Optimised synthesis of microstrip branch-line couplers taking dispersion, attenuation loss and T-junction into account

An optimized synthesis technique is presented for microstrip branch-line couplers with any number of branches, taking dispersion, conductor attenuation, and T-junction effects into account. It is noted that while microstrip dispersion affects coupler performance particularly at higher frequencies, T-junction effects make performance worse also at very low frequencies (1 GHz). Two 3-branch, 3-dB couplers were designed and manufactured on an alumina substrate, for operation at 1.6 and 20 GHz. The good agreement between measured and computed S-parameters of the 3-branch couplers demonstrates the model reliability up to 20 GHz. A brief analysis of sensitivity versus geometrical dimensions of the coupler is made.<>