Electronics
Transmission Lines
Close a switch on a two-wire line and watch where the energy really goes: not down the copper, but through the space between the wires, riding the fields as a wave at v = 1/√(LC). Change the load and the wave bounces back doubled, inverted, or not at all.
E (from the charge, ∝ V)B (from the current, ∝ I)energy flow S = E × Hshading on the plane
Drag to orbit · scroll to zoom · dots on the wires drift with the current
Power into the load—
—
Source
Load
Show
Shade
Draw it as
Plane
Arrows
Values
time t—
ΓL = (RL − Z0)/(RL + Z0)—
V at the load—
I at the load—
power into the load—
work done by the source—
heat in Rs—
stored in the line ½∫(CV² + LI²)dx—
delivered to the load—
What to observe
- Close the switch and the load stays dark until the wavefront has crossed the line. The glowing fog is the energy: it fills the space between the wires, not the copper, and the gold S arrows show it flowing to the load.
- An open end sends the wave back with the same sign: the voltage doubles and the current stops behind it. A short sends it back inverted, so V drops to zero.
- With a matched load nothing comes back: the pulse is swallowed whole and the line is empty again.
- Drive a mismatched line with AC and the two waves add into a standing wave: V has nodes exactly where I peaks.
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