MFJ-986Differential-TTuner Instruction Manual
11
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 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.
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 open wire lines, keep
the lines around these lengths. The worst possible line lengths are shown in parenthesis:
Dipole: Recommended: Avoid:
160 meters;
35-60, 170-195 or 210-235 ft.
(130, 260 ft)
80 meters;
34-40, 90-102 or 160-172 ft.
(66, 135, 190 ft)
40 meters;
42-52, 73-83, 112-123 or 145-155 ft.
(32, 64, 96, 128 ft)
Some slight trimming or adding of feedline may be necessary to accommodate the 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 halfwave dipole (or shorter) antenna on 160 meters can be a
disaster for both your signal and the tuner. The best way to operate 160 meters 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.
SWR Meter Calibration
The MFJ-986 has been calibrated at the factory. If it should ever need to be recalibrated, then follow
this procedure:
Equipment Needed
1. Transmitter capable of supplying enough power to obtain ½ to full-scale reading at 14 or 21
MHz.
2. 50-ohm dummy load that is capable of handling full transmitter output power and has better than
a 1.15:1 SWR.
3. Power meter of know accuracy. The calibration will only be as good as the standard reference
meter.
4. 50-ohm cables capable of handling the power. RG-58/u is recommended. DO NOT USE RG-59
or RG-11.
