Why 50 ohm?

A couple of our engineers recently had a long discussion while trying to decide what range of impedances to include for the design of a coaxial feed for one of our antennas. The question that quickly arose was: ??why are most coaxial cables that are commonly available 50 ohm or 75 ohm??. I?m sure you must have asked yourself the same question before.

While looking into this, we discovered a 1955 IEE paper, titled ?THE CHOICE OF IMPEDANCE FOR COAXIAL RADIO-FREQUENCY CABLES? by WT Blackband. It is an excellent study considering various factors like attenuation, voltage, cable thickness and thermal characteristics of coaxial cables of various impedances and made with various materials. Though each performance measure for coaxial cables suggests a different optimal impedance, the general conclusion is basically: ??The best choice of impedance is 75 ohms for low-loss air-spaced cables, and 50 ohms for general-purpose thermoplastic cables.?

A definite must read if you ever wondered what the best coaxial cable impedance might be for a specific application ? and why 50 Ohms and 75 Ohms seem to be so popular!

Author: Robert Kellerman

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