There's Nothing Magic about 50 Ohms

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  • Author: Gary Breed
Once in a while I am reminded that some seem-ingly important technical “standards” are noth-ing more than convenient, more-or-less arbi-trary, choices. The deeply-entrenched 50-ohm standard impedance for RF circuits came to mind as I was reviewing and researching this month’s tutorial on recent work in coaxial cables. Where did this important number come from? The most common story is that 50-ohm high power coaxial lines were first made using standard sizes of copper pipe, such as 3/4 inch for the inner conductor and 2 inch for the outer conductor. While this may explain why certain transmission lines are 52 or 53 ohms versus 50 ohms, I don’t think this is the entire story of how 50 ohms became the most common standard. Being curious, I went digging through my small collection of old reference books.