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Number 12 on your Feedbackeard
73 Review by Bill Clarke WA4BLC
The MFJ Differential-T
Antenna Tuner
MFJ Enterprises, Inc .
PO Box 494
Miss. State, MS 39762
601-323-5869
800-647-1800
Price Class: $240
A simpler, surer tuner.
Over the years, various configurations
have appeared on the market and in
magazine construction articles. Each was alleged
to be an improvement over all the others.
Some may have been, but the majority
just plain worked, at least after a fashion.
Antenna tuners can be thought of as variable
transformers that compensate for di fferences
between your antenna's actual RF
impedan ce and the impedance you r
transceiver would like to see at its coax connector.
They are the great equalizers in the
game of reducing the SWR seen at the rig .
In manual tuners, the operator must make
manual adjustments to controls on the antenna
tuner for it to do its job. Generally, tuner
adjustment consists of selecting an appropriate
inductance from a multi-position switch
and adjusting two variable capacitors until the
SWR indication at the transmitter is at 1:1 (or
as low as you can get it). SWR is indicated by a
meter that must be switched between forward
and reflected power to obtain comparative
readings.
New Version
A few mo nths ago, MFJ introduced the Model
986 Differential-T Antenna Tu ner. It is a 3
kW unit, providing very simplified (only two
controls) broadband t uning and incorporates
all the AF output metering you'll usually ever
need.
This tuner uses a single differential va riable
capacitor and a roller inductor in a 'r-Network
circuit. The usual tuner configuration consists
of two variable capacitors and a switch-selected
inductor in a Pi-network. The first obvious
improvement on this tuner is that there is one
less variable control for the operator to adjust.
Merely crank the variable inductor and turn
the capacitor control until the SWA is maximally
dipped.
Photo B. Rear panel provides teeanne.
ground, and 12 volt DC connectors.
42 73AmateurRadio . June,1989
A cross-needle SW R
meter is built into the
unit , giving si m u ltaneous
readi ngs of forward
and refl ected power.
A novel inclu sion is a
peak reading meter circuit
for SSB power output.
T his is a f eature
many SSB operators
want. Before now, that
meant the purchase of a
separate meter costing as much as $300.
Features
The Mod el 986 has:
• Continuous 3 kW fro m 1.8 through 30
MHz.
• A dual-range (200/2000 Watts) back-lighted
cross-needle meter providing SWR,
power, and peak reading functions.
• A g-positicn ceramic switch allows serection
between two coax feeds, a bal anced
line, or a dummy position (which could be
used for a bypassed antenna or dummy
load).
• An internal balu n lor balanced feedlines.
• Small size (10.75" x 4.5 " x 15.0").
Inside the Model 986
Before operating the MFJ-986, I removed
the covers. Inside I found a large roller inductor
with 62 turns on it, with an attached beltdriven
turns co unter. The capacitor is of high
quality construction with adequate spacing for
all legal ham output powe r (and then some).
The antenna switch is ceramic and the balun
is constructed 01 two st acked toroids wit h
windings positioned in a manner thai should
preclude any flash-over problems. Soldering
and mechanical work shews itself well , with no
obvious defects or poor quality workmanship
noted.
u••
The instructio n manual in cludes a small
chart for preliminary settings lor each band.
Using the recommended settings, I tuned my
75 meter loop for 3947 kHz. Tuning was done
by turning th e roller inductor crank until the
counter indicated 046. I then applied a little
power and continued inductor adjustment until
I located the lowest reflect ed power indication
point. Afterwards I adjusted the capacitor
for further reduction . I then fine tuned with
Photo A. Front view ofthe MFJ·986 showing the two tuning controls,
antenna selector switch, and the cross-needle meter.
these two controls. Total time for the first tuning
was nine seconds. I then logged the capacitor
setting from its vernier sca le and the
inductor setting from the inductor's counter.
This information is good as a starting point
when retuning the same antenna system for
the sa me resonant frequency later on .
I then tuned the same antenna lor each ham
band. There were no problems and all settings
were logged . Using the loop has presented a
problem for my othertuneron 1Oand 15 meters,
but the MFJ-986 tuned it with no problems.
The last operational test was to reset the
tuner for each band per my notes based on the
previous settings. The unit proved to be very
resetable, although some fine tweaking was
needed on 75 and 160 meters.
Varied Comments
The turns counter is rel ative only, but it's
completely accurate for logging and resetting.
Havin g the built-in cross-needle forward/
reflected meter is a real advantage over other
tuners. The peak reading meter is ideal for
tnose of us attempting 10 run a full gallon.
The antenna selector switch should be selfgrounding.
As it's set up, feedlines not in use
are not grounded.
Due to the design 01 the differential-T circuit,
it is possible to gel a low SWR at only one
combination of the controls. No more worry
about too m uch inductance.
Highly Recommended
Pricewlse, the unit is a bargain. Yo u get a
cross-needle meier thai includes simultaneous
forwardfreflected indications and selectable
peak reading, an antenna selector
switch, and a 3 kW tuner all in one box. Yet the
unit sells for over $100 less than its nearest
competition, which is the MFJ-989B.
Do I recommend this tuner? Yes! It is
economical, solid, and easy to use. DI