the line endlessly, the line is infinitely long, and a certain finite value of

impedance across points K and L is finally reached.

3-55. In this discussion of transmission lines, the effect of conductance (G) is

minor compared to that of inductance (L) and capacitance (C), and is

frequently neglected. In figure 3-16, view C, G is omitted and the inductance

and resistance of each line can be considered as one line.

3-56. Let us assume that the sections of figure 3-16, view C continue to the

right with an infinite number of sections. When an infinite number of

sections extends to the right, the impedance appearing across K and L is Z0.

If the line is cut at R and S, an infinite number of sections still extends to the

right because the line is endless in that direction. Therefore, the impedance

now appearing across points R and S is also Z0, as illustrated in view D. You

can see that if only the first three sections are taken and a load impedance of

Z0 is connected across points R and S, the impedance across the input

terminals K and L is still Z0. The line continues to act as an infinite line, as

illustrated in view E.

3-57. Figure 3-17, view A illustrates how the characteristic impedance of an

infinite line can be calculated. Resistors are added in series parallel across

terminals K and L in eight steps, and the resultant impedances are noted. In

step 1 the impedance is infinite; in step 2 the impedance is 110 ohms. In

step 3 the impedance becomes 62.1 ohms, a change of 47.9 ohms. In step 4 the

impedance is 48.5 ohms, a change of only 13.6 ohms. The resultant changes

in impedance from each additional increment become progressively smaller.

Eventually, practically no change in impedance results from further additions

to the line. The total impedance of the line at this point is said to be at its

characteristic impedance; which, in this case, is 37 ohms. This means that an

infinite line constructed as indicated in step 8 could be effectively replaced by

a 37-ohm resistor. View B shows a 37-ohm resistor placed in the line at

various points to replace the infinite line of step 8 in view A. There is no

change in total impedance.

3-58. In figure 3-17, resistors were used to show impedance characteristics

for the sake of simplicity. Figuring the actual impedance of a line having

reactance is very similar, with inductance taking the place of the series

resistors and capacitance taking the place of the shunt resistors. The

characteristic impedance of lines in actual use normally lies between 50 and

600 ohms.

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