“…The characteristic impedances of the directly fed line Z L11 , Z L12 and coupled line Z L21 , Z L22 are equal to the impedances found from (1) for the assumed operational bandwidth and terminating impedances Z T1 , Z T2 = Z T3 , Z T4 . As it was shown in [7, 8], the achievable transformation ratio in a coupled‐line section is limited where C jj and L jj ( j = 1, 2) are the per‐unit‐length self‐capacitance and self‐inductance of line j , C m and L m are per‐unit‐length mutual capacitance and inductance. Therefore, in case of a two‐section 3 dB directional coupler, the impedance transforming ratio R needs to be lower than 2, since in such a directional coupler coupling k > 0.707 (>3 dB) is required for the tightly coupled section [11, 12].…”