A tunable diplexer composed of asymmetric hairpin resonators using varactors with constant passband bandwidth and high isolation characteristics has been realized. A symmetric hairpin resonator loaded with a (Ba x Sr 1-x )TiO 3 (BST) varactor is proposed as a tunable resonator. It is matched to the frequency characteristics of bandpass filter (BPF) design parameters, namely, external quality factor and coupling coefficient. This resonator has a higher unloaded quality factor below 2 GHz than one using a silicon varactor diode. Also, beyond 2 GHz, the transmission characteristics of the proposed hairpin resonator loaded with a BST varactor are more stable. A tunable second-order BPF that maintains a constant bandwidth is configured using the proposed resonator. Those frequency characteristics are realized with only two BST varactors and a DC voltage supply. Finally, a configured and fabricated tunable diplexer using the proposed resonators is shown.