Wideband bandpass filtering branch‐line balun with high‐isolation

The wideband bandpass filtering branch‐line balun with high isolation is presented in this paper. The proposed balun can be designed for wideband performances by choosing a proper characteristics impedance of input vertical transmission line and odd‐mode impedance of parallel‐coupled lines. The proposed balun was designed at a center frequency (f₀) of 3.5 GHz for validation. The measured results are in good agreement with the simulations. The measured power divisions are ?3.31 dB and ?3.24 dB at f₀ and ?3 ± 0.17 dB within the bandwidth of 0.95 GHz (3 GHz to 3.95 GHz). The input return loss of 24.09 is measured at f₀ and higher than 20 dB over the same bandwidth. Moreover, the measured output losses are better than 11 dB within a wide bandwidth. The isolation between output ports is 20.32 dB at f₀ and higher than 13.2 dB for a broad bandwidth from 1 GHz to 10 GHz. The phase difference and magnitude imbalance between two output ports are 180° ± 4.5° and ± 0.95 dB, respectively, for the bandwidth of 0.95 GHz.     

Wideband filtering power dividers using single‐ and double‐layer periodic spoof surface plasmon polaritons

In this article, four wideband power dividers (two are filtering power dividers) using single‐ and double‐layer periodic spoof surface plasmon polaritons (SSPPs) are proposed. Double‐sided parallel‐strip line is used to realize the wideband and low loss for the double‐layer SSPPs. T‐shaped SSPPs power dividers with large bandwidth, wideband isolation, and low loss using double‐layer SSPPs to single‐layer SSPPs transition are realized. Conventional coplanar waveguide is used as the output ports for the single‐layer SSPPs. Moreover, two new SSPPs power dividers with filtering performance are realized with adding via holes in the double‐layer SSPPs. The theoretical analysis, parametric study, and design procedure for these wideband power dividers are illustrated. In addition, for validity demonstration, four wideband SSPPs power dividers are fabricated in microstrip technology and characterized. Good agreements can be observed between the measured and simulated results, indicating good potential applications in the integrated plasmonic devices.     

Wideband balun filter on slot‐line resonator with intrinsic balanced performance in magnitude and phase

In this article, a novel wideband balun filter on the coupled slotline‐line resonator topology with intrinsic balanced performance in magnitude and phase performance is presented. The proposed wideband balun filter consists of four uniform half‐wavelength slot‐line resonators (SLRs) in middle, one single‐ended unbalanced port, and one balanced port with two terminals. By utilizing the distinct field conversion from the slotline to the microstrip line feeding line, the proposed technique can realize good balanced performance inherently in magnitude and phase over a wideband frequency range. In order to demonstrate the proposed scheme and support the design conception, one circuit prototype is designed, fabricated, and tested. All of the measured results of the frequency response, magnitude, and phase imbalance performance agree well with the simulated one, and these results have effectively demonstrated its good filtering property, intrinsic balanced performance in magnitude and phase over a wide band frequency.     

Balanced power dividers with improved bandwidth using folded lines

Two novel wideband balanced‐to‐unbalanced power dividers based on transmission lines and coupled lines are proposed in this article. Multifunction can be realized without cascading single devices with common mode suppression. The desired power divider configurations can be obtained using the even/odd‐mode characteristic impedance of the coupled lines and characteristic impedance of the transmission lines. A resistor in the middle of two single‐ended ports is used to realize isolation between output ports. Two prototypes of wideband balanced‐to‐unbalanced power dividers operating at 2.3 GHz with differential‐mode bandwidths of 79.6 and 134.1% are designed and fabricated. The theoretical and measured results are in good agreement and show good in‐band performances.     

Harmonics and size reduced microstrip branch‐line baluns using shunt open‐stubs

This article presents the design of harmonic suppressed compact microstrip balun, using shunt open‐stubs. Two approaches using Π‐shaped and T‐shaped open‐stub configurations are investigated. Because of the stopband effect of the open‐stubs, the harmonic passband responses are suppressed effectively. A compact‐size balun is realized because both kinds of structures have slow wave characteristics. To validate theoretical predictions, two prototype baluns operating at f₀ = 1 GHz, showing size reduction up to 75% and no spurious passband up to 4f₀ compared with the conventional design are fabricated in microstrip form. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2011.     

Compact wideband bandpass filter based on double‐T‐shaped stub loaded resonator and loading technique for zero‐voltage point

In this article, a double‐T‐shaped stub centrally loaded uniform impedance resonator (UIR) is introduced and its resonant characteristics are well clarified, which provided a simple approach for triple‐mode wideband bandpass filter (BPF) design. The double‐T‐shaped stub consists of a T‐shaped stub at the center of UIR and two shunt uniform‐impedance stubs at the T‐shaped stub. Furthermore, loading technique for zero‐voltage point is employed to guide design procedure from UIR to the proposed resonator. The resonant frequencies of the first three modes for the resonator can be free to adjust by the length of the UIR and the two kinds of stub. Finally, a compact wideband BPF is designed, fabricated, and measured. The measured results are in good agreement with the full‐wave simulation results. The realized wideband filter exhibits a 3 dB fractional bandwidth of 69.1% with good in‐band filtering performance, wide stopband, and sharp out‐of‐band rejection skirt.     

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 f_0d. The experienced results are in good agreement with the theoretical and simulated results.     

Epsilon‐shaped circularly polarized strip and slot‐loaded ultra‐wideband antenna for Ku‐band and K‐band

A compact epsilon‐shaped (ε) ultra‐wideband (UWB) antenna for dual‐wideband circularly polarized (CP) applications has been investigated in this article. It consists of a stepped stub loaded modified annular ring‐shaped radiator and modified CPW ground plane. The ground plane is loaded with two semicircular notches and a spiral‐shaped slot. The impedance bandwidth (IBW) is 97.02% (10.4‐30 GHz) along with an overall footprint of 20 × 20 mm². The fractional axial ratio bandwidth (3‐dB ARBW) for two wide bands is 38.50% (13.30‐19.64 GHz) and 6.45% (26.25‐28.00 GHz), respectively. The proposed antenna is left‐hand circularly polarized with a peak gain of about 5.09 and 5.14 dB in both 3‐dB ARBW bands. The proposed antenna is dominating other reported CP antenna structures in terms of number of CP bands, 3‐dB ARBW, IBW, peak gain, and dimensions.     

Design of a wideband filtering power divider with good in‐band and out‐of‐band isolations

In this article, a wideband filtering power divider with good in‐band and out‐of‐band isolations is designed based on a hybrid Wilkinson and Gysel structure. To achieve a good in‐band response, two additional in‐band transmission poles can be introduced by installing the coupled‐line structures at each port. By mounting a stepped impedance open stub at the input port, two transmission zeros are generated near the passband to improve the passband skirt. Furthermore, the out‐of‐band rejection and isolation are achieved by the other two transmission zeros, which are produced by the open stub and the three coupled‐line sections mentioned above. Additionally, a good in‐band isolation is realized by the isolation resistor between output ports. For the demonstration, a wideband filtering power divider centered at 1.5 GHz with a 56% fractional bandwidth and 20‐dB isolation is designed and fabricated. The simulated and measured results are in good agreement with each other.     

Directional‐coupler‐based microwave sensors for differential angular‐displacement measurement

The novel application of microwave directional couplers to develop angular‐displacement microwave sensors is reported. The proposed sensor approach employs as stator a branch‐line‐type coupler arranged in transversal mode by loading its direct and coupled ports with two distinct‐length open‐ended stubs. Thus, by taking the isolated port of the coupler as the stator output node, a bandpass filtering transfer function with transmission zeros (TZs) is created. Then, a rotor made up of an angularly‐moveable open‐ended stub is attached to a curved section of the longest loading stub of the stator through physical contact, so that their interconnection point varies with the angular‐displacement of the rotor. In this manner, the sensor transfer function is altered with the stub rotation through TZ reallocation, angular‐displacement sensing capabilities are achieved. The theoretical operational foundations of the conceived branch‐line‐coupler‐based microwave angular‐displacement sensor, which features single/multi‐band sensing properties in terms of inter‐TZ spacing and stop band attenuation levels, along with design examples and curves are provided. The extrapolation of this sensor principle to other classes of power‐distribution circuits, such as the rat‐race‐type directional coupler, is also demonstrated. Finally, for experimental‐validation purposes, two 920 MHz microstrip prototypes of the conceived branch‐line‐coupler‐based angular‐displacement microwave‐sensor approach are built and measured.     

Quintuple-mode wideband bandpass filter based on stub-loaded circular resonator

In this paper, novel compact wideband filters have been implemented using a stub-loaded microstrip resonator based on a circular open-loop configuration for the first time. To offer an alternative wideband filter configuration, the quadruple?/quintuple-mode wideband bandpass filters (BPFs) using stub-loaded circular microstrip resonator (SLCMR) and parallel-coupled microstrip line (PCML) feed configuration are proposed. They are realized by employing a single open stub and two open stubs, respectively. A half-wavelength $\left(\lambda /2\right)$ circular microstrip resonator (CMR) loaded with two open stubs excites three resonant modes, while the one with a single stub produces two resonant modes. One of the modes is odd-mode generated by a $\lambda /2$ CMR, the others are even-modes excited by the two loading stubs. The PCML with enhanced input/output (I/O) coupling degree excites also two modes in the passband. Two loading stubs not only creates transmission poles in the passband, they also create two transmission zeros in the lower and upper stopbands. The quadruple?/quintuple-mode wideband BPFs with fractional bandwidths FBW of 60% and 62% are successfully realized by using the SLCMR and PCML feed configuration together. Both filters are designed, fabricated, and tested where excellent agreement between simulated and measured results is observed.     

Compact dual‐band single‐ended‐to‐balanced power dividers with open/short‐ended stubs

In this article, two novel topologies of compact‐size dual‐band single‐ended‐to‐balanced power dividers that are loaded with open‐ and short‐ended stubs are presented. Quarter‐wavelength open‐ended stubs and half‐wavelength short‐ended stubs are respectively exploited in the proposed dual‐band power‐divider configurations to incorporate the dual‐band functionality into them for flexibly‐adjustable dual‐frequency‐ratio specifications. Each engineered five‐port power‐divider circuit features high in‐band input/output power‐matching levels, high in‐band power‐isolation levels between the two differential‐mode outputs, and high common‐mode‐rejection levels in a broad spectral range. Two microstrip prototypes designed at 0.9/1.8 GHz (GSM bands) and 1.57/2.45 GHz (GPS and WLAN bands) are constructed and characterized for experimental‐demonstration purposes.     

Dual‐band bandpass filters with multiple transmission zeros using λ/4 stepped‐impedance resonators

Two novel dual‐band microstrip bandpass filters (BPFs) with multiple transmission zeros are proposed in this article. The dual‐band BPFs with second‐order bandpass responses are due to two λ/4 stepped‐impedance resonators (SIRs). Two passbands (center frequency ratio f _s/f₀ is 2.36) are realized based on the asymmetric SIRs. The transmission zeros near the passbands can be adjusted conveniently using the stopband transmission characteristic of the open/shorted coupled lines. Two planar microstrip dual‐band BPFs (ε _r = 2.65, h = 0.5 mm) with four and six transmission zeros are designed and fabricated. High selectivity and good in‐band performances can be achieved in the proposed filters.     

High selectivity wideband BPF with very wide stopband characteristic

This paper describes the design and evaluation of a highly selective wideband microstrip bandpass filter with a near brick‐shaped transmission response and a very wide stopband characteristic. The proposed filter structure excites multimode resonances that combine to realize a wideband filter response and excited too are transmission zeros that create a highly selective filter with wideband suppression in the upper and lower stopbands. The filter configuration comprises electromagnetically coupled resonators that are stub loaded. The input and output feedlines are interdigitally coupled to the resonators. Measured results confirm the low‐loss and via‐free wideband filter exhibits an elliptical response with a wide stopband with a rejection greater than 30 dB. The selectivity factor and stopband performance of the proposed filter is better than that obtained with the high‐temperature superconductor (HTS) filters. Design of the filter is relatively simple and easy to manufacture using standard PCB technology. There is good correlation between the simulation and measured results. The proposed wideband bandpass filter is suitable for applications in high interference environments and cognitive radio systems.     

A 1‐to‐n way single‐ended‐to‐balanced substrate integrated waveguide filtering power splitter

An approach to 1‐to‐n (n = 3, 4…) way single‐ended‐to‐balanced filtering power splitter (SETBFPS) is proposed. The properly placed balanced ports with 0.5λ_g (λ_g is the substrate integrated waveguide [SIW] guided wavelength at f₀) space make the TE32nd 103 and TE32nd 105 modes of n 32nd‐mode SIW multimode resonators form differential‐mode (DM) passband of the SETBFPS. Compared with the state‐of‐art single‐ended‐to‐balanced power splitters, the proposed approach has all the functions of 1‐to‐n way, filtering, and common‐mode (CM) suppression. A 1‐to‐3 way prototype is exemplified at 3.5 GHz with the minimum insertion loss (IL) of 0.09 dB, a fractional bandwidth (FBW) for a 15‐dB return loss of 35%, and a FBW for 15‐dB CM suppression of 52%. Low IL and wide bandwidth can be observed.     

A low‐profile dual port UWB‐MIMO/diversity antenna with band rejection ability

A band notched ultra‐wideband (UWB) antenna is presented in this article as a good prospect for multiple‐input multiple‐output (MIMO)/diversity application. The proposed MIMO antenna is constituted of two modified rectangle‐shaped patch antenna elements. A stepped stub is extended from the modified ground plane as a decoupling element between the radiators to realize a good isolation level between them. A band rejection response is obtained by connecting an open resonant stub to each of the radiators. The simulated prototype is fabricated and tested for verification. Results reveal that the proposed prototype provides a 10 dB return loss bandwidth from 3.08 to 10.98 GHz with band notch characteristics from 4.98 to 5.96 GHz, and a good port isolation level (S₂₁ ≤ 20). Diversity performances are ensured in terms of total active reflection coefficient, envelope correlation coefficient (<0.013 except notch band), diversity gain (≈9.51 dB), mean effective gain ratio (≈1), and channel capacity loss (≤0.35 bps/HZ except notch band). It evidences that the presented band notched UWB antenna can be a good prospective for MIMO/diversity applications.     

Wideband and wide stopband superconducting bandpass filter using asymmetric stepped‐impedance resonators connected to open stub

A wideband wide stopband filter is designed using asymmetric stepped‐impedance resonators (ASIRs) connected to a large open stub. The capacitive open stub and the parallel‐coupled microstrip line are used to achieve the strong couplings for large fractional bandwidth (FBW). For a wide‐stopband performance, the proposed filter uses ASIRs to improve the high‐order spurious resonant frequency. The first and last resonators of the proposed filter are further optimized to suppress the spurious resonant frequency caused by open stub. The final filter has a 70% FBW centered at 4.87 GHz with 20‐dB‐rejection stopband up to 15.78 GHz (approximately 3.24 f₀). The measured insertion loss is less than 0.15 dB and the return loss is better than 17 dB.     

Filtering balanced‐to‐single‐ended power dividing network

In this article, the filtering balanced‐to‐single‐ended power dividing networks are proposed. Except the fundamental functions of differential‐mode transmission, common‐mode suppression, and out‐of‐phase single‐ended output ports with isolation, the proposed designs show the advantages of wide controllable range of differential‐mode bandwidth, multiple transmission zeros (TZs), and wide bandwidth for high out‐of‐band suppression. The frequencies of TZs, bandwidth, isolation, and common‐mode suppression can be controlled by the parameters. For demonstration, three prototypes (Deigns I, II, and III) with two, four, or six TZs are implemented. The measured results show that design I (II and III) has an insertion loss of 0.38 dB (0.7 dB and 0.8 dB), an operating bandwidth of 12.5% (7.5% and 6.9%), and a bandwidth for 30‐dB out‐of‐band suppression of 0.06f₀ (0.09f₀ and 0.14f₀). The isolation and common‐mode suppression inside the passbands of the three prototypes are all larger than 17 and 38 dB, respectively.     

Compact dual‐polarized dipole antenna with high isolation using hybrid balun circuit

In this article, a crossed dipole antenna with compact size (43 × 43 mm²) and high isolation (42 dB) is proposed using a hybrid balun circuit. The hybrid balun consists of a tapered balun and a Marchand balun. The tapered balun transforms the microstrip line into the parallel feeding lines for one pair of dipoles, and the Marchand balun transforms the stripline transmission line into a couple of out‐of‐phase feeding lines for the crossed pair of dipoles. Due to the low coupling between two different baluns, a high isolation of the antenna can be achieved when the crossed dipole antenna is cascaded with the hybrid balun. Moreover, these two baluns can improve impedance matching and therefore further miniaturize the size of the crossed dipole antenna. The impedance matching of two baluns can be controlled and adjusted independently. As a result, the proposed dual‐polarized antenna has the bandwidth of 1.7 to 2.8 GHz for S11 < ?15 dB with compact size and high isolation. The proposed antenna with the hybrid balun circuit are fabricated, and the simulated and measured results meet well.     

Novel single-layer dual-band half-mode substrate integrated waveguide filter and filtering power dividers with very compact sizes

In this article, a novel single-layer dual-band (DB) half-mode substrate integrated waveguide (HMSIW) filter and three equal/unequal DB HMSIW filtering power dividers (FPDs) with super-compact sizes have been proposed. In order to design the proposed devices, the evanescent mode technique and the metamaterial concept have been used, simultaneously. Two passbands have been generated below the cut-off frequency of the HMSIW structure by etching a new compact DB metamaterial unit cell with effective negative permittivity in two different frequencies on the metal surface of the HMSIW structure. The center frequencies of these two passbands can be simply controlled by changing the size of the DB metamaterial unit cell. To confirm the design concept, three prototypes of the DB structures working at 2.4 and 3.5 GHz are simulated, constructed, and measured. The overall dimension of the designed DB HMSIW filter and DB HMSIW FPDs are approximately 0.12 λ_g × 0.11 λ_g. Compared to other existing devices, the performance of the proposed structures is very satisfactory. Compact size, easy integration, easy fabrication process, low cost, low loss, and high selectivity are the advantages of the designed structures.