b. Artificial transmission lines also present opposition to RF energy
called characteristic resistance. We use the term characteristic resistance
to indicate the impedance of an artificial transmission line.
The symbol
for characteristic resistance is Rc. As in a real line, the opposition is
equal to the square root of the inductance divided by the capacitance. The
formula is the same as for a real line:
4.
Transmission lines delay voltage.
a. When voltage is applied to the input terminals of a real line, a
definite amount of time passes before the voltage appears at the output.
This is known as time delay (TD). In a real line, the time delay depends
upon the distributed inductance and capacitance per unit length of time and
the length of line used.
The time delay of a real transmission line is
equal to the length of the line times the square root of L and C.
The
formula for determining the TD of a real line is as follows:
b. Transmission line manufacturers use one foot as a unit length.
Since the values of L and C per foot are very small, it takes many feet of
real line to delay a pulse just a fraction of a second. For example, let's
use the TD formula to find out how long it takes a pulse to travel along
1,000 feet of line that has a distributed inductance of .2 microhenries, and
a distributed capacitance of 20 uuf per foot. Using the formula, we get:
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