and simple structure is proposed. The proposed filter is fed by microstrip-slotline transition structures. A UWB differential-mode (DM) passband is achieved by employing H-type sandwich slotline structure. The fractional bandwidth of proposed balanced BPF could reach as much as 132%. In order to improve the selectivity, a source-load coupling is introduced. Meanwhile, one more layer of substrate is employed to reduce the DM passband insert loss. In addition, due to the symmetrical structure, a high UWB common-mode suppression without influencing the DM passband is realized. Therefore, the filter design method can be simplified significantly. In order to validate the effectiveness of the design method, a balanced UWB BPF is fabricated and the simulated results agree well with the measured ones.

High Selectivity Dual-Wideband Balun Filter Utilizing a Multimode T-line Loaded Middle-Shorted CSRR

This brief presents a dual-band balun filter based on a novel slotline multi-mode T-line loaded middle-shorted complementary split-ring resonator (TLMS-CSRR) with wide bandwidth and high selectivity. Coupled with ${{\tau }}$ -shaped and ${{\pi }}$ -shaped slotline stub as input and output microstrip-slotline transition structure respectively, high rejection between two passbands and at harmonic frequencies are realized. With the TLMS-CSRR, the proposed balun filter obtains five transmission poles (TPs) and four transmission zeros (TZs) which broaden the bandwidth and sharpen the roll-off skirt significantly. Besides, a high-Q folded slotline coupled with input feeding line is introduced in ${{\tau }}$ -shaped structure to enhance the quality factor of TZ, resulting in more than 16 dB suppression between passbands. The ${{\pi }}$ -shaped slotline structure enhances harmonic rejection by 15 dB@ up to $8.7{f} _{1}$ (2.3 GHz). The fabricated balun filter demonstrates two passbands operating from 1.9 GHz to 2.7 GHz and 4.8 GHz to 5.6 GHz respectively. The measured magnitude imbalances are 0.4 dB and 0.6 dB while phase imbalances are 2.5° and 3°.

Wideband and spurious‐suppressed differential bandpass filter based on strip‐loaded slot‐line structure

In this article, a wideband and spurious-suppressed differential bandpass filter based on strip-loaded slot-line structure is presented. By means of the differential microstrip-slot-line-microstrip transition, the proposed filter has a wideband bandpass filtering response. Simultaneously, the utilization of the strip-loaded slot-line extends its upper stop-band. The proposed bandpass filter has wider upper-stopband, wideband bandpass response, and intrinsic high common-mode (CM) suppression. To verify the design concept, one filter example has been designed, fabricated, and measured. It has a differential-mode (DM) 3-dB fractional bandwidth of 157% with a low 0.82 dB minimum insertion loss. What's more, it shows a very wide 20 dB DM stop-band bandwidth of 6.5 f0d. The experienced results are in good agreement with the theoretical and simulated results.

Wideband Linear-to-Circular Polarizer Based on Orthogonally Inserted Slot-Line Structures

This letter presents a printed wideband linear-to-circular polarizer for X -band applications. The proposed polarizer consists of a periodic array of orthogonally inserted slot-line structures, which are treated as two independent transmission paths for two orthogonal components of a linearly polarized incident plane wave, respectively. In order to effectively realize linear-to-circular polarization conversion, the orthogonal slot-line structures are loaded with inductive strips and pairs of open-circuited strips, respectively. This arrangement can maintain equal magnitude and 90° phase difference between these two transmission paths in a wideband frequency range. With the aid of equivalent circuit models, an operation principle of the proposed polarizer is extensively explained, and a design example is then conducted based on the prescribed specifications. Finally, the designed polarizer is fabricated and measured. The measured results confirm the theoretical predictions that the insertion loss is less than 1 dB from 8.27 to 13 GHz, and the axial ratio is less than 1 dB over entire band ranged from 7.92 to 12.16 GHz. This kind of polarizer indeed features great design flexibility, low insertion loss, and wide polarization conversion bandwidth.

Slotline-Based Balanced Filter With Ultra-Wide Stopband and High Selectivity

Slotline-based balanced filter with ultra-wide stopband is proposed in this brief. The proposed balanced filter integrates split-ring slotline resonators (SRSRs) and $\pi $ -type slotline structures, which not only increase the selectivity, but also enhance suppression in a stopband range of 7.1 times as large as center frequency. The SRSRs are implemented by two symmetrically folded stepped-impedance resonators and used as a second-order resonant circuit to obtain high-selectivity bandpass response. Moreover, the $\pi $ -type slotline structure enhances harmonic rejection significantly. The fabricated balanced filter demonstrates an insertion loss of 1.07 dB @ ${f} _{0}$ (3.5 GHz), a differential-mode (DM) stopband suppression of 20 dB @ up to 7.1 ${f} _{0}$ , a common-mode (CM) suppression of 40.5 dB over the passband. To the best of the authors’ knowledge, with maintaining competitive in-band performances, the proposed balanced filter achieves the widest stopband suppression.

A Compact and Vialess Vertical Microstrip-to-Microstrip Differential Transition

Abstract In this paper, a new compact, vialess and vertical microstrip-to-microstrip differential transition is presented based on half-wavelength slotline structure. The equivalent-circuit model is constructed, and theoretical analyses based on equivalent circuit model reveal that good differential mode (DM) signal transmission between the top- and bottom-layer differential microstrip lines can be achieved over a wide frequency range determined while the common mode (CM) signal is rejected. Moreover, the systematic design method is presented to extract the values of electrical parameters from prescribed performance in terms of its operating bandwidth and ripple levels. In addition, owing to the employment of half-wavelength slot, the proposed differential transition has a compact size. Finally, the proposed differential transition and design theories are well vindicated by an exemplary circuit centered at 2.45 GHz with the measured relative frequency bandwidth (FBW) of 114 %.

Frequency Selective Rasorber with Low Insertion Loss and Dual-Band Absorptions Using Planar Slotline Structures

This letter proposes a dual-polarized frequency selective rasorber (FSR), which exhibits bandpass filtering response together with absorption performance at both sides of the passband. Although the design concept is modified from a three-dimensional FSR, the proposed FSR is implemented by cross-inserting a few identical printed circuit board pieces etched with planar slotline structures, thus releasing difficulties in fabrication and assembly. The bandpass characteristic is provided by lossless resonators constructed in the slotlines with center-loaded U-type strips, whereas the absorption bands are realized by lossy resonators formed in short-circuited slotlines with lumped resistors. Surface current distributions are illustrated to explain the operation mechanism. A prototype of the designed FSR is fabricated and measured. The measured results show that the minimum insertion loss in the passband is about 0.3 dB, and the absorption bandwidths are ranged from 1.6 to 2.3 GHz and from 3.6 to 4.6 GHz, respectively. To further demonstrate the design concept, another FSR with wider absorption bands is developed by introducing more lossy resonators. Moreover, the proposed FSRs feature stable performance under oblique incident angles.

A Compact Ultra-Wideband Planar Printed Bow-Tie Quasi-Yagi Antenna with Rhombus Director and Tapered CPS Connection Structure

ABSTRACTA compact ultra-wideband (UWB) planar printed bow-tie quasi-Yagi antenna is proposed and experimentally studied in this paper. It is composed of three wideband configurations: micro-strip-line to slot-line transition structure with a shorting vias (balun), a pair of bow-tie driver, and rhombus director, which aims at breaking the bandwidth limitation of the conventional micro-strip (MS) quasi-Yagi antenna. To further improve the impedance matching, the tapered co-planar strip-line is adopted between the bow-tie driver and the balun as bridge. The overall size of the proposed antenna is only 30 mm × 27 mm. A prototype UWB quasi-Yagi antenna is fabricated and measured to verify the validity of this design. The measured impedance bandwidth, defined by −10 dB reflection coefficient, can reach an operating bandwidth of 101% from 3.62 to 11 GHz, over which the antenna exhibits excellent end-fire radiation with a front-to-back ratio of greater than 19 dB and a moderate gain of better than 3.81 dBi. In contrast to the conventional micro-strip quasi-Yagi antenna design, our proposed antenna features a smaller size and wider −10 dB reflection coefficient bandwidth.

Analysis and Design of Wideband Microstrip-to-Microstrip Equal Ripple Vertical Transitions and Their Application to Bandpass Filters

This paper first presents two types of wideband microstrip-to-microstrip equal ripple vertical transitions designed with the uniform and folded slotline structures, respectively. By solving the characteristic function with differential calculation, the condition for realizing the constant equal ripple frequency response but different bandwidths (BWs) for the proposed transition is determined with resorting to the characteristic impedances of the coupled open-circuited microstrip line and slotline. Then, a vertical transition with the uniform slotline is synthetically analyzed and designed with Chebyshev polynomial function and another similar vertical transition with the same in-band ripple level but different BWs is also comparatively presented. By modifying the normalized impedances of the proposed transition to make the theoretical result close to the simulated one with the initial normalized impedances, the transformer turns ratios of the microstrip-to-slotline intersection is exactly calculated by the impedance ratios between two corresponding sets of normalized impedances. Due to the degraded high-frequency performance caused by the radiation loss of the uniform slotline, two vertical transitions with the folded slotline structure are further analyzed and the related turn ratios are extracted with the proposed approach. To verify the effectiveness on the reduction of radiation loss for the folded slotline structure, a six-pole wideband bandpass filter is presented with high-frequency performance greatly improved. In final, wideband vertical transitions with different slotline structures and a six-pole wideband bandpass filter are synthetically designed, simulated, fabricated, and measured.