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This paper demonstrates two broadband air-filled micro-coaxial 4:1 (2-24 GHz) and 2.25:1 (2-22 GHz) impedance transformers. The 4:1 transformer converts 50 to 12.5 Ω and the 2.25:1 device transforms 50 to 22.22 Ω. The circuits are fabricated on silicon with PolyStrata technology, and are implemented with 650 μm × 400 μm air-filled micro-coaxial lines. Back-to-back circuits and single structures with geometrical tapers are designed for systematic characterization. Simulation and measurement results are in excellent agreement. The return loss for both transformers is better than 15 dB over the design bandwidth.
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Ehsan et al. (2010) studied this question.
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