MFJ MFJ-1785 User Manual

80/40/20 Meter Rotatable Dipole

Model MFJ-1785

INSTRUCTION MANUAL

CAUTION: Read All Instructions Before Operating Equipment

MFJ ENTERPRISES, INC.

300 Industrial Park Road

Starkville, MS 39759 USA

Tel: [phone] Fax: [phone]

VERSION 0B

COPYRIGHT

C

2007 MFJ ENTERPRISES, INC.

MFJ-1785 Rotatable Dipole

80, 40, 20 Meters

		Ceramic		3/16”	cable
		Insulator
		T3	T2		T1
	20M		Tubing
	40M	T1	1 1/2” x 72”
80M
		T2	1 3/8” x 72”
		T3	1 1/8” x 60”

Drawing not to scale.

Boom support U-bolt

T1

T2

1 1/4” x 20” Fiberglass rod

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 ¼ x 20” solid fiberglass rod	1	811-1785-4
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
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.

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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 TeflonTM wire and capacitance hats.

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

Band		Power
	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

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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

z5/16” nut driver, for tightening the spokes and hose clamps

z3/8” wrench for securing the balun u-bolt

zTwo 7/16” wrenches for tightening the ¼ bolts on the mast mount assembly

z1/2” wrench or deep well socket for tightening the u-bolts to the mast

zHeavy wire cutters/Lineman's Pliers for spoke trimming or a hack-saw

zCable cutters for trimming the boom support cables.

zPliers for holding the spoke ends as they are trimmed

zSafety 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:

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