MFJ MFJ-1792, MFJ-1793 User Manual
MFJ-1792/1793 Vertical Antenna Instructions
MFJ-1792/1793 Vertical Antenna
Table of Contents:
Introduction ..................................................................................................................... 1
Description ...................................................................................................................... 1
Bandwidth, SWR, and Power Handling.......................................................................... 2
Choosing a Location for the Antenna.............................................................................. 3
Grounding........................................................................................................................ 3
Maintenance .................................................................................................................... 4
Tools and Time Required for Assembly ......................................................................... 4
MFJ-1792 Parts List........................................................................................................ 5
Assembly and Installation Procedure.............................................................................. 6
Raising the Antenna ........................................................................................................ 11
Tuning the Antenna......................................................................................................... 12
Technical Assistance ....................................................................................................... 13
MFJ-1792/1793 Vertical Antenna Instructions
MFJ-1792/1793
INSTRUCTION MANUAL
INTRODUCTION
On frequencies below 7 MHz, where vertical polarization and low wave angles are
desired, the antenna must be very high above ground or vertically polarized. On low
frequencies , the height of a vertical half wave dipole antenna is usually impraticle to
construct. A half wave vertical (Hertz) on 80 meters, for example, is approximately 133
feet high. This is a very difficult height to achieve. If a vertical antenna is grounded, the
height only needs to be a quarter wave length. The ground system acts as an "electrical
mirror". The missing half of the antenna is supplied by the "mirror image". The
directional characteristics of a grounded quarter wave antenna over perfect ground are the
same as a half wave antenna in free space. A grounded quarter wave vertical has a
circular or omni-directional pattern in the horizontal plane, and radiates maximum signal
at low angles above the horizon.
A grounded vertical antenna can be shorter than a quarter wavelength and be made
resonant by loading it with inductance, capacitance (at or near the top), or both. Loading
the antenna can be done several different ways, but not every way has the same
efficiency. In general, the best efficiency is obtained by top loading the antenna.
The efficiency of a quarter wave or a shorter vertical antenna can easily exceed 50% if
special care is taken in constructing of the ground system and the loading method. To
attain an efficiency comparable with a 1/2 wave antenna in grounded antennas having
heighths of 1/4 wavelength or less, great care must be taken to reduce ground resistance
and the resistance of any required loading inductors. MFJ 1792 / 1793 uses the best
technique available to provides maximum efficiency on the 80, 40 and 20 meter bands.
On 40 meters the antenna is a 33 foot (full quarter wave) large diameter antenna. On the
20 meter band, MFJ uses decoupling stub technology for maximum efficiency. The
decoupling stub is a full quarter wave. Top loading technology, the most efficient form of
loading, is used on 80 meters.
DESCRIPTION
The MFJ-1792 / 1793 operates as a full quarter wave on 40 and 20 meters, and a top
loaded 1/8 wavelength antenna on 80 meters. When used with a good ground system, the
MFJ-1792 / 1793 has excellent efficiency and bandwidth on 40 and 20 meters. No
loading coils are used on these bands. On 80 meters, the antenna is a very efficient top
loaded antenna. Top loading is the most efficient loading method known. It is top loaded
with a loading coil, wound on fiberglass with Teflon
diameter capacitor hat (4.5 foot). Top loading provides a uniform distribution of current
on the antenna element. Top loading guaranties maximum efficiency and performance.
The 80 meter radiation comes from the entire length of the 33 foot tall, large diameter
TM
coated wire. It also uses a large
1
MFJ-1792/1793 Vertical Antenna Instructions
aluminum tubing. The antenna has a swivel base to allow easy mounting and access to
the top for tuning the 80 meter band. The swivel base is constructed with heavy duty
aluminum plate, and a large diameter, super strong, solid fiberglass insulator. The
antenna can be mounted on a short mast with 2" maximum diameter. MFJ supplies UBolts for use with 1.5" maximum diameter masts. You can buy your own 2" diameter UBolts and install them in the holes provided on the base swivel mount. Since the antenna
is 35 feet tall it requires guying. Guying anchors and guy lines are not supplied with the
antenna. For best results, non-conductive marine type nylon cords can be used.
BANDWIDTH, SWR, AND POWER HANDLING
- The 40 meter band has a very wide bandwidth
band is covered at less than 1.5:1 SWR.
- The 80 meter band is top loaded with a high-Q loading coil. The SWR on this band is
very low (less than 1.4:1). The bandwidth is approximately 80 KHz at 2:1 SWR points.
An L-network at the feed point on the base of the antenna ensures the lowest SWR.
- The 20 meter band has a very low SWR
approximately 300 KHz. The 20 meter operating frequency is easily adjusted by
increasing or decreasing the length of the decoupling stub at the bottom of the antenna.
MFJ- 1792/1793 is capable to of handling full legal PEP power as follows:
BAND SSB CW
80 meter 1500 1500
40 meter 1500 1500
20 meter 1500 1500
(approximately 800 KHz)
(less than 1.4:1)
. The bandwidth is
. The entire
SAFETY PRECAUTIONS:
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.
2
MFJ-1792/1793 Vertical Antenna Instructions
3.) FOLLOW THE GUIDELINES FOR ANTENNA
INSTALLATION AS RECOMMENDED BY THE US
CONSUMER PRODUCT SAFETY COMMISSION.
CHOOSING A LOCATION FOR THE ANTENNA
The ideal antenna mounting location is an open area more than 70 feet away from other
large conducting objects or antennas. If such a location is
obstructed
This antenna should be mounted on a rigid support. The mounting U-Bolts are supplied
with this antenna. The antenna accepts masts between one and 1.5 inches outside
diameter. Additional holes in the base 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-1792 / 1793 will perform quite well in ground mounted installations with a
good ground system of either welded mesh wire fence or 8 or more 33 foot long (or
longer) radials. The antenna can be mounted a few feet above level with ground plane
drooping down until it reaches the ground. If the antenna is ground mounted, it
be surrounded with a protective non-metallic fencing. This will increase the safety and
lessen the likelihood of damage to the antenna.
location should be selected.
available, the
not
least
MUST
GROUNDING
Unlike some other verticals, this antenna requires an RF ground system. However, it is a
good idea to have some sort of earth connection to avoid lightning damage to 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.
Grounds should consist of a solid copper rod driven into the soil (at least 6 feet deep).
Never use woven flexible braiding for ground connections unless
absolutely necessary
.
Braiding has a 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 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
3
MFJ-1792/1793 Vertical Antenna Instructions
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 connector 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". It 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!
WARNING:
- Keep this antenna out of reach of adults, children, and animals.
- Any contact with this antenna while transmitting will cause severe RF
burns and voltages that kill.
- Never place this antenna close to electric power lines or utility wires.
Tools and Time Required for Assembly
The estimated assembly time for this antenna is two to three hours. An additional one
hour is required to adjust the antenna on all the bands.
The required tools are:
1/4" standard screwdriver
3/8" open end wrench ( if you purchased the 1793 )
5/16" nut driver
7/16" nut driver
7/16" open end wrench
Large wire cutters
#1 or #2 Philips screwdriver
Eye protection
Flat ground or at least 3 saw horses.
SAFETY PRECAUTIONS!!
1.) THIS ANTENNA IS AN ELECTRICAL CONDUCTOR.
2.) ANY CONTACT BETWEEN THE ANTENNA, FEEDLINE OR
SUPPORTING STRUCTURE AND POWER LINES WILL RESULT
4
Loading...