MFJ-925 Super-Compact IntelliTuner Automatic Antenna Tuner
Instruction Manual
©
2006 MFJ Enterprises, Inc.
14
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 (60 to 70 feet) open
wire line is used to feed a half-wave (100 to 140 feet) 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 feet) 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 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.
Summary of Contents for MFJ-925
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