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CHAPTER 2 - Conveying Power at Radio Frequency 48 Figure 2.5-5 Voltage approaches the open end of the line, which can be seen as a capacitor As the voltage reaches the end of the line, the last capacitor, which represents our open- circuit termination, also charges to E/2 through the last inductor (Figure 2.5-5). Now there's no voltage difference across the terminals of the last inductor (Figure 2.5-5) and hence no further current will flow from the battery into the inductor. As we know, inductors oppose sudden changes in current while capacitors oppose sudden changes in voltage. So, just like a capacitor would try to maintain the voltage across its terminals and discharge gradually until the field and the voltage between the plates have dropped to zero, the inductor will try to maintain the current flow, in the same direction as the initial current, by means of the energy stored in its magnetic field (Figure 2.5-6). Figure 2.5-6 The end of the line is reached, current from the battery stops, but last inductor keeps current flowing However, just as in the case of the capacitor, where the electric field eventually dies away, the magnetic field of the inductor collapses until no energy is left in it and current stops flowing into the end capacitor. What happened to such energy? It has been used to charge the capacitor further (Figure 2.5-6 and Figure 2.5-7)! But how much further? Well, since the energy stored in capacitors and inductors is the same 16 , the capacitor's electric field ends up storing twice this energy and hence the voltage across its terminals doubles (Figure 2.5-7). 16 The transmission line has a characteristic impedance √ We can therefore write ⇒ i.e. E E/2 E/2 E/2 E/2 E/2 I E E/2 E/2 E/2 E/2 → E Conquer Radio Frequency 48 www.cadence.com/go/awr

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