A real transmission line is one constructed for the principal purpose of moving signal information, and an artificial transmission line is a unit constructed to exhibit electrical characteristics resembling a real transmission line. The artificial line does not require great physical length, and may be constructed to withstand high Voltages, but is inferior in frequency response, which can be an advantage. The most familiar real transmission line is the coaxial cable, and those characteristics of greatest interest to the technician are (1) the characteristic impedance, (2) inductance per unit of length, (3) capacitance per unit of length, (4) loss per unit of length, (5) a frequency at which the line becomes reactive, (6) Voltage rating, and (7), the velocity factor, a decimal number or percentage used to calculate and increase the signal propagation time from the speed of light, 1.015 nanosecs per foot.

Some examples of common real transmission lines are listed in the following table. A very few rough and general rules of application are (1) near-50-ohm cables are generally used for rf transmission (2) near-75-ohm cables are generally used for video transmission, and (3) cables with larger center-conductor diameters may be used for receiver circuits to reduce losses, or for transmitter circuits to increase power-handling capability.