A wideband coupler with wide suppression band using coupled stub

Planar microstrip branch line couplers are simple but have narrow bandwidth and are unable to reject out-of-band signals adequately. This paper proposes a new feeding network for branch line couplers, using this feeding network the bandwidth of the coupler has a five-fold increase in bandwidth and with bandpass characteristics. The flat response is obtained by impedance matching from the feeding network, which also rejects the second harmonic. This is the result of the open-circuit termination while the third harmonic is rejected by the short-circuit termination due to the coupled stubs. The proposed circuit was built for verification purposes and measured results agree well with simulation.