Compact coupled-line balun with complex impedances transformation and high isolation

A planar microstrip circuit is proposed to design a kind of coupled-line balun with good ports matching and high output-ports isolation for complex source to complex load impedance transformation. This balun prototype essentially consists of three coupled-line sections, a tapped open transmission-line stub (TOTS) on the left-hand side, and a tapped complex equivalent-impedance part (TCEP) on the right-hand side. The TCEP is the key to realise high output isolation and all ports matching. Based on the traditional even-odd mode technique and the transmission-line theory, closed-form mathematical equations for circuit electrical parameters are obtained. To demonstrate this proposed design theory, a practical microstrip coupled-line balun is designed, simulated and measured to operate at approximately 2 GHz, indicating high isolation and balanced output. Good agreements between the simulated and measured results verify this design.