the ratio of the maximum signal strength (incident
voltage plus reflected voltage) in the transmission line to
wave-modulated rf signal with a constant output level is
the minimum signal strength (incident minus reflected
obtained from the signal source and fed through the
voltage) in the line. This ratio is called the voltage
standing wave detector to the unknown load. When the
standing wave ratio (VSWR).
frequency knob is set to the frequency of the signal
(3) In this test set two methods are used to measure
source, a radial field is produced in the upright section of
VSWR: the first method, used below 1 GHz, measures
the standing wave detector. When the ANGLE OF
VSWR using a standing wave detector; the second
method, used above 1 GHz, measures the magnitude of
attached to a crystal detector is rotated through the radial
field in order to sample and detect the maximum and
VSWR as follows:
minimum points of the standing wave. As in a slotted
transmission line, the ratio between the maximum and
VSWR = 1 + IrI or IrI = VSWR - 1
minimum standing wave voltages is the VSWR. The
1 + Irl
VSWR + 1
output of the crystal detector is fed to the SWR meter
which measures this ratio.
b. VSWR Measurements Below 1 GHz.
(1) The test setup diagram for VSWR measurement
Figure 2-23. VSWR Measurements Below I GHz, Block Diagram.
(2) For measurement of VSWR of a multiport
that the ratio between the applied rf power and the
network, it is necessary to terminate the output port or
detected dc voltage is not constant as the rf power
ports by a matched load. When the outputs are properly
increases beyond a certain level. This error can be
terminated, the multiport network can be treated as a
minimized by reducing the power input to the detector by
one-port device.
means of the variable attenuator in the test setup.
(3) Errors, inherent in VSWR measurement, arise
essentially from the following sources:
nonlinearity of the SWR meter amplifier, inaccuracies in
(a) Detector error due to deviation from the
its range switching circuits and inaccuracy of the
square-law
behavior
is
due
to
the
fact
indicating meter.
(4) The accuracy of the VSWR measure-
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