Dual-band Microstrip Balun with Different Complex Load Impedances in Frequency Bands

The paper proposes a circuit and calculation method for dual-band microstrip balun that along with equal-amplitude and counterphase distribution of signal between outputs in two bands, ensures the transformation of complex load impedances, which are different in these bands, into a real value of input impedance of the source. In this case, the matching of device input and outputs is achieved with a high decoupling level of the latter. The proposed structure consists of cascade connection of four sections of coupled lines, two reactive elements that are implemented by stubs and the decoupling links of outputs. Three variants of circuits are presented for implementing the specified structure. The presence of elements in the circuit, the parameters of which can be specified, ensures the flexibility of designing process without any restrictions on values of loading impedances. In addition, the calculation process takes into account the difference of phase velocities of modes of microstrip coupled lines. For checking the properties of the proposed circuit and its designing technique, a microstrip balun for two working bands of frequencies with average values of 1.4 GHz and 2.8 GHz at different complex load impedances was manufactured and experimentally tested. The results of measurements well agree with the results of electromagnetic simulation; they also show that in both bands, the unbalance of amplitudes and phases at the balun outputs does not exceed 1 dB and 10 degrees, respectively, confirming the expediency of using the proposed circuit and its designing method.