10.1.3 Characteristic Impedance
The ratio of voltage to current at any point along a transmission line is called its impedance. The ratio of voltage to current for a wave travelling along an infinitely long transmission line is known as the characteristic impedance, Zo. For an ideal line—one with uniform geometry, uniform dielectric, and negligible loss— Zo is purely resistive and constant along its length.
The characteristic impedance is a fundamental parameter because an infinitely long line absorbs all incident energy: none is reflected and all is transmitted onward. Although an infinite line cannot be constructed, a finite line can be made to behave as though it were infinite by terminating it in a purely resistive load equal to its characteristic impedance. When the termination resistance equals Zo, all of the incident power is absorbed by the load, and no reflections occur.
If the termination differs from Zo, a portion of the incident energy is reflected back toward the source. These reflections interfere with the forward wave to form standing waves along the line, as discussed in the following section.
For a lossless line, Z0 may be expressed in terms of the distributed inductance L and capacitance C per unit length as Z0 = √(L/C), a result derived from transmission-line theory.
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