MFJ-1792/1793 Instruction Manual
Vertical Antenna
1
MFJ-1792/1793
INSTRUCTION MANUAL
INTRODUCTION
In cases where vertical polarization and low wave angle is desired, at frequencies below 7
MHz, the antenna must a vertical. At these low frequencies, the height of a vertical half
wave dipole antenna is beyond the construction reach. A half wave on 80 meters, for
example, is approximately 133 feet high, which is beyond the doubt very difficult to make.
If the lower end of a vertical antenna is grounded, the antenna height need to be only a
quarter wave length to resonate at the same frequency as the ungrounded half-wave
antenna. The reason for this is that ground having high conductivity acts as an electrical
mirror, and the missing half of te antenna is supplied by the mirror image. The directional
characteristics of a grounded quarter wave antenna is the same as that of a half wave
antenna in free space. Thus, a grounded quarter wave vertical has a circular or omni-
directional pattern in the horizontal plane.
A grounded vertical antenna can be made much shorter than a quarter wavelength and still
be made resonant at the same frequency by loading it with inductance, capacitance, or
both. Loading the antenna can be done several different ways, but not every way yields
the same efficiency. The best efficiency is obtained by top loading the antenna.
The efficiency of a quarter wave or a shorter vertical antenna
(shorter than a quarter
wave)
can go over 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 the 80 meter band.
DESCRIPTION
The MFJ-1792 / 1793 operates as a full quarter wave on 40 and 20 meters, and a top
loaded antenna on 80 meters. Over a good ground system the MFJ-1792 / 1793 has
excellent efficiency on 40 and 20 meters, because no loading coils are used on these bands.
Also, 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
TM
coated wire, and a large diameter capacitor hat (4.5 foot). Top-
loading is the most efficient loading method known. When loaded on top, the distribution
of current on the antenna element ofa short quarterwave in slmost uniform. Top loading
guaranties the maximum radiation intensity as well as the best efficiency and performance.
