MFJ 1789, Instruction Manual

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MFJ-1789 Rotatable Mini-Dipole Instruction Manual
11
WARNING: You will be trimming the outer ends of the spokes. The sharp sends can cause
a hazard until they are covered with the plastic caps. Therefore it is VERY IMPORTANT
to wear safety glasses when trimming the spokes, or when working near the antenna.
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.
1
) The antenna spokes should be tuned 40 first and 10 last. This is to prevent the larger spokes
from affecting the tuning of the higher bands. With all the spokes installed, several swr dips
may be seen on a swr analyzer. You must make sure that the dip you are trying to tune is the actual
one relative to the spoke you are cutting. If your not sure you which dip is the correct one, remove
one of the spokes and see if that dip moves. If it doesn’t, you have the wrong one. You may also use
an analyzer to determine which spoke is associated with a dip by touching that spoke with your hand
and watching the effect on the analyzer. Touching any spoke will cause the dip to move but touching
the spoke that is associated with that dip will cause a change that is much more drastic than the
others. It is a good idea to do a rough tuning through all the bands to begin with. The first rough
tuning should get the dips just below each band. The second run through will have less interaction
between bands since the spokes are already trimmed close to their final length.
2) Begin tuning by trimming one 40 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.
40 M: 1" trimmed off a pair of spokes equals
approximately
25 KHz
30 M: 1" trimmed off a pair of spokes equals approximately 45 KHz
20 M: 1" trimmed off a pair of spokes equals approximately 100 KHz
17 M: 1" trimmed off a pair of spokes equals
approximately
125 KHz
15 M: 1" trimmed off a pair of spokes equals approximately 175 KHz
12 M: 1" trimmed off a pair of spokes equals approximately 200 KHz
10 M: 1" trimmed off a pair of spokes equals
approximately
250 KHz
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. 20 meters must be the
second HF band adjusted, 15 meters the third, and 10 meters last. After adjusting 10 meters go back
and check the other bands. Re-check the tightness of all spoke screws.
5)
The six and two meter bands are tuned by lengthening or shortening the threaded portion of the stubs.
This adjustment is very coarse. If the frequency is still too low with the stub element extension at a
minimum, the unthreaded ends of the stubs can be trimmed. Once again the lower frequency stub (6
meters) should be adjusted first.
NOTE : The six meter stub covers from 50 to 54 MHz as it is adjusted. Be careful because the third
harmonic resonance of the six meter stub will show up as a low SWR on the third harmonic of it's
setting, usually between 150 and 160 MHz. The range of the two meter stub allows frequencies as