A Compact Planar Quint-Passband Lowpass-Bandpass Filter Using Open-End Folded Stub CRLH-TLs

This paper proposes compact quint-passband filter (QPF) having a Low passband (LP) and four Bandpass passbands (BPs). The QPF is implemented using four Composite right/left-handed open-end folded stubs (CRLH-OEFS) loaded on the main transmission line. In the LP region, the measured insertion loss is less than 0.56 dB. Four BPs show measured insertion loss less than 0.9 dB, 0.88 dB, 0.93 dB and 1.25 dB. Measured 3 dB cut-off frequency of the LP region is 0.61 GHz. The four BPs with center frequency 1.87, 3.61, 5.3 and 6.5 GHz have 3 dB fractional bandwidth of 44, 10.8, 4.5 and 4.46, respectively. The transmission zeros (TZs) observed at 0.67 GHz, 0.95 GHz, 2.65 GHz, 3.83 GHz, 3.98 GHz, 6.11 GHz and 7.3 GHz have insertion loss of 31 dB, 47 dB, 22 dB, 31 dB, 43 dB, 45 dB and 21 dB, respectively. A parametric analysis of the QPF’s physical parameters and modelling of the filter’s equivalent LC circuit is presented in this paper. The proposed QPF shows a small footprint (0.098 λg × 0.051 λg, where λg is the guided wavelength of the 50 Ω microstrip line at 0.61 GHz), good rejections with insertion loss of 31 dB, 47 dB, 22 dB, 31 dB, 43 dB, 45 dB and 21 dB at respective TZs and a sharp transition due to the creation of seven TZs between LP and BP responses.