MFJ MFJ-974 Instruction Manual Download Page 4

MFJ-974/974H Balanced Line Antenna Tuner 

Instruction Manual 

When necessary to increase the tuner’s matching range, set the 

ANTENNA

 or 

TRANSMITTER 

control to a 

higher number, or dial in a lower letter on the 

INDUCTOR

 switch, but remember that this lowers the efficiency 

and power handling capability of the tuner. 

 

Adjustment  Procedure 

 

1. 

If the exciter or transmitter must be tuned, do it into a 50-ohm dummy load. 

2. 

Set the 

ANTENNA, INDUCTANCE, 

and 

TRANSMITTER 

controls at the settings in the Suggested 

Tuning Chart on Page 9. At these settings signals (or at least some noise) should be audible in your 
receiver. 

3. 

Place the 

POWER

 switch in the 

LO

 position. 

4. 

Transmit at low power (20 or 25 watts should suffice).  

5. 

Rotate the 

ANTENNA 

and

 TRANSMITTER 

controls for maximum forward power and minimum 

reflected power (ideal). If full forward power and zero reflected power cannot be obtained, turn the 

INDUCTOR 

one letter lower in the alphabet and try again. If this does not achieve a match, turn the 

INDUCTOR 

control 

two

 letters lower and try.

 

Note:   

Never

 

change the 

INDUCTOR 

setting while power is applied to the tuner! 

6. 

Once full forward power and zero reflected power are achieved, 

always

 try advancing the 

INDUCTOR

 

setting by one letter (higher) and tune for a match. If a match can be achieved at two different 

INDUCTOR 

settings, the “higher letter” (lower inductance) setting is better. 

7. 

Adjust transmitter to full output and touch up the tuner settings if needed. 

8. 

For quick retuning of the tuner, record the 

INDUCTOR

 and 

CAPACITOR

 settings for each band (see 

the Logged Tuning Chart, Page 10). 

Note:   

Maximum power handling is achieved when both the 

ANTENNA

  and 

TRANSMITTER

 

controls are set at the lowest possible number, and the 

INDUCTOR

 control is set at the highest possible 

letter that permits matching the antenna. Following this guideline will ensure maximum power handling 
capability and efficiency, and the smoothest tuning. 

 
 
 
 
 
 
 
 

 
 
Operating Notes

 

 

While this tuner is designed to have as large a tuning range as possible, there are limits to the tuning range of the 
capacitors.  Some antennas may require more or less capacitance than is available.  In these cases, the SWR may 
not be reduced to 1:1.  If the SWR is higher than the limits of your rig, try changing the length of the antenna or 
feedline to bring the impedance within the tuning range of the tuner. 
 

WARNING: 

• 

Never

 operate the tuner with the top removed. Contact with the components 

inside the tuner while transmitting will result in painful RF burns. 

• 

Never

 rotate the INDUCTANCE switch while transmitting. Doing so may 

permanently damage the switch. 

• 

Locate the tuner so that the rear terminals are 

not accessible

 during operation. 

The balanced line connectors may have high voltage on them while transmitting.

• 

Disconnect

 all antennas from the tuner during lightning storms. 

• 

Always

 tune with low power (i.e. less than 100 watts).  Apply maximum power 

only after tuning up. 

• 

Be sure to adjust the SWR before transmitting at full power.  Do 

not

 transmit 

with a high SWR for extended periods. 

Summary of Contents for MFJ-974

Page 1: ... arrays random wires and many other antennas The MFJ 974H has rear panel connectors for coaxial and single or two wire feedlines The built in balun works with balanced open wire twinlead or twin axial feedlines Understanding Power Ratings There are no standardized power rating systems for tuners The names used i e 3 kW Tuner carry over from the time when amplifiers were rated by peak power input a...

Page 2: ...the true peak envelope power Forward power is displayed on the left hand FORWARD meter scale This scale is calibrated from 0 to 300 watts In the HI position each picket scale mark represents 5 watts below 10 watts 10 watts between 10 and 100 watts and 25 watts between 100 and 300 watts In the LO power position full scale meter reading is 30 watts divide the meter reading by 10 for the correct valu...

Page 3: ...al line is connected to the BALANCED LINE terminals 4 A ground post is provided for an RF ground connection Operation The MFJ 974H uses three inductors for maximum efficiency A 2 inch diameter high Q air wound inductor is tapped for flexibility and is used on frequencies from 3 5 to 30 MHz For 160 Meters MFJ 974H only an additional inductance made of Teflon insulated wire wound on two powdered iro...

Page 4: ...t and touch up the tuner settings if needed 8 For quick retuning of the tuner record the INDUCTOR and CAPACITOR settings for each band see the Logged Tuning Chart Page 10 Note Maximum power handling is achieved when both the ANTENNA and TRANSMITTER controls are set at the lowest possible number and the INDUCTOR control is set at the highest possible letter that permits matching the antenna Followi...

Page 5: ... 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 good DC and RF grounds It is particularly important to have a good RF ground when using a single wire feeder When using a single wire feeder the tuner needs something to pu...

Page 6: ...e 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 same 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 feet Th...

Page 7: ... Starkville MS 39759 by Facsimile FAX to 662 323 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 WARNING To avoid problems a dipole antenna should be a full half wavelength on the lowest band On 160 Meters an 80 or 40 Meter antenna fed the normal way will be ...

Page 8: ...MFJ 974 974H Balanced Line Antenna Tuner Instruction Manual 8 ...

Page 9: ... 1 5 E 3 25 3 75 2 E 2 50 3 75 2 E 1 5 600 4 0 0 F 0 25 4 0 1 F 0 50 4 0 1 F 1 600 7 15 0 I 2 5 25 7 15 1 I 0 50 7 15 1 5 I 1 5 600 10 1 0 K 2 5 25 10 1 1 K 0 50 10 1 0 K 1 600 14 2 3 L 4 25 14 2 4 L 3 50 14 2 4 L 3 600 18 1 6 5 L 7 5 25 18 1 6 L 6 50 18 1 7 L 7 600 21 2 7 5 L 7 5 25 21 2 7 5 L 7 5 50 21 2 7 L 7 5 600 24 9 8 5 L 8 5 25 24 9 8 5 L 8 5 50 24 9 9 K 8 600 28 5 8 5 L 9 25 28 5 9 L 9 50...

Page 10: ...a Tuner Instruction Manual 10 Logged Tuning Chart Use the chart below to log values for your station You may want to copy this chart and post it by your tuner Frequency MHz ANTENNA INDUCTOR TRANSMITTER 1 8 3 5 7 10 1 14 18 21 24 28 50 ...

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