Comments on “Compact Dual-Band Branch-Line Coupler With Dual Transmission Lines”

The synthesis and design method for the dual-band, dual transmission line branch-line coupler in [1] involves equivalent circuit transformations that are single-banded in nature and not suitable for the dual-band operations. The proposed equivalent dual transmission line transform has been applied only in the middle of the dual-band operation frequencies and its validity has not been confirmed at the two band frequencies. As the result, the proposed circuit equivalence can be quite poor and the coupler design in [1] based on it can be quite erroneous at the dual-band operation frequencies.