The choice of impedance for coaxial radio-frequency cables

The choice of characteristic impedance of a coaxial cable is discussed. Among the cable properties considered are attenuation per unit length, voltage rating and power ratings based upon thermal and voltage limitations. A method is given for determining the optimum proportioning to satisfy a specification of two or more cable properties. It is shown that the criterion of cable diameter leads to conclusions little different from those based upon the criteria of dielectric or conductor cross-section. The best choice of impedance is 75 ohms for low-loss air-spaced cables, and 50 ohms for general-purpose thermoplastic cables.