MFJ-976 Legal Limit Fully Balanced Antenna Tuner
Instruction Manual
8
Antenna System Hints
Location
For the best performance, an end-fed long wire antenna should be at least one quarter-wavelength long at the
operating frequency. Horizontal center-fed antennas should be at least a half-wavelength long and located as high
and clear as possible. While good RF grounds help the signal in almost any transmitting installation, it is
extremely important to have good RF grounds with long wire or other Marconi style antennas.
Matching Problems
Most matching problems occur when the antenna system presents 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 very high impedance at the tuner. A similar problem occurs if the antenna has
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 length (60 to 70 feet) of 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 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 any 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.
