MFJ-945D Instruction Manual
MFJ-945D Mobile Tuner
Instruction Manual
Introduction
The MFJ-945D tuner is a 300 watt PEP output (175 watts on 160 m) antenna tuner. It is
designed to match 50 ohm output amplifiers, transmitters or transceivers to an antenna in a
mobile installation. Forward power, reflected power, and SWR are displayed on the
wattmeter's illuminated cross-needle meter.
The MFJ-945D uses a "T" matching network. It continuously tunes all frequencies from 1.8
through 30 MHz. It will match dipoles, inverted-vee's, verticals, mobile whips, beams, random
wires, and many other antennas. The MFJ-945D has a single connector for coaxial output.
SWR/Wattmeter
The cross-needle meter measures the peak or average FORWARD power, REFLECTED power,
and SWR. The meter's full scale forward and reflected power range is controll ed by the left
POWER switch that selects 300W (HI) or 30W (LO). If your transmitter runs more than 30
watts of output power, set this switch to the 300W HI (in) position. If your transmitter has less
than 30 watts of output, set this switch to the 30W LO switch position (out).
Forward power is displayed on the left-hand FORWARD meter scale. This scale is calibrated
from 0 to 30 watts and is read directly in the 30 watt position. In the 300W (HI) position the
forward power scale must be multiplied by 10. Reflected power is read on the right-hand
REFLECTED meter scale. This scale indicates 5 watts full scale when the 30W power
sensitivity is selected, and 50 watts full scale when the 300W power scale is selected. This
scale is also multiplied by 10 when using the 300W power position.
The most accurate power readings occur in the upper half of the meter scales. When trying to
measure power with a less than perfect match, the reflected power should be subtracted from
the forward power readings.
The SWR is read directly from red SWR curves that range from 1:1 to infinity. SWR is
measured by observing the point where the forward and reflected power needles cross. The
SWR is indicated by the red curve closest to the needle crossing point. No cumbersome or time
consuming SWR sensitivity adjustments are required with this meter.
The wattmeter has an internal lamp that backlights the meter scale. The lamp circuit requires
power from an external 12 Vdc source. The rear panel jack accepts a 2.5 mm plug with a
positive tip polarity. The negative lead is grounded inside the tuner. The METER LAMP ON /
OFF switch turns the meter lamp off and on.
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MFJ-945D Instruction Manual
Interconnections
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MFJ-945D Instruction Manual
Installation
SAFETY NOTE:
OPERATION.
1.
Locate the tuner in a convenient location at the operating position.
2.
Install the tuner between the transmitter and the antenna as shown in the diagram at left.
Use a coaxial cable (such as RG-8/U) to connect the transmitter (or amplifier) to the
connector marked TRANSMITTER on the rear of the tuner.
3.
Use a coaxial cable (such as RG-8/U) to connect the connector marked ANTENNA on the
rear of the tuner to the antenna. If use of more than one antenna is required, install a coax
switch between the tuner and the antennas.
For wire antennas an appropriate balun should be used between the coax from the tuner to
the wire feed to the antenna.
4.
A ground post is provided for an RF ground connection.
LOCATE THE TUNER SO THE REAR IS NOT ACCESSIBLE DURING
Using The MFJ-945D
CAUTION:
In any conventional "T" network tuner, maximum power handling and smoothest tuning occur
when the capacitance in the network is as large as possible. In this tuner the TRANSMITTER
and ANTENNA controls have maximum capacitance at position 0 (fully meshed), and
minimum capacitance at position 10 (fully open). Use the highest capacitance possible (lowest
number on the front panel scale) for each band for the smoothest tuning, highest efficiency, and
greatest power handling capability. The chart in the tuning instructions shows typical capacitor
settings that can be used for each amateur band.
Never Change The Antenna or Inductor Selector Switch Position While
Transmitting! Never Apply More Than 300 Watts To The MFJ-945D.
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