MFJ MFJ-1742 User Manual

MFJ-1742 20 Meter Double Extended Zepp Instruction Manual

INTRODUCTION

The MFJ-1742 is center fed wire antenna that, when properly erected and fed
with balanced feedline, will give up to three decibels (dB) power gain on the 20­Meter amateur band, along with directivity broadside to the antenna. In addition,
with a proper antenna tuner it will also give good results on all amateur bands
from 5 to 50 MHz. It also will work acceptably well on the 3.5 MHz band,
although it is somewhat shorter than a half wavelength. This antenna will handle
1500 watts of RF power (with a suitable antenna tuner). Its total length is
approximately 84 feet.

Center-fed wire antennas, when extended beyond a half wavelength in length,
begin to exhibit gain and directivity over a halfwave dipole. Such antennas are
called collinear arrays, and their gain and directivity increase as their length
increases, up to a point.

The simplest of these collinear
arrays is a centerfed antenna one
wavelength long, called “two half
waves in phase” or a “Double
Zepp.” Such an antenna has about

1.6 dB of gain over a dipole.

An extended double Zepp (figure
at right) is longer: 0.64
wavelengths per side, or about 5/8
wavelength per side. The formula to determine the length of each side in feet is
599 divided by the frequency in MHz.

This antenna has some 3 dB of gain over a dipole on 20 Meters. A technical
discussion of how this antenna works is beyond the scope of this manual, but, in
simple terms, at 0.64 wavelength per side the current points of the wires are
moved out from the feedpoint, providing electrical spacing between the two
sides (elements) of the antenna. This results in gain and directivity.

Unfortunately, increasing the leg lengths beyond 0.64 wavelength makes the

WARNINGS: Always mount antennas so that they are out of

the reach of adults and children. Contact with any part of

this antenna can cause RF burns or other injuries.

Constructing or erecting antennas where they may contact

electrical power lines can result in injury or death.

1

MFJ-1742 20 Meter Double Extended Zepp Instruction Manual

whole system look like a “long wire” antenna of random length, and the gain
decreases.

The gain of the double extended
zepp is contained in two large
“lobes” at right angles to the
antenna (figure at right). Minor
lobes exist in other directions,
giving the antenna about the same
gain (i.e. 1) as a dipole in these
directions.

Because a double extended zepp is
not resonant at its design
frequency, its input impedance is
complex and very high, so it must
be fed with balanced feedline through an antenna tuner.

Assembly

This antenna comes assembled and ready to install. The lengths of the antenna
wires are quite critical and should not be changed. Nylon ropes are needed to
support the ends of the antenna from suitable structures or trees.

Installation

The best location for this antenna is as high and far away as possible from utility
wires, other antennas, and other structures. It is impossible to find a perfect
location, so the best compromise must be accepted. In order to perform properly
the antenna must be installed as a horizontal antenna, using two tall supports
separated by more than 85 feet.

Suspend the antenna with at least a 50-pound working load nylon rope or
equivalent strength weather resistant non-metallic rope. Never use wire or wire
core rope to support the ends of any antenna. Attach the rope to the end
insulators through the empty holes.

Try to keep the antenna as horizontal (level) as possible. The antenna should be
at least 35 feet above ground to give acceptable performance and as high as
possible for the best overall performance. The feedline should drop vertically
from the horizontal section of the antenna as far as is practical.

If you are using trees for end supports, be sure to allow enough slack or use
some type of pulley and counterweight system to prevent the antenna

2