Standing-Wave Current vs Traveling-Wave Current
A followup:
If we have a transmission line which is an integral number of half
wavelengths long, open circuited at the far end, and driven by a perfect
voltage source of Vs*sin(wt) in series with *any* non-zero resistance:
The amplitude of the wave reflected from the source will decrease each
time, resulting in convergence at the following steady state conditions:
vf(t, x) = (Vs/2) * sin(wt - x)
vr(t, x) = (Vs/2) * sin(wt + x)
Where x is the position from the source in electrical degrees or
radians, and vf and vr are the totals of all forward and reverse
traveling waves respectively.
The total voltage along the line at any time and position is:
v(t, x) = vf(t, x) + vr(t, x) = Vs * sin(wt) * cos(x)
This clearly shows that the total voltage at any point along the line is
sinusoidal and in phase at all points. The "standing wave" is the
description of the way the peak amplitude of the sine time function
differs with position x.
So the amplitude of the voltage at both ends of the line (where |cos(x)|
= 1) will equal the source voltage. The number of reflections (length of
time) it will take for the system to converge to within any specified
closeness of the steady state depends on the amount of source
resistance. If the source resistance is the same as the line Z0,
convergence is reached in a single round trip. The time increases as the
source resistance gets greater or less than this value. If the source
resistance in zero or infinite, convergence to this steady state will
never be reached, as shown in the earlier posting.
Roy Lewallen, W7EL
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