Grounding Considerations; Maintenance - MFJ 1789 Instruction Manual

MFJ-1789 Rotatable Mini-Dipole
low as 130 MHz to be covered with the stub fully extended and 148 MHz with the stub fully
contracted.
NOTE: The 2-meter resonance is affected by other resonances in the overall antenna structure. You
may find a fixed resonance close to the 2-meter band that has a very low SWR, but the resonant
frequency of this resonance will not be adjustable. Ignore this point and look for a higher SWR
resonance that is adjustable. Typically, you may find a fixed, low SWR resonance at 144 MHz, and
an adjustable resonance at 147 MHz with a 2:1 SWR. This higher frequency, higher SWR resonance
is the desired resonance point. Note that between these two resonant points, the SWR should be 2:1
or less across most of the 2-meter band.
6) Final frequency adjustments can be made by trimming spokes on one of the loading coil assemblies
only if desired, though symmetrical trimming is preferred. The minor adjustment of one side only
without the symmetrical trimming of the other side is perfectly acceptable so long as the frequency is
changed less than 30 KHz on 40 meters and 60 KHz on the higher bands during final adjustment.
Moving the resonant frequency too far with only the spokes on one loading coil assembly will
increase the SWR of the antenna at the resonant frequency.
NOTE: The spokes can also be bent toward another spoke in the same mounting ring to raise the
frequency slightly without cutting.
7) When all tuning is complete, install the plastic end caps on the outer ends of all spokes. Note that the
plastic end caps will lower the resonant frequency on all HF bands by 30-50 kHz. The plastic end
caps should fit snugly on the spokes. Should a cap fit too loosely, add a drop of glue to the spoke
end before sliding on the cap.

GROUNDING CONSIDERATIONS

Although this antenna is designed to operate efficiently without the requirement of an earth ground,
safety grounding must still be provided to protect equipment, property and persons from the hazards of
lighting strikes and other weather related electrical discharges. In addition, the coaxial cable feeding the
antenna should have the shield grounded to eliminate the risk of any indoor equipment failure allowing
hazardous voltages that could create a shock hazard.
Adequate protection can be accomplished by grounding the shield of the coax to a good earth ground
where it enters the building, or directly burying the cable in the earth for several feet before it enters the
building. For maximum lightning protection, the coaxial cable should be totally disconnected from the
station during threatening weather conditions.
A less effective method of protecting station equipment is to install an in-line coaxial lightning arrestor
with a heavy duty ground wire to a suitable earth ground, or a safety switching system as part of the basic
ham station equipment.

MAINTENANCE

Your antenna is constructed of heavy-duty non-corrosive materials and should withstand normal climates
for many years. The use of some type of coaxial connector moisture protection is recommended at the
balun coax connection, and also around the center-feed connections, especially in coastal areas where
salty mist is commonplace.
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Instruction Manual