MFJ-931 Instruction Manual
Artificial RF Ground
Failure to Obtain a Ground Reading
If a ground current reading cannot be obtained at full transmitting power, the counterpoise or
ground may not generate sufficient current at the operating frequency. If no RFI problems exist,
set the CAPACITANCE control to zero, and the INDUCTANCE control to "A" and operate. If
you experience RF in the shack or other problems, alter the counterpoise or add a different
length of wire. The wire for a counterpoise should be a quarter wavelength at the lowest
frequency used. Additional short lengths may be placed for other bands if needed. More radials
will give a stronger current reading.
You should be aware of typical antenna current behavior at different wavelengths. An antenna
which is a half wavelength will not show any ground current at exactly the resonant frequency.
An antenna which is a quarter wavelength at the resonant frequency will show maximum ground
current. The amount of current through the ground wire depends entirely on the antenna system.
The odd wavelengths give maximum current reading, and the even wavelengths are minimum or
zero. Zero current draw shows a zero meter reading. This does not mean that the meter is not
working. Try the same antenna on at least an eighth of a wavelength difference in frequency,
which should show some ground current.
In the case of a halfwave antenna, the meter is not usable for measuring ground current, so it is
very difficult to tune the artificial ground. You may use a field strength meter, such as the MFJ-
812B, to tune the ground for maximum radiated signal. This also means that the receiving
station S-units will go to maximum for this antenna system. You would also use the field
strength meter when tuning a counterpoise system.
Using the artificial ground to tune the ground may not be obvious if you are a new user. You
must visualize and account for all sources of ground current. If the artificial ground is
connected into the ground lead of the transmitter, and a ground wire is also connected to the
ground lead of the amplifier or tuner, then the artificial ground is effectively shorted out, and no
ground current will show on the meter. You must install the MFJ-931 in series with the ground
system.
Visual tool:
A station is positioned a mile higher than the ground rod. A ground wire runs
from the ground rod to the station. The ground wire will show very high resistance and radiation
instead of conducting current to the ground rod. The MFJ-931 tunes the wire for low
resistance at the antenna system resonant frequency so that the antenna system may develop
maximum current. This in turn provides maximum radiated power from the antenna system.
Without the MFJ-931, the wire makes a better antenna than a ground wire.
