MFJ-948 Versa Tuner II
MFJ-948 Versa Tuner II
Instruction Manual
General Information
The MFJ-948 is a 300 watt RF output power antenna tuner that will match any transmitter
or transceiver to virtually any antenna. Peak or average forward and reflected power and
SWR can be read on the illuminated cross-needle meter.
The MFJ-948 uses a "T" matching network and covers all bands between 160 and 10
meters. This network will tune dipoles, inverted-vees, verticals, mobile whips, beams,
random wires, and many other antennas. The MFJ-948 has rear panel connectors for
coaxial, single wire or two wire feedlines. A built-in 4:1 balun allows the use of balanced
open wire, twinlead, or twin-axial feedlines.
An internal eight position antenna-selector switch selects an external 50 ohm dummy load,
two separate coaxial line outputs, or a single wire line-balanced line output. All of these
functions can be selected in tuned (with tuner's "T" network in line) or bypassed (no tuning
circuit) configurations.
Peak Reading SWR/Wattmeter
The illuminated cross-needle meter measures the peak or average FORWARD power,
REFLECTED power, and SWR. The wattmeter is active in all the ANTENNA
SELECTOR positions. To use the wattmeter without the tuning circuit select one of the
ANTENNA SELECTOR positions under BYPASS.
The meter's full scale forward and reflected power range is controlled by the left METER
switch that selects 300W or 30W. If your transmitter runs more than 30 watts of output
power, set this switch to 300W (in position). If your transmitter has less than 30 watts of
output, set this switch to the 30W switch position (out).
Peak envelope power (PEP) is measured when the PEAK or AVERAGE METER 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 PEP power is
twice the average power with SSB two tone test modulation, and may be any amount larger
than the average power with SSB voice signals.
Forward power is displayed on the left-hand FORWARD meter scale. This scale is
calibrated from 0 to 300 watts and is read directly in the 300 watt position. Each picket
(scale mark) represents 25 watts between 300 and 100 watts, 10 watts between 100 and 10
watts, and has a single 5 watt picket below 10.
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MFJ-948 Versa Tuner II
In the 30W position the forward power scale must be divided by 10. Each picket
represents 1/2 watt below 1 watt, 1 watt from 1 to 10 watts, and 2.5 watts from 10 to 30
watts.
The 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 5 watts above 10
watts and at each watt below 10 watts. This scale is also divided by 10 when using the
30W switch position.
When trying to measure power with a less than perfect match, the reflected power should
be subtracted from the forward power readings. The most accurate peak power readings
are obtained only with a sustained carrier, voice or two tone test modulation. During
normal voice modulation the wattmeter will typically indicate 70% of the true peak power.
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-1312C power
supply. The rear panel jack accepts a 2.5 mm coaxial plug with a positive center pin
polarity. The METER LAMP ON / OFF switch turns the meter lamp off and on.
Antenna Selector
The ANTENNA SELECTOR switch has eight positions. From counter-clockwise to
clockwise the positions are: EXT. DUMMY LOAD, BALANCED or SINGLE WIRE
LINE, COAX 1, and COAX 2 with the tuner matching circuits in line, and the reverse
sequence from COAX 2 back to EXT. DUMMY LOAD with the antenna tuning circuits
bypassed.
NOTE: An external 50 ohm dummy load should be connected to the tuner when the
ANTENNA SELECTOR switch is in the EXT. DUMMY LOAD position.
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MFJ-948 Versa Tuner II
Interconnections
Installation
1. Locate the tuner in a convenient location at the operating position. Avoid placing the
tuner near microphones, speech processors, computers, TNC's or other RF sensitive
devices.
WARNING: If random wire or balanced feeders are connected directly to this tuner,
position the tuner so the rear panel terminals can not be accidentally
contacted by persons or conductors. When transmitting with random wire
or balanced lines, the rear panel feed-through insulators can operate with
high RF voltages. These voltages may cause serious RF burns. These
high RF voltages may also damage anything contacting or within a half
inch of the terminals.
2. Install the MFJ-948 between the transmitter and antenna as shown in the diagram
above. Use a 50 ohm coaxial cable to connect the transmitter or transceiver to the SO239 (UHF female) labeled TRANSMITTER on the back of the tuner.
3. Coaxial feedlines attach to the SO-239 (UHF female) connectors labeled COAX 1 and
COAX 2.
4. A random wire (longwire) antenna may be connected to the five-way binding post
marked WIRE. See the ANTENNA HINTS section for detailed suggestions on
bringing
a
single
wire
feeder
into
the
operating
position.
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MFJ-948 Versa Tuner II
5. A balanced feedline (twin lead, open wire, or twin-axial line) may be connected to
the two binding posts marked BALANCED LINE. Connect a jumper wire from the
WIRE binding post, as indicated by the dotted line on the MFJ-948, to one of the
BALANCED LINE posts. This connection activates the internal 4:1 balun.
NOTE: Do not connect wire and balanced antenna's to the MFJ-948 at the same time,
unless you want to apply power to both antennas at the same time. If a longwire
or single wire feeder is used, be sure to remove the WIRE to BALANCED LINE
jumper connection.
Using The MFJ-948
CAUTION: Never change the ANTENNA or INDUCTOR selector switch position
while transmitting! Never apply more than 300 Watts to the MFJ-948!
In any conventional "T" network tuner, maximum power handling and the smoothest
tuning occurs when the capacitance in the network is as large as possible. In this tuner the
TRANSMITTER and ANTENNA MATCHING controls have maximum capacitance at
position 0 (fully meshed), and minimum capacitance at position 10 (fully open). Be sure to
use the highest possible capacitance for each band. This will provide the smoothest tuning,
highest efficiency, and greatest power handling capability. The chart in the tuning
instructions shows typical capacitor settings that can be used for each amateur band.
The INDUCTANCE switch in the MFJ-948 has maximum inductance in position "A", and
minimum inductance in position "L". Less inductance is needed as the frequency is
increased. If too little inductance is used, the tuner may not match the load properly. If too
much inductance is used, the tuner will be "touchy" and power handling will be
compromised. The chart in the tuning instructions shows typical INDUCTANCE switch
settings for each amateur band.
NOTE: If your transmitter uses an adjustable output circuit it must be properly tuned into
a 50 ohm load at the operating frequency. Proper tuning can be accomplished by
placing the ANTENNA SELECTOR switch in the fully clockwise BYPASS
EXTERNAL DUMMY LOAD position. Adjust the transmitter according to the
manufacturer's instructions into the 50 ohm dummy load before adjusting the
tuner.
Most modern solid state transceivers do not require adjustments. If the transceiver has a
built in antenna tuner, be sure it is turned off or disabled.
After properly preparing the transmitter, place the MFJ-948 ANTENNA SELECTOR
switch in the desired antenna position in the BYPASS area. If the SWR is low (very little
or no reflected power), the tuner can be left in this position.
If the SWR is higher than desired, place the ANTENNA SELECTOR switch in the proper
TUNED area that selects the desired antenna. Adjust the tuner as described below to
obtain the best SWR. Do NOT change the transmitter's tuning (plate) or loading (antenna)
controls until after the tuner has been fully adjusted. The transmitter can be "touched up"
(if necessary) after the MFJ-948 is fully tuned.
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MFJ-948 Versa Tuner II
Adjustment Procedure
When using the MFJ-948 in receive only applications, adjust the MFJ-948 for the highest
"S" meter or signal level. The Tuning Chart can be used as a starting reference. To use the
MFJ-948 for transmitting, follow the steps below:
1. Select the 30W (out) METER switch scale. Place the PEAK AVG button in the AVG
(out) position. Turn the transmitter's power control fully down.
2. Position the TRANSMITTER and ANTENNA MATCHING controls and the
INDUCTOR SELECTOR switch in the bottom Tuning Chart position for the operating
frequency.
Tuning Chart
Freq. MHz
1.8
1.8
2.0
2.0
3.5
3.5
3.5
3.75
3.75
3.75
4.0
4.0
4.0
7.15
7.15
10.1
10.1
14.2
18.1
18.1
21.2
21.2
24.9
28.5
Transmitter
4-1/2
1
5-1/2
3
9
6
4
7
5
3
7
6
4
8
6
9
8
9-1/2
9-1/2
8-1/2
9-1/2
7
9
9-1/2
Inductor
A
B
A
B
B
C
D
C
D
E
C
D
E
F
G
G
H
H
I
J
J
K
K
K
Antenna
4-1/2
1
5-1/2
3
9
6
4
7
5
3
7
6
4
8
6
9
8
9-1/2
9-1/2
8-1/2
9-1/2
7
9
9-1/2
3. Apply just enough power on CW (or AM / FM / RTTY) to obtain noticeable deflection
on the reflected power meter.
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MFJ-948 Versa Tuner II
4. Carefully adjust the TRANSMITTER and ANTENNA MATCHING controls for the
lowest reflected power.
NOTE: These controls interact. Adjust the TRANSMITTER control for minimum, then
adjust the ANTENNA control for minimum SWR. Go back and forth between
these adjustments as many times as required until the lowest reflected power (best
SWR) is obtained.
5. If a perfect or very low SWR can not be obtained, STOP transmitting. Try the row of
chart settings just above the last one tried. Repeat from Step 3.
NOTE: If an SWR of 1:1 can not be obtained, move the switch towards the end of the
alphabet one position at a time. Repeat Steps 3 and 4 for each new INDUCTOR
switch position. Always use the lowest alphabetical setting possible.
CAUTION: Never transmit while changing the INDUCTOR SELECTOR.
6. Increase the transmitter power until the Forward power level is full scale (30 watts) and
observe the reflected power or SWR. If the reflected power and SWR are not
satisfactory, adjust the MATCHING controls again.
NOTE: If your transmitter can not reach 30 watts, set it to the maximum power available.
7. After a low SWR is obtained, the transmitter power may be increased to any value up
to 300 watts carrier or 300 watts PEP.
Your VERSA TUNER II will reduce the SWR of most antenna systems to 1:1. In some
cases, a perfect 1:1 SWR may not be obtainable. If this is the case, the length of the
antenna or the feedline can be changed slightly until a low SWR can be obtained. See the
antenna hints section.
In Case Of Difficulty
If this tuner fails to tune, please double check all connections and follow the tuning
procedures again. Be sure you are using enough inductance (lowest letter usable for band)
and have the capacitors open far enough (highest front panel number).
If this tuner arcs at the rated power levels, please double check all connections and follow
the tuning procedures again. Be sure you are using the least amount of inductance and the
greatest capacitance possible to match the load on the operating frequency.
NOTE: If this tuner arcs when operating on the 160 meter band, it may be necessary to
reduce transmitter output power.
If you are still unsuccessful, but the tuner does adjust and operate when switched to the
TUNED EXT. DUMMY LOAD position or another antenna, please read the Antenna
Hints text.
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MFJ-948 Versa Tuner II
Grounding Hints
To minimize RFI, single wire feedlines (such as used with Windom or longwire antennas)
should be kept away from other wiring. Radiation will be minimized if the single wire
feeder runs parallel and reasonably close to the wire that connects the tuner to the outdoor
ground. The antenna feed wire should be adequately insulated to prevent arcing or
accidental contact.
For safety, please use both dc and RF grounds. It is particularly important to have a good
RF ground while using a single wire feeder. When using a single wire feeder, the tuner
needs something to "push" against in order to force current into a single wire feedline. If a
good RF ground is not available, RF will usually find it's way back into the power line
(RFI), transmitter audio circuits (RF feedback), or the operator (RF burns).
Water pipes and ground rods provide good dc and ac safety grounds, but they are often
inadequate for RF grounding because they are single conductors. RF grounds work much
better when "spread out" over a large area with multiple connections directly to the
equipment ground point. Water pipes, heating ducts, and fences may work (especially if
they are all connected together with jumper wires), but the best RF grounds are radial
systems or multi-wire counterpoises that provide large low resistance surfaces for RF
energy. Ground rods by themselves are almost useless for dependable RF grounding.
RF and lightning travels on the surface of conductors. Braided or woven conductors have
high surface resistance to lightning and RF. Ground leads for RF and lightning should
have wide smooth surfaces. Avoid the use of woven or braided conductors in RF and
lightning grounds unless the lead needs to be flexible.
Antenna Hints
WARNING: For operator safety a good outside earth ground or water pipe ground
should ALWAYS be installed and connected to the case of the MFJ-948.
Make certain the safety ground also connects to the transmitter and other
station accessories. A wing nut post marked GROUND is provided for
ground connection(s).
Location
For the best performance, an end-fed longwire wire antenna should be at least one quarterwavelength long at the operating frequency. Horizontal antennas should be at least a half
wave long and high and clear of surrounding objects. While good RF grounds help the
signal in almost any transmitting installation, it is extremely important to have good RF
grounds with long wire or other Marconi antennas.
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MFJ-948 Versa Tuner II
Matching Problems
Most matching problems occur when the antenna system presents an extremely high
impedance to the tuner. When the antenna impedance is much lower than the feedline
impedance, an odd quarter-wavelength feedline converts the low antenna impedance to a
very high impedance at the tuner. A similar problem occurs if the antenna has an
extremely high impedance and the transmission line is a multiple of a half-wavelength.
The half-wavelength line repeats the very high antenna impedance at the tuner. Incorrect
feedline and antenna lengths can make an antenna system very difficult or impossible to
tune.
This problem often occurs on 80 meters if an odd quarter-wave (60 to 70 foot) open wire
line is used to feed a half-wave (100 to 140 foot) dipole. The odd quarter-wave line
transforms the dipole's low impedance to over three thousand ohms at the tuner. This is
because the mismatched feedline is an odd multiple of 1/4 wavelength long. The line
inverts (or teeter-totters) the antenna impedance.
A problem also occurs on 40 meters with this antenna example. The feedline is now a
multiple of a half-wave (60 to 70 foot) and connects to a full-wave high impedance antenna
(100 to 140 foot). The half-wave line repeats the high antenna impedance at the tuner. The
antenna system looks like several thousand ohms at the tuner on 40 meters.
The following suggestions will reduce the difficulty in matching an antenna with a tuner:
1. Never center feed a half-wave multi-band antenna with a high impedance feedline that
is close to an odd multiple of a quarter-wave long.
2. Never center feed a full-wave antenna with any feedline close to a multiple of a halfwave long.
3. If a tuner will not tune a multi-band antenna, add or subtract 1/8 wave of feedline (for
the band that won't tune) and try again.
4. Never try to load a G5RV or center fed dipole on a band below the half-wave design
frequency. If you want to operate an 80 meter antenna on 160 meters, feed either or
both conductors as a longwire against the station ground.
To avoid problems matching or feeding any dipole antenna with high impedance lines,
keep the lines around these lengths. The worst possible line lengths are shown in
brackets:
160 meter dipole; 35-60, 170-195 or 210-235 feet.
(Avoid 130, 260 ft)
80 meter dipole; 34-40, 90-102 or 160-172 feet.
(Avoid 66, 135, 190 ft)
40 meter dipole; 42-52, 73-83, 112-123 or 145-155 feet. (Avoid 32, 64, 96, 128 ft)
Some trimming or adding of line may be necessary to accommodate higher bands.
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MFJ-948 Versa Tuner II
WARNING: To avoid problems, a dipole antenna should be a full half-wave on the
lowest band. On 160 meters, an 80 or 40 meter antenna fed the normal
way will be extremely reactive with only a few ohms of feedpoint
resistance. Trying to load an 80 meter (or higher frequency) antenna on
160 meters can be a disaster for both your signal and the tuner. The best
way to operate 160 with an 80 or 40 meter antenna is to load either or
both feedline wires (in parallel) as a longwire. The antenna will act like a
"T" antenna worked against the station ground.
Technical Assistance
If you have any problem with this unit first check the appropriate section of this manual. If
the manual does not reference your problem or your problem is not solved by reading the
manual, you may call MFJ Technical Service at 601-323-0549 or the MFJ Factory at 601323-5869. You will be best helped if you have your unit, manual and all information on
your station handy so you can answer any questions the technicians may ask.
You can also send questions by mail to MFJ Enterprises, Inc., 300 Industrial Park Road,
Starkville, MS 39759; by FAX to 601-323-6551; or by email to mfj@mfjenterprises.com.
Send a complete description of your problem, an explanation of exactly how you are using
your unit, and a complete description of your station.
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MFJ-948 Versa Tuner II
Schematic
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