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In this paper, a new design method is proposed for multi-mode two-band bandpass filters based on combining a square loop resonator that has a capacitive perturbation element and short-circuited stubs. This concept is a new multi-mode two-band filter design having Nth and 2Nth order for the first and second passbands that appear in f0 and 3f0 center frequencies, respectively. It has been shown that the number of transmission modes in each passband can easily be increased by periodically cascading one unit filter cell including one square loop resonator and two short-circuited stubs. This also provides a reconfigurable filter property, namely the obtaining of two different filtering characteristics in the second passband by means of the reverse and straight placing of the cascaded unit filter cells in the horizontal axis. Two 2nd/4th order two-band bandpass filter prototypes are fabricated and tested to validate the proposed design method. The total surface areas of these fabricated prototypes are 26.0 mm × 9.0 mm and 24.4 mm × 16.8 mm, and the measured insertion losses in the first/second passband are about 0.63 dB/0.96 dB and 0.59 dB/0.83 dB. It is observed that the filter prototypes have the advantages of low insertion loss, compact size, and flexible filtering characteristics. The designed filters are suitable for operation in the 5G spectrum, which includes a range of radio frequencies sub-6 GHz.

In this paper, an electronically switchable quad-band microstrip bandpass filter is designed using varactor perturbed dual-mode resonators. For this purpose, four meandered dual-mode square loop resonators (DMSLRs) having reference patches are nested and varactor diodes are used to serve as perturbation elements. The designed resonator is analyzed by deriving even/odd mode resonance conditions. S-parameters are also calculated according to the even and odd mode input impedances. Stub loaded feedlines are used to couple the DMSLRs to Input/Output (I/O) ports. According to the capacitances of varactor diodes, each passband can be independently turned on and off. Coupling scheme and coupling matrix of the designed filter are introduced to realize the coupling descriptions. Switching operations are also investigated over the frequency responses obtained from coupling matrix. Passbands of the filter are adjusted to 1.4, 1.7, 2.25, and 2.7 GHz. The designed filter was also fabricated and measured results show a good agreement with the simulated and theoretical results. © 2017 Informa UK Limited, trading as Taylor & Francis Group.

WOS: 000369166100041, Design of a novel ultra-wideband (UWB) microstrip bandpass filter is presented by using a square loop quintuple-mode resonator (QMR) having open-circuited stubs. The proposed resonator has five resonant modes in UWB band range and is coupled to Input/Output (IO) ports by parallel-coupled feed lines. The first three resonance modes are resulted from the stub loaded square loop resonator, whereas the remaining two modes can be formed by means of the coupling between the open-circuited stubs and the resonator. The designed UWB filter has totally seven poles including two poles resulting from the nature of the coupling sections and five poles from the QMR. Theoretical analysis of the resonator is realized by defining the equivalent circuit model. Coupling scheme and coupling matrix of the designed filter are also extracted to clarify the filter topology. In addition, two patch perturbation elements are used to control the in-band return loss levels. A seven pole UWB bandpass filter is designed, analyzed, and fabricated. Measurement results show a good agreement with the predicted results. (c) 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:662-666, 2016

In this letter, a dual-mode dual-band microstrip bandpass filter design with center frequency control methodology is presented by using dual-mode meandered loop resonator. Center frequencies of the passbands can be controlled by changing the connection points of internally located square loop loading elements without changing the total surface area. These loading elements are also useful to obtain a compact configuration. In addition, while the first passband has two reflection zeros, the second passband has one reflection zero, as it is obtained from the harmonic of the first passband. Therefore, degenerate modes of the first passband can be excited by a perturbation element. Using patch or corner cut perturbation elements, elliptical and linear phase filtering characteristics can be obtained in the first passband, respectively. Two dual-mode microstrip bandpass filters having elliptical and linear phase filtering characteristics were designed, fabricated, and measured. Measured results are in a very good agreement with the simulated results. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:639–642, 2015

In this paper, a novel microstrip diplexer with tunable bandwidths and switchable channels is presented by using dual-mode square loop resonators (DMSLRs). Two resonators at different electrical lengths are coupled to input and output ports with lumped capacitors, and the isolation between the output ports is optimized by means of these capacitors. In order to obtain tunable bandwidth in each channel, varactor diodes are located at the orthogonal corners of the proposed resonators. In addition, the proposed resonators have reference patch elements at the lateral arms to provide convenience of varactor diode capacitances, such as perturbation elements. Depending on the capacitances of the varactor diodes, the bandwidths of each channel can be tuned. Furthermore, each channel can be switched off. The designed diplexer has two channels at the center frequencies of 1.8 and 2.6 GHz, which are allocated to 4.5G applications. Three-dB bandwidths can be tuned between 60 and 160 MHz for the first channel and between 150 and 350 MHz for the second channel. Isolation between the output ports is obtained as better than 18 dB in each channel. The designed diplexer was fabricated and measured, and the measured results were consistent with the predicted results. © TUBITAK.

In this paper, design of a tunable dual-mode dualband microstrip bandpass filter is presented. The designed filter is constructed by using two nested dual-mode resonators (DMRs) having two patch capacitances located at the lateral arms of the resonators as reference elements. Varactor diodes are used instead of perturbation elements in order to excite the degenerate modes. Linear phase and quasi elliptical filtering characteristics can be independently obtained in each passband and the band-widths can also be tuned at both filtering characteristics. For the experimental verification of the designed structure, a dual-mode dual-band microstrip bandpass filter was fabricated and measured. Center frequencies of the passbands were adjusted to 1.78-2.65 GHz and 1.83-2.7 GHz for linear phase and quasi elliptical filtering characteristics, respectively. Insertion losses in each passband were observed as better than 3 dB in measurements. © 2017 IEEE.

Design and fabrication of compact dual-mode dual-band microstrip bandpass filters with a novel feed scheme is presented. The proposed filters have two square loop resonators for two passbands, and each resonator is fed by the feedlines located at the upper corners of the resonators. By using the perturbation elements such as patches or corner-cuts, two degenerate modes of each passband can be easily excited, so mode frequencies and transmission zeros of the passbands can be controlled independently. Also, the proposed feed scheme provides the harmonic suppression for the first passband. In addition, theoretical results have been obtained by using an equivalent circuit model, and a coupling scheme is also presented. The designed filters have an electrical length of 0.25λg × 0.25λg for the quarter wavelength resonator, where λg is the guided wavelength at 1.8 GHz. Two bandpass filters having elliptical and linear phase filtering characteristics in each passband have been designed, fabricated, and tested. Measured results are in an excellent agreement with simulated and calculated ones. In all frequency responses, return losses are better than 15 dB and insertion losses are at acceptable values for both passbands. © 2012 Taylor & Francis.

Asia-Pacific Microwave Conference (APMC) -- DEC 05-09, 2016 -- New Delhi, INDIA, WOS: 000405560500065, In this paper, a novel microstrip bandstop filter with tunable center frequency and bandwidth is introduced. A dual-mode square loop resonator and three varactor diodes are used to achieve the tunability operation. The dual-mode square loop resonator is coupled to a straight transmission line that connects input and output ports. One of the varactors is used as a perturbation element on the symmetrical axis of the resonator and even mode resonance frequency can be tuned by changing the capacitance of its. Remaining two varactors are located at the lateral arms of the resonator as reference elements and they have task on the tunability of the odd mode resonance frequency. Depending on the capacitance changes of the varactors, both of center frequency and bandwidth of the stopband can be tuned. The designed filter was also fabricated and measured for the experimental verification of the predicted results., IEEE, MTTS, IEEE Delhi Sect, European Microwave Assoc, Scientific and Technological Research Council of Turkey (TUBITAK) [215E099], This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under Grant 215E099.

44th European Microwave Conference (EuMC) -- OCT 05-10, 2014 -- Rome, ITALY, WOS: 000392912200265, A new design of dual-mode bandstop filters with four poles employed by single dual-mode microstrip square loop resonators are proposed. Two fourth order filters with two pairs of reflection zeros at real or imaginary finite frequencies are demonstrated with simulations and measurements. It is shown that the location of reflection zeros and degenerated modes can be easily controlled by simply changing the perturbation element size. It is shown that one of fourth order filters has two finite-frequency reflection zeros on only left side of the stopband, while the other one has two pairs of finite-frequency reflection zeros in the lower and upper passbands. Good agreement between simulated and measured results is achieved., APS, European Microwave Assoc, IEEE MTT S, horizon house

43rd European Microwave Conference (EuMC) -- OCT 07-10, 2013 -- Nuremberg, GERMANY, WOS: 000330768700010, A novel microstrip dual-mode bandstop filter with Coplanar Waveguide (CPW) fed structure is proposed by using a microstrip square loop resonator. Two types of coupling between the degenerate modes are described by changing the perturbation's sizes as in conventional ones. The difference between the proposed filters and conventional ones is resonator fed structures. In the proposed filter design method, CPW-fed structure is used to excite the degenerate modes of the resonator with degenere modes of the square loop resonator. Finally, two dual-mode bandstop filters are designed and realized with using degenerate modes of the microstrip square loop. It is shown that the size of the perturbation element determines the location of the reflection zeros of the filter according to the coupling type of the resonator., APS

Bu çalışmada, dual mod karakteristiğine sahip kare halka mikroşerit rezonatörler kullanılarak, birinci bandı eliptik ikinci bandı lineer faz, birinci bandı linner faz ikinci bandı eliptik, iki bandı da eliptik ve iki bandı da lineer faz karakteristiğine sahip olmak üzere dört adet yeni dual band band geçiren filtre örneği sunulmaktadır. Kullanılan perturbasyon elemanlarıyla dejenere modlar uyarılmakta ve böylece mod frekansları ve iletim sıfırları kontrol edilebilmektedir. Önerilen bu yapılar bandların mod frekansları ve transmisyon sıfırları anlamında birbirinden izole olarak kontrol edilebilmelerine imkan sağlar. Önerilen yapılara ait filtre karakteristikleri de perturbasyon elemanlarının kesik ya da yama tipinde olmasına göre değişim göstermektedir. Tasarımda farklı bir besleme konfigürasyonu kullanılmıştır. Bu yeni besleme konfigürasyonu kullanılarak, yapıların geniş band harmonikleri ek bir yükleme elemanına gerek duyulmadan bastırılmıştır.Önerilen bu dört bandgeçiren filtre imal edilmiş ve simulasyon sonuçlarının ölçüm sonuçlarıyla uyumlu olduğu görülmüştür.Anahtar kelimeler: Perturbasyon elemanı, dual mod rezonatör, dual mod filtre, dual band filtre, saçılma parametreleri In this work, by using microstrip square loop resonators having dual mode characteristics four novel dual band, bandpass filters with elliptical filter characteristics for both of the bands, with linear phase filter characteristics for the first band and elliptical filter characteristics for the second band, with elliptical filter charactersitics for the fist band and linear phase charactersitics for the second band, with linear phase filter characteristics for both of the bands are presented. By means of used perturbation elements, degenere modes are excited and by this way mode frequencies and transmission zeros can be controlled. Proposed structures allow controlling each passband separately in terms of mode frequencies and transmission zeros. Filter characteristics of proposed structures varies according to the type of perturbation elements are cut or patch. In the design a novel feed scheme is used. By using this novel feed scheme, wideband harmonics of the structures can be suppressed without using any loading element. Four proposed bandpasss filters have been fabricated and it has been observed that simulated results are compatible with measured results.Key words: Perturbation element, dual mode resonator, dual band filter, dual mode filter, scattering parameters. 136

In this paper, by using microstrip resonators having dual-mode characteristics, a novel dual-band bandpass filter is presented. Designed resonator structures have been formed by placing a couple of open-circuited stubs to the corner points of a square loop resonator. By means of movement of the gaps between the open ends to obtain a perturbation effect, two degenerate modes are excited, mode frequencies and transmission zeros can be controlled. A novel perturbation arrangement without using an extra perturbation element is formed. On the other hand, location of these gaps allows for creating either an elliptical or a linear filter characteristic in the related passband without shifting the center frequency and changing the bandwidth. Two resonators which represent two different passbands are fed by only one feed-line. Designed dual-band microstrip filters have totally 14.2mm × 14.2mm surface area and so, such a configuration has been satisfied a compact structure. Four dual-band filters using dual-mode resonators are demonstrated with their simulation and measurement results. © 2011 EUROPEAN MICROWAVE ASSOC.

Asia Pacific Microwave Conference 2009 -- DEC 07-10, 2009 -- Singapore, SINGAPORE, WOS: 000279924300235, A novel simple and understandable topology is proposed for dual-mode dual-band bandstop filters with asymmetric frequency response. Two microstrip square loop resonators are used to obtain the dual-band filter characteristic. Each resonator uses a perturbation element to excite dual-modes. Distinct characteristics of new filters are investigated using full-wave electromagnetic simulations. It is also found that there are five reflection zeros. Three of them are inherently associated with the proposed topology, whereas the other reflection zeros near stopbands are associated with the perturbation. They may be easily adjusted by changing the size of the perturbation element. Four dual-band filters using dual-mode resonators are demonstrated with different asymmetric responses, which result from different locations of adjustable reflection zeros. Both simulated and measured results are presented and exhibit dual-mode dual-band bandstop responses with high sharpness., Scientific and Technological Research Council of Turkey (TUB TAK) [107E117]; Turkish Academy of Sciences (TUBA), This work was supported by the Scientific and Technological Research Council of Turkey (TUB TAK) under Grant 107E117 and Turkish Academy of Sciences (TUBA).

A novel feed scheme is presented to design a dual-mode dual-band microstrip bandpass filter. The proposed filter has two square loop resonators for both passbands and each resonator are fed by a feedline placed on the upper corners of resonators. Proposed structure allows controlling each passband separately in terms of mode frequencies and transmission zeros. By means of the perturbation elements such as patches or cuts, two degenerate modes can be excited. Also, wideband harmonic suppression can be achieved through this novel feed scheme. Two bandpass filters having elliptical and linear phase frequency responses have been fabricated for experimental verification of the predicted results. Measured results are in good agreement with simulated results. © 2011 Chamber of Turkish Electric.