Enhanced Harmonic Suppression for a Miniaturized Hairpin-Line Bandpass Filter with Meander Spurline

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Abstract
The design of a fourth-order centrally corrugated double-folded hairpin line bandpass filter with a broad stopband and enhanced selectivity is demonstrated in this article. Initially, a double-folded hairpin-line bandpass filter (DFHLBF) is designed from a fourth-order conventional hairpin-line bandpass filter (CHLBF), which is centred at 2.5 GHz and has a fractional bandwidth of 3%. This results in a 68% size reduction. With this folding mechanism, the filter's skirt characteristic is only enhanced at the upper passband edge, resulting in an attenuation level of 40 dB at 2.65 GHz. The capacitive loading has then been incorporated by periodic rectangular corrugations that have disturbed the folded arms. Because of the high capacitive coupling between the folded arms, a symmetrical passband with an attenuation level better than 40 dB at both edges has been observed. Besides, a size reduction of 14% than that for the DFHLBF has been achieved. However, because of the imbalance of the modal phase velocities in the inhomogeneous microstrip filter construction, the stopband's attenuation level climbs to 8 dB. Rectangular meander spurlines have been added between the connected arms of the neighbouring centrally corrugated cells of the filters as a way to lower the attenuation levels of the harmonics. This creates a slow-wave effect between the odd- and even-modes of the propagating signals. This results in an overall size reduction of 81% over the standard hairpin-line filter and an enlarged stopband of up to 4.36f0 with a rejection level of 42 dB.

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