A broadband and compact branch-line coupler using series capacitances and short-circuited stubs

To have broadband branch-line couplers with matching networks even at high frequencies such as X-band, a circuit construction using series capacitances and short-circuited stubs freeing from shunt capacitances is employed. Replacing the λ/4 transformers with lumped elements and being absorbed the caused shunt capacitances into adjacent λ/4 transmission lines segments, it is realized, and the lines become short. The branch-line coupler fabricated in multi-layer LTCC substrates is described. In addition to the design method, the designed and experimental results, indicate good performance: the amplitude imbalance does not exceed 0.5dB, the distributed phase difference error varies within 1.4°, the return loss and isolation are below -20dB over 23% of the bandwidth in X-band, are presented. Also, the size is compact compared with conventional couplers. On the basis of the results, it is verified that the proposed coupler is efficient.