Matching Using Only 50-Ohm and 75-Ohm Transmission Lines
High Frequency Electronics
atching using common coaxial cables. The most common coaxial cable impedances are 50 and 75 ohms, so this note looks at the range of impedances that can be matched using only these two types of coax. For simplicity, non-reactive impedances are used in these examples, but this method can match a wide range of reactive impedances, as well. Transmission lines are easy to analyze on a Smith® chart, and there are several handy software-based tools that avoid the need for paper charts, compass, and other manual methods. winSmith 2.0 was used for this note; see the References for other options. The highest impedance that can be matched using these lines is 112.5 ohms, using a 1/4-wavelength 75-ohm line. The low-impedance range goes to near zero, since low impedance lines can be made by connecting multiple lines in parallel. In practice, any impedance between a few ohms and 112.5 ohms can be readily matched with the proper combinations of lines.
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