By exploiting bended microstrip lines and tapered resonators, a new microstrip dual‐band bandpass filter (DBBPF) with tunable center frequency and compact size is proposed, analyzed, and fabricated. The LC equivalent circuit of the basic resonator is introduced to present an analytical description. The proposed filter has two passbands at the center frequencies of 2.4 and 5.7 GHz. The measured data show <0.22 and 0.56 dB insertion losses in the first and second bands, respectively, which are considered as a marked advantage. Furthermore, without increasing the circuit size, the second band can be tuned between 3.85 and 7.5 GHz. Finally, an acceptable similarity is observed between the simulated and measured S‐parameters.

In this letter, a compact dual‐band bandpass filter (BPF) is designed, analyzed, and fabricated. A loop resonator loaded by two modified T‐shaped resonators, double T‐shaped resonators, and open bended stubs are applied to form the proposed configuration. The coupling spaces between the loop resonators and open bended stubs result in two passbands with the center frequencies of 2.4 GHz and 5.7 GHz. The LC equivalent circuit of the main resonator is introduced to present an analytical description. Without needing to increase the circuit size, the upper passband can be adjusted between 5.72 GHz and 6.93 GHz. The measured data indicates less than 0.64 dB and 0.76 dB insertion losses in the first and second passbands, respectively. Adjustable second center frequency, compact size, low insertion loss, good suppression level, and a symmetrical structure are the marked features of the proposed BPF. Finally, a good agreement between the simulated and measured results is observed.

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In this paper, a microstrip dual-band bandpass filter (DBBPF) based on an octagonal loop resonator (OLR), tapered resonators and open bended stubs (OBSs) is designed and analysed. The proposed structure produces two passbands with the centre frequencies of 3.65 and 5.67 GHz. The marked advantages of the proposed filter are as follows: Two centre frequencies can be individually tuned. The bandwidth of the upper passband can also be controlled. Furthermore, the DBBPF benefits from an ultra-wide upper stopband from 5.9 up to 21 GHz with an attenuation level of higher than 20 dB and a small size of 0.21 λg × 0.26 λg, where λg is the guided wavelength at 3.65 GHz. The designed filter is horizontally and vertically symmetrical leading to a reciprocal S matrix. Other remarkable specifications of the proposed filter are the insertion loss < 0.62 dB, the return loss > 20.2 dB and sharp response. To provide an analytical description, the LC equivalent circuits of initial and main resonators are presented. Acceptable similarity between simulated and measured results verifies the design process.

One of the most interesting topics in bio-optics is measuring the refractive index of tissues. Accordingly, two novel optical biosensor configurations for cancer cell detections have been proposed in this paper. These structures are composed of one-dimensional photonic crystal (PC) lattices coupled to two metal–insulator–metal (MIM) plasmonic waveguides. Also, the tapering method is used to improve the matching between the MIM plasmonic waveguides and PC structure in the second proposed topology. The PC lattices at the central part of the structures generate photonic bandgaps (PBGs) with sharp edges in the transmission spectra of the biosensors. These sharp edges are suitable candidates for sensing applications. On the other hand, the long distance between two PBG edges causes that when the low PBG edge is used for sensing mechanism, it does not have an overlapping with the high PBG edge by changing the refractive index of the analyte. Therefore, the proposed biosensors can be used for a wide wavelength range. The maximum obtained sensitivities and FOM values of the designed biosensors are equal to 718.6, 714.3 nm/RIU, and 156.217, 60.1 RIU−1, respectively. The metal and insulator materials which are used in the designed structures are silver, air, and GaAs, respectively. The finite-difference time-domain (FDTD) method is used for the numerical investigation of the proposed structures. Furthermore, the initial structure of the proposed biosensors is analyzed using the transmission line method to verify the FDTD simulations. The attractive and simple topologies of the proposed biosensors and their high sensitivities make them suitable candidates for biosensing applications.