Dual-band rat-race balun structure using transmission-lines and lumped component resonators

This paper presents a new dual-band rat-race balun structure for microwave circuit designs and applications. The proposed design methodology for the dual-band rat-race calls for conventionally designing a balun at the higher frequency, f₁, by using distributed components. To achieve good performance also at the second lower frequency, f₂, resonators connected to open-circuited stubs are added to the initial design. This leads to small physical dimensions of the entire circuit, especially if the two frequency bands are far apart, because the main body of the balun is dimensioned for the smaller wavelength. A dual-band rat-race balun has been designed and fabricated at 2.14 and 3.6 GHz. Measurement and 2.5D electromagnetic simulation results are found to be in good agreement.