Voltage Standing-Wave Ratio
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TERMINATION IN A RESISTANCE LESS THAN Z0
SUMMARY - 14182_156
Neets Module 10-Introduction to Wave Propagation, Transmission Lines, and Antennas
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3-47
Voltage Standing-Wave Ratio
The ratio of maximum voltage to minimum voltage on a line is called the VOLTAGE STANDING-
WAVE RATIO (vswr). Therefore:
The vertical lines in the formula indicate that the enclosed quantities are absolute and that the two
values are taken without regard to polarity. Depending on the nature of the standing waves, the numerical
value of vswr ranges from a value of 1 (Z
_{L}
= Z
_{0}
, no standing waves) to an infinite value for theoretically
complete
reflection
. Since there is always a small loss on a line, the minimum voltage is never zero and
the vswr is always some finite value. However, if the vswr is to be a useful quantity, the
power losses
along the line must be small in comparison to the transmitted power.
Power Standing-Wave Ratio
The square of the voltage standing-wave ratio is called the POWER STANDING-WAVE RATIO
(pswr). Therefore:
This ratio is useful because the instruments used to detect standing waves react to the square of the
voltage. Since power is proportional to the square of the voltage, the ratio of the square of the maximum
and minimum voltages is called the power standing-wave ratio. In a sense, the name is misleading
because the power along a
transmission line
does not vary.
Current Standing-Wave Ratio
The ratio of maximum to minimum current along a
transmission line
is called CURRENT
STANDING-WAVE RATIO (iswr). Therefore:
This ratio is the same as that for voltages. It can be used where measurements are made with loops
that sample the
magnetic field
along a line. It gives the same results as vswr measurements.
Q28.
At what point on an open-circuited rf line do voltage peaks occur?
Q29.
What is the square of the voltage standing-wave ratio called?
Q30.
What does vswr measure?
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