Current Distribution on an Open-Circuited Lossless Line

Figure 7. Curves for electric field and voltage relations on a short-circuited lossless line 5/4 λ in length. For (b) and (d) the resultant wave lies along the X axis. At certain points along the line the voltage is always zero, and at other points it is as shown by the curve "instrument readings." These curves also apply to magnetic field and current relations on open-circuited lossless lines. (See acknowledgment, Fig. 6.).

The impressed voltage will, of course, force a current into the line. The initial current component of the electromagnetic wave will travel down the line as indicated in Fig. 7(a) Because the line is open circuited, the current at the distant end must be zero. For this to be possible it is necessary that a 180° phase shift of the magnetic field and current component of the electromagnetic wave occur at the open-circuited end of the line. Then, the reflected component will cancel the oncoming initial component, and the resultant magnetic field and the current will be zero.

If the relations between the initial and the reflected components are studied for other input phase positions, Figs. 7(b), (c), and (d) result. If thermomilliammeters are inserted at various points along the line, the effective value of the current readings will plot as shown in Fig. 7(e). This curve is called a current standing wave.