MFJ MFJ-974, MFJ-974H User Manual

MFJ Balanced Line Tuner

Introduction

The MFJ-974H 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 – 1.8 through
54 MHz (3.5 through 54 MHz for the MFJ-974), continuous. It is rated at 300 watts SSB PEP transmitter output
and 150 watts CW transmitter output.

The MFJ-974H 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-974H's illuminated cross-needle
meter.

The MFJ-974H is a fully balanced wide range T-Network using four gear driven 1000 volt air variable capacitors
and a high-Q air wound tapped inductor for 80 through10 Meters. Separate inductors are used for 6 and 160
Meters. The tuning components are mounted symmetrically to 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-974H 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-974H 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.

Special Note: This manual applies in all

respects to both the MFJ-974H and MFJ-974,

except that in the case of the MFJ-974 all

references to 160 Meters should be ignored.

MFJ-974/974H Balanced Line Antenna Tuner Instruction Manual

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, 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 (300
watts) or LO (30 watts). If your transmitter runs more than 30 watts of output power, set this switch to the 300
watt HI (in) position. If your transmitter has less than 30 watts of output, set this switch to the 30 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 unmodulated 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.

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MFJ-974/974H Balanced Line Antenna Tuner Instruction Manual

Installation

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

1. Place the tuner in a convenient location at the operating position. With random wire or balanced feedlines,
the 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 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.

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

A. Coaxial feedlines connect to the coax connector labeled ANTENNA (a jumper wire is required between

the lower, red binding post and the ground terminal).

B. Random wire or single wire line antennas should be connected to the top red connector on the back of

the unit (jumper wire required between the lower, red binding post and the ground terminal).

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

Note

CAUTION

C. Balanced feedline (open wire, twinlead, or twin-axial line) is connected to the BALANCED LINE

terminals.

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

Transceiver or

Transmitter

MFJ-974H/974

-

Block Diagram

Figure 1

RF ground

Operation

The MFJ-974H uses three inductors for maximum efficiency. A 2½-inch diameter high-Q air-wound inductor is
tapped for flexibility and is used on frequencies from 3.5 to 30 MHz. For 160 Meters (MFJ-974H only) an
additional inductance, made of Teflon™ insulated wire wound on two powdered-iron cores, is switched in by the
front panel 160 METER pushbutton. Front panel inductor switch position L automatically selects a 6-Meter only
inductance (bypassing the larger air-wound coil). Inductance switch position A is the greatest inductance,
position L the least. The inductance decreases as the knob is rotated clockwise.

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.

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