MFJ MFJ-17754 User Manual

MFJ-17754 40/20 Meter Dipole Instruction Manual

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The MFJ-17754 is a two band, 40- and 20-Meter center-fed dipole
antenna capable of handling 1500 watts. Its total length is 42 feet. Each
side of the dipole is broken up by a coil that presents a high impedance at
20 meters and acts as a loading coil on 40 meters, allowing the antenna
to resonate on 40 meters even though it is physically shorter than a half­wavelength (approximately 65 feet at 7.1 MHz).

Because this antenna acts as a dipole on both bands its impedance is in
the vicinity of 50 to 70 ohms, permitting it to be fed with coaxial cable.
The use of a current balun between the coaxial feedline and the antenna
is highly recommended, to help prevent stray RF from traveling down
the coaxial cable’s outer shield. This is easily accomplished with an
MFJ-915 RF Isolator and a “double male” coaxial connector.

Assembly

This antenna comes assembled and ready to install, but may require
adjustment of wire lengths once in place. Ropes are needed to support
the ends of the antenna from suitable structures or trees.

The antenna comes with the outer ends of the two sides of the antenna
“loose.” For initial installation, wrap these two ends loosely around the
provided ceramic end insulators and back on themselves, using about six
inches of wire on each end. Do not cut these wires; they are provided
extra-long to allow for all possible installation situations. Wrapping the
wire back on itself without cutting it shortens the electrical length of the
wire while saving the full length for possible use later. Be sure to install
your RF isolator or balun, if you choose to use one.

Installation

The best location for this antenna is as high and far away as possible
from utility wires, other antennas, and other structures. It is impossible to
find a perfect location, so the best compromise usually must be accepted.
The antenna can be installed in three basic ways:

WARNINGS: Always mount antennas so that they are out of

the reach of adults and children. Contact with any part of

this antenna can cause RF burns or other injuries.

Constructing or erecting antennas where they may contact

electrical

ower lines can result in injury or death.

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MFJ-1775C 40/20 Meter Dipole Instruction Manual

Horizontal Antenna: Requires two tall supports separated by more than
42 feet. Suspend the antenna with at least a 50 pound working load
nylon rope or another equivalent strength weather resistant non-metallic
rope. Never use wire or wire core rope to support the ends of any
antenna. Attach the rope to the end insulators through the empty holes.

Try to keep the antenna as horizontal (level) as possible. The antenna
should be at least 35 feet above ground to give acceptable performance
on 40 Meters, and as high as possible for the best overall performance.
The coaxial cable should drop vertically from the horizontal section of
the antenna as far as is practical.

If you are using trees for end supports, be sure to allow enough slack or
use some type of pulley and counterweight system to prevent the antenna
or rope from breaking when the trees sway in the wind. Try to keep the
ends of the antenna at least five feet from metallic supports.

Inverted "V" Antenna: This method requires only one tall support and
also places the least mechanical strain on the antenna. Hang the center
insulator of the antenna from the support using nylon or other non­conductive rope tied to the center hole of the center insulator. The center
insulator will be the highest point of the antenna. If possible, position the
two ends of the antenna so the inside angle they form is at least 120
degrees, and never less than 90 degrees. Secure the antenna ends with
nylon or weather resistant non-metallic rope to suitable supports.

Sloper Antenna: This antenna also can be suspended as a sloping dipole.
This requires one tall support and one short support. The center (feed
point) of the antenna should be at least 30 feet above the ground in this
configuration. A sloping dipole radiates mainly in the direction of the
downward slope. The optimum angle of "slope" will vary with the
desired coverage and the frequency of operation, but will almost always
be somewhere between 45 degrees and almost vertical.

Tuning

This antenna comes with the inner sections of wire (between the
feedpoint and the coils) cut to resonate near the center of the 20-meter
band. For operation near only one end of 20 Meters, the 20-meter wires
can be lengthened or shortened to lower or raise the resonant frequency.
This should be done before adjusting the lengths of the 40 meter wires
(the wires from the coils to the ends of the antenna). Do not cut the 20­meter wires until you have temporarily installed the antenna and
determined its 20-meter resonant point.

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