MFJ Versa Tuner II 941E
User Manual: Pdf MFJ--941E Versa Tuner II
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MFJ-941E Versa Tuner II
1
MFJ VERSA TUNER II
GENERAL INFORMATION:
The MFJ-941E is designed to match virtually any transmitter to any antenna, including dipoles,
inverted-vees, verticals, mobile whips, beams, random wires, and others fed by coax lines,
balanced lines or a single wire. An eight-position antenna-selector switch provides versatile
antenna selection. The MFJ-941E will handle up to 300 Watts of transmitter RF output power.
The MFJ-941E employs a cross-needle meter so forward power, reflected power and SWR may
be read simultaneously.
CROSS-NEEDLE SWR/WATTMETER:
The meter on the MFJ-941E may be used alone or with the tuner. Set the ANTENNA
SELECTOR to COAX 1 DIRECT or COAX 2 DIRECT to use the meter without the tuner. The
MFJ-941E utilizes a cross-needle meter so FORWARD power, REFLECTED power and SWR
may be read simultaneously in two ranges. FORWARD power may be read by setting the power
range switch to HI (300 Watts) or LO (30 Watts). Next read the power level on the FORWARD
SCALE. REFLECTED power is shown simultaneously on the REFLECTED SCALE. SWR is
determined by observing the intersection point of the two needles. No SWR sensitivity adjustment
is needed to read SWR. The HI range is 300 Watts FORWARD and 60 Watts REFLECTED.
The LO range is 30 Watts FORWARD and 6 Watts REFLECTED. The difference between the HI
and LO scales readings is a factor of 10.
The meter lamp can be powered by a 12 Vdc source, such as the optional MFJ-1312B power
supply. Use a 2.1mm coaxial plug with the center conductor of the plug connected to the positive
and the sleeve connected to ground. The METER LAMP ON/OFF switch will activate the meter
lamp.
ANTENNA SELECTOR:
The ANTENNA SELECTOR switch utilizes eight positions. They are DUMMY LOAD, BALANCE
LINE, COAX 1, and COAX 2 positions for both BYPASS (direct) and TUNED (through) the tuner
operation. An external 50 Ohm dummy load can be connected to the EXT LOAD connector
located at the rear of the tuner. Do not continuously key into the dummy load for more than 2
minutes at a time.
CAUTION: Never use the MFJ-941E for OVER 300 Watts of RF output power, even in the
DIRECT or DUMMY LOAD positions.
MFJ-941E Versa Tuner II
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INSTALLATION:
1. The tuner should be placed in a location where operation will be convenient. The ceramic
feed through insulators will have high RF voltages if random wire or balanced line operation
is used. These voltages can cause serious RF burns if touched when transmitting.
NOTE
: Locate the tuner so the rear is not accessible during operation.
2. The MFJ-941E should be installed between the transmitter and antenna. A coaxial line
should be connected to the transmitter and the SO-239 coax TRANSMITTER connector on
the back of the tuner.
3. One or two coax-fed antennas may be connected to the SO-239 coax connectors marked
COAX 1 or COAX 2. Coax 1 and Coax 2 antennas may be connected directly to the
transmitter, bypassing the tuner, by setting the ANTENNA SELECTOR switch to COAX 1
DIRECT or COAX 2 DIRECT, respectively.
4. A random wire antenna may be connected to the five-way binding post marked WIRE. The
random length wire should be long, high, and as clear of surrounding objects as possible.
For optimum operation, the wire antenna should be a quarter wave-length or longer at the
operating frequency. Do NOT ground the random wire antenna. The tuner should be well-
grounded to the transmitter. A binding post marked GROUND is provided for ground
connections.
5. A balanced line-fed antenna may be connected to the two five-way binding posts marked
BALANCED LINE. A jumper wire from the WIRE binding post, as indicated by a dotted line
on the MFJ-941E, should be connected to one of the posts of the BALANCE LINE. This
couples the MFJ-941E to the balanced line through a 4:1 balun.
NOTE
: Either a balanced line or a random wire antenna may be connected to the MFJ-941E at
one time. If a random length wire is used, care should be taken to assure that no
jumper wire is between the WIRE binding post and the BALANCED LINE.
6. An external 50 Ohm dummy load may be connected to the EXT. DUMMY LOAD connector
located at the rear of the tuner.
MFJ-941E Versa Tuner II
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MFJ-941E Front Panel
USING THE MFJ-941E:
The INDUCTOR switch on the MFJ-941E represents maximum inductance at position "A" and the
minimum inductance at position "L". Lower inductance is needed at higher frequencies than at
low frequencies for the same impedance. The TRANSMITTER and ANTENNA controls represent
maximum capacitance at position 10. For optimum operation of the MFJ-941E, the transmitter
must be tuned to a 50 Ohm output impedance at the frequency of operation. The ANTENNA
SELECTOR switch should be set to DUMMY LOAD for tuning up the transmitter.
NOTE: The transmitter should always be tuned at a low output power.
After the transmitter is properly tuned, the ANTENNA SELECTOR should be set to the desired
antenna and the tuner adjusted for a minimum SWR as described below. DO NOT readjust the
transmitter loading control setting after loading it to 50 Ohms.
NOTE: When using the MFJ-941E for receiving only, tune as described in Steps 1 and 2.
TUNER ADJUSTMENT:
1. Set the TRANSMITTER and ANTENNA controls to 5. In this position the capacitors are
half-open.
2. Rotate the INDUCTOR control until maximum noise is obtained with the transceiver in the
receiving mode.
CAUTION: Do not operate the ANTENNA selector switch while transmitting!.
3. While transmitting a steady state carrier (CW) alternately adjust TRANSMITTER and
ANTENNA controls for minimum SWR. Since both controls interact, the two controls can
best be adjusted by turning the TRANSMITTER control one small increment at a time, then
rotating the ANTENNA control for minimum SWR. Repeat this process until minimum
SWR is obtained.
MFJ-941E Versa Tuner II
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4. If a SWR of 1:1 is not achieved, the INDUCTOR control should be increased or decreased
and Step 3 repeated. If arcing should occur between capacitor plates, the INDUCTOR
control should be increased or decreased by one position, and Step 3 repeated. If SWR of
1:1 cannot be achieved, Step 3 should be repeated for each INDUCTOR control position.
5. After a minimum SWR is achieved, transmitter power may be increased to 300 Watts. The
VERSA TUNER II will reduce the SWR of most feed systems to 1:1. In some cases, a 1:1
SWR is not achievable. In such cases, the length of the antenna may be increased or
decreased to improve SWR.
6. SWR of 1:1 may occur at more than one set of control settings on the MFJ-941E. When
an SWR of 1:1 is obtained, make sure that the transmitter power is relatively high. If
transmitter power has decreased substantially, try another INDUCTOR control setting and
repeat Step 3.
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. If you are still
unsuccessful, please read the Antenna Matching Problems text below.
NOTE: If this tuner arcs when operating on the 160 meter band, it may be necessary to reduce
transmitter output power.
Antenna 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.
MFJ-941E Versa Tuner II
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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 half-wave
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
d
35-60, 170-195 or 210-235 feet. [ Avoid 130, 260 ft ]
80 meter
d
34-40, 90-102 or 160-172 feet. [ Avoid 66, 135, 190 ft ]
40 meter
d
42-52, 73-83, 112-123 or 145-155 feet. [ Avoid 32, 64, 96, 128 ft ]
NOTE: Some trimming or adding of line may be necessary to accommodate higher bands.
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 601-323-5869. 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 Facsimile to 601-323-6551; or by email to techinfo@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.
Schematic:
