MFJ-962E Instruction Manual
10
One example where this problem occurs is on 80 meters when an odd quarter-wave (60 to 70
foot) 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 foot). 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:
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 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 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.
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