MFJ MFJ-921 User Manual

INTRODUCTION

Thank you for purchasing the MFJ-921 Dual Band VHF Antenna Tuner. The
MFJ-921 covers a frequency range from 144 MHz to 220 MHz. It will handle up
to 300 watts PEP of output power. It is designed to match a wide range of
impedances for coax fed antennas. The built-in SWR/Watt meter is usable for
both the 144 MHz and 220 MHz bands.

INSTALLATION

1. Connect your coax line fed from the antenna to the coax connector on the
MFJ-921 marked ANTENNA.

2. Connect a short coax line from the coax connector on the MFJ-921 marked
TRANSMITTER to the antenna connector of your transceiver.

3. Connect a ground wire from the GROUND wing-nut of the MFJ-921 to the
transceiver ground or earth ground of your station.

OPERATION

SWR/ WATT METER

1. SWR Measurement:

a. Set the Push button switch to SET position (IN).
b. Transmit a continuous carrier and set the SWR/POWER control for a

full-scale meter deflection.

c. Set the push button switch to SWR position (OUT) and read the SWR

on the SWR scale of the meter.

2. Power Measurement (144 MHz):

a. Set the push button switch to SET position (IN).
b. For 30 watts range - - rotate the SWR/POWER control to the 30 mark

and read the power level on the 30 watt scale.

c. For 300 watts range - - rotate the SWR/POWER control to the 300

mark and read the power level on the 300 scale.

Note: The SWR/POWER control on the MFJ-921 is factory calibrated for the

30 watt range. Do not reset the knob on this control. However, due to
component tolerance when precision reading is desired, the 300 watt
range can be recalibrated as follows.

a. Set the push button switch to SET position (IN).

b. Rotate the SWR/POWER control to the 30 mark and read the power

level on the 30 watt scale.

c. Rotate the SWR/POWER control counter-clockwise until the power

level reading on the 300 watts scale is the same as it is on the 30 watts
scale in Step b above.

d. Mark the control setting for the recalibrated 300 watt range.

3. Power Measurement (220 MHz):
Power measurement on the 220 MHz operation is the same as in the 144 MHz
operation as in Step 2. However, instead of using the 30 and 300 marks, use
the mark to the left of the 30 for 30 watt range and to the left of the 300 for the
300 watt range

ANTENNA OPERATION

The ANTENNA and TRANSMITTER controls on the MFJ-921 present a
minimum capacitance at "0" and a maximum capacitance at "10".

After properly installing the MFJ-921, use the tuner to tune for minimum SWR as
described below:

1. Set both the TRANSMITTER and ANTENNA controls to "0".

2. Apply enough transmitter power to have an adequate indication on your SWR/

WATT meter. Do not apply full power while tuning for minimum SWR.

3. While transmitting, alternately adjust the TRANSMITTER control and the

ANTENNA control for minimum SWR. Since the TRANSMITTER and
ANTENNA controls interact, the two controls can best be adjusted by
turning the TRANSMITTER control at a small increment at a time and then
rotating the ANTENNA control for the minimum SWR. Repeat this process
until the minimum SWR is achieved.

4. After minimum SWR is achieved, full output power, up to 300 watts PEP, may

be applied to the MFJ-921.