MULTIFOLDED BANDWIDTH BRANCH LINE COUPLER WITH FILTERING CHARACTERISTIC USING COUPLED PORT FEEDING

A new uni-planar structure branch-line coupler with broad bandpass response is proposed. Single section branch line couplers (SSBC) are popular due to their simplicity and ease of use but suffer from narrow return loss bandwidth and poor out of band rejection characteristics. The work presented here overcomes these limitations with the use of coupled port feeding. Through the study of input impedance of feeding network and single section branch line coupler, the bandwidth of the new coupler increased by almost 6-times. In addition, the coupler exhibited band-pass filtering characteristics. Measured results exhibited low insertion loss (≤4-dB), small magnitude difference (≤1-dB), good return loss and isolation (≥10-dB) and small phase variation (90°±5) within the passband. A measured bandwidth of 58% was achieved with this single section coupled port fed branch line coupler at a centre frequency of 1-GHz. Its two output ports achieved rejection levels better than 25 dB in the stopband.