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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.

Содержание MFJ-931

Страница 1: ...wire to the red binding post labeled To Counterpoise Wire or Ground Connection Wire on back of the MFJ 931 Be sure not to reverse these wires The random wire should be a quarter wave length or less at the operating frequency Be sure to throw the random wire along the floor and to tape up the open end of the wire to avoid RF burn To obtain maximum RF ground current for a low impedance RF ground alt...

Страница 2: ...ner side by side or one on top of the other to insure the ground of each are as close to each other as possible Connect your existing ground wire to the red binding post labeled To Counterpoise Wire or Ground Connection Wire on the back of the MFJ 931 Do not reverse these wires After making these connections follow the same procedure for maximizing RF ground current as described in the Creating Ar...

Страница 3: ...velength 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 antenn...

Страница 4: ...ver 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...

Страница 5: ...k the appropriate section of this manual If the manual does not reference your problem or your problem is not solved by reading the manual you may call MFJ Technical Service at 662 323 0549 or the MFJ Factory at 662 323 5869 You will be best helped if you have your unit manual and all information on your station handy so you can answer any questions the technicians may ask You can also send questi...

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