Varactor‐tuned bandpass filter using a microstrip stepped‐impedance combline filter and a new J‐inverter

The stepped‐impedance combline band pass filter (BPF) with novel input and output networks based on a new proposed J‐inverter, which increases the center frequency tuning range with constant bandwidth (BW), is discussed. The even and odd mode analysis shows that the stepped resonators provide the necessary conditions for the appropriate coupling coefficient to keep the BW constant. In order to create an appropriate quality factor, the new proposed J‐inverter is used. In the frequency response of the stopband region, three transition zeros (TZs) are generated, which two of TZs are controllable. The proposed tunable BPF has been fabricated on RO4003 substrate with a dielectric constant of 3.38 and 0.813 mm thickness. The size of the filter is compact and is only , where is the guided wavelength at the lower center frequency. The center frequency is equal to 1.723 GHz and a frequency tuning range from 1.2 GHz to 2.246 GHz with 100 MHz constant 3 dB absolute BW, the insertion loss of 1.4–3.17 dB and return loss of 17.8–25.9 dB is achieved.