CHAPTER 6 Transmission Techniques: Wire and Cable becomes critical. With smaller distances, patch cords, patch panels, and eventually the connectors themselves become just as critical as the cable. The impedance of these parts, especially when measured over the desired bandwidth, becomes a serious question. To be truly accurate, the quarter-wavelength numbers in Table 6.28 need to be multiplied by the velocity of propagation of each cable. So, in fact, the distances would be even shorter than what is shown. It is quite possible that a cable can work fine with lower-bandwidth applications and fail when used for higher-frequency applications. The characteristic impedance will also depend on the parameters of the pair or coax cable at the applied frequency. The resistive component of the characteristic imped- ance is generally high at the low frequencies as compared to the reactive component, falling off with an increase of frequency, as shown in Figure 6.19. The reactive component is high at the low frequencies and falls off as the frequency is increased. The impedance of a uniform line is the impedance obtained for a long line (of infinite length). It is appar- ent, for a long line, that the current in the line is little affected by the value of the terminating impedance at the far end of the line. If the line has an attenuation of 20 dB and the far end is shortcircuited, the char- acteristic impedance as measured at the sending end will not be affected by more than 2%. TWISTED-PAIR IMPEDANCE For shielded and unshielded twisted pairs, the characteristic impedance is: 101670 C ( V P ) (6.15) Z o