Input-Reflectionless 2.4–3.8GHz Balun with Low Phase/Amplitude Imbalance Using Stacked-Coupled Microstrip Line

In this paper, a compact input-reflectionless balun with low phase/amplitude imbalance based on stacked-coupled microstrip line is proposed, which consists of reflectionless- and balun-sections. The reflectionless-section is a particular two-stage coupled transformation structure. The absorbing resistor is located at the end of 90° stacked-coupled microstrip line, which can introduce theoretically ideal all-band reflectionless characteristics. Meanwhile, the balun is composed of two symmetrical couplers, where two output ports have 180° phase difference with equal power division. By cascading these two sections, a prototype of input-reflectionless balun is achieved. To verify the operation mentioned above, a compact balun operated at 2.4–3.8 GHz with 3-dB factional bandwidth (FBW) of 45% is proposed and fabricated. The minimum in-band insertion loss is 1.1 dB with reflectionless level of -10 dB from DC to 8.2 GHz.