A Novel Tri-Band Branch-Line Coupler With Three Controllable Operating Frequencies

A novel branch-line coupler with three controllable operating frequencies is presented for tri-band operation. The tri-band operation is achieved by attaching a matching circuit to each port of the conventional branch-line coupler. Closed-form design equations are derived. For verification, a microstrip 3 dB branch-line coupler operating at 0.9/2/2.45 GHz is designed, fabricated and measured. The measured results are in good agreement with the simulated results. When the return loss and isolation are better than 15 dB and 20 dB simultaneously, the measured bandwidths of three operating bands are all almost 40 MHz, while the amplitude imbalance is below 0.8 dB and the phase difference is within $-90^{\circ}\pm 3^{\circ}$.