MFJ-998RT Legal Limit IntelliTuner
TM
Automatic Remote Antenna Tuner Instruction Manual
© 2011 MFJ Enterprises, Inc
MFJ-998RT Version 1A
18
to 140 feet). The halfwave 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 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 a 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.
WARNING
To avoid problems, a dipole antenna should be a full half-wave on the lowest band. On 160 meters, an 80
or 40 meters antenna fed the normal way will be extremely reactive, with only a few ohms of feedpoint
resistance. Trying to load an 80 meters half-wave 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 meters
antenna is to load either or both feedline wires (in parallel) as a longwire. The antenna will act like a “T”
antenna worked against station ground.
