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Second, there’s also an unavoidable difference that is a function of transmission-line attributes. First, those two resistance values are close to the natural value that impedances have using a coaxial transmission line of convenient dimensions – the first such lines were made of copper plumbing pipe of different diameters, with one inserted into the other. The reason for both 50 and 75 ohms is both technical and historical. Q: We sometimes see an RF system with a nominal impedance of 75 ohms – what’s the story there?Ī: While most RF designs use resistive 50-ohm impedances as their nominal value, video systems have traditionally used 75 ohms and still do. The 50-ohm value is a goal and often used as a normalized or target value. As a result, designers have to learn to live with and work with the reactive impedances, even if they are not at 50 ohms, even if they are purely resistive. This is a function of the physics of the construction, strays and parasitics, electromagnetic theory, and other considerations that cannot be controlled or changed. While designers wish everything was a resistive-only 50 ohms, the reality is that the output and input impedances of many components of circuits are not 50 ohms and not purely substance. Q: Why all this worry? Aren’t RF systems designed to be at 50-ohms with no reactive component?Ī: Only in dreams. This part looks at the closely related topic of nominal impedance and VSWR. Part 1 of this article looked at the basics of impedance matching and power transfer.
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Understanding the role and implications of impedance in RF transmission lines and antennas is vital to successful system performance.