AbstractTwo novel cascaded microstrip open‐loop resonators with loading varactors are presented to design tunable filter. In two coupled resonators, the coupled sides can provide both electrical and magnetic coupling. A capacitance loaded at input/output feed line is used to control the external quality. By adjusting the parameters of coupling side and the value of the capacitance, coupling coefficients and external quality can be tuned to meet the requirement of constant absolute bandwidth. A two‐pole filter is demonstrated a tuning range from 607 to 942 MHz with the insertion loss 2.5–3.2 dB. Especially, the 3‐dB bandwidth is approximately 63 ± 7 MHz with the center frequency from 642 to 942 MHz. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1178–1181, 2014