5-3
coupler. This detected rf energy is measured across a known impedance to obtain either incident or
reflected power.
Operating the Wattmeter
Always de-energize and tag the rf power source before measuring incident power. Insert the proper
coupler-detector for the rf power being measured into the wattmeter case. Remove the wire shunt (not
shown in figure 5-1) from the meter terminals, then connect the wattmeter into the transmission line,
either at the load or the rf source. Ensure that all connections are tight.
Position the POWER RANGE knob to a value higher than the rated power of the rf source.
CAUTION
If the rated power to be measured is not known, place the POWER RANGE
knob in the highest power position before turning on the power source.
Rotate the coupler-detector so that the arrow indicating power flow points toward the load. Turn on
the rf power source. Rotate the POWER RANGE knob to the proper range for measuring and observe the
point at which the indicating meter peaks.
Q-1. To measure incident power, you must rotate the coupler-detector of the wattmeter so that the
arrow indicating power flow points toward which end of the transmission line?
Reflected power is measured in the same manner as described for incident power, except that the
coupler-detector is rotated so that the arrow points toward the rf source.
After completing power measurements, de-energize the rf source, disconnect the wattmeter from the
transmission line, and place the wire shunt on the meter terminals.
Interpreting Power Measurements Made by the Wattmeter
The rf power measurements made by the wattmeter are used to determine the voltage standing wave
ratio (VSWR) of the load and the power absorbed by the load. (VSWR is covered in NEETS, Module 10,
Introduction to Wave Propagation, Transmission Lines, and Antennas.) The VSWR can be determined
from a chart provided in the wattmeter technical manual, or it can be calculated (as shown in the
following example for a UHF transmitter) by the formula below (Pi is the incident power, and Pr is the
reflected power as measured by the wattmeter):
Where:
Pi = 30 watts
Pr = 0.5 watts
