A UHF Compact Complex Impedance-transforming Balun with High Isolation

Background: A compact complex impedance-transforming balun for UHF frequencies, which is based on a coupled-line structure that matched all ports and provided high output port isolation, was designed in this paper. Methods: A lumped component transformation was used to minimize circuit size. The implemented circuit operated at 433 MHz with the reflection coefficients less than -16 dB at all ports, 0.22 dB amplitude balance and 180° phase balance at the output ports. The signal coupling between the output ports was -16.8 dB. The circuit size is small at 0.032λ. Results: Complex impedance-transforming baluns were designed to operate at 433 MHz. The source impedance at port 1 was set at Zs = 12 - j12Ω and the load impedances at port 2 and 3 were set at ZL = 80 + j30Ω. Conclusion: A compact complex impedance-transforming balun at UHF frequency, with all ports matched and high isolations, was designed and illustrated in this paper.