The increasing use of electrical pulses in the transmission of data by cable has resulted in a need for a better understanding of the electrical characteristics of a cable. Many system specifications state that the cables used shall have a certain characteristic impedance, specified in ohms. Any cable-maker's catalogue will list the characteristic impedance values of most coaxial cables, which usually range from 50 to 95 ohms. But impedance information on the more common types of shielded pairs is not readily available to the cable user. Because there are too many variations of applications involved with these cables.
Phase refers to the instantaneous relationship between voltage and current.
Phase relationship between voltage and current. This is because an inductance resists a change in current. In reality, perfect inductors and capacitors do not exist. They contain some resistance, so the phase angle is always less than 90 o , whether it be a positive or negative 90 o . The equivalent circuit of a cable is shown in Figure 2.