MFJ 1777 Manuel utilisateur
MFJ All Band Doublet Instruction Manual
1
MFJ-1777 All Band Doublet
INTRODUCTION
The MFJ-1777 is a multi-band center-fed dipole antenna capable of handling
1500 watts. It consists of a 102-foot “flat top” antenna fed with a length of 450
ohm balanced “ladder line.”
This antenna requires the use of a suitable matching network (antenna tuner)
with provisions for balanced-feed antennas, for interface with the 50-ohm
unbalanced antenna outputs of modern transceivers and amplifiers.
This antenna will work as supplied, as a centerfed antenna, on all amateur bands
80 through 10 Meters. On 160 Meters it can be used as top loaded vertical
antenna, with the dipole element acting as a capacitance hat on 160 meters (see
“160 Meter Operation,” page 3). Operation on 160 Meters in this manner
requires a ground screen or radial ground system, which is discussed on page 4.
This antenna will not work effectively in its center-feed configuration on 160
Meters.
TOOLS AND TIME REQUIREMENTS
This antenna requires no assembly. The time needed for installation will vary
with your skill and equipment. No special tools are required to install this
antenna. Nylon ropes are needed to support the ends of the antenna from
suitable structures or trees If trees are used, you will need a way to get ropes
over it. The most common method is to use a weighted arrow or a fishing rod
and a sinker to place a small line over a tree. A larger line is then pulled up and
used to pull the support rope over the tree. The ARRL Antenna Handbook has
detailed suggestions for installing wire antennas.
WARNING: 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.
See the Safety Precautions on page 7.
MFJ All Band Doublet Instruction Manual
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INSTALLING THE ANTENNA
The best location for this antenna is as high and far away as possible from utility
wires, other antennas, and other structures. It is difficult to find a perfect
location, so the best compromise usually must be accepted. The antenna can be
installed in three basic ways:
Horizontal Antenna: Requires two tall supports separated by more than 102
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 the antenna. Attach the rope to the end
insulators through the empty holes.
Try to keep the antenna as horizontal as possible. The antenna must be more
than 35' above ground level to give acceptable performance, and as high as
possible for the best overall performance. The ladder line should drop vertically
from the horizontal section for at least 20 feet and should be kept several inches
from conductive objects. If the antenna's ladder line has to be installed near
conductive objects, space the line at least 6" from the object with non-
conductive supports (for example: PVC pipe standoff supports 6" long). Use
nylon cable ties to secure the feedline to the supports.
Be sure to allow enough slack or use some type of pulley and counterweight
system to prevent the antenna or rope from breaking if the trees move 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: This method requires only one tall support and also
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 should be the highest
point of the antenna. The antenna ends should be less than 25.5 feet below the
center insulator. This avoids an inside angle of less than 120 degrees between
the antenna's legs. The antenna ends should be secured with nylon or weather
resistant non-metallic rope to suitable supports.
Sloper Antenna: This antenna can also be suspended as a sloping dipole. This
requires one tall support and one short support. The center of the antenna must
IMPORTANT: Use weather resistant rope rated at
at least 50 pounds working load to support this antenna.
MFJ All Band Doublet Instruction Manual
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be at least 30 feet above the ground in this configuration. The antenna radiates
mainly in the direction of the downward slope. The optimum angle of "slope"
will varying with the desired coverage distance and the frequency of operation,
but will almost always be somewhere between 45 degrees and almost vertical.
160 METER OPERATION AND GROUNDING
The MFJ-1777 will can be used on the 160 meter band as a “Marconi Antenna.”
Operation on 160 requires a tuner and a good RF ground system. Do not try to
use this antenna in its supplied, center-feed configuration on 160 Meters, as you
risk damage to your antenna tuner and/or transceiver.
To use the MFJ-1777 as a Marconi antenna, connect together the bottom ends of
the feedline (ladder line) at your tuner. This allows the vertical feedline to
become a radiating element, working against the station ground. The feedline
should be kept as vertical and as clear of metallic objects as possible. Obviously,
an “indoor” configuration such as this has complications, namely, that the
feedline is “hot” inside. Take suitable precautions.
A good way to visualize an RF ground is to picture a large conductive "mass"
that the antenna pushes against. The "mass" should have a short connection to
the ground point and extend as far as possible in every direction. The ideal
ground system for 160 meters would have 120 straight evenly spaced radials at
least 140 feet long. A marginally acceptable ground system requires a
minimum of four or more long (60 foot or more) straight radials. Other large
metallic masses (such as fences or water lines) should also be "tied in" to the
ground system.
The earth and fresh water are poor electrical "masses" because of their poor
conductivity (1 to 30 milliohms/meter). Although salt water is several hundred
times (5000 milliohms/meter) more conductive than dirt, copper is several
hundred times (millions of milliohms/meter) better than saltwater. Copper
radials obviously offer a much better ground than saltwater.
Ground rods almost never suffice for RF grounding in applications like this
because they connect to the lossy dirt. This is why radials are required when
feeding this antenna as a Marconi. Radials will improve your signal even if the
earth is wet or saturated with salt water.
Elevated counterpoise systems are also effective, but they also require as much
"electrical mass" as possible. A supplemental earth connection is always
required for lightning and low frequency ac grounding.
MFJ All Band Doublet Instruction Manual
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The earth ground should consist of at least one copper ground rod driven into
the earth a minimum of 6 feet. Multiple ground rods and buried wires are
superior to a single rod for lightning and RF protection. 160 meter operation
requires special grounding considerations. These considerations are discussed
in the "160 Meter Operation" section.
Never use woven flexible braiding for ground connections unless absolutely
necessary. Braiding has high resistance to RF and lightning. Copper flashing,
wide copper foil, or large gauge solid copper wires are the proper materials for
use in RF and lightning grounding applications. Never ground the feedline on
the antenna side of the balun.
In-line coaxial lightning arrestors offer a minimal improvement in lightning
protection. The best method of protecting station equipment is to disconnect the
feedline outside the building.
TUNER REQUIREMENTS
As noted earlier, this antenna requires an antenna tuner, ideally one that is able
to tune balanced feedlines. If you have a tuner that tunes only unbalanced, 50-
ohm loads (such as the internal tuner in some transceivers) it is still possible to
use this antenna. Connect a suitable 4-to-1 current balun (such as the MFJ-912
“W9INN Balun Box”) between your balanced feedline and your transceiver
through a short piece of 50-ohm coax.
TUNING THE ANTENNA
This antenna comes with 100 feet of high-quality balanced feedline. In general
any length of feedline will tune on all bands with this antenna. You may add
more feedline if necessary to span the distance from your tuner to the antenna,
or you may shorten the feedline if it is much too long (there is nothing magic
about the 100 feet supplied). Should you find that it is impossible to tune your
antenna on one or more bands, after you have exhausted the recommendations
in your tuner manual, then try changing the length of the feedline, by adding or
subtracting, say, five percent, or three to six feet, of feedline length.
MAINTENANCE
MFJ All Band Doublet Instruction Manual
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This antenna is constructed of heavy duty materials and should withstand
normal climates for many years. General Electric makes a pure silicone grease
called "silicone dielectric compound" that can be applied sparingly to the
soldered connections at the antenna’s center insulator. This is the same type of
sealer that commercial antenna installers and CATV companies use with
great success.
A less desirable but adequate sealer is the automobile seam sealer commonly
marketed as "coax seal". This is a pliable black sealing compound.
TECHNICAL ASSISTANCE
If you have any problem with this unit first check the appropriate section of this
manual. If the manual does not reference your problem or your problem is not
solved by reading the manual you may call MFJ Technical Service at 662-323-
0549 or the MFJ Factory at 662-323-5869. You will be best helped if you have
your unit, manual and all information on your station handy so you can answer
any questions the technicians may ask.
You can also send questions by mail to MFJ Enterprises, 300 Industrial Park
Road, MS 39759; by FAX to 662-323-6551; or by email to
techinfo@mfjenterprises.com. Send a complete description of your problem, an
explanation of exactly how you are using your unit, and a complete description
of your station.
MFJ All Band Doublet Instruction Manual
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SAFETY PRECAUTIONS
• This antenna is an electrical conductor.
Contact with power lines can result in death or
serious injuries. Do not install this antenna
where there is any possibility of contact with
power cables or the service drop. The antenna
should not be close to power lines during
installation, operation, or removal.
• Follow the guidelines for antenna installation
recommended by the U.S. Consumer Product
Safety Commission.
• Keep this antenna out of reach of adults,
children, and animals.
• Any contact with this antenna while
transmitting can cause RF burns.
• Never place this antenna close to electric
power lines of utility wires.
• Install the antenna away from living areas to
reduce levels of electromagnetic fields.
• Never operate this antenna near RF sensitive
medical devices such as pacemakers.
• Keep the ladder line away from buildings and
metallic objects. Ladder line is a high
impedance line that can develop high RF
voltages with high transmitter power levels.
• Use a minimum of 50-pound working load
weather resistant rope to support this antenna.
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