MFJ MFJ-974 Instruction Manual Download Page 5

MFJ-974/974H Balanced Line Antenna Tuner 

Instruction Manual 

When adjusting the tuner, use the lowest number on the 

ANTENNA

 and 

TRANSMITTER

 controls and the 

highest letter on the 

INDUCTOR

 control that produces a good SWR. This will reduce tuner losses and increase 

the power rating of the tuner. 

 

In Case Of Difficulty

 

 

If the tuner fails to tune, please 

double check

 all connections and follow the tuning procedures again.   

 
Be sure you are using 

enough inductance

 (low enough inductance letter) and have the 

capacitors open far enough 

(higher front panel numbers). 
 
If the tuner arcs

 

at the rated power levels, please 

double check

 all connections and follow the tuning procedures 

again. The power rating of this tuner is 300 watts PEP RF power.  Be sure you are using the 

least amount of 

inductance

 (highest number) and the 

greatest capacitance

 (lowest number) possible that still allows matching 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 used with a dummy load or another 
antenna, please read the 

Antenna System Hints

 section. 

 

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 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 "push" against in order to 
force current into the single wire feedline. If a good RF ground is not available, RF will usually find its 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. Ground rods by themselves are almost useless for dependable RF 
grounding. 
 
RF grounds work much better when "spread out" over a large area, especially when they employ multiple 
connections directly to the equipment ground point. Water pipes, heating ducts, and fences may work (especially 
if they are connected together with multiple wires), but the best RF grounds are radial systems or multi-wire 
counterpoises. Radials and counterpoises provide large, low resistance surfaces for RF energy. 
 

Caution 

 

For operator safety a good outside earth ground or water pipe 

ground should always be installed and connected to the case of the 

MFJ-974H/974. Make certain the safety ground also connects to 

the transmitter and other station accessories. A wing nut post 

marked GROUND is provided for ground connections. 

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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