MFJ-991 / MFJ-994 IntelliTuner Automatic Antenna Tuners
Instruction Manual
©
2004 MFJ Enterprises, Inc.
19
SWR Bridge Calibration
To calibrate the SWR Bridge, you will need a transmitter capable of 100 watts output, a precise calibrated
wattmeter, a 50-ohm dummy load, three 50-ohm SO-239 coax cables, a Phillip screwdriver, and a tuning
tool or small flat blade screwdriver. Make sure the tuner’s meter is calibrated.
WARNING: Do not touch anything inside the tuner during operation! Serious, painful RF
burns can result.
WARNING: Never operate the MFJ-991/4 with its cover removed; dangerous voltages and
currents can be present during operation. Never exceed tuner specifications.
Note: The trimmers involved in this calibration are located on the main circuit board in front of the
TRANSMITTER connector; do not confuse these with the SWR/wattmeter trimmers.
1. Turn off the power to the tuner and the transmitter.
2. Remove the cover from the tuner (12 screws) with a Phillips screwdriver.
3. Connect the 50-ohm dummy load to the ANTENNA connector; connect the wattmeter between the
transmitter and the TRANSMITTER connector on the tuner.
4. Turn on the power to the transmitter. Select a frequency in the middle of an HF band, such as 7.253
MHz, for calibration.
5. Press and hold
both
the [TUNE] and [L-UP] buttons while turning the tuner power on. Both meter
needles will bounce. Release the buttons.
6. Key the transmitter to output 100 watts and adjust the trimmer capacitor C91 for minimum reflected
power. Notice the reflected meter movement is exaggerated for easier calibration.
7. Key the transmitter to 100 watts output and adjust the FWD trimpot R72 until the forward power
needle indicates 100 watts.
8. Press the [TUNE] button and observe that the reflected power needle goes to full scale.
9. Turn off the transmitter and reverse the ANTENNA and TRANSMITTER connections; that is,
connect the 50-ohm dummy load to the TRANSMITTER connector and connect the
transmitter/wattmeter to the ANTENNA connector.
10. Turn on the transmitter’s power.
11. Key the transmitter to 100 watts output and adjust the REF trimpot R71 until the forward power
needle (yes, the forward needle) indicates 100 watts; the reflected meter should be at full scale.
12. Press the [TUNE] button to end calibration.
13. Turn off the power to the tuner and the transmitter.
14. Remove the transmitter/wattmeter and the 50-ohm dummy load from the tuner.
15. Secure the cover back onto the tuner.
16. Connect your transmitter to the TRANSMITTER connector and connect your antenna to the
appropriate antenna connector on the tuner.
Summary of Contents for IntelliTuner -991
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