MFJ MFJ-976 User Manual

MFJ Legal Limit Balanced Line Tuner

Introduction

The MFJ-976 balanced line antenna tuner is a fully balanced true, balanced line antenna tuner, providing superb current balance throughout a very wide matching range – 12 to 2000 ohms – and frequency range of 1.8 through 30 MHz, continuous. It is rated at 1500 watts PEP SSB and CW.

The MFJ-976 is designed to match 50 ohm output transmitters or transceivers to virtually any antenna. Peak and average forward power, reflected power, and SWR are displayed on the MFJ-976's illuminated cross-needle meter.

The MFJ-976 is a fully balanced wide range T-Network using four 500pF air variable capacitors for efficient operation on 160 meters. The MFJ-976 also provides excellent performance on 10 meters using MFJ’s exclusive

Self-Resonance Killer

ensure electrical balance and are centrally located in the cabinet for better isolation.

A 1:1 current balun is placed on the low impedance 50 ohm input (transmitter) side to convert the balanced TNetwork to unbalanced operation. The balun is made of 50 ferrite beads on RG-303 Teflon™ coax to give exceptional and efficient isolation. It stays cool even at the tuner’s maximum rated power.

The MFJ-976 will match virtually any balanced-line-fed antenna: dipoles, inverted-vees, verticals, mobile whips, yagi beams, quad loops, horizontal loops, sterba curtain arrays, random wires, and many other antennas. The MFJ-976 has rear panel connectors for coaxial and single or two wire feedlines. The built-in balun works with balanced open wire, twinlead, or twin-axial feedlines.

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 to 600 watts CW, and 600 to 900 watts PEP SSB.

Load conditions and control settings also greatly affect the power handling capability of tuners. 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.

™

and low minimum capacitiance. The tuning components are mounted symmetrically to

MFJ-976 Legal Limit Fully Balanced Antenna Tuner Instruction Manual

Peak Reading SWR/Wattmeter

The cross-needle meter measures the peak or average FORWARD power, REFLECTED power, and SWR, and is always operating since the tuner is always in line.

The meter's full scale forward and reflected power range is controlled by the POWER switch that selects HI (1500 watts) or LO (300 watts). If your transmitter or amplifier runs more than 300 watts of output power, set this switch to the 300 watt HI (in) position. If your transmitter or amplifier has less than 300 watts of output, set this switch to the 300 watt LO (out) switch position. Peak envelope power (PEP) is measured when the PEAK or AVG power push button is placed in the PEAK (in) position. Peak power and average power values are equal with steady un-modulated carriers, FSK, or FM. The meter reading on these modes will be the same whether the PEAK/AVG button is in or out. 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 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 300 watts. In the HI position, each picket (scale mark) represents 5 watts below 10 watts, 10 watts between 10 and 100 watts, and 25 watts between 100 and 300 watts. In the LO power position, full-scale meter reading is 30 watts; divide the meter reading by 10 for the correct value.

Reflected power is read on the right-hand REFLECTED meter scale. This scale indicates 60 watts full scale when the 300W power sensitivity is selected, and 6 watts full scale when the 30W power scale is selected. This scale has a picket every watt below 10 watts and every 5 watts above 10 watts.

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-1312D 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 LAMP ON/OFF switch turns the meter lamp off and on.

MFJ-976 Legal Limit Fully Balanced Antenna Tuner Instruction Manual

Installation

ocate the tuner so the rear is not accessible during operation.

1. Place the tuner in a convenient location at the operating position. The terminals on the rear of the tuner will have high RF voltages present during operation. These voltages can cause serious RF burns if the terminals are touched while transmitting. Be sure to locate the tuner so these terminals cannot accidentally be contacted during operation.

2. Install the tuner between the transmitter and the antenna. Use a 50-ohm coaxial cable (such as RG-8/U) to connect the transmitter to the connector marked TRANSMITTER on the rear of the tuner (see Figure 2).

3. Connect the antenna feedline to the tuner as follows:

NOTE: The MFJ-976 can be used to tune antennas fed with a balanced or unbalanced feedline. Two jumpers are installed at the factory for unbalanced operation. These must be removed before the tuner is used to tune an antenna fed with balanced feedline line.

A. Balanced feed line

terminals on the left rear panel of the tuner (see Figure 2).

i. Loosen the wing nuts and remove the jumpers on the right side of the rear panel. Figure 2

shows the location of these jumpers with dashed lines. The jumpers may remain connected to the ground terminal but make certain they cannot make contact with one of the connection terminals on the fiberglass plate. See Figure 2.

ii. Connect your balanced feedline to the balanced line terminals located on the left side of the

rear panel. See Figure 2.

(open wire, twin lead, or twin-axial line) is connected to the BALANCED LINE

CAUTION

: Route all single and random wire antennas safely to prevent RF burn hazard.

Note B. Coaxial (un-balanced)

connected to the tuner with coax operate as unbalanced antennas. Two jumpers must be installed on the rear panel for operation. These jumpers are installed at the MFJ Factory. Figure 2 indicates the location of these jumpers with dashed lines on the right side of the rear panel.

C. Random Wire or Single Wire Line

located on the left of the rear panel as shown in Figure 2.

i. Connect a random or long wire to the top left terminal shown in figure 2.

ii. Connect the ground terminal to a suitable earth ground.

4. A ground post is provided for an RF ground connection.

feedlines connect to the coax connector labeled ANTENNA. Antennas

fed antennas must be connected to the top balanced line terminal

Transmitter

Figure 1 Block Diagram

MFJ-976

RF ground

MFJ-976 Legal Limit Fully Balanced Antenna Tuner Instruction Manual

ANTENNA

BALANCED LINE

RANDOM WIRE

Connect the jumpers across the terminals to the right for

unbalanced operation. The

broken lines indicate the

jumper connections

POWER

12VDC

GROUND

TRANSMITTER

Operation

The roller inductor has maximum inductance at about 000 and minimum inductance at 125 on the reference counter. The capacitors have maximum capacitance at 0 and minimum capacitance at 10. In simple language, as the frequency is increased, the normal control positions rotate clockwise just like on other equipment.

Note: Always use the most capacitance (settings closest to 0) for the most power handling and the least

loss. Use the smallest possible inductance (the lowest number possible) also.

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.

Increase the number the controls are set at (on a given frequency) to INCREASE the matching range. Remember, this LOWERS the efficiency and power handling capability of the tuner.

1. Tune the exciter into a dummy load (most solid state transmitters are "pre-tuned" to 50 ohms and do

not require adjusting with the dummy load).

2. Position the ANTENNA and TRANSMITTER controls at the following settings:

The ANTENNA and TRANSMITTER capacitors have maximum capacitance at setting 0 and minimum capacitance at setting 10.

Maximum tuner efficiency is achieved with the most capacitance (settings closest to 0) and the least possible inductance (higher letters of the alphabet). This efficiency translates into best power handling, broadest bandwidth, and least power loss. The goal is always to operate the tuner at the lowest Q for the LC (inductance/capacitance) circuit.

When necessary to increase the tuner’s matching range, set the ANTENNA or TRANSMITTER control to a higher number, or dial in a higher number on the INDUCTOR counter, but remember that this lowers the efficiency and power handling capability of the tuner.

MFJ-976 Legal Limit Fully Balanced Antenna Tuner Instruction Manual

Adjustment Procedure

1. If the exciter or transmitter must be tuned, do it into a 50-ohm dummy load.

2. Set the ANTENNA, INDUCTANCE, and TRANSMITTER controls at the settings in the Suggested

Tuning Chart on Page 9. At these settings signals (or at least some noise) should be audible in your receiver.

3. Place the POWER switch in the LO position.

4. Transmit at low power (20 or 25 watts should suffice).

5. Rotate the ANTENNA and TRANSMITTER controls for maximum forward power and minimum

reflected power (ideal). If full forward power and zero reflected power cannot be obtained, turn the

INDUCTOR one letter lower in the alphabet and try again. If this does not achieve a match, turn the INDUCTOR control two letters lower and try.

Note

: Never change the INDUCTOR setting while power is applied to the tuner!

6. Once full forward power and zero reflected power are achieved, always try advancing the INDUCTOR

setting by one letter (higher) and tune for a match. If a match can be achieved at two different

INDUCTOR settings, the “higher letter” (lower inductance) setting is better.

7. Adjust transmitter to full output and touch up the tuner settings if needed.

8. For quick retuning of the tuner, record the INDUCTOR and CAPACITOR settings for each band (see

the Logged Tuning Chart, Page 10).

: Maximum power handling is achieved when both the ANTENNA and TRANSMITTER controls

Note

are set at the lowest possible number, and the INDUCTOR control is set at the highest possible letter that permits matching the antenna. Following this guideline will ensure maximum power handling capability and efficiency, and the smoothest tuning.

• Never operate the tuner with the top removed. Contact with the components inside the tuner while transmitting will result in painful RF burns.

• Never rotate the INDUCTANCE switch while transmitting. Doing so may permanently damage the switch.

• Locate the tuner so that the rear terminals are not accessible during operation.

WARNING:

The balanced line connectors may have high voltage on them while transmitting.

• Disconnect all antennas from the tuner during lightning storms.

• Always tune with low power (i.e. less than 100 watts). Apply maximum power

only after tuning up.

• Be sure to adjust the SWR before transmitting at full power. Do not transmit with a high SWR for extended periods.

Operating Notes

While this tuner is designed to have as large a tuning range as possible, there are limits to the tuning range of the capacitors. Some antennas may require more or less capacitance than is available. In these cases, the SWR may not be reduced to 1:1. If the SWR is higher than the limits of your rig, try changing the length of the antenna or feed line to bring the impedance within the tuning range of the tuner.

When adjusting the tuner, use the lowest number on the ANTENNA and TRANSMITTER controls and the highest letter on the INDUCTOR control that produces a good SWR. This will reduce tuner losses and increase the power rating of the tuner.

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