A fourth-order bandpass filter with dual-wide passbands is proposed in this article. The novelty of the proposed work is the realization of a dual-band bandpass filter with closely spaced passbands, wide passband bandwidth, high rejection between the passbands, using a novel combination of open stub tapped series coupled lines (CLs) and shunt transmission line loaded with end connected CLs. The characteristics of the proposed dual-wideband bandpass filter are investigated by adopting even–odd mode analysis. The transmission zeros generated by the transmission line loaded with an end-connected CL and open stub are utilized to achieve a high skirt rate. A rejection better than 55 dB is achieved with a single transmission zero between the two passbands indicating high isolation and close passbands. The dual-wideband filter is designed and manufactured to operate at 1.33 and 2.32 GHz. The experimented 3-dB fractional bandwidths of the two pass frequency bands are 40.6% and 24.13%. The proposed filter’s tested frequency responses agree well with simulated results. The proposed device can be used in various applications such as telecommunications, satellite communications, radar systems, imaging, and spectroscopy.

An Nth-order tunable bandpass filter (BPF) structure with extended passband bandwidth has been proposed in this paper based on the novel tunable resonator and coupling structure. The designed tunable resonator is composed of two coupled lines and one varactor diode, while the presented tunable coupling structure is constructed by two cascaded transmission lines and one short-circuited varactor diode. Moreover, the even-(odd-) method and lumped-element equivalent circuit are employed to analyze the operating mechanisms of the designed Nth-order tunable BPF. Specifically, the frequency tuning range is determined by the characteristic impedance and electrical length of coupled line when the varactor diode is within the fixed tuning range. Then, the design procedure for the Nth-order tunable BPFs is proposed. To demonstrate the presented idea, the second- and third-order fully tunable BPFs have been designed and simulated. Finally, a second-order tunable BPF with the compact size of 0.03λg × 0.13λg has been fabricated and measured, and the measured center frequencies are ranging from 0.91 to 1.46 GHz with the passband bandwidths wider than 11.2%.

This paper outlines a new class of quad-band amplifier (QBA) realized using extended composite right- and left-handed coupled lines. The design procedure to reach the QB CLs with predefined frequency operations and characteristic impedance of their ports is clearly presented. Designed and fabricated QBA operation frequencies are $f_{b_{1}} = 2.9\,GHz$ and ${\rm \;} f_{b_2} = 4.3\,GHz$ (output port B) ${\bi \;} \; f_{f_1} = 3.8\,GHz\; $ and $\; \; f_{f_2} = 5\,GHz\; $ (output port C) and the proposed structure exhibits a matching (−10 dB) bandwidth of over 300 MHz in each operation frequency bands. In addition, the insertion loss is smaller than 1 dB. The length and the width of the proposed QBA are approximately 7.7 and 2.4 cm, respectively. To further demonstrate the idea, a QBA is successfully designed and fabricated. The simulated and measured results of the proposed QBA are presented to verify the proposed idea.

In this paper, a microstrip diplexer composed of two similar resonators is designed. The proposed resonator is consisting of four microstrip cells, which are connected to a coupled lines structure. In order to select a suitable geometric structure, first, all cells are assumed as undefined structures where there is a lack of basic information about their geometry and dimensions. Then, an equivalent LC circuit of the coupled lines is introduced and analyzed to estimate the general structure of the resonator respect to a requested resonance frequency. The proposed diplexer is designed to operate at 2.36 and 4 GHz for wireless applications. The insertion losses (S21 and S31) are decreased significantly at the resonance frequencies, so that they are 0.2 and 0.4 dB at 2.36 and 4 GHz, respectively. The designed diplexer is fabricated and measured and the measurement results are in a good agreement with the simulations.