Figure 1: A transmission line can be seen as a series of "buckets" of capacitance charged to a voltage by the signal as it "walks the line." |

2. Signals are dynamic, and once launched, cannot be prevented from propagating down the transmission line.

### Be the Signal

To illustrate the dynamic nature of signals, imagine a very simple, 1 ns long, 50 Ω impedance transmission line. As a 1 V signal is launched into the transmission line and propagates, at each step along the way it asks "What's the impedance of the environment?" at its leading edge. That is the *instantaneous impedance*, notated as Z. Impedance is always defined as the ratio of a voltage to a current. We know the voltage of this signal (1 V), but how do we find the current at the edge?