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In this paper, a size-reduced aperture-coupled tunable hairpin bandpass filter which has improved selectivity and has constant bandwidth is proposed. We used an aperture coupling between first and second stage of the filter to reduce the size of the filter. Varactors are loaded to open end of the quarter-wave resonators to tune the center frequency. The open stub of the filter makes transmission zero which enhances the selectivity of the filter. Furthermore, we also loaded varactors to end of the open stubs to control the notch frequency. Thus, we could obtain the constant bandwidth over the tuning range. The varactor loaded open stub of the hairpin resonator which works as a K-inverter makes possible to meet the wideband coupling relationship in overall frequency tuning range. The measurement results show the proposed aperture-coupled tunable hairpin bandpass filter not only has small size but also enhanced selectivity and constant bandwidth. The proposed filter has 19.4% tuning range from 1.4 GHz to 1.7 GHz and constant bandwidth within the tuning range. The proposed filter is designed and tested and 50% size reduction was achieved.