MFJ MFJ-1785 User Manual
80/40/20 Meter Rotatable Dipole
Model MFJ-1785
INSTRUCTION MANUAL
VERSION 0B
CAUTION: Read All Instructions Before Operating Equipment
MFJ ENTERPRISES, INC.
300 Industrial Park Road
Starkville, MS 39759 USA
Tel: 662-323-5869 Fax: 662-323-6551
COPYRIGHT 2007 MFJ ENTERPRISES, INC.
C
MFJ-1785 Rotatable Dipole
80, 40, 20 Meters
Boom support U-bolt
Ceramic
Insulator
T3
20M
40M
80M
T1
T2
T3
Drawing not to scale.
Tubing
1 1/2” x 72”
1 3/8” x 72”
1 1/8” x 60”
T2
3/16” cable
T1
T1
1 1/4” x 20”
Fiberglass rod
T2
T3
20M
40M
80M
MFJ-1785 Rotatable Dipole Instruction Manual
MFJ-1785 Parts List
As you unpack your antenna you should find the parts in the following list.
Part Description QTY MFJ Part No.
Hardware Bag #1
Balun 1 80-1785-2
3/16 cable, 30 feet 1 13-1785-1
AV-640 Parts Pack 1 758-9200
Boom Bracket #1 1 735-1642
Boom Bracket #2 1 735-1644
5/16 U-bolt 2 758-9224
Ceramic Insulators, 3/16 2 MFJ-17A01
U-BOLT, 5/16 X 3 1/2 1 758-9220
Hardware Bag #2
1/4-20 x 1” bolt 6 662-1000s-HH
1/4-20 x 2”bolt 2 662-200s-HH
1/4-20 hex nut 8 705-2520s
1/4-20 split lock washer 8 711-2537s-SL
5/16” Split Lock Washer 6 711-3144s-SL
5/16” Hex Nuts 6 705-3118s
3/16” cable thimbles 4 713-9156
Boom Clamp 2 735-1785
Plastic spoke safety caps 42 765-1002
6-32 x 3/8” screws 50 656-0375s
6-32 kep nuts 50 705-0632s
#16 hose clamps 6 745-3116s
S.S. Strip, 2” x 5/8” 4 737-1785
U-Bolt saddle 1 758-9196
Tubing and Fiberglass
1 1/8 x 60” aluminum tube (T3) 2 810-1796-2
1 ¼ x 72” aluminum tube (T2) 2 20-1792-4
1 3/8 x 72” +1 ½ “x 48”aluminum tube (T1) 2 810-1785-2
1 ¼ x 20” solid fiberglass rod 1 811-1785-4
Loading Coils and Spokes
MFJ-1785 Loading coil assembly 2 811-1785-1C1
36” spokes 18 810-1798-5
30 3/4” spokes 16 810-1796-3c
12 3/4” spokes 8 810-1796-6
For installation, you will need some items not supplied with the antenna installation kit:
• A rigid mast or other mounting pipe between 1 3/4" and 2" outside diameter. (suitable materials
include TV mast sections, galvanized iron pipe, or heavy duty rigid conduit.)
• Quality low-loss 50-Ohm coax with a PL-259 from antenna to transmitter.
An Antenna Analyzer (MFJ-259B or similar), or SWR meter and transceiver.
1
MFJ-1785 Rotatable Dipole Instruction Manual
MFJ-1785 80-40-20 Meter Rotatable Dipole
Introduction
The MFJ-1785 legal limit Rotatable Dipole operates on the 20, 40, and 80 Meter bands. The
antenna is thirty-three feet in length and can be turned by a medium duty antenna rotator.
WARNING: Improper installation and assembly can be hazardous! Read these instructions
thoroughly before attempting to assemble, install or operate this product! High power transmitting
devices produce voltages that can cause severe RF burns. Make certain the Antenna is installed out of
reach.
SPECIFICATIONS
The MFJ-1785 is 1/2 wavelength on the 20 meter band. The antenna uses efficient end loading coils with
capacitance hats for operation on the 40 and 80 meter bands. The end-loading coils are wound on
fiberglass forms with Teflon
The MFJ-1785 boom is made of 6063 T-6 aircraft strength aluminum tubing. The center insulator is 1.25
inch solid fiberglass rod. The boom is supported by 3/16” cable isolated from the boom with ceramic
insulators.
Assembly will require approximately one afternoon. The MFJ-1785 will radiate a maximum of 1500
watts PEP on 80-40-20 meters.
Electrical Specifications
Power
Band CW SSB 2:1 SWR BW
80m 1500 1500 25KHZ
40m 1500 1500 150KHZ
20m 1500 1500 1 MHz
Mechanical Specifications
Mast Size: 1 3/4 to 2” diameter
Overall Length: 34 feet
Turning Radius: 17 feet
TM
wire and capacitance hats.
2
MFJ-1785 Rotatable Dipole Instruction Manual
CHOOSING A LOCATION FOR THE ANTENNA
For best performance on receiving and transmitting, mount the antenna in a clear location above or away
from buildings, towers, feedlines, utility wires, and other antennas. While your own ingenuity and
particular circumstances will determine the final mounting method, we'll pass along a few ideas for both
permanent installation and portable operation.
• Never mount this antenna in a location that will permit unsuspecting people to come in contact with
the loading coil spokes or any other part of the antenna.
• Never mount this antenna where a mechanical failure might allow the antenna to contact power lines
or other utility wires.
• Always ground the feedline at the point where it enters a building to a good earth ground for
lightning protection.
• Always ground the mast or tower base to a suitable ground rod.
WARNING: Always mount this antenna so that it is out of the reach of adults and children.
The capacitance elements can cause injury and/or severe RF burns.
Permanent Installation
The ideal installation is a rigid pole, roof mount, or tower that puts the antenna completely in the clear. If
the ideal installation is not possible, choose the best compromise. Heavy Duty TV mast, two inches in
outside diameter, heavy-duty rigid electrical conduit, and steel water pipes are suitable mast materials.
This antenna will mount on masts between 1- ¾” and 2” outside diameter. The use of soft or thin wall
masts is not recommended. These soft/ thin masts may fail and result in damage to property and possible
injury.
Note: The MFJ-1785 mast must extend 4 feet or more above the radiating element. This
extra height will give the support cables enough angle to provide sufficient support.
TOOLS AND TIME REQUIRED FOR ASSEMBLY
The estimated assembly time is two hours. Antenna assembly requires the following hand tools:
z #2 Phillips Screwdriver, for securing the 6-32 spoke screws
z 5/16” nut driver, for tightening the spokes and hose clamps
z 3/8” wrench for securing the balun u-bolt
z Two 7/16” wrenches for tightening the ¼ bolts on the mast mount assembly
z 1/2” wrench or deep well socket for tightening the u-bolts to the mast
z Heavy wire cutters/Lineman's Pliers for spoke trimming or a hack-saw
z Cable cutters for trimming the boom support cables.
z Pliers for holding the spoke ends as they are trimmed
z Safety Glasses
You also need two stable supports at least 36" tall (saw horses, trash cans, etc) during assembly, and a
short (10-12') temporary mast (1 3/4 to 2" OD) for temporary mounting during tuning..
SAFETY PRECAUTIONS:
3
Loading...