2.3 Transmission Lines – an Introduction
39
Figure 2.3-5 Electric and Magnetic fields as signal propagates down a transmission line
Assuming that there is air between our two conductors, voltages and currents will propagate down
our transmission line at nearly the speed of light, progressively charging the distributed capacitance
and drawing a constant current of limited magnitude from the battery. This is shown in Figure 2.3-6.
Since the wires are infinitely long, their distributed capacitance will never fully charge to the source
voltage and this pair of wires will continue to draw a constant current from the source so long as the
switch is closed, behaving as a constant load. When we consider our "wires" under these conditions
we cannot just see them as a pair of conductors, we must treat them as a transmission line.
So our infinitely long transmission line behaves as a constant load and its response to the applied
voltage is resistive rather than reactive, despite the fact that the line comprises solely of inductance
and capacitance (assuming that the wires have zero resistance). This is because, from the battery's
perspective, there is no difference between a resistor eternally dissipating energy and an infinite
transmission line eternally absorbing energy. The ratio of the battery voltage and the constant
current that an infinite line would draw is called the characteristic impedance (
) of the line, and it
is determined by the values of our distributed capacitance and inductance, which in turn are fixed by
the geometry of the two conductors.
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