MFJ MFJ-1762 Manual de usuario

MFJ-1762 Instruction Manual

Introduction: Thank you for purchasing the MFJ-1762 three-element six-meter Yagi.

The MFJ-1762 is a light-weight directional antenna especially designed for installation

with readily-available TV-type masts, mounts, and hardware. The driven element employs

a "no-tune" hairpin impedance matching system, and all elements are cut to exact length at

the factory. No adjustment should be needed for coverage of the 50-MHz SSB band (see

page 8 for FM-segment tuning instructions). Because of its compact size, light weight,

and unique construction, the MFJ-1762 is also a excellent choice for six-meter portable or

"rover" operation. In weak-signal DX applications, two MFJ-1762s may be stacked for

increased capture area and 3-dB additional gain. The MFJ-1762 has been computer

modeled on both ELNECand YAto confirm design integrity, and has been thoroughly

field tested in northern climates for winter survivability.

Typical Specifications:

Boom length......................6'

Turning Radius..................5' 8"

Longest Element...............117-1/2"

Weight..............................2.5 Lbs.

Feed Impedance...............50Ω

Resonant Frequency..........50.3 MHz

1.5:1 VSWR Bandwidth...1.7 MHz

Forward Gain....................6.0 dBd (8.2 dBi)

Front-to-Rear Ratio........>18 dB measured

Maximum Power...............150 Watts

Safety Precautions: Before assembling, please read the following safety notice:

This antenna is an electrical conductor--do not handle or mount near power lines, service

entrances, or other dangerous power sources. Mount out of the reach of adults, children,

and animals. Antenna elements develop lethal voltages and may cause severe RF burns

during transmitter operation. For lightning protection, always ground your supporting

mast to two or more outdoor ground rods. Disconnect the feedline from your radio when

not in use. Do not expose pacemakers or other bio-medical equipment to strong RF

radiation. To avoid personal injury or damage to the antenna, plan all mechanical aspects

of your installation carefully. Avoid handling heavy or unwieldy masts by yourself, and

make sure a second person is available to assist you in an emergency. Never work on a

roof or climb a tower alone!

MFJ-1762 Parts List: To begin, confirm that all necessary parts have been supplied with

your antenna package:

[ ] 1 - 6' x 1-1/8" OD boom

[ ] 2 - 54" x 1/4" OD element section (director)

[ ] 2 - 54-1/2" x 1/4" OD element section (driven element)

[ ] 2 - 59-1/4" x 1/4" OD element section (reflector)

[ ] 1 - 5/16" x 1/4" OD spacer

[ ] 1 - hairpin inductor

[ ] 2 - "L" bracket

[ ] 1 - #8 x 1/2" sheet-metal screw

[ ] 6 - 1/2" x 10-32 screw

[ ] 2 - 1/2" x 6-32 screw

[ ] 2 - 6-32 nut

[ ] 2 - 6-32 flat washer

[ ] 2 - 6-32 lock washer

[ ] 4 - Insulated polyethylene element-mounting grommets

[ ] 2 - #6 solder lug

[ ] 1 - 1 - U-bolt mounting hardware kit (U-bolt, mast clamp, nuts)

[ ] Tube of Locktite 

If any parts are damaged or omitted, please contact MFJ at (601) 323-0549 to arrange for

replacement.

Tools: You'll need a selection of common household tools to construct your antenna:

Phillips-head screw driver (#2)

Straight-blade screw driver

Pliers

Utility knife or Ex-actoknife with #11 blade

Soldering Set-up

Suggested Antenna Materials:

RG8-M Coax (also RG59 if stacking antennas)

Sealant (Seal-All ) or equivalent

Plastic electrical tape

Suggested Test Equipment:

MFJ-259 RF Analyzer or VSWR bridge

Construction:

[ ] Begin by laying the boom on a flat surface. Find the driven-element holes and the two

sets of mast-mount holes (one for vertical and one for horizontal mounting). The mast-

mount holes are toward the reflector end of the antenna. Identify the director and

reflector ends of the boom before starting assembly.

[ ] Find two (2) 54" director element sections and identify the tapped end of each. Insert

the tapped ends into the 1/4" holes drilled at the director end of the boom. Secure each in

place with 10-32 screws. If you do not anticipate disassembling your beam, apply a drop

of Locktite to each screw. Grip the element section gently with pliers when tightening.

Director Driven Element Reflector

Hairpin Inductor

35-1/4" 35-1/4"

Boom

Mounting Clamp

#10 Screw

107" tip-to-tip 108-1/4" tip-to-tip 117-1/2" tip-to-tip

Driven Element

Mounting Holes Mast

[ ] Find two (2) 59-1/4" reflector element sections and repeat the same mounting

procedure at the back end of the antenna.

Element

#10 screw

Boom Boom

Element

"L" Bracket

#10 Screw

Insulating Grommet

Driven Element Mounting Detail

Director, Reflector Mounting Detail

[ ] Find four (4) black polyethylene insulating grommets and snap one into each driven-

element mounting hole (do not confuse these holes with the U-bolt mounting holes).

[ ] Locate the two remaining element sections (54-1/2" each) and slip the tapped end of

each through the driven-element insulating grommets.

[ ] Using 10-32 screws, install a "L" bracket on each driven element section as shown in

the construction detail. Apply Locktite to the screws and secure firmly in place, using

pliers to gently grip the element.

[ ] Locate the aluminum hairpin inductor and install as shown below, using a #8 sheet

metal screw and the 5/16" x 1/4" OD spacer. Note that the end mounting holes on the

hairpin should align with the driven element "L" brackets.

Hairpin Inductor

Element

#6-32 Mounting Screw

"L" Bracket

Boom

Insulating Grommet

#8 x 1/2" SM Screw

5/16" spacer

Hairpin Mounting Detail

Boom

Hairpin

5/16" Spacer

Element

#6-32 Mounting Screw

#8 x 1/2" SM Screw

[ ] Temporarily fasten each "L" bracket to an end of the hairpin inductor with #6

hardware. The coaxial feedline will be connected to these points later on.

[ ] Temporarily install the mast-mounting hardware (U-bolt can go on either way).

This completes initial mechanical assembly. Check all fasteners for tightness.

Feedline Installation:

For coax runs up to 70 feet, we recommend using RG8M (or mini-8) 1/4" diameter foam

coax. This light-weight cable is available through most amateur radio dealers and is

available in Radio Shack stores nationwide. RG8M performs well at 50-MHz and will

handle 150 Watts with a reasonable margin of safely. Use the following steps to prepare

your feedline:

[ ] Strip back the outer insulation 1" and prepare the two conductors as pigtails--as

shown in the diagram below.

RG8M Lugs

Sealant

[ ] Install a lug on each pigtail (crimp-type lugs should be crimped and soldered). Pigtail

length should not exceed 1-1/4"--including the lug. Excessively-long pigtails will

detune the matching system.

[ ] Apply 2-3 thin coats of Seal-Allto the exposed braid to retard water migration into

the cable. If Seal-All is not available, use a rubberized contact cement or a silicon sealer.

Allow sealant to dry thoroughly before handling the feedline.

[ ] Connect each coax pig-tail to a feedpoint stud using the #6 hardware provided (do not

use Locktite on these connections). Note that the hairpin is installed on the bottom (or

earthward) side of the boom. When installing the MFJ-1762, the hairpin should be on the

ground side and your coax should drop directly below the hairpin.

Balun: The MFJ-1762 driven element is balanced. In order to prevent unwanted

radiation from the feedline, we suggest installing a balun at the feedpoint. This need not

be an elaborate or expensive addition--you can make a simple and effective choke-type

balun using the feedline itself. To do this, form a 5-turn coil of RG8M just below the

feedpoint using about 6' cable. Tape the coils together tightly with electrical tape and

secure them to the mast. As an alternative, install three (3) FB43-5621 ferrite cores on the

feedline at the feedpoint. Be sure to secure your feedline to the mast with electrical tape

to provide stress relief at the feedpoint.

Horizontal Mounting: The MFJ-1762 presents about the same mast-loading as a small

TV antenna. This means you can erect it high and in the clear using standard TV mast,

roof or chimney-mounting hardware, and a light-duty rotator. To avoid property damage

or injury, plan your installation carefully and follow the same safety guidelines that apply

to any TV-antenna installation. Be especially careful not to bend or mis-align elements

when raising the mast. If you stack-mount the MFJ-1762 with other antennas, allow 5'

vertical spacing between higher-frequency Yagis and 10' spacing between lower-frequency

Yagis. Mounting the MFJ-1762 (or any Yagi) too close to existing antennas may distort

the pattern--or raise the launch angle of your signal and compromise performance.

Vertical Mounting: The MFJ-1762 may also be mounted vertically for FM (or for West

Coast SSB) operation. When mounting a single antenna vertically, precautions are needed

to ensure that the mast and feedline do not upset the antenna's VSWR and radiation

pattern. In order to prevent the mast from detuning the driven element, we recommend

using a 8' fiberglass "painter's pole" as a mast. This will provide a strong RF-insulated

support. Fiberglass poles are available at most large home-care centers. A wooden closet

pole will also work--if weather-proofed with a uV-resistant finish. To prevent the feedline

from detuning the driven element, run your coax down the boom to the reflector end, then

let it drop down behind the reflector and below the rotor. If needed, use a short length of

wooden dowel to extend the boom--this will keep the coax from hitting (and detuning) the

reflector. Note that these precautions are not needed if you use a stacked pair of vertical

beams. In this case, the vertical mast will be located mid-way between the antennas and

will not interfere. A stacked pair is always preferable for vertical installations.

Single Vertical Mount Stacked Vertical Mount

Fiberglass Pole

Stacking Two MFJ-1762s: For best stacking performance, use 5/8-wave antenna

separation. In most installations, a single 12' x 1-1/4" length of thin-wall 6061 aluminum

mast may be used to support both antennas. For horizontal stacks, the first antenna

should be mounted as close as possible to the rotor, and the second mounted at the top of

the mast. This will provide approximately 11' 6" of spacing. In vertical installations, a 10'

mast may be substituted with only a minor loss in gain with both antennas mounted at the

extreme ends of the pipe.

In order to feed two antennas with 50Ωline, you'll need to make a simple phasing and

matching harness. The harness is made from readily-available 75Ωline. Either solid or

foam-dielectric RG59 is suitable for this purpose. Prepare your harness as follows:

[ ] Cut one (1) electrical-wavelength of RG59. Be sure to use the correct velocity factor

for your particular cable (155-5/16" for 0.66 VF solid dielectric and 186" for 0.79 VF

foam cable). If your cable has a different VF (velocity Factor), use the formula:

Length in Inches

11800

x VF

Freq

in MHz

[ ] Install three (3) FB43-5621 ferrite sleeves at each end of the cable.

[ ] Prepare pigtails with lugs at each end (see previous instructions for RG8M).

[ ] Cut the RG59 1/4-way down its total length and install PL-259's (at 38'3/4" for 0.66

VF solid or at 46-1/2" for 0.79 VF foam).

[ ] Splice the RG59 back together using a coaxial type "T" adapter--this becomes the

antenna array's 50-Ohm feedpoint.

[ ] Connect pigtail ends to the antennas. Note that this "off-center fed" harness shifts

signals 180-degrees out of phase. To correct for this, be sure to transpose pigtail

connections at the second antenna. If you don't do this, the yagis will be fed out-of-

phase and signals will cancel rather than add! You must install the harness as

shown below:

50-Ohm Feed

RG59

RG8X

Top Antenna

Bottom Antenna

Harness

Portable Operation with the MFJ-1762: Although designed for permanent roof-top

installations, your MFJ-1762 makes an ideal portable antenna. Its unique construction

permits you to remove the elements in less than a minute for easy storage or travel. To

prepare for transport, simply remove the six #10-32 element mounting screws and detach

each element sections from the boom. The feedline and hairpin assembly may remain

attached. If you remove the U-bolt assembly, elements may be stowed inside the boom.

Tuning the MFJ-1762 for FM Sub-band Operation:

The MFJ-1762 is easily retuned for operation in the FM portion of the six-meter band. To

do this, simply re-cut the element sections as shown below using a hacksaw or tubing

cutter. Be careful not to cut the threaded ends when shortening elements!

[ ] Reflector sections, cut for 56-3/4" each.

[ ] Driven element sections, cut for 52-3/8" each.

[ ] Director sections, cut for 51-3/4" each.

This will provide minimum VSWR near the 52.525-MHz FM-simplex calling frequency,

and should provide low VSWR on most 6-Meter FM-repeater input frequencies.

In Case of Difficulty: If you experience difficulty with your MFJ-1762, please check out

the following possibilities before contacting the factory:

High VSWR or Intermittent operation:

[ ] Inspect all connectors for a broken lead, water, ice, severe corrosion, or dirt.

[ ] Inspect coax for kinks, chaffing, crushed areas, breaks in jacket, water infusion, etc.

[ ] Inspect the antenna for loose or broken elements, broken coax pigtail, or debris.

[ ] Check in-line devices such as amplifiers, coax switches, VSWR meters, etc.

[ ] Check balun sleeves for damage from excessive RF heating.

No Sensitivity, Poor directivity:

[ ] Improper phasing (stacked beams only).

[ ] Extreme build up of ice and snow.

IMPORTANT NOTE: Rain, snow, or ice accumulating on antenna elements normally

increases VSWR by lowering the antenna's resonant frequency. Precipitation detuning is

normal for all VHF Yagis, and the MFJ-1762 is cut approximately 200 kHz high in

frequency to help compensate for this effect in wet weather.

The MFJ-1762 is covered by MFJ's exclusive "no matter what" 1 year warranty. If your

antenna does not work properly and a thorough inspection fails to identify the problem,

please contact the factory for further assistance. To contact MFJ's Tech Hotline, call

(601) 323-0549. Be sure to have the manual and a complete description of the difficulty

handy so that you can answer any questions.

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