Tuning The Antenna - MFJ -1775 Instruction Manual

MFJ-1775 Rotatable Mini-Dipole
raised to the final height. However, when the plastic end caps are installed on the
ends of the spokes, the resonant frequency on all bands is lowered 30-50 kHz

Tuning the Antenna

1) Measure and record the frequency of lowest SWR for each band. The lowest SWR should be at or
below the bottom end of each HF band. The SWR should be below 2:1 at resonance on each band.
The following is a typical chart for initial measurements of a new antenna before tuning:
40 M 1.7:1 at 6.99 MHz
20 M 1.2:1 at 13.9 MHz
15 M 1.2:1 at 20.85 MHz
If you are using a transceiver and SWR meter, the initial resonant frequency may be lower than you can
measure. If so, remove one spoke from each loading coil assembly to raise the resonant frequency.
Measure this resonant frequency and calculate the approximate resonant frequency as if the spokes were
in place using the chart on the next page.
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.
The following tips will help you obtain a better SWR on 40 meters than the tuning method used on other
bands. On 40 meters the lowest SWR is usually not obtained with left and right loading spokes adjusted
to the same frequency.
Low End of 40 Meters: If you desire operation below 7.1 MHz, the lowest SWR will be obtained by
adding two long spokes to ONLY ONE of the 40 meter loading coil assemblies. It is best to add the
spokes on opposite sides of the coils. Medium 40 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 40 Meters: If you desire operation above 7.2 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 40 Meters: For operation between 7.1 and 7.2 MHz, add ONE long spoke to one of the
loading coil assemblies only.
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
10 M 1.7:1 at 27.97 MHz
6 M 1.2:1 at 50 MHz
2 M 2.0:1 at 146 MHz
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Instruction Manual