Wide Stopband Centrally Corrugated Folded Bandpass Filter with Trapezoidal Spurline

The present article describes the design of a compact fourth-order centrally corrugated hairpin line bandpass filter with an improved skirt characteristic and an extended stopband. At first, the compactness of 52% 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 centrally corrugated double folded hairpin line (CCDFHL) cells in a hybrid configuration. Accordingly, a symmetrical passband with attenuation levels of more than 40 dB at the edges and sharp skirt characteristics have been occurred due to the strong capacitive coupling between the corrugated folded arms of each double folded hairpin-line (DFHL) cell. However, the harmonics with the attenuation level of 10 dB have limited the applications of such filter for mixers and frequency synthesizers in WLAN. Subsequently, the coupled arms of two adjacent DFHL cells have been perturbed by a trapezoidal-shaped meander spurline with optimum dimensions to achieve modal phase velocity compensation. As a result, an extended stopband with a rejection level of 35 dB up to
$3.88f_{0}$
and 25 dB up to
$6f_{0}$
has been obtained. Besides, an overall size reduction of 59.6% compared to the conventional filter has been achieved accordingly.