3-46
approaches an infinite value, ER increases and approaches Ei in value. When RL is smaller than Z0, ER has
a negative value. This means that the reflected voltage is of opposite polarity to the incident wave at the
termination of the line. As RL approaches zero, ER approaches Ei in value. The smaller the value of ER, the
smaller is the peak amplitude of the standing waves and the higher are the minimum values.
TERMINATION IN A RESISTANCE GREATER THAN Z0
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. Since 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.
TERMINATION IN A RESISTANCE LESS THAN Z0
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.
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 no energy is absorbed by the load. 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.
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.
Q26. A nonresonant line is a line that has no standing waves of current and voltage on it and is
considered to be flat. Why is this true?
Q27. On an open line, the voltage and impedance are maximum at what points on the line?
STANDING-WAVE RATIO
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.
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 Z
0. 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 (Z
0).
