MFJ MFJ-17758 User Manual

MFJ-17758 80/40 Meter Dipole Instruction Manual

The MFJ-17758 is a two band, 80- and 40-Meter center-fed dipole
antenna capable of handling 1500 watts. Its total length is 85 feet. Each
side of the dipole is broken up by a coil that presents a high impedance at
40 meters and acts as a loading coil on 80 meters, allowing the antenna
to resonate on 80 meters even though it is physically shorter than a half­wavelength (approximately 130 feet at 3.7 MHz).

Because this antenna acts as a dipole on both bands its impedance is
around 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 (MFJ-7702).

Assembly

This antenna comes assembled and ready to install, but may require
adjustment of wire lengths once in place. Nylon 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 power lines can result in injury or death.

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MFJ-1775B 80/40 Meter Dipole Instruction Manual

Horizontal Antenna: Requires two tall supports separated by more than
85 feet. Suspend the antenna with at least a 50 pound working load
nylon rope or 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 80 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. It is also
possible to use masts, towers, or other tall structures for supports. Try to
keep the ends of the antenna at least five feet from metallic supports.

Inverted "V" Antenna: Requires only one tall support and places the
least strain on the antenna. Hang the center insulator of the antenna from
the support using a nylon rope or other non-conductive 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 not 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 of the
antenna must be at least 40 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 varying 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 40-meter
band. For operation at only one end of 40 Meters, the 40-meter wires can
be lengthened or shortened to lower or raise the resonant frequency. Do
this before adjusting the lengths of the 80 meter wires (the wires from the
coils to the ends of the antenna) – and after you temporarily install the
antenna and determine its resonant point on 40 Meters.

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