MFJ-931 Instruction Manual
Artificial RF Ground
Using a Counterpoise
If you are unable to connect the station to any type of ground wire, then you may still use the
counterpoise to increase your signal. As more and more wire is installed underneath an
antenna, the signal strength will increase. This will continue until a maximum occurs. Only one
wire installed along the floor is enough to make a very noticeable improvement in signal
strength. The counterpoise reflects radiated energy, which would normally be absorbed by the
ground as heat, back above the ground plane. This redirection of energy combines with the
primary radiated signal as usable field strength. Received energy bounces off of the counterpoise
into the antenna along the reverse path.
Visual tool:
A lighted candle on a table in a dark room emits a given amount of energy in all
directions. Light which reaches the walls will have a given brightness. You could say that this
would be signal strength. Energy is broadcast in all directions around the candle, up and
down. If a mirror is inserted underneath the candle, energy previously absorbed by the table
(ground) is radiated upward. The result is that the amount of light on the walls increases.
The transmitted energy is not changed, simply redirected. The amount of signal increase will
depend on the area of reflection. Beyond a certain area, no further increase will be noticed. A
sliver of mirror placed on only one side of the candle would give an increase only in that
direction. A ground plane underneath a HT rubber ducky will tremendously increase radiated
field strength from the radio.
Radio Frequency Interference
In may cases, providing an adequate ground to the antenna system will reduce or stop
unwanted radiation outside the operating frequency. Harmonics can be directed to
ground, or simply become part of the main radiated energy. This gives the appearance of a
cleaner signal on the air.
RFI can also increase by use of a grounded antenna system. A common belief, which is not
true, is that an artificial ground will remove all RFI from the station. RFI suppression is a side
effect of a clean signal, not a universal solution.
The most common type of RFI is commonly called front-end overload. This results whenever a
semiconductor junction comes into contact with an AC (RF) signal and rectifies it. Any
diode, transistor, or IC can rectify RF and mix it internally with any other signal present. More
signal present raises the level of mixing in the semiconductor junction. The higher signal
present can be a result of the increased field strength of the antenna. The only way to remove
this type of RFI is by shielding and filtering of the received RF in the receiving device.
Grounding of the receiving device case can help. If the case is non-metallic, then RF energy is
free to invade the internal circuitry. Any wires attached to the device may act as antennas.
Installing an RF suppression kit, such as the MFJ-701, can greatly reduce the interference
traveling up these lines.