MFJ-1775W Rotatable Mini-Dipole
Instruction Manual
9
CAUTION: Use a pair of pliers to hold the outer piece of each spoke that is to be cut off so
as to keep the cut piece from flying off and causing injury.
The following tips will help you obtain a better SWR on 60 meters than the tuning method used on other
bands. On 60 meters the lowest SWR is usually not obtained with left and right loading spokes adjusted
to the same frequency.
Low End of 60 Meters:
If you desire operation below 5.5 MHz, the lowest SWR will be obtained by
adding two
long
spokes to ONLY ONE of the 60 meter loading coil assemblies. It is best to add the
spokes on opposite sides of the coils. Thirty-six inch 60 meter spokes from both coils can then be
trimmed equal amounts to resonate the antenna. This will leave one loading coil assembly with two
spokes that are longer than those on the other loading coil assembly.
High End of 60 Meters:
If you desire operation above 5.4 MHz the lowest SWR will be obtained by
trimming the spokes on one of the loading coils only until the antenna is resonant approximately 15 KHz
below the operating frequency.
Middle of 60 Meters:
For operation between 5.4 and 5.5 MHz, add
ONE
long spoke to one of the
loading coil assemblies only.
2)
Begin tuning by trimming one 60 meter spoke
from each end of the antenna
by cutting off small,
equal sections from the end of the spoke until the antenna resonates approximately 15 KHz below the
desired operating frequency. Use the chart below to
approximate
the amount of spoke to be trimmed.
Trimming only one spoke from the top or bottom results in half the frequency change.
60 M: 1" trimmed off a pair of spokes equals
approximately
25 KHz
30 M: 1" trimmed off a pair of spokes equals
approximately
100 KHz
17 M: 1" trimmed off a pair of spokes equals
approximately
175 KHz
12 M: 1" trimmed off a pair of spokes equals
approximately
250 KHz
Note:
Typical final spoke lengths for the low end of each HF band are shown in the table below.
3)
If the SWR is acceptable at the desired operating frequency, the adjustment for that band may be
skipped.
4)
Now tune progressively higher frequency bands by trimming pairs of spokes on each of the two
loading coil assemblies. Keep each pair of capacitance spokes equal in size. 30 meters must be the
second HF band adjusted, 17 meters the third, and 12 meters last. After adjusting 12 meters go back
and check the other bands. Re-check the tightness of all spoke screws.
5)
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 60 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.
