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3.4 Mismatched lines - A Physical viewpoint 135 3.4 Mismatched lines - A Physical viewpoint In this section we will explain the physical phenomena that occur in a transmission line when the line is mismatched with terminations greater ( ) and lower ( ) than its characteristic impedance. It is recommended that the reader reviews section 2.5, since some of the concepts explained in it, which are essential to understanding this section, will not be repeated here. As we did in section 2.5 we will be examining our circuits by using both charge profiles and transmission line models. 3.4.1 Z L > Z 0 , Z L =100Ω As the switch is closed, the voltage wavefront will make its way down the line, just as in the case of an open-circuited line (Figure 2.5-23), since the generator does not know in advance what termination it will encounter! As the end of the line is reached however, unlike the case of an open- circuit (Figure 2.5-26), current is now allowed to flow through the termination and into the return wire. However there is a similarity with the open-circuit termination. An open-circuit caused charges to bunch up since they had nowhere to go. In this case also, since , some of the charges will not be able to go through the load as they encounter greater resistance than they did along the line. These charges will bunch up at the end of the line as shown in Figure 3.4-1. Also, since not all the charges get through the termination, the charges that do travel through it will not flow at the same rate as the rate at which the battery draws charge from the bottom wire. The combination of these effects, creates a greater electric field and hence a larger voltage across the load. The generator does not know what has happened yet and hence keeps pumping charges down the line thereby extending the region of charge accumulation in the top wire and reducing the charge density in the return wire. This in turn extends the region of higher voltage back towards the generator as shown. Comparative Charge Spacing Figure 3.4-1 Charge distribution and voltage profile for mismatched line with + _ Zero Volts Vs Vs + 1 3 __ Vs V S Conquer Radio Frequency 135 www.cadence.com/go/awr