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How can stepped-impedance transmission line sections be used to design a UWB matching network without discrete inductors or capacitors?

I need to match a 50Ω feed line to a low-input-impedance UWB planar monopole (≈15-25Ω) over 3.1–10.6 GHz. Lumped elements are too narrowband and lossy. I've read about multi-section quarter-wave transformers, but they are inherently narrowband. Are stepped-impedance lines (like a high-Z section followed by a low-Z section) effective for UWB matching? What's the design methodology—is it based on a tapered line approximation, or is there a synthesis technique for a specific number of sections? How do I determine the required characteristic impedances and lengths?

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By Vipin Kumar Answered 1 year ago

Yes, stepped-impedance lines are an excellent distributed solution for UWB matching. The key is to move beyond the single-section λ/4 transformer. You need a multi-section, tapered impedance profile. I typically use a 3 or 4-section Chebyshev transformer designed for an equal-ripple response across the band. The synthesis involves calculating the characteristic impedances of each section (e.g., from 50Ω down to 20Ω in steps) and their lengths. Critically, for UWB, the sections are not λ/4 at the center frequency; their lengths are a significant fraction of the wavelength at the highest frequency, often optimized via simulation. I start with a Klopfenstein taper as the ideal continuous template, then discretize it into 3-4 steps for fabrication. This approach provides a smooth impedance transition, minimizing reflections across the entire decade bandwidth.

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