MFJ -974H Instruction Manual

MFJ Balanced Line Tuner
Introduction
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.
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 T-
Network 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.