Dual-Band Design Theory for Dual Transmission-Line Transformer

In this letter, a novel dual-band (DB) design theory for dual transmission-line (TL) transformer is introduced, and it only consists of two parallel TLs with different physical lengths. Design equations for these two characteristic impedances and their physical lengths are derived in closed form. Through mathematical analysis, two different DB design expressions are newly summarized and proved under the condition of even and odd physical length ratios, respectively. It is proven that the proposed topology could realize around eight times lower impedance transformation ratio than that in conventional works. For verification purposes, two 5-50 Ω transformers with different frequency ratios have been simulated, fabricated, and measured. Measured and simulation results are matched very well.