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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.

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.

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.