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This study aimed to design and fabricate a lowpass‐bandpass (LP‐BP) diplexer with high isolation for telecommunication applications including wireless communications. The results revealed that the −3 dB cutoff frequency of the lowpass filter (LPF) was equal to 0.82 GHz. The advantages of the LPF section include a very high suppression factor (SF) parameter (about 4.4), very sharp roll‐off‐rate (ROF or ζ) parameter (687), and a very high figure of merit (FOM) (about 233491). The bandpass filter (BPF) section was designed applying dual‐mode resonators with triband. The central frequencies of these tribands were equal to 2.38, 3.93, and 5.65 GHz. In the following, an SMV‐1247‐079LF SMD varactor diode is used to adjust the proposed LPF. The tuning range for the proposed LPF is 0.3‐0.72 GHz. In the proposed diplexer, the isolation between the output ports was higher than 45 dB. The results of measurements were in good agreement with those of the simulation.

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In this article, a well‐defined tunable lowpass‐bandpass (LP‐BP) diplexer with proper performance is provided using stepped impedance lines and square‐looped resonators for telecommunication applications. Our proposed scheme is the first tunable LP filter BP filter diplexer that provides good results. The proposed diplexer has the ability to adjust the cut‐off frequency of the lowpass filter (LPF) and to adjust the center frequency of the bandpass filter (BPF). The varactor diodes in reverse bias were used to make the proposed diplexer tunable. The tuning range of the cut‐off frequency for the LPF is from 0.46 to 0.96 GHz. An important feature of the LPF is a high suppression factor of about 45 dB in the stopband and a sharp roll‐off rate (ROF). The adjustability of the BPF part for the first channel ranges from 1.8 to 2.4 GHz; for the second channel, the ranges from 2.8 to 3.2 GHz, and for the third channel, the ranges from 5.2 to 5.6 GHz. The proposed diplexer is designed, simulated, fabricated, and finally tested and measured. The isolation between the output ports is about 34 dB. Simulation results are in good agreement with the test results and the measured values.

Conventional Wilkinson power dividers (WPDs) can provide acceptable performance close to the nominal center frequency. However, these WPDs can also exhibit poor out-of-band performance while requiring a large footprint. In order to improve on the current state of the art, a modified microstrip WPD is proposed that exhibits a substantially improved stopband and high isolation. A lowpass filter (LPF) structure is utilized in both branches of the power divider to provide harmonic suppression. According to the obtained results, the input return loss (|S11|), output return loss (|S22|), output insertion loss (|S21|) and isolation (|S32|) are better than 34.2 dB, 26.2 dB, 3.52 dB and 31.2 dB, respectively. The proposed modified WPD has a wide 20 dB stopband (from 2.54 GHz to 13.48 GHz) and filters the second to seventh harmonics with attenuation levels of greater than 20 dB. The proposed WPD has a small size of 33.8 mm × 27 mm (0.42 λg × 0.33 λg), where λg is the guided wavelength at the operating frequency of 1.8 GHz. The WPD has been fabricated and tested and shows good agreement between simulated and measured results and the proposed design has desirable characteristics for LTE and GSM applications.