MFJ MFJ-986 User Manual
MFJ-986 Instruction Manual
MFJ-986 Differential-T
Introduction
The MFJ-986 roller inductor tuner is a 3000 watt PEP input, 1500 watt PEP output antenna tuner. These
power ratings are for load impedances of 35-500 ohms. The MFJ-986 is designed to match 50 ohm
output amplifiers, transmitters or transceivers to virtually any antenna. Peak and average forward power,
reflected power, and SWR are displayed on the MFJ-986's illuminated cross-needle meter.
The MFJ-986 Differential-T 3kW Tuner uses a differential capacitor to make tuning easy. It
continuously tunes all frequencies from 1.8 through 30 MHz, including MARS and all the WARC
bands. It will match dipoles, inverted-vee's, verticals, mobile whips, beams, random wires, and many
other antennas. The roller inductor lets you tune down your SWR to the absolute minimum. The MFJ986 also features a peak reading cross-needle, SWR/Wattmeter, and a new current balun that will work
with balanced open wire, twinlead, or twin-axial feedlines.
An internal six position antenna-selector switch selects two coaxial line outputs, a single wire linebalanced line output, or an external dummy load connection. The coaxial line outputs can be selected in
tuned (with tuner's matching network in line) or direct (no matching circuit) configurations.
Understanding Power Ratings
There are no standardized power rating systems for tuners. The names used (i.e. 3 kW Tuner) carry over
from the time when amplifiers were rated by peak power input, and not the true RF power output. For
example, the one thousand watt Johnson Matchbox was rated to handle a 1000 watt plate modulated AM
transmitter (four kilowatts PEP transmitter input and 3000 watts PEP RF output). The Heathkit SB-220
was called a two kilowatt amplifier, and the rated CW output was approximately 600 watts. Matching
tuners were called 2 kilowatt tuners, and these tuners safely handled 600 watts of CW power and 1200
watts PEP SSB.
Tuner
The FCC has changed the power rating system of amplifiers, and tuners no longer follow amplifier
power ratings. Most typical 1500 watt tuners remain able to safely handle 400-600 watts CW, and 600900 watts PEP SSB.
Load conditions and control settings also greatly affect the power handling capability of the tuner. Tnetworks typically handle more power on higher frequency bands into higher load impedances. The
worst operating condition for T-network tuners are low impedance capacitive reactance loads. Tnetwork tuners always handle the least power when operated on 160 meters into low impedance
capacitive reactive loads.
Follow the guidelines in this manual to avoid exceeding the ratings of this tuner.
Peak Reading SWR/Wattmeter
The cross-needle meter measures the peak or average FORWARD power, REFLECTED power, and
SWR. The illuminated cross-needle meter operates with the antenna tuning circuit in or out of line. The
wattmeter can be used without the tuning network by setting the ANTENNA SELECTOR to either of
the two COAX DIRECT positions of the antenna selector. The wattmeter is active in all positions of the
ANTENNA SELECTOR.
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MFJ-986 Instruction Manual
The meter's full scale forward and reflected power range is controlled by the left POWER swi tch that
selects 2000W (HI) or 200W (LO). If your transmitter runs more than 200 watts of output power, set
this switch to the 2000W HI (in) position. If your transmitter has less than 200 watts of output, set this
switch to the 200W LO switch position (out).
Peak envelope power (PEP) is measured when the PEAK or AVG POWER push button (right-hand
side) in placed in the PEAK (in) position. Peak power and average power values are equal with steady
unmodulated carriers, FSK, or FM. The meter reading on these modes will be the same whether the
PEAK / AVG button is pressed or released. On SSB, the PEP meter reading should be twice the average
power with two tone test modulation.
On SSB, the ratio of PEP to average power varies with different voice characteristics. With most voices,
the PEP reading is three to five times higher than the average voice power reading. The most accurate
peak envelope power readings are obtained only with sustained carrier, voice or two tone test
modulation. During normal voice modulation the wattmeter will typically indicate only 70% of the true
peak envelope power.
Forward power is displayed on the left-hand FORWARD meter scale. This scale is calibrated from 0 to
200 watts and is read directly in the 200 watt position. Each picket (scale mark) represents 5 watts
below 40 watts and 10 watts between 40 and 200 watts.
In the 2000W (HI) position the forward power scale must be multiplied by 10. Each picket represents 50
watts below 400 watts and 100 watts from 400 to 2000 watts.
The reflected power is read on the ri ght-hand REFLECTED meter scale. This scale indicates 50 watts
full scale when the 200W power sensitivity is selected, and 500 watts full scale when the 2000W power
scale is selected. This scale has a picket every watt below 20 watts and every 5 watts above 20 watts.
This scale is also multiplied by 10 when using the 2000W 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
reading.
The SWR is read directly from eleven 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, such as the optional MFJ-1312B power supply. The rear panel jack
accepts a 2.1 mm coaxial plug with the center conductor positive (+) and the sleeve negative (-). The
negative lead is grounded inside the tuner. The METER LAMP ON / OFF switch turns the meter lamp
off and on.
Antenna Selector
The ANTENNA SELECTOR allows you to select 2 rear panel SO-239 coaxial connectors, either direct
or through the tuner, an external dummy load connection, or a balanced feedline. The External Dummy
Load position lets you connect a dummy load, like the MFJ-250 Oil-filled VERSALOAD or the MFJ264 DRY LOAD for tuning.
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MFJ-986 Instruction Manual
Current Balun
The MFJ-986 uses a CURRENT balun to force equal currents into the two antenna halves so you get
minimum balanced currents. Minimum unbalanced current reduces field pattern distortion which
concentrates your power for a stronger signal.
Installation
CAUTION: Locate the tuner so the rear is not accessible during operation.
1. Locate the tuner in a convenient location at the operating position. If random wire or balanced line
operation is used, the ceramic feed through insulators may have high RF voltages. These voltages
can cause serious RF burns if the terminals are touched when transmitting. Be sure to locate the
tuner so these terminals can not be accidentally contacted during operation.
2. Install the tuner between the transmitter and the antenna. 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. If you are using a linear amplifier, then connect the output of the transceiver to the input of
the amplifier and the output of the amplifier to the MFJ-986.
3. Connect the antenna(s) to the tuner as follows:
A. Coaxial feedlines from your antenna(s) connect to the coax connectors COAX 1 and/or COAX
2. Coax lines may be fed direct or through the tuner as selected by the ANTENNA
SELECTOR switch.
B. Any antenna fed with a single wire should be connnected to the center hole of COAX 1 or
COAX 2.
C. Any balanced feedline (open wire, twinlead, or twin-axial lines) is connected to the ceramic
BALANCED LINE terminals on the back of the unit.
D. Use the GROUND post to ground the tuner to the transceiver, amplifier, and earth ground.
E. A dummy load could be connected to the EXTERNAL DUMMY LOAD connector. This will
give you the convenience of not having to connect or disconnect your dummy load from the
MFJ-986 every time you tune your radio. Use coax cable such as RG-8/U.
Operation
The roller inductor has a minimum inductance when the knob is rotated fully clockwise. When rotated
fully counter-clockwise, it has maximum inductance.
Note:
The MFJ
Air Core
™ Roller Inductor is designed with an exclusive
Self-Resonance Killer
™ that
keeps potentially damaging self-resonances away from your operating frequency. This feature is
switched in and out of the circuit with a built-in switch in the roller. Therefore, as you turn the
roller up and down, you may feel a bump. This is normal and you should not be alarmed.
1. First, rotate the knob fully counter clockwise. Then, use a small screwdriver or a pencil and push
the reset lever through the hole beside the counter. The counter should reset to "000"; this is the
maximum inductance. Turning the knob clockwise will now increase the numbers and decrease the
inductance of the MFJ-986.
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