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MFJ-986 |
Instruction Manual |
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 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.
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. 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.
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 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 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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