Isolation circuit of impedance-transforming 3-dB compact baluns for perfect output matching and isolation

In this paper, a new structure of isolation circuit of an impedance-transforming 3-dB compact balun is proposed for perfect matching at all ports and perfect isolation. This isolation circuit is composed of very small length of transmission-line sections and lumped elements, with which a compact balun can be constructed together with a set of coupled-transmission line sections. A microstrip compact balun to transform 50 ohm into 40 ohm at a center frequency of 1.5 GHz was designed and measured. The measured results show almost perfect matching at each port, power division with 180° out of phase and perfect isolation between two output ports around 1.5 GHz.