temperature changes. A RANGE switch on the power
the thermistor mount and the power source. A CALIB.
meter provides for full scale power measurements from
FACTOR control provides for compensating, in the
10 sew to 10 mw. Power levels above 10 mw can be
metering circuit, for the efficiency and VSWR of the
measured by inserting a suitably calibrated attenuator
thermistor mount being used.
between
* USED ONLY IF RF POWER LEVEL IS GREATER THAN 10 MW
Figure 2-22. Rf Power Measurement, Block Diagram.
b. Several sources of error are inherent to this method
of measuring rf power. Some of these errors can be
]
[
1 - 4(SL SG)
determined accurately and compensated for, while
Max Loss (%) =
]
others are known, or can be found, to exist between
X100
definite limits.
[
c. The major compensable errors are those due to
]
Min Loss (%) =
1 - 4(SL SG)
]
power dissipated in the transmission
SL + SG)2
line connecting the source to the thermistor mount and
X100
those due to the efficiency of the thermistor mount. The
first kind is corrected by determining the power lost in the
where:
transmission line and adding it to the power meter
reading. The error due to the efficiency of the thermistor
SL - VSWR of the thermistor mount.
mount is corrected automatically (to with +2%) by setting
SG - VSWR of the source.
the CALIB. FACTOR control on the power meter to the
e. The instrumentation error of the power meter is
setting determined from the efficiency-vs-frequency chart
about +1 % of full scale reading. The overall accuracy of
mounted on the thermistor mount.
the power measurement is +6%o (plus the mismatch
d. The error due to mismatch loss results from
errors, if not corrected).
VSWR's at the source and at the thermistor mount. If
the magnitude and phase angle of both VSWR's (or
2-14. VSWR Measurements
equivalently the reflection coefficients) are known, a
a. General.
unique value of mismatch loss may be determined.
(1) An rf signal in a perfectly uniform, infinitely
Generally, the VSWR's are either known or readily
long transmission line has a constant signal strength
measurable but the phase angle is difficult to determine.
along the line, except for the resistive losses in the
Working with the VSWR's only, it is possible to
material of the line. The signal in such an ideal
determine a range of errors (between which the actual
transmiss3ion line is propagated without reflections. If
error must exist) corresponding to the two extreme cases
an irregularity (mismatch) exists in the line, part of the
of phase angle relationship. This range of error may be
signal is reflected back along the line. This produces
determined
from
the
following
equations:
periodic variations of signal strength,
called standing waves, along the line.
(2) Since power reflected back toward the signal
source is lost, it is desirable to reduce the reflected
power as much as possible. A measure of the amount of
power reflected is
2-27
