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 antenna system very difficult or impossible to
tune.
This problem often occurs on 80 meters if 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 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.
MFJ-949E Versa Tuner II

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 halfwave
long.
3. If a 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 lines,
keep the lines around these lengths. The worst possible line lengths are shown in
brackets:
160 meter dipole; 35-60, 170-195 or 210-235 feet. (Avoid 130, 260 ft)
80 meter dipole; 34-40, 90-102 or 160-172 feet. (Avoid 66, 135, 190 ft)
40 meter dipole; 42-52, 73-83, 112-123 or 145-155 feet. (Avoid 32, 64, 96, 128 ft)
Some trimming or adding of line may be necessary to accommodate 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 (or higher frequency) antenna on
160 meters can be a disaster for both your signal and the tuner. The best
way to operate 160 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.

from MFJ

K1WTX