3-142. When RL is greater than Z0, the end of the line is somewhat like an

open circuit; that is, standing waves appear on the line. The voltage

maximum appears at the end of the line and also at half-wave intervals back

from the end. The current is minimum (not zero) at the end of the line and

maximum at the odd quarter-wave points. Because part of the power in the

incident wave is consumed by the load resistance, the minimum voltage and

current are less than for the standing waves on an open-ended line. Figure

3-34, view G, illustrates the standing waves for this condition.

3-143. When RL is less than Z0, the termination appears as a short circuit.

The standing waves are shown in figure 3-34, view H. Notice that the line

terminates in a current loop (peak) and a voltage node (minimum). The

values of the maximum and minimum voltage and current approach those for

a shorted line as the value of RL approaches zero.

3-144. A line does not have to be any particular length to produce standing

waves; however, it cannot be an infinite line. Voltage and current must be

reflected to produce standing waves. For reflection to occur, a line must not

be terminated in its characteristic impedance. Reflection occurs on lines

terminated in opens, shorts, capacitances, and inductances, because the load

absorbs no energy. If the line is terminated in a resistance not equal to the

characteristic impedance of the line, some energy will be absorbed and the

rest will be reflected.

3-145. The voltage and current relationships for open-ended and shorted

lines are opposite to each other, as shown in figure 3-34, views C and D. The

points of maximum and minimum voltage and current are determined from

the output end of the line, because reflection always begins at that end.

3-146. The measurement of standing waves on a transmission line yields

information about equipment operating conditions. Maximum power is

absorbed by the load when ZL = Z0. If a line has no standing waves, the

termination for that line is correct and maximum power transfer takes place.

3-147. You have probably noticed that the variation of standing waves shows

how near the RF line is to being terminated in Z0. A wide variation in voltage

along the length means a termination far from Z0. A small variation means

termination near Z0. Therefore, the ratio of the maximum to the minimum is

a measure of the perfection of the termination of a line. This ratio is called

the standing-wave ratio (SWR) and is always expressed in whole numbers.

For example, a ratio of 1:1 describes a line terminated in its characteristic

impedance (Z0).

Integrated Publishing, Inc.
6230 Stone Rd, Unit Q
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