MFJ -976 Instruction Manual page 9

MFJ-976 Legal Limit Fully Balanced Antenna Tuner
To avoid problems, a dipole antenna should be a full half-wavelength on the lowest
band. On 160 Meters, an 80- or 40-Meter antenna fed the normal way will be
extremely reactive, with only a few ohms of feedpoint resistance. Trying to load an
80-Meter halfwave dipole (or shorter) antenna on 160 Meters can be a disaster for
both your signal and the tuner. The best way to operate 160 Meters with an 80- or
40-Meter antenna is to load either or both feedline wires (in parallel) as a longwire.
The antenna will act like a "T" antenna worked against station ground.
SWR Meter Calibration
The MFJ-976 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 14 or 21
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. Refer to Figure 1 for the Test Setup and refer to the PCB layout in Figure 2 for trim pot location.
2. Remove the top of the MFJ-976.
3. Connect the Test Setup equipment as shown in Figure 1. Use a 50-ohm dummy load for the
antenna. Set the Transmitter to the 14 MHz in the 20-meter band.
4. With the 3000/300 Watts button pushed out for the 300 Watt scale, Transmit 100 Watts as
indicated on the reference meter. Adjust the LO FWD trim pot to set the forward power scale to
100 Watts.
5. With the 3000/300 Watts button pushed in for the 3000 Watt scale, Transmit 1000 Watts as
indicated on the reference meter. Adjust the HI FWD trim pot to set the forward power scale to
1000 Watts.
6. 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.
7. With the 3000/300 Watts button pushed out for the 60 Watt reflected scale, Transmit 10 Watts
as indicated on the reference meter. Adjust the LO REF trim pot to set the reflected power scale
to 10 Watts.
WARNING!
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Instruction Manual