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.