1. Theoretically, characteristic impedance is the impedance that would be simulated by a given two-conductor or coaxial line of uniform construction, if that line were of infinite length. This value is determined by the materials used for the two conductors, the dielectric used to insulate the two conductors, the diameters of the conductors, and the spacing between them.
2. In practice, for a transmission line or waveguide terminated with a load that produces no standing waves on the line, the ratio of radio-frequency (RF) voltage to RF current. This ratio is the same at all points along the length of a perfectly matched line, and depends on the physical construction of the line. Coaxial lines typically have Z0 between 50 and 100 ohms. Twinlead is available with 75-ohm and 300-ohm Z0 values. Open-wire line has Z0 between 300 and 600 ohms, depending on the spacing between the conductors, and also on the type of dielectric (insulating material) employed to keep the spacing constant between the conductors.
3. Experimentally, the value of impedance that, if it terminates a transmission line or waveguide, results in no reflected power from the load end of line. This is always a pure resistance; that is, it contains no reactance.
The Symbol, Z0.