A center frequency agile coupled line bandpass filter (BPF) with constant absolute bandwidth is reported in this paper. Reported BPF employ a pair of coupled half wavelength resonators with varactor diode (SMV1232-079LF) at one end to change the resonator's effective electric length and hence to change the center frequency. To keep the absolute bandwidth constant as the center frequency is tuned, mixed electric and magnetic coupling is reported here. High Frequency Structure Simulator (HFSS) in conjunction with Agilent's Advance Design System (ADS) is used to design and simulate the bandpass filter structure. For validation purpose, an experimental prototype is realized. The design methodology and experimental results are presented in the paper.