If you have a 50-ohm coax cable (or microstrip line or any other 50 ohm transmission line) that isn't connected to anything on either end, and connect an ohmmeter across it, you might expecting to read 50 ohms if you were new to microwave engineering. What is this thing called characteristic impedance, also known as "Z-zero"? Let's start with a common misconception. There is no better way to say this, than "what an idiot!" Bad advice is continuous, we won't provide any more of it. A reading of zero indicates a potential short between the inner cable and its shield. A reading of infinite resistance indicates a break somewhere along the the inside of the cable. A reading around the 50 ohm or 75 ohm rating of the cable indicates that the cable is working properly across its length. Test the length of the cable by placing a multimeter probe on each end of the cable, touching the center wire emerging from the coaxial connector.Ĥ. Disconnect both ends of the cable from their connections.ģ. Coaxial cable is typically rated to operate at 50 or 75 ohms so the scale should be at or greater than these ratings.Ģ. A higher scale works, but avoid moving into a scale close to 1k ohms or higher. Set your multimeter to an ohms scale around 100 ohms. They sure have a lot of sponsored links though!ġ. There is a lot of misinformation on the world wide web, you have to be careful. Measuring Z0 with time domain reflectometry (separate page) Can you measure Z0 with an ohm-meter?īelow is some text we found on the "E-how" web site in 2012, that shows how to test a 50 or 75 ohm cable, the content has since been taken down. Measuring Z0 using reflection coefficient Only one solution (so far) is capable of measuring Z0 over frequency such that dispersion can be observed. About the only thing they have in common is they all use electrical measurements of transmission lines, as opposed to electromagnetic or closed-form simulations. This page will tie together some different attempts of measuring characteristic impedance (Z0). Click here to go to our main measurement pageĬlick here to go to our page on characteristic impedanceĬlick here to go to our time domain reflectometry pageĬlick here to go to our page on quarter-wave transformers
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