MFJ MFJ-945E User Manual

MFJ-945E Mobile Tuner

MFJ-945E MOBILE TUNER

General Information:

The MFJ-945E Mobile Tuner is a compact tuner designed for m obile operation in cars, trucks,
RV's, and boats. Covering
mobile whip antenna. It will also match almost any antenna for hom e station operation such as
dipoles, inverted-vees, verticals, beam s , random wires, and other s fed by coax lines or single wire.
The MFJ-945E employs a cross-needle m eter so forward power, reflected power, and SWR may
be read simultaneous ly. T he meter lamp can be powered by a 12Vdc, 300mA s our ce, s uc h as the
MFJ-1312B for hom e-station operation. The meter lamp
lamp. Use a 2.1mm coax ial plug with the center conductor of the plug connected to the positive
and the sleeve connected to ground. Optional mobile mounting bracket (MFJ-20) is available.

Cross-Needle SWR/Wattmeter:

The MFJ-945E utilizes a cross-needle meter to read FORWARD power, REFLECTED power and
SWR simultaneously in either HI (300 Watts) or LO (30 Watts) power ranges. Set the power

range switch to HI or LO. Next read the FORWARD power on the Forward Scale. REFLECTED
power is shown at the same time on the Reflected Scale. SWR is read by observing where the two
needles cross. No SWR sensitivity adjustment needed to read SWR. The HI range is

FORWARD and 60 Watts REFLECTED. The LO range is 30 Watts FORWARD and 6 Watts
REFLECTED. The MFJ-945E is factory calibrated on the LO scale at 10Watts FORWARD and

Watts REFLECTED, and on the HI scale at

2

1.8

to

30 MHz

, it will match virtually any transmitter to almost any

ON/OFF

Watts FORWARD and 20Watts REFLECTED.

100

switch will activate the meter

Watts

300

Installation:

Place the tuner in a convenient location at the operating position. Avoid placing the tuner

1.

near microphones, speech processors, TNC's or other RF sensitive devices.
Install the MFJ-945E between your transmitter and antenna. Connect a coax line from your

2.

transmitter to the SO-239 connec tor labeled
Connect your antenna to the SO-239 connector labeled
tuner. A random wire antenna may be connected to the center connec tion of the
connector. 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
or longer at the operating frequency. Do
the tuner is well grounded to the transmitter! A post labeled
ground connection(s).

NOTE:

If random wire operation is desired, position the tuner so that the rear panel terminals
can not be contacted by persons or conductors! W hen transmitting with random wire,
the rear panel connectors can operate with high RF voltages. These voltages may
cause serious burns and may also damage anything contacting or within 1/2" of the
terminals!

TRANSMITTER

ground the random wire antenna. Ensure that

not

on the rear panel of the tuner.

ANTENNA

on the rear panel of the

ANTENNA

1/4-wavelength

GROUND

is provided for

1

MFJ-945E Mobile Tuner

MFJ-945E Front Panel

Using the MFJ-945E:

The

INDUCTOR

minimum inductance at position L. Less inductanc e is needed at higher f requencies than at low
frequencies for the same impedance. The
represent maximum capacitance at position 10. For optimum operation of the MFJ-945E, the
transmitter must be tuned to a 50 Ohm output impedance at the operating frequency.

tune the transmitter at a low output power!

setting after loading it to a 50 Ohm load! After properly tuning the transmitter, tune the MFJ-945E
for a minimum SWR as described below:

NOTE
:

1.

When using the MFJ-945E for receiving only, tune as described in Steps 1 and 2.

Set the
opened at this setting.
Rotate the

2.

receiving mode.

WARNING:

While transmitting a steady state carrier (CW), alternately adjust the

3.
TRANSMITTER

controls can best be adjusted by turning the
time, then rotating the
minimum SWR is obtained.
If an SW R of

4.

Step 3. If arcing should occur between the capacitor plates, increase or decrease the

INDUCTOR

repeat Step 3 for each

switch on the MFJ-945E represents maximum inductance at position A and a

TRANSMITTER

INDUCTOR

and

ANTENNA

and

ANTENNA

TRANSMITTER

Do

readjust the transmitter loading control

not

controls to position 5. The tuning capacitors are half

control until maxim um noise is obtained with your tranceiver in the

controls both

Never transmit while changing the INDUCTOR setting!

ANTENNA

controls for minimum SW R. Since both of these c ontrols interact, the two

TRANSMITTER

ANTENNA

is not obtained, increase or decrease the

1:1

control for m inimum SWR. Repeat this procedure until

control one position, then repeat Step 3.

INDUCTOR

control position.

NOTE:

control a sm all increment at a

INDUCTOR

If you can't obtain a

control and repeat

1:1

Always

and

SWR,

2

MFJ-945E Mobile Tuner

After minim um SWR is achieved, transm itter power m ay be increased up to 300 Watts. Your

5.

MFJ-945E will reduce the SWR of m ost feed systems to 1:1. In some cases , a 1:1 SWR is
not possible. Increase or decrease the length of your antenna to improve SWR.
An SWR of 1:1 may occur at more than one set of control settings on your MFJ-945E. When

6.

an SWR of 1:1 is obtained, check transmitter power. Ensure that 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,

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

at the rated power levels, please

arcs

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

Note
:

If this tuner
transmitter output power.

please

double check all connections

Antenna Matching Problems

when operating on the

arcs

and follow the tuning procedures

double check all connections

text below.

160 meter

band, it may be necessary to reduce

and follow the

Antenna Matching Problems:

Most matching problem s occur when the antenna system presents an extremely high impedance
to the tuner. W hen the antenna impedance is muc h
quarter-wavelength feedline converts the low antenna impedance to a

the tuner

. A similar problem occurs if the antenna has an extremely high impedance and the
transmission line is a m ultiple of a half -wavelength. The half -wavelength line repeats the very high
antenna impedance at the tuner.

system very difficult or

impossible

Incorrect feedline and antenna lengths can make an antenna

to tune.

This problem of ten 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 f eedline 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 f oot) and connects to a f ull-wave high impedanc e antenna (100 to 140 foot).
The half-wave line r epeats the high antenna impedance at the tuner. The antenna system look s
like several thousand ohms at the tuner on 40 meters.

than the feedline impedance, an odd

lower

very high impedance at

3

MFJ-945E Mobile Tuner
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 f eed a full-wave antenna with any feedline close to a multiple of a half-wave

long.

3. If a tuner will not tune a m ulti-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 m atching 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:

80 meter dipole:

40 meter dipole:
NOTE:
WARNING:

Some trimming or adding of line may be necessary to accommodate higher bands.

To avoid problem s, a dipole antenna should be a full half-wave on the lowest band.

35-60, 170-195 or 210-235 feet.
34-40, 90-102 or 160-172 feet.
42-52, 73-83, 112-123 or 145-155 feet.

Avoid 130, 260 ft ]

[

Avoid 66, 135, 190 ft ]

[

Avoid 32, 64, 96, 128 ft ]

[

On 160 meters, an 80 or 40 m eter antenna fed the norm al way will be extremely reactive with only
a few ohms of feedpoint r esistance. Trying to load an 80 meter (or higher frequency) antenna on
160 meters can be a disaster for both your signal and the tuner. T he best way to operate 160 with
an 80 or 40 m eter 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 referenc e your problem or your problem is not solved by reading the m anual, you
may call MFJ at

601-323-0549

manual and all inform ation 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., P.O. Box 494, Mississippi State,
MS 39762; by FAX to

[e-mail] . Send a complete description of your problem, an explanation of
exactly how you are using your unit, and a complete description of your station.

4

or at

601-323-5869

601-323-6551

. You will be best helped if you have your unit,

; through Compuserve at 76206,1763; or by email to