TC 9-64 _________________________________________________________________________
TERMINATION IN A RESISTANCE GREATER THAN Z0
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.
TERMINATION IN A RESISTANCE LESS THAN Z0
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.
STANDING-WAVE RATIO
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).
3-46
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