MFJ 945E MOBILE TUNER. Antenna Tuner Manual

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MFJ-945E Mobile Tuner

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MFJ-945E MOBILE TUNER

General Information:

The MFJ-945E Mobile Tuner is a compact tuner designed for mobile operation in cars, trucks,

RV's, and boats. Covering 1.8 to 30 MHz, it will match virtually any transmitter to almost any

mobile whip antenna. It will also match almost any antenna for home station operation such as

dipoles, inverted-vees, verticals, beams, random wires, and others fed by coax lines or single wire.

The MFJ-945E employs a cross-needle meter so forward power, reflected power, and SWR may

be read simultaneously. The meter lamp can be powered by a 12Vdc, 300mA source, such as the

MFJ-1312B for home-station operation. The meter lamp ON/OFF switch will activate the meter

lamp. Use a 2.1mm coaxial 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 300 Watts

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

2Watts REFLECTED, and on the HI scale at 100Watts FORWARD and 20Watts REFLECTED.

Installation:

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

near microphones, speech processors, TNC's or other RF sensitive devices.

2. Install the MFJ-945E between your transmitter and antenna. Connect a coax line from your

transmitter to the SO-239 connector labeled TRANSMITTER on the rear panel of the tuner.

Connect your antenna to the SO-239 connector labeled ANTENNA on the rear panel of the

tuner. A random wire antenna may be connected to the center connection of the ANTENNA

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 1/4-wavelength

or longer at the operating frequency. Do not ground the random wire antenna. Ensure that

the tuner is well grounded to the transmitter! A post labeled GROUND is provided for

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! When 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!

MFJ-945E Mobile Tuner

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MFJ-945E Front Panel

Using the MFJ-945E:

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

minimum inductance at position L. Less inductance is needed at higher frequencies than at low

frequencies for the same impedance. The TRANSMITTER and ANTENNA controls both

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. Always

tune the transmitter at a low output power! Do not readjust the transmitter loading control

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

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

1. Set the TRANSMITTER and ANTENNA controls to position 5. The tuning capacitors are half

opened at this setting.

2. Rotate the INDUCTOR control until maximum noise is obtained with your tranceiver in the

receiving mode.

WARNING:

Never transmit while changing the INDUCTOR setting!

3.

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

TRANSMITTER controls for minimum SWR. Since both of these controls interact, the two

controls can best be adjusted by turning the TRANSMITTER control a small increment at a

time, then rotating the ANTENNA control for minimum SWR. Repeat this procedure until

minimum SWR is obtained.

4. If an SWR of 1:1 is not obtained, increase or decrease the INDUCTOR control and repeat

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

INDUCTOR control one position, then repeat Step 3. NOTE: If you can't obtain a 1:1 SWR,

repeat Step 3 for each INDUCTOR control position.

MFJ-945E Mobile Tuner

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5. After minimum SWR is achieved, transmitter power may be increased up to 300 Watts. Your

MFJ-945E will reduce the SWR of most 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.

6. An SWR of 1:1 may occur at more than one set of control settings on your MFJ-945E. When

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, 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.

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.

MFJ-945E Mobile Tuner

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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 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 ]

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 at 601-323-0549 or 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., P.O. Box 494, Mississippi State,

MS 39762; by FAX to 601-323-6551; through Compuserve at 76206,1763; or by email to

76206.1763@Compuserve.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.