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A Compact wide‐stopband planar diplexer based on rectangular dual spiral resonator (RDSR) is proposed. The proposed diplexer is composed of three parts: (1) two RDSR bandpass filters (BPFs) with right bend open‐circuited stubs coupling feed structure, (2) open‐circuited stub to reduce the second harmonic in upper stopband, and (3) T‐junction configuration to combine two BPFs circuits for one planar diplexer. Based on the structure, the fabricated wide‐stopband planar diplexer prototype with compact dimension is achieved to operate at the WLAN frequencies (i.e., 3.65/5.2 GHz). Both the simulated and measured results indicate that satisfactory impedance matching and high isolation between two channels are achieved. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:174–178, 2015

A compact dual-band bandpass filter using stub-loaded stepped impedance resonator (SLSIR) with cross-slots is presented. The symmetric SLSIR is analyzed using even- and odd-mode techniques. Design equations are derived and they are used to guide the design of the circuits. Two passbands can be easily tuned by cross-slots and open stubs. Transmission zeros among each passbands are created, resulting in high isolation and frequency selectivity. An experimental circuit is fabricated and evaluated to validate the design concept. The fabricated filter is compact with 19.76 × 12.7 mm2. The measurement results are in good agreement with the full-wave simulation results.

A dual closed-loop stepped impedance resonator (DCLSIR) is investigated and used in designing a compact microstrip bandpass filter (BPF). The proposed DCLSIR is symmetrical; as a result, the symmetric characteristics of the resonator have been used. The design equations are derived and used to support the circuit design. The center frequency, position of transmission zeros, and fractional bandwidth (FBW) are easily tuned by changing the physical dimensions of the resonator. Three transmission zeros are generated to improve the performance in the upper stopband. A DCLSIR prototype BPF is fabricated with a center frequency of 9.3 GHz, and evaluated to validate the design concept. The measured FBW is 9.25%, the insertion loss is 1.58 dB, and the return loss is over 17 dB. The measurement results agree well with the simulation results.

A compact microstrip diplexer based on dual closed loop stepped impedance resonator (DCLSIR) is proposed. The proposed microstrip diplexer is composed of the combination of two DCLSIR bandpass filters (BPFs), which are designed for X-band application. For the demonstration, a dual-channel diplexer has been designed and fabricated using microstrip and printed circuit board (PCB) technologies, respectively. The fabricated diplexer, operating at 8.3/10 GHz for X-band application, has compact size (15.17 mm × 2.69 mm). The measured results are in good agreement with the full-wave simulation results. Good isolation between two channels is achieved.