MFJ MFJ-974 Скачать руководство пользователя страница 6

MFJ-974/974H Balanced Line Antenna Tuner 

Instruction Manual 

RF and lightning travel on the surface of conductors. Braided or woven conductors have high surface resistance to 
lightning and RF.  Ground leads for RF and lightning should have wide smooth surfaces. Avoid the use of woven 
or braided conductors in RF and lightning grounds unless the lead needs to be flexible. 

 

Antenna System Hints 

 
Location

 

For the best performance, an end-fed longwire wire antenna should be at least one quarter-wavelength long at the 
operating frequency. Horizontal center-fed antennas should be at least a half-wavelength long and located as high 
and clear as possible. While good RF grounds help the signal in almost any transmitting installation, it is 
extremely important to have good RF grounds with long wire or other Marconi style antennas. 

 

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 otherwise perfect antenna system very difficult or impossible to tune. 
 
One example where this problem occurs is on 80 meters when an odd quarter-wave length (60 to 70 feet) of 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 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).  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. 
 
This places enormous strain on the balun and the insulation in the tuner, since voltages can reach several thousand 
volts.  This can cause component arcing and heating. 
 
The following suggestions will reduce the difficulty in matching an antenna with a tuner: 

• 

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. 

• 

Never center feed a full-wave antenna with any feedline close to a multiple of a half-wave long. 

• 

If this 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. 

• 

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 open wire lines, 
keep the lines around these lengths.  [The 

worst possible

 line lengths are shown in brackets]: 

 

160 meters; dipole 

35-60, 170-195 or 210-235 feet 

[Avoid 130, 260 ft] 

80 meters; dipole: 

34-40, 90-102 or 160-172 feet 

[Avoid 66, 135, 190 ft] 

40 meters; dipole: 

42-52, 73-83, 112-123 or 145-155 feet 

[Avoid 32, 64, 96, 128 ft] 

 
Some slight trimming or adding of feedline may be necessary to accommodate the higher bands. 

Содержание MFJ-974

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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 ...

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

Страница 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...

Страница 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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