Unequal termination branch-line balun with high-isolation wideband characteristics

This article presents a design of an unequal termination impedance branch-line balun with wideband high isolation. The wideband and high isolation characteristics are obtained only in the under-matched region. For an experimental verification, the proposed balun was designed at a center frequency (f₀) of 2.6 GHz. The measured results are in good agreement with the simulations. From the measurement, the power divisions are -3.29 dB and -3.3 dB. The return loss at f₀ is 22.3 dB and is higher than 20 dB over a bandwidth of 0.51 GHz. The isolation between output ports is higher than 19 dB for the bandwidth of 0.64 GHz. The phase imbalance between two output ports is 180° ± 5° for the bandwidth of 0.64 GHz.