In this letter, an ultrawideband (UWB) bandpass Wilkinson power divider (WPD) is introduced. By using filter synthesis theory and proposed optimization algorithm, all the S-parameters (S₁₁, S₂₁ = S₃₁, S₂₂ = S₃₃ and S₃₂) of the proposed topology could provide an equal-ripple response, and their equal-ripple level and bandwidth can be controlled, respectively. For verification purposes, a prototype UWB WPD has been simulated, fabricated, and measured. The measured and simulated results are matched reasonably well.

To accurately design wideband filters composed of three‐coupled‐lines, an improved equivalent circuit is derived by taking the cross‐coupling between non‐adjacent lines into consideration. Since the cross‐coupling between non‐adjacent lines is also dependent on the coupling between adjacent lines, conventional synthesis techniques cannot be used directly. Instead, an improved synthesis scheme based on successive iterations is proposed to synthesise the filters. As examples, two 2‐stage microstrip three‐coupled‐line filters [bandwidth: 2.67–5.33 GHz (fractional bandwidth: 67%)] with a reversed connection sequence are synthesised, simulated, and experimentally realised to validate the proposed theory. The frequency responses of the designed filters using the authors’ new model and synthesis theory compare well with those by simulation and experiment, whereas an error of about 17% is found in bandwidth if the authors use the conventional model that neglecting the cross‐coupling between non‐adjacent lines. In addition, the asymmetric property of the three‐line unit is revealed.

In this brief, a tunable Wilkinson power divider (WPD) with port match and isolation is proposed based on a reconfigurable compact structure. The reconfigurable unit is composed of one varactor cascaded by a transmission line (TL) with specific characteristic impedance and electrical length. Each unit can be alternately connected with two cascaded varactors to constitute a T-type structure, where the operating frequency can be selected by providing proper bias voltages for the three varactors. By adjusting the characteristic impedance and electrical length of the transmission line in each unit, the continuous tuning range can be easily extended to exceed 120% tunability. The reconfigurable units can also be flexibly configured to work in designated discrete tuning range as well. An experimental reconfigurable WPD with wide continuous tuning range of 0.96-1.73 GHz (57.3% tunability) is designed and fabricated. The measured results show that more than -20dB return losses and isolations can be realized in the entire tuning range.

A novel compact ultra-wideband (UWB) bandpass filter is proposed to meet federal communications commission (FCC)'s spectrum mask. Shunt short-circuited stubs and the parallel coupled stepped-impedance resonators (SIRs) are used to achieve a fraction passband from 3.1 GHz to 10.6 GHz. The equivalent circuit of the newly proposed filter is established, while the corresponding chebyshev-type transfer function is derived for filter synthesis. Compared with formerly proposed UWB filter without short-circuited stubs, the new one significantly improves the selectivity and out-of-band characteristics to completely satisfy the FCC spectrum mask. In addition, the use of the short-stubs effectively enlarged the coupling gaps (from 0.05 mm to 0.15 mm) to successfully alleviate the requirement on fabrication precision. For verification, the proposed filter is synthesized, simulated and fabricated in microstrip line. The good agreement between the measured and predicted results validates the effectiveness of newly proposed synthesis theory. Meanwhile, the designed filter exhibits very compact size, good characteristics of low insertion loss, sharp skirts, flat group delay and good stopband (especially in lower one) as well.

In this paper, a parallel transmission-line transformer with complex terminal loads is introduced for dual-frequency design. Through analyzing the different cases of complex terminal loads conditions, there are four different mapping patterns of designable regions on the Electrical Length-Electrical Length (EL-EL) plane. By adding an extra vertical Frequency Ratio (FR) axis, a novel 3D cube can be created on the FR-EL-EL coordinate system, then the designable ranges of frequency ratio can be easily drawn in this 3D cube. Therefore, the visible 3D mapping approach is newly reported for matching the complex terminal load of parallel transmission-line transformer at two different frequencies. For the certain pattern, a clearly boundary could be drawn to distinguish the designable and undesignable ranges. For verification, two examples are designed, fabricated and measured. Simulated and measured results are matched very well.

A UWB filter with improved skirt selectivity and compact size was implemented by a new efficient design scheme. Firstly, based on synthesis theory, short-circuited stubs are utilized together with a 5lambda0/4 stepped-impedance resonator (SIR). Furthermore, keeping other parameters unchanged, two-section stepped-impedance open-circuited stubs are theoretically designed to replace the short-circuited ones. As a result, a pair of controllable attenuation poles is introduced to effectively improve selectivity of the filter. To validate the proposed techniques, the designed UWB filter was simulated, fabricated and measured. The measured frequency responses of the UWB filter agree well with the theoretical ones and successfully satisfy outdoor limit of FCC. In addition, the fabricated filter also exhibits compact size, low return loss in passband, flat group delay and good stopband property as well. Moreover, to be highlighted here, the application of the stubs successfully enhances the coupling coefficient of parallel-coupled structure, and the coupling gap size in this filter is enlarged to 0.16 mm from 0.06 mm in conventional one without the stubs.