MFJ-17758 80/40 Meter Dipole Instruction Manual
1
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 half-
wavelength (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.
