Multi-Folded Bandpass Filter with Harmonic Suppression by Meander Spurline

The design of a fourth-order compact bandpass filter using multi-folded hairpin resonators with an improvement in the skirt characteristics and an extended stopband is presented in this article. At first, the compactness of 40% has been achieved for a fourth-order bandpass filter centered at 2.5 GHz and fractional bandwidth of 5% over the conventional hairpin-line filter by cascading the multi-folded hairpin line (MFHL) cells in a hybrid configuration. Accordingly, the skirt characteristics have been improved by the introduction of a deep transmission zero of attenuation level of 56 dB at the upper edge of the passband. However, the presence of the spurious harmonics with the attenuation level of 10 dB has overshadowed the benefits of the multi-folding of the hairpin lines. In this context, a meander spur line with optimum dimensions was incorporated symmetrically in each coupled section of adjacent MFHL cells to achieve modal phase velocity compensation. As a result, an extended stopband with a rejection level of 25 dB up to 4.8f0 and 33 dB up to 4.16f0 has been obtained. Besides, an overall size reduction of 49% has been achieved accordingly.