MFJ-842 Mobile VHF/UHF SWR/Wattmeter
Instruction Manual
4
Calibration
The MFJ-842 has been calibrated at the factory. If it should ever need to be recalibrated,
then follow this procedure:
Equipment Needed
1. Transmitter capable of supplying enough power to obtain ½ to full-scale reading
at 140 or 525 MHz.
2. 50-ohm dummy load that is capable of handling full transmitter output power and
has better than a 1.15:1 SWR.
3. Power meter of know accuracy. The calibration will only be as good as the
standard reference meter.
4. 50-ohm cables capable of handling the power. RG-58/u is recommended. DO
NOT USE RG-59 or RG-11.
Meter Calibration
1. In order to calibrate the meter, you must remove the rear panel of the meter by
removing the screw located in between the ANT. and TX. connectors. The trim
pots used to set the needles are located on the bottom. Refer to
Figure 2
for the
Test Setup and refer to the Trim Pot Location in
Figure 3
for trim pot location.
2. Connect the Test Setup equipment as shown in
Figure 2
. Use a 50-ohm dummy
load for the antenna. Set the Transmitter to a mid-band frequency.
3. With
the
POWER RANGE
button pushed
in
for the 150 Watt scale, transmit 150
Watts as indicated on the reference meter. Adjust the HI FWD trim pot to set the
forward power scale to 150 Watts.
4. With
the
POWER RANGE
button pushed
out
for the 15 Watt scale, Transmit 15
Watts as indicated on the reference meter. Adjust the LO FWD trim pot to set the
forward power scale to 15 Watts.
5. To set the reflected power, interchange the TRANSMITTER and ANTENNA
coax cables so that the transmitter is connected to the ANTENNA connector and
the dummy load is connected to the TRANSMITTER connector.
6. With
the
POWER RANGE
button pushed
in
for the 50 Watt reflected scale,
Transmit 50 Watts as indicated on the reference meter. Adjust the HI REF trim
pot to set the reflected power scale to 50 Watts.