5.25 Power Measurement Using AN/USM-177 Test Set
NOTE: Do not turn radar transmitter on until the test set has been nulled and zeroed.
1. Select a fixed attenuator of the proper size and frequency range to ensure that the thermistor dissipates no more
than 10 mW (10 DBM). Use the largest attenuator if in doubt. Use 20 dB pad in incident jack.
2. Turn radar to standby and perform any adjustments which affect transmitter power, i.e., PRF, magnetron filament
3. Connect the fixed attenuator (if necessary) to the "incident power" jack on the bidirectional coupler.
4. Connect the thermistor mount to the fixed annetuator, if used, otherwise directly to the "incident power" jack on
the bidirectional coupler.
5. Using the supplied cable connec the thermistor mount to the meter.
6. Select the proper "mount res". This information is on the thermistor mount.
7. Plug in the test set and turn to "line on".
8. Select the proper "calib factor". This information is found on the graph on the thermistor mount.
9. Turn the range selector switch to the "0.1 mW" position.
10. Adjust the "zero" and "vernier" controls until the meter reads 1/3 to 2/3 full scale deflection.
11. Turn the range switch to "null".
12. Using a screwdriver, turn the null adjustment until the meter nulls.
13. Select: the proper setting on the range switch for the power being measured. If in doubt start with the highest
scale (10 DBM) and work down.
14. Zero the meter using the "zero" and "vernier" controls.
15. Turn the radar to transmit.
16. Adjust the range switch on the test set so that the meter indication is 1/3 to 2/3 full scale deflection. Turn the
radar to standby when switching meter ranges.
17. Add the following losses:
a. Bidirectional coupler
b. Range switch
________________________________ +DBM 5-122