CRLH-TL Based Compact Planar Dual Band Lowpass-Wide Bandpass Filter

The compact dual band lowpass-wide bandpass filter (DB LWBF) proposed in this study is achieved utilising four Composite right/left-handed (CRLH) meandering shaped stubs (MSSs) connected to the main transmission line. In the lowpass band, the proposed filter has a measured -3 dB cut-off frequency of $1.09 ~\mathrm{GHz},~\left|S_{21}\right|$ better than 0.25 dB, and $\left|S_{11}\right|$ of less than 38 dB. The measured $\left|S_{21}\right|$ in the bandpass passband is 0.25 dB, with $\left|S_{11}\right|$ better than 28 dB and a fractional bandwidth of $79 \%$ at center frequency of 3.28 GHz. From 1.21 GHz to 1.57 GHz, the filter has greater than 17 dB stopband rejection level, and from 5.01 GHz to 7.25 GHz, it has a more than 20 dB stopband rejection level. For the proposed DB LWBF, electromagnetic simulations, Equivalent LC circuit representations, parametric analysis and measurement results are presented for the proposed DB LWBF. The proposed DB LWBF shows a small circuit size $\left(0.083 ~\lambda_{g} \times 0.048 ~\lambda_{g}\right)$, where $\lambda_{g}$ is the guided wavelength at 1.09 GHz), low insertion loss, wide out-of-band frequency range and high out-of-band rejection level.