Compact Dual‐/Tri‐/Quad‐Band banpass filters using shorted stub loaded stepped‐impedance resonator

In this article, the shorted stub loaded stepped‐impedance resonator (SSLSIR) with the individually tunable first even resonant mode and first odd resonant mode is applied to design dual‐, tri‐, and quad‐band bandpass filters (BPFs). The SSLSIR dual‐band BPF with asymmetrical coupling is realized using the first even resonant modes and the first odd resonant modes of a set of SSLSIRs. Then, the high‐impedance feeding lines of SSLSIR dual‐band BPF is modified to produce a new passband, and thus a new tri‐band BPF is realized. The proposed quad‐band BPF consists of two sets of SSLSIRs with symmetrical coupling. Each of the designed circuits occupies a very compact size and has a good in‐band out‐of‐band performance. Good agreements are observed between the simulated and measured results. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:601–609, 2015.     

A balanced dual‐band bandpass filter with a wide stopband and controllable differential‐mode frequencies and fractional bandwidths

In this article, a balanced microstrip dual‐band bandpass filter (BPF) is designed. The proposed filter is achieved by employing a microstrip U‐shape half‐wavelength resonator, a folded stub‐loaded resonator and balanced microstrip/slotline transition structures. The center frequencies and the fractional bandwidths of the two differential‐mode (DM) passbands can be controlled independently by changing the physical lengths of the two resonators and the gaps between each resonator, respectively. The balanced microstrip/slotline transition structures can achieve a wideband common‐mode (CM) suppression. Meanwhile, the DM passbands are independent from the CM responses, which significantly simplify the design procedure. In addition, a wide DM stopband is also realized. In order to validate the design strategies, a balanced dual‐band BPF centered at 2.57 and 3.41 GHz was fabricated and a good agreement between the simulated and measured results is observed.     

Design of wideband bandpass filter with high and wide stopband rejection using asymmetrical‐loaded stepped‐impedance resonator

This study presents a wideband bandpass filter (WBBPF) with wide and high stopband suppression by loading a stepped‐impedance resonator (SIR). The prototype of WBBPF is composed of an inverted π‐shaped resonator with T‐shaped resonator and open stub loaded, centrally. Odd‐/even‐mode analysis technique is employed to characterize the resonant properties of this prototype. Then, a SIR is loaded to this filter, asymmetrically, to improve the out‐of‐band performance. For experimental validation, a WBBPF is designed, fabricated, and tested. The measurement results show that the center frequency of WBBPF is located at 5.095 GHz, and the 3‐dB fraction bandwidth is about 71%. Plus, the out‐of‐band suppression with 30‐dB rejection level can be extended to 18.17 GHz.     

Compact dual‐band bandpass filter and diplexer using hybrid resonant structure with independently controllable dual passbands

In this article, a compact dual‐band bandpass filter (BPF) is developed using a hybrid resonant structure, which consists of a microstrip stub‐loaded dual‐mode resonator and a slotline stub‐loaded dual‐mode resonator. These two resonators, both having two controllable resonant modes and one transmission zero (TZ), are analyzed and used to construct two desired passbands of a dual‐band BPF. Multiple TZs are generated by introducing a source‐load coupling, thus improving the selectivity of the passbands. Then, the dual‐band BPF is reshaped to configure a compact diplexer. The inherent TZs of the two proposed resonators are designed to improve the frequency property and port isolation of the diplexer. Finally, a dual‐band BPF and a diplexer with the lower and upper passbands centered at 2.45 and 3.45 GHz, respectively, are designed, fabricated, and measured to verify the proposed structure and method.     

Coupled‐line band‐pass filter with T‐shaped structure for high frequency selectivity and stopband rejection

A coupled‐line band‐pass filter (BPF) with T‐shaped stub structure is presented. Five transmission poles within the passband and eight deep transmission zeros (TZs) from 0 to 2f₀ (f₀ denotes filter's center frequency) are realized through input impedance calculations. With the simple T‐shaped structure, the positions of six TZs can be appropriately adjusted to achieve high frequency selectivity and stopband rejection. For demonstration, a BPF prototype centered at 2.05 GHz is designed and fabricated, whose measured rejection levels are of over 45.5 dB at lower stopband and better than 19.5 dB at upper stopband. The simulation and measurement results are in good agreement, which validates the design idea.     

Design of high selectivity tri‐band bandpass filter with wide upper stopband

This article presents a dual‐plane structure high selectivity tri‐band bandpass filter (BPF) which consists of a pair of T‐shaped microstrip feed lines with capacitive source‐load coupling as well as spur lines embedded, and three resonators, i.e., a dual‐mode stub‐loaded stepped impedance resonator and two nested dual‐mode defected ground structure resonators. Using the intrinsic characteristics of the resonators and feed lines, nine transmission zeros near the passband edges and in the stopband can be generated to achieve high selectivity. An experimental tri‐band BPF located at 2.4/5.7 GHz [wireless local area networks (WLAN) application] and 3.5 GHz [worldwide interoperability for microwave access (WiMAX) application] has been simulated and fabricated. Good agreement between the simulated and measured results validates the design approach. © 2012 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2013.     

Multi‐mode resonators with quad‐/penta‐/sext‐mode resonant characteristics and their applications to bandpass filters

In this article, a new class of dual‐/tri‐band and ultra‐wideband (UWB) bandpass filters (BPFs) using novel multi‐mode resonators are proposed. The classical even‐/odd‐mode method is applied to analyze the resonant characteristics of the proposed resonators, which exhibit controllable resonant modes with different dimension parameters under the same configuration. According to the analysis, three resonators with quad‐/penta‐/sext‐mode resonant characteristics are obtained by choosing the specific dimension parameters. Then, the quad‐mode resonator is used to design a dual‐wideband BPF centred at 2.39/5.14 GHz with 3‐dB fractional bandwidths (FBWs) of 36.9%/18.9%, and the penta‐mode resonator is utilized to design an UWB BPF with 3‐dB FBW of 102.2%, whereas the sext‐mode resonator is applied to design a tri‐band BPF with centre frequencies of 2.09/3.52/5.46 GHz and 3‐dB FBWs of 11.3%/20%/12.1%. All these three filters are fabricated and measured, and the measured results are in good agreement with the simulated ones.     

Independently controllable dual‐band microstrip bandpass filter using quadruple‐mode stub‐loaded resonator

In this article, a quadruple‐mode stub‐loaded resonator (QM‐SLR) is introduced and its four modes are excited using a simple approach, which can provide a dual‐band behavior. By changing the length of the loaded stubs, independently tunable transmission characteristics of the proposed quadruple‐mode stub‐loaded resonator were extensively described for filter design. Moreover, microwave varactors were adopted to represent the length variation of the loaded stubs for the dual‐band tunability. The equivalent circuit modeling of the open stub with microwave varactor was given and discussed. Then, adopting the compact quadruple‐mode stub‐loaded resonator with three varactors, an independently controllable dual‐band bandpass filter (BPF) was designed, analyzed, and fabricated. Its separated bandwidths and transmission zeros can be tuned independently by changing the applying voltage of the microwave varactors. A good agreement between simulated and measured results verified the design methodology. The proposed filter possesses compact size, simple structure, and excellent dual‐band performances. © 2016 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:602–608, 2016.     

A balanced dual‐band BPF with independently controllable frequencies and bandwidths

A balanced second‐order dual‐band bandpass filter (BPF) with independently controllable center frequencies and bandwidths based on coupled stepped‐impedance resonators (SIRs) is designed in this article. To obtain a dual‐band differential‐mode (DM) response, two pairs of SIRs with different resonant frequencies are employed in the design. The bandwidths of the two DM passbands can be independently tuned by adjusting the coupling gaps and coupling lengths of the corresponding resonators. In addition, three transmission zeros are realized to enhance the selectivity of the DM passbands. The microstrip‐slotline transition structure is utilized to achieve a wideband common‐mode (CM) suppression. Moreover, the DM responses are independent of the CM ones, which significantly simplify the design procedure. Finally, a balanced dual‐band BPF is designed to validate the design method and a good agreement between the simulated and measured results is observed.     

Compact UWB bandpass filter with dual‐notch bands using asymmetric tri‐section stepped impedance resonator

In this study, a novel high selective UWB band pass filter (BPF) with dual notch band is presented. UWB BPF is realized using stub‐loaded multiple‐mode resonator (MMR). The MMR is constructed by loading a quintuple mode open stub at the centre in an asymmetric tri‐section stepped impedance resonator (ATSSIR). Five modes, including two odd modes and three even modes, placed within UWB band. Two transmission zeros generated by the fractal stub improve the passband selectivity greatly. Two half wavelength long fractal Hilbert resonators are embedded near I/O line to achieve notch bands at 5.1 and 5.9 GHz. Aperture‐backed interdigital coupled‐lines are implemented to improve the coupling. The proposed prototype is fabricated and tested. The measured insertion loss is observed to be within 1.5 dB over the passband. By virtue of two transmission zeros (TZs), on either side of the passband, at 5.1 and 5.9 GHz, respectively, the passband selectivity is achieved with measured roll‐off factor at around 34 dB/octave. The out‐of‐band rejection of the filter is greater than 22 dB up to 18 GHz. The simulated results are in good agreement with the measured responses.     

Dual‐band band pass filter using stub‐loaded open‐loop resonators with wide controllable bandwidths

Two dual‐band band pass filters (BPF) using stub‐loaded open‐loop (SLOL) resonator are presented in this article. A novel coupling tuning method by changing the relative coupling position of the resonators is proposed to control the bandwidth of each passband in a wide range. Transmission zeros are created to improve the selectivity by source‐load coupling. Because of the large ratio of two bandwidths, a novel dual‐band matching method is proposed to match the different load impedances at two passband frequencies to the same source impedance. Hence, relax the fabrication requirement of gap. The proposed dual‐band band pass filter is designed and fabricated. The measured 3 dB fractional bandwidths (FBWs) of two 2.45/5.25 GHz dual‐band BPFs are 6.5%/14.5% and 9.8%/5.5%, respectively. The results are in good agreement with the simulation. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:367–374, 2014.     

Direct synthesis of compact wideband differential bandpass filter on composite triple‐mode resonator

In this paper, a direct synthesis approach is proposed for design of a compact wideband differential‐mode (DM) bandpass filter (BPF) on a composite triple‐mode resonator. By virtue of intrinsic common‐mode (CM) suppression in slotline portion, only the DM transmission performances need to be focused on in our design, so as to facilitate the design process. Consequently, a synthesis approach based on an equivalent simplified network is established to design this DM BPF, resulting to directly determine all the circuit element values. A wideband DM BPF is then designed to validate effectiveness of proposed synthesis method. Finally, two circuit prototypes have been designed, fabricated, and measured. The synthesized, simulated and measured results agree well with each other over a wide operating band, which has demonstrated the proposed DM BPFs' attractive performances, such as wide bandwidth of operation, sharpened frequency selectivity, and good CM suppression.     

Compact wideband microstrip bandpass filter with wide upper stopband based on coupled‐stub loaded resonator

A novel wideband microstrip bandpass filter (BPF) based on a coupled‐stub loaded resonator (CSLR) is presented in this article. The CSLR is constructed by attaching one short‐circuited parallel coupled microstrip line (PCML) in shunt to a high impedance microstrip line. The filter bandwidth can be conveniently controlled via reasonable adjusting of the impedance of PCML. Moreover, new defected microstrip structures (DMSs) introduced in the PCML functions as a means of adjusting the positions of transmission zeros, created by the PCML. The resonant mode and transmission zero chart are given, indicating that the higher modes could be suppressed by the transmission zeros. Finally, to validate the proposed method, two wideband BPF filters with and without DMSs centered at 3 GHz with 3 dB fractional bandwidth of 87% are designed and fabricated. The measured results show that both the return losses are better than 15.8 dB, while the BPF with DMSs has a ?19.4 dB isolation wideband from 1.57 to 4.23 . The measured results are in excellent agreement with full‐wave electromagnetic simulation results. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:122–128, 2015.     

A tunable balanced dual‐band BPF with quasi‐independently controlled center frequency and bandwidth

In this paper, a balanced dual‐band bandpass filter (BPF) with high selectivity and low insertion loss performance is presented by employing stub loaded resonators (SLRs) and stepped impedance resonators (SIRs) into balanced microstrip‐slotline (MS) transition structures. The balanced MS transition structures can achieve a wideband common‐mode (CM) suppression which is independent of the differential‐mode (DM) response, significantly simplifying the design procedure. Six varactors are loaded into the resonators to achieve the electrical reconfiguration. The proposed balanced dual‐band BPF can realize quasi‐independently tunable center frequencies and bandwidths. A tuning center frequency from 2.48 to 2.85 GHz and a fractional bandwidth (20.16%‐7.02%) with more than 15 dB return loss and less than 2.36 dB insertion loss are achieved in the first passband. The second passband can realize a tuning center frequency from 3.6 to 3.95 GHz with more than 12 dB return loss and less than 2.38 dB insertion loss. A good agreement between the simulated and measured results is observed.     

Fork‐shaped patch printed ultra‐wideband slot antenna with dual band‐notched characteristics using metamaterial unit cells

In this article, a miniaturized fork‐shaped patch ultra‐wideband (UWB) planar wide‐slot antenna with dual band‐notched characteristics is proposed. With fork‐shaped patch, ultra‐wideband impedance matching from 3.1 to 13.2 GHz is easily achieved. Then, two novel and simple methods are applied to solve the difficulty for UWB slot antennas with fork‐shaped patch to realize band‐notched characteristics. By etching one pair of I‐shaped resonators on both branches of the fork‐shaped structure and adding a rectangular single split‐ring resonator in the rectangular openings of fork‐shaped patch, the wireless local area network (WLAN) band from 5.5 to 6.1 GHz and the International Telecommunication Union (ITU) 8 GHz band from 7.9 to 8.7 GHz are rejected, respectively. The coplanar waveguide‐fed UWB antenna is successfully designed, fabricated, and measured. The measured and simulated results show a good agreement. The antenna provides nearly stable radiation patterns, high gains and high radiation efficiency.     

Dual‐mode resonator with modified dual‐square‐Loop for WLAN and WiMAX quad‐band filter design

We propose the improved configurations with dual‐mode dual‐square‐loop resonators (DMDSLR) for quad‐band bandpass filter (BPF) design. The modified DMDSLR filter employs two sets of the loops. The square loop is designed to operate at the first and third resonated frequencies (2.4/5.22 GHz) and the G‐shaped loop is employed at the second and fourth resonated frequencies (3.59/6.6 GHz). The resonant frequency equations of DMDSLR are introduced for simply designing quad‐band BPF. Resonant frequencies can be controlled by tuning the perimeter ratio of the square loops. A systematic design procedure with the design map is applied for accuracy design. To obtain lower insertion loss, higher out‐of‐band rejection level and wider bandwidth of quad‐band, the miniaturized DMDSLR with meander‐line technique is proposed. The proposed filters are successfully simulated and measured showing frequency responses and current distributions. It can be applied to WLAN and WiMAX quad‐band systems. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:332–340, 2014.     

Compact tri‐band BPF applying novel multi‐mode microstrip resonator

This article presents a novel multi‐mode microstrip resonator. Using the even‐odd‐mode method, its resonance characteristics are analyzed and the design graphs are given. Each mode equivalent circuit is a λ/4 stepped impedance resonator (SIR), so the proposed resonator has a compact size and the higher harmonics can be tuned in a wide range. Stub–stub coupling is introduced to split two identical modes and produce two transmission zeros (TZs). Then a tri‐band filter operating at 1.5, 2.4, and 3.8 GHz is designed using the proposed resonator. The three center frequencies and bandwidths can be independently controlled. By tuning the impedance and length ratios of the stubs, wide upper stopband is achieved. Finally, the designed filter is fabricated and measured, and the measured results agree well with the simulated ones. © 2016 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:559–564, 2016.     

Compact sub‐6 GHz 5G‐multiple‐input‐multiple‐output antenna system with enhanced isolation

A compact two‐element multiple‐input‐multiple‐output (MIMO) antenna system with improved impedance matching and isolation is presented for future sub‐6 GHz 5G applications. The two identical tapered microstrip line fed modified rhombus‐shaped radiating elements are placed in the same orientation at a compact substrate area of 0.24λ₀ × 0.42λ₀ (where, λ₀ at 3.6 GHz) on a shared rectangular ground. A remodeled T‐shaped ground stub is placed between a pair of radiating element to achieve improved impedance bandwidth and isolation. Further, a split U‐shaped stub connected to center of each radiating element to achieve the desired resonant frequency of 3.6 GHz. The proposed antenna covers a ?10 dB operating band of 3.34 to 3.87 GHz (530 MHz) with more than 20 dB isolation between a pair of elements. MIMO performances are also analyzed and experimentally validated. The measured performances of a prototype are found in good agreement with simulated performances. Further, the simulation study is carried out to see the effect of housing and extended ground plane on two‐element MIMO antenna for practical application. An idea of realization of 12‐element MIMO is also studied using the proposed two‐element MIMO antenna.     

Band‐notched ultra‐wideband antennas using nonperiodic composite right/left‐handed resonators

This article investigates the characteristics of a single/double‐cell composite right/left‐handed (CRLH) resonator and its application on multiple band‐notched ultra‐wideband (UWB) antennas while suggesting an accurate design procedure. Periodicity assumption and calculating a dispersion diagram allow band‐notched frequencies to adequately predict prior to antenna design. Zeroth‐order resonance (ZOR) frequency due to the CRLH characteristics of a mushroom resonator and higher‐order resonance frequencies are predictable through the hypothetical dispersion diagram. To demonstrate this method, compact, printed, ultra‐wideband circular monopole antennas with four/five‐band notched characteristics using a single/double mushroom resonator are presented. The effects of mushroom cell size on ZOR and the other band notched resonant frequencies are also investigated. The numerical simulations show that the asymmetrical unit cell provides the capability to tune both ZOR and band notched frequencies. Comparison between the simulation and measurement results shows reasonable agreement.     

A new dual‐band single‐ended‐to‐balanced filtering power divider

This article proposes a new dual‐band single‐ended‐to‐balanced (SETB) filtering power divider (FPD), which shows the excellent characteristics of wideband common‐mode (CM) suppression and good selectivity. By employing the structure of double‐sided parallel‐strip line with a mid‐inserted conductor and a T‐shaped defected ground structure etched in the mid‐inserted conductor, out‐of‐phase behavior and high CM suppression can be achieved successfully. Besides, to realize dual‐wideband filtering performance and high selectivity, two pairs of step impedance stubs (SIS) loaded quarter‐wavelength central line‐terminal‐shorted three parallel‐coupled microstrip lines structure are adopted. Meanwhile, two pairs of resistors are introduced so as to realize excellent isolation. To verify effectiveness of the design method, a prototype of dual‐band SETB FPD which operates at 3.2 and 4.9 GHz is designed, fabricated, and tested. Final results exhibit that the new dual‐band SETB FPD possess high selective dual‐band differential mode response, wideband CM suppression, and excellent isolation between the balanced output ports.