___________________________________________________ Principles of Transmission Lines
Assume that the line shown in figure 3-23 is 1,000 feet long. A 100-foot
(approximately 30.5 meter) section is measured to determine L and C. The
section is found to have an inductance of 0.25 millihenry and a capacitance
of 1000 picofarads. Find the characteristic impedance of the line and the
velocity of the wave on the line. The characteristic impedance is--
Z 0 = LC
-3
0.25x10
Z0 =
-12
1000x10
6
Z 0 = 0.25x10
Z 0 = 0.5x10
3
Z 0 = 500Ω
If any other unit length had been considered, the values of L and C would be
different, but their ratio would remain the same as would the characteristic
impedance.
The formula for T is:
T = LC
-3
-12
T = 0.25x10
x1000x10
-12
T = 0.25x10
-6
T = 0.5x10
second
T = 0.5 microsecond
The formula for the velocity of a wave is:
D
V=
T
100 feet
V=
-6
0.5x10
second
6
V = 200x10 feet/second
V = 200,000,00 0 feet/secon d
REFLECTIONS ON A TRANSMISSION LINE
3-88. Transmission line characteristics are based on an infinite line. A line
cannot always be terminated in its characteristic impedance because it is
sometimes operated as an open-ended line and at other times as a short-
circuit at the receiving end. If the line is open-ended, it has a terminating
impedance that is infinitely large. If a line is not terminated in characteristic
impedance (Z0), it is said to be finite.
3-89. When a line is not terminated in characteristic impedance, the incident
energy is not absorbed but is returned along the only path available--the
3-27
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