MFJ 1798 Manuel utilisateur

NOTE: This Drawing Is Not

To Scale!

MFJ-1798 Vertical Antenna Instructions

MFJ-1798 Vertical Antenna

INTRODUCTION and THEORY OF DESIGN

The MFJ-1798 is an 80 through 2 meter ground independent vertical antenna. This antenna has

several unique features including an inverted radiating element, a fully elevated counterpoise,

lumped constant end loading, and linear stub decoupling. These unique features eliminate the

traditional problems encountered in ground independent multi-band vertical designs.

Established antenna theory dictates that maximum radiation occurs in the high current portion of

the antenna. While conventional antennas generally place the highest current at the bottom or

center of the radiating element, the MFJ-1798's unique design places the highest current and the

maximum radiation point at the very top of the radiating element. This feature produces the

lowest wave angle possible for a given antenna height, and elevates the radiating section away

from the lossy earth.

It is a well-documented fact that bending or folding a radiating element at a high current point

reduces antenna system efficiency. The MFJ-1798 eliminates all bending or folding of the

important high current areas by using linear decoupling stubs on frequencies above 14 MHz.

These linear decoupling stubs provide low loss operation without compromising bandwidth.

Efficient lumped constant end loading produces the highest efficiency possible on frequencies

below 14 MHz.

Counterpoises must be an appreciable fraction of a wavelength above ground if losses are to be

minimized. The MFJ-1798 design places the counterpoise at the top of the radiating element.

This provides maximum separation between the counterpoise and the lossy earth. For example, a

ground mounted MFJ-1798 has a counterpoise elevation of one half wavelength on 10 meters.

DESCRIPTION

The MFJ-1798 is a multi-band vertical that operates on every amateur band between 80 and two

meters. The inverted radiating element is constructed from heavy duty 6061-T6 aluminum

tubing. This antenna performs as a full size inverted ground plane on all bands above 14 MHz,

and as an efficient end loaded vertical on 30, 40, and 80 meters.

The 12 feet diameter counterpoise is constructed by mounting four six feet long tubes on a

fiberglass insulated plate at the top of the antenna. The tubing has a wire that circles the

perimeter of the counterpoise to stabilize the support and increase the capacitance of the hat.

A 1/4 wave two meter vertical is mounted above the counterpoise.

Four linear decoupling stubs are tuned for 10, 12, 15, and 17 meters. These stubs are constructed

from 3/16" solid aluminum rod and supported by fiberglass insulators.

MFJ-1798 Vertical Antenna Instructions

A small adjustable "T" section near the bottom allows adjustment of the resonant frequency of

the full size radiating element on 20 meters.

A three section loading coil with capacitance hats for 30, 40, and 80 meters is mounted next to

the 20 meter "T" adjustment near the lower end of the antenna.

The base section contains the air core high power choke balun. This balun uses Teflon insulated

coaxial cable. The base section normally mounts on masts below 1-1/2" diameter, but additional

holes are provided to accept masts up to 2" diameter with user supplied clamps.

10 1500 2500 1250 2.0MHz

6 300 750 300 2.4MHz

2 200 200 200 7MHz

CHOOSING A LOCATION FOR THE ANTENNA

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 burns or other injuries.

For the best performance on receiving and transmitting, mount this antenna in a clear location at

least a few feet above or 70 feet away from buildings, towers, feedlines, utility wires, and other

antennas. While your own ingenuity and particular circumstances will determine the final

mounting method, we will pass along few rules that can not be neglected.

Never mount this antenna in a location that will permit unsuspecting people to come in contact

with the loading spokes or any other part of the antenna.

The power rating of this antenna varies from band to band. The PEP ratings are primarily

determined by the voltage breakdown of the components, while the CW ratings are generally

determined by component heating.

The following chart lists the power rating and the 2:1 VSWR bandwidth of this antenna:

Band Power Bandwidth

CW SSB RTTY

80 1000 1500 750 35KHz

40 1250 1500 1000 25KHz

30 1500 1500 1500 370KHz

20 1500 2500 1500 700KHz

17 1500 2500 1500 680KHz

15 1500 2500 1500 800KHz

12 1500 2500 1250 750KHz

MFJ-1798 Vertical Antenna Instructions

Never mount this antenna where a mechanical failure might allow the antenna to contact power

lines or other utility wires.

Always ground the feed line at the point where it enters a building to a good earth ground for

lightning protection.

MOUNTING LOCATION

WARNING: Always mount this antenna so that it is out of the reach of adults as well as

children. Contact with any part of this antenna can cause injury and / or severe RF burns.

The ideal antenna mounting location is an open area either above or more than 70 feet away from

other large conducting objects or antennas. If such a location is not available, the least obstructed

location should be selected.

This antenna should be mounted on a rigid support. The mounting clamps supplied with this

antenna accept masts between one and 1.5 inches outside diameter. Additional holes in the mount

of this antenna will accept mast clamps up to 2" OD. Soft or thin wall masts should not be used

to mount this antenna to any supporting structure.

The MFJ-1798 will perform quite well in ground mounted installations. The lower end of the

antenna should be at least 5 ft above ground level. If the antenna is ground mounted, it MUST be

surrounded with a protective non-metallic fencing. This will increase the safety and lessen the

likelihood of damage to the antenna.

GROUNDING

Unlike most verticals, this antenna does not require an RF ground system. However, some sort of

earth connection is a good idea to avoid lightning damage to the station equipment and to

improve operator safety.

The feedline's shield should be grounded at the entrance point of the building before it reaches

the operating position. Failure to do so can result in lightning damage to the equipment. This will

also help prevent voltage from being applied to the antenna support if the station equipment ever

develops a problem.

Additional lightning protection can be achieved by burying the feedline directly in the soil for a

minimum of 10 feet before the cable enters the building. It is also an excellent idea to ground the

mast or supporting structure.

Grounds should consist of multiple rods driven into the soil. Several long straight buried wires

will assist in dissipating lightning strikes. Leads from the ground system to the antenna mast or

coaxial cable should be as short and direct as possible. Sharp bends in the ground leads should be

avoided. It is an excellent idea to connect all the grounds together outside the building with

MFJ-1798 Vertical Antenna Instructions

smooth, wide, heavy conductors. This includes connecting the power line grounding rod to the

feedlines ground.

Never use woven flexible braiding for ground connections unless absolutely necessary. Braiding

has very high resistance to both 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.

The use of an in-line coaxial lightning arrestor will only offer a minimal improvement in

lightning protection. The best method of protecting the station equipment is to disconnect the

feedline outside the building or, at the minimum, use a safety switch to disconnect and ground

the feedline.

MAINTENANCE

Your antenna is constructed of heavy duty non corrosive materials and should withstand normal

climates for many years. The use of some type of coaxial connector moisture protection is

recommended at the bottom coax connection and also around the matching network feed point,

especially in coastal areas where salty mist is commonplace.

GE makes a pure silicone grease called "SILICONE DIELECTRIC COMPOUND" that can be

applied SPARINGLY to the threaded area of the female connector. This compound, or even a

clear silicone heat sink compound, will prevent moisture from entering the connector through the

threads and protect the connectors from corrosion. 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 sold as "coax seal".

This is a pliable black sealing compound.

When installing any coax sealer, NEVER completely cover the barrel of the coax connector. The

sealer should ONLY be placed near the junction of the threaded part of the chassis connector and

the knurled area of the male connector. This will leave the bottom of the male connector's outer

sleeve open and permit the connector to "breathe" so it does NOT collect moisture!

IMPORTANT FOR COASTAL AREA OPERATORS: It is advisable to use some type of

silicon spray (circuit board type from Radio Shack) on the capacitor rings to prevent corrosion

between the aluminum and the stainless steel screws. Corrosion on the capacitor rings tend to

make it difficult to remove the spokes to retune the antenna.

TOOLS AND TIME REQUIRED FOR ASSEMBLY

The estimated assembly time for this antenna is four to six hours. An additional one to two hours

is required to adjust the antenna on all the bands. The most convenient place to adjust this

antenna is on a ground mounted temporary mast or support.

MFJ-1798 Vertical Antenna Instructions

The required tools are:

1/4" standard screwdriver

3/8" open end wrench

5/16" nut driver

7/16" nut driver

7/16" open end wrench

Large wire cutters

#1 Philips screwdriver

#2 Philips screwdriver

Eye protection

Temporary mast (5-8').

Step ladder or saw horses.

One soldering iron

MFJ-1798 PARTS LIST

NOTE All screws and nuts are stainless steel.

As you unpack this antenna you should find the following parts.

In the event any parts are missing, please contact us

immediately.

Group A

Packed loose in box or inserted inside each other:

[ ] One 30" long, 1" dia. fiberglass loading coil assembly

[ ] One bundle of 14 long capacitance hat spokes

[ ] One bundle of 4 short capacitance hat spokes

[ ] One 6' long 6060-T6 aluminum tubing 1-3/8" OD.

[ ] One 6' long 6061-T6 aluminum tubing 1-1/4" OD.

[ ] One 50'' long 6061-T6 aluminum tubing 1-1/8" OD.

[ ] Four 6' long 6061-T6 aluminum tubing 1/4" OD.

The following bundle of 3/16" diameter aluminum rods:

Threaded rods Non-threaded rods

[ ] Four 55" [ ] Four 72"

[ ] One 19" [ ] One 66.5"

[ ] One 48"

[ ] One 41"

[ ] One 15"

[ ] One fiberglass base insulator mounted on a heavy aluminum bracket

MFJ-1798 Vertical Antenna Instructions

[ ] One balun and feedline assembly with small coil and capacitor

network

Group B (Hardware Bag #1)

[ ] One bag of hardware containing the following parts groups:

(1) Five #4 self tapping screws (4) Twenty-nine 6-32 nuts

Five 4-40 x 3/8" screws Two 10-32 split ring washers

Five 1/4-20 x 1-3/4" bolts

Four #4 flatwashers

Five 1/4" split ring washers

(2) Thirty 6-32 x 1/4" screws (5) Six 3/16" ID aluminum couplers

Twelve 10-32 nuts Six black cable ties

Two white nylon insulators

(3) Twenty-seven 6-32 x 5/8'' screws One red plastic cap

Two 10-32 x 1-3/4" screws

Five 1/4-20 nuts

Group C (Hardware Bag #2)

[ ] One bag of large hardware containing the following parts groups:

(1)Three small aluminum "L" brackets (3) Four counterpoise clamps with no flange

Three triangular aluminum brackets Four counterpoise clamps drilled flange

One rectangular aluminum bracket One 20 meter adjustment "U" clamp

(2)Five flat rectangular fiberglass stub insulators

One set of fiberglass and aluminum

counterpoise support assembly plates

One hollow center, square aluminum plate

Group D (Hardware Bag #4)

[ ] One bag containing the following:

Seven hose clamps (4 clamps are pre-installed on the angle

bracket in this bag.

3 clamps are packed loose in the bag to be sed in

step 9, 10,

and 11 on page 11.)

Two "U" bolts with saddle brackets, nuts, and split wahers to fit

masts under 1-1/2" OD

35" (approximately) of small tinned wire

MFJ-1798 Vertical Antenna Instructions

One large angle bracket (with four host clamps) with slots for

mounting loading coil

ADDITIONAL USER SUPPLIED MATERIALS

Please be sure you have the following additional items available:

[ ] A 6'-8' rigid mast or other mounting pipe between 1" and 1.5" outside diameter (suitable

materials include a heavy duty TV mast section or galvanized steel pipe).

[ ] Quality low-loss 50-Ohm coax with PL-259 connectors to reach from antenna to transmitter.

[ ] A SWR meter or Analyzer (MFJ-207, MFJ-249 or equivalent).

[ ] Enough 1/8 to 3/16" nylon rope to reach the ground from the antenna's final location.

SAFETY PRECAUTIONS!!

Please remember the following important points:

1.) THIS ANTENNA IS AN ELECTRICAL CONDUCTOR.

2.) ANY CONTACT BETWEEN THE ANTENNA, FEEDLINE, OR

SUPPORTING STRUCTURE AND POWER LINES WILL RESULT IN

DEATH OR SERIOUS INJURY.

For safety please:

1.) DO NOT INSTALL THIS ANTENNA WHERE THERE IS ANY

POSSIBILITY OF CONTACT WITH UTILITY LINES.

2.) DO NOT LOCATE THE ANTENNA SUPPORT WHERE IT CAN

CONTACT POWER LINES IN THE EVENT OF MECHANICAL

FAILURE OR ACCIDENT.

3.) FOLLOW THE GUIDELINES FOR ANTENNA INSTALLATIONS AS

RECOMMENDED BY THE US CONSUMER PRODUCT SAFETY

COMMISSION.

ASSEMBLY PROCEDURE

During assembly, refer to the drawings throughout this manual and

the overview drawing on Page 1. Follow these instructions in the

exact sequence listed to avoid errors and achieve maximum

performance from this antenna.

MFJ-1798 Vertical Antenna Instructions

After the antenna is assembled, you must double check the

hardware for proper tightening and assembly. The resonant

frequency and SWR can be tested with the antenna only a few feet

above ground on a temporary support before raising it to the

final tower or rooftop position.

NOTE: Wear safety glasses whenever working near or on this

antenna.

WARNING! You can be killed if the antenna, feed line, or the

equipment used to install the antenna accidentally contacts any

utility lines. Never install an antenna near power lines!

1. Be especially careful while moving or installing this antenna. This antenna is bulky enough to

cause a loss of balance if handled by an inexperienced person, or if the counterpoise or a

capacitance spoke accidentally becomes snagged on a gutter, a tree limb, or any other object.

2. Mount the antenna high enough to place it completely out of reach, or protect it with a fence.

The ends of the capacitance spokes and other areas of the antenna can cause eye injury, serious

RF burns, or both.

3. Be sure the mast is sturdy enough to support 20 pounds of weight and a wind load of

approximately 3 square feet.

Step By Step Assembly Procedure

1- Prepare a temporary 5'-6' ground-level mounting mast. This temporary mast will simplify the

initial testing and adjustment of this antenna. It is also a good idea to have one or two stable

supports (table, saw horses, etc.) and a 6' step ladder available at the assembly location.

2- Be sure the parts are separated in the groups listed earlier. Be sure all of the parts are available

BEFORE beginning the actual antenna assembly.

3- Gather the following tools for the basic assembly. At minimum, these consist of:

•- #1 Philips screwdriver for capacitance spokes screws

•- #2 Philips screwdriver for other 6-32 and 10-32 screws

•- 1/4" standard screwdriver or 5/16" nut driver for hose clamp

•- 3/8 open end wrench for stub adjustments

•- Two 7/16 open end wrenches or one wrench and a 7/16" nut driver for

U bolts and coupling bolts

•- Wire cutter for trimming capacitance spokes and counterpoise wire

•- Safety glasses

•- One soldering iron

MFJ-1798 Vertical Antenna Instructions

NOTE - A small adjustable crescent wrench can substitute for the 3/8" and 7/16" open

end wrenches.

IMPORTANT: Do not use a high torque electric screwdriver to mount the

capacitance spokes or any of the #4 through #10 hardware.

The hardware may be damaged if excessive or uncontrolled

torque is applied during assembly.

Locate the following parts. The abbreviation "Gp" precedes the Group the part is located in:

One fiberglass base insulator and mounting bracket assembly (Gp A)

One balun and feedline assembly (Gp A)

Two 1/4-20 x 1-3/4" bolts (Gp B1)

Four 6-32 x 5/8" screws (Gp B3)

Two 1/4-20 nuts (Gp B3)

Two 1/4" split ring washers (Gp B4)

Four 6-32 nuts (Gp B4)

Two white nylon insulators (Gp B5)

U-bolt

Referring to figure 1:

4- Install the U-bolts on the open face of the base insulator mounting bracket.

Mount the saddle clamps with the flat side against the plate, and the toothed side facing the

rounded U-bolt area. These clamps will secure the antenna to the mast.

5- Put the lock washers over the 1/4-20 bolts and slide one of the bolts through the upper balun

mounting hole on the side bracket of the base mounting plate. The upper mounting hole is

located at the end of the bracket where the fiberglass insulator sticks out.

6- Use a white nylon insulator to space the balun away from the bracket, and be sure the balun

hole closest to the end of the balun with the very long lead is bolted to the top bracket hole.

Install the nut inside the balun form. Finger tighten only.

7- Attach the bottom balun hole to the bottom hole in the mounting assembly as explained in step

5. Snug both bolts until the split ring washer just collapses with a 7/16" wrench.

8- Bolt the coax connector housing to the base mounting assembly with four 6-32 x 5/8" screws

and 6-32 nuts.

Figure

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