Design of broadband impedance transformer using coupled microstrip transmission lines

In this paper, a novel broadband impedance transformer combining with a symmetric coupled microstrip line and an interconnecting line is presented. It has been proved that a fractional bandwidth of about 90% at -20 dB reflection level can be achieved with this structure, and the insert loss is less than 2 dB in the matching bandwidth. The measurement results of actual circuit are well agreed with the simulation. Its bandwidth is 2.5 times larger as compared with the traditional quarter-wavelength impedance transformer, and its length is less than half of the traditional one with the help of bending the structure.