Cable characteristic impedance

For signal transmission along a coaxial line from the source to the load to be carried out with maximum efficiency, the cable must operate in the traveling wave mode. In this case, reflections of RF energy from the load are minimal, and the reactive power is zero. Typically, RF path components have an input impedance of 50 Ohms or 75 Ohms. Most coaxial cables for radio communication have a characteristic impedance of 50 Ohms, and for television — 75 Ohms.

Since the inner diameter of the cable braid is equal to the diameter of the polyethylene insulation, the characteristic impedance of the cable can be determined with sufficient accuracy by calculating, after measurement, the ratio of the diameter of the polyethylene insulation to the diameter of the central conductor. If this ratio is between 3.3 and 3.7, the cable has a characteristic impedance of 50 Ohms; if it is between 6.5 and 6.9, the characteristic impedance is 75 Ohms.