MFJ Differential-T Tuner -986 User manual

INSTRUCTION MANUAL
CAUTION: Read All Instructions Before Operating Equipment
COPYRIGHT 2013 MFJ ENTERPRISES, INC.
C
VERSION 7A
MFJ ENTERPRISES, INC.
300 Industrial Park Road
Starkville, MS 39759 USA
Tel: [phone] Fax: [phone]
Requires 9VDC Battery or 12VDC Source for Meter Operation
Model MFJ-986

MFJ-986Differential-TTuner Instruction Manual
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DISCLAIMER
Information in this manual is designed for user purposes only and is not intended to supersede
information contained in customer regulations, technical manuals/documents, positional handbooks, or
other official publications. The copy of this manual provided to the customer will not be updated to
reflect current data.
Customers using this manual should report errors or omissions, recommendations for improvements, or
other comments to MFJ Enterprises, 300 Industrial Park Road, Starkville, MS 39759. Phone: (662) 323-
5869; FAX: [phone]. Business hours: M-F 8-4:30 CST.

MFJ-986Differential-TTuner Instruction Manual
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MFJ-986 Differential-T ™ Tuner
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 MFJ-
986 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 line-
balanced 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.
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 600-
900 watts PEP SSB.
Load conditions and control settings also greatly affect the power handling capability of the tuner. T-
networks 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. T-
network 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.

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Power Requirement
The upgraded MFJ-986 requires a power source to run its SWR/Wattmeter circuitry. Power may come
from one of two sources:
1.) 9-Volt Battery:To install a 9-volt battery, access the internal holder and snap clip by removing the
12 cabinet screws and lifting off the tuner cover. The holder is mounted on the upper right corner of the
front panel. We strongly recommend installing a battery, even if you intend to use an external power
source. Doing so provides backup voltage to operate the meter and also prevents the battery snap clip
from floating loose inside the cabinet where it could contact high RF potentials. If you elect to omit the
battery, make sure the snap clip is anchored solidly to the battery holder where it was wedged for
shipping.
To extend battery life, the tuner has built-in RF-sensing that applies power to the meter electronics only
when a transmit signal is applied. After a short period of transmitter inactivity, power is removed
automatically.
Important Note: The 9-V battery powers only the meter electronics. It does not power the meter LEDs.
You must supply external DC to illuminate the SWR meter.
2.) 12-VDC External Power:The Meter Lamp power jack is located above the Transmitter connector
on the rear panel. Apply power from any filtered dc wall adapter (MFJ-1312D or equivalent), or use
your station's 13.8-Vdc supply. A cord fitted with a 2.1-mm plug is supplied with the tuner for this
purpose. If you make your own supply cord, connect +12 volts to the center pin and ground the outer
sleeve, as shown below:
+
-+
-2.1 mm
Important Note: You don't need to disconnect the battery when using external dc power. It is
protected from back-feed and won't be damaged by the other source.
Important Warning: Never connect an ac-voltage or positive-ground source
to the Meter Lamp jack or permanent damage may result!
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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The meter's full scale forward and reflected power range is controlled by the left POWER switch 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 right-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 MFJ-
264 DRY LOAD for tuning.

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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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2. Tune the exciter into a dummy load (most solid state transmitters are "pre-tuned" to 50 ohms and do
not require loading with the dummy load).
3. Select the desired antenna with the ANTENNA SELECTOR.
4. Position the capacitor control at the following settings:
160 M:
80 M:
75 M:
40 M:
30 M:
20 M:
17 M:
15 M:
12 M:
10 M:
Then place the POWER switch in the LO and AVG meter positions.
5. Starting from the minimum INDUCTOR position (highest number), turn the INDUCTOR control to
add more inductance while applying a slight amount of power (less than 25 watts). Tune for the
lowest reflected power and maximum forward power (lowest SWR). The lowest reading should
occur somewhere above the following recommended inductor positions.
Tuning Chart
Frequency (MHz) Capacitor Value Inductor (counter indicator) Load (ohms)
1.8 50
1.8 600
2.0 50
2.0 600
3.5 50
3.5 600
3.75 50
3.75 600
4.0 50
4.0 600
7.15 50
7.15 600
10.1 50
10.1 600
14.2 50
14.2 600
18.1 50
18.1 600
21.2 50
21.2 600
24.9 50
24.9 600
28.5 50
28.5 600

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6. Adjust the CAPACITOR control for lowest reflected power (and SWR).
7. If the SWR did not reach 1:1 in step 5, turn the INDUCTOR control clockwise (to a higher number)
again and repeat Step 5. For maximum power handling and efficiency, always adjust the capacitor
to the lowest front panel number that allows proper antenna matching. This insures maximum
power handling and lowest power loss in the tuner.
8. Advance the power (do not exceed 100 watts) and, if necessary, touch up the CAPACITOR and
INDUCTOR controls for minimum reflected power and maximum forward power (lowest SWR).
Remove the power.
9. After adjusting the tuner for minimum SWR, the amplifier may be turned on. The METER switch
should be placed in the HI position, and the amplifier tuned according to the manufacturer's
instruction.
10. For quick retuning of the tuner, record the INDUCTOR and CAPACITOR settings for each band.
Note: Maximum power handling occurs when both the CAPACITOR AND INDUCTOR are set at
the lowest front panel numbers for capacitance and highest inductance that permit matching
the antenna. Following this guideline will insure maximum power handling capability and
efficiency, and the least critical tuning adjustments.
Operating Notes
1. While this tuner is designed to have as large a tuning range as possible, there are limits to the tuning
range of the capacitor. Some antennas may require more or less capacitance than the controls have.
In these cases, the SWR may not be reduced to 1:1. If the SWR is higher than the limits on your
rig, try changing the length of the antenna or feedline to bring the impedance within the tuning
range of the tuner.
2. When adjusting the tuner, use the lowest amount of CAPACITANCE and INDUCTANCE that
produces a good SWR. This will reduce tuner losses and increase the power rating of the tuner.
3. If the INDUCTOR counter slips out of calibration, turn the INDUCTOR fully counter clockwise.
Then, with a small screw driver or pencil, push the reset button through the hole to the right of the
counter. This resets the counter to "000".
4. The following table can be used as a guide to tune your MFJ-986 to a specific operating band.
When the MFJ-986 is properly tuned, the counter reading should approximately match the number
on the table. If the counter is too far from the number on the table, then the tuner is not properly
tuned. If this is the case, repeat the tuning process.
BAND INDUCTOR
160 M
80 M
40 M
20 M
15 M
10 M
INITIAL TUNE-UP INDUCTOR SETTINGS

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WARNING:
1. Never operate the tuner with the top removed. Contact with the components inside the tuner while
transmitting will result in painful RF burns.
2. Never rotate the ANTENNA SELECTOR switch while transmitting. Doing so may permanently
damage the switch.
3. Locate the tuner so that the rear terminals ARE NOT accessible during operation. The single wire
and balanced line connections may have high voltage while being used.
4. Disconnect all antennas from the tuner during lightning storms.
5. Always tune with low power (i.e. less than 100 watts). Apply maximum power only after tuning up.
6. Be sure to adjust the SWR before transmitting at high power (above 100 watts). Do Not transmit
with a high SWR for extended periods of time.
In Case Of Difficulty
If the tuner fails to tune, please double check all connections and follow the tuning procedures again.
Be sure you are using enough inductance and have the capacitors open far enough.
If the tuner arcs, please double check all connections and follow the tuning procedures again. The
power rating of this tuner is 1500 watts PEP RF power. If this tuner arcs at these power levels, make
sure you are using the least amount of inductance and the greatest capacitance possible that still allows
matching the load on the operating frequency.
If you are still unsuccessful, but the tuner does adjust and operate when switched to a dummy load or
another antenna, please read the ANTENNA HINTS section.
Grounding Hints
To minimize RFI, single wire feedlines (such as used with Windom or longwire antennas) should be
kept away from other wiring. Radiation will be minimized if the single wire feeder runs parallel and
reasonably close to the wire that connects the tuner to the outdoor ground. The antenna feed wire
should be adequately insulated to prevent arcing or accidental contact.
For safety, please use good dc and RF grounds. It is particularly important to have a good RF ground
while using a single wire feeder. When using a single wire feeder, the tuner needs something to "push"
against in order to force current into the single wire feedline. If a good RF ground is not available, RF
will usually find it's way back into the power line (RFI), transmitter audio circuits (RF feedback), or the
operator (RF burns).
Water pipes and ground rods provide good dc and ac safety grounds, but they are often inadequate for
RF grounding because they are single conductors. Ground rods by themselves are almost useless for
dependable RF grounding.

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RF grounds work much better when "spread out" over a large area, especially when they employ
multiple connections directly to the equipment ground point. Water pipes, heating ducts, and fences
may work (especially if they are connected together with multiple wires), but the best RF grounds are
radial systems or multi-wire counterpoises. Radials and counterpoises provide large, low resistance
surfaces for RF energy.
RF and lightning travels on the surface of conductors. Braided or woven conductors have high surface
resistance to lightning and RF. Ground leads for RF and lightning should have wide smooth surfaces.
Avoid the use of woven or braided conductors in RF and lightning grounds unless the lead needs to be
flexible.
CAUTION:For operator safety, a good outside earth ground or water pipe ground should always be
installed and connected to the case of the MFJ-986. Make certain the safety ground also
connects to the transmitter and other station accessories. A wing nut post marked
GROUND is provided for ground connection(s).
Antenna System Hints
For the best performance, an end-fed longwire wire antenna should be at least one quarter-wavelength
long at the operating frequency. Horizontal dipole antennas should be at least a half-wavelength long
and located as high and clear as possible. While good RF grounds help the signal in almost any
transmitting installation, it is extremely important to have good RF grounds with long wire or other
Marconi style antennas.
Matching Problems
Most matching problems occur when the antenna system presents an extremely high impedance to the
tuner. When the antenna impedance is much lower than the feedline impedance, an odd quarter-
wavelength feedline converts the low antenna impedance to a very high impedance at the tuner. A
similar problem occurs if the antenna has an extremely high impedance and the transmission line is a
multiple of a half-wavelength. The half-wavelength line repeats the very high antenna impedance at the
tuner. Incorrect feedline and antenna lengths can make an otherwise perfect antenna system very
difficult or impossible to tune.
One example where this problem occurs is on 80 meters when an odd quarter-wave (60 to 70 foot) open
wire line is used to feed a half-wave (100 to 140 foot) dipole. The odd quarter-wave line transforms the
dipole's low impedance to over three thousand ohms at the tuner. This is because the mismatched
feedline is an odd multiple of 1/4 wavelength long. The line inverts (or teeter-totters) the antenna
impedance.
A problem also occurs on 40 meters with this same antenna example. The feedline is now a multiple of
a half-wave (60 to 70 foot) and connects to a full-wave high impedance antenna (100 to 140 foot). The
half-wave line repeats the high antenna impedance at the tuner. The antenna system looks like several
thousand ohms at the tuner on 40 meters.
This places enormous strain on the balun and the insulation in the tuner, since voltages can reach several
thousand volts. This can cause component arcing and heating.