N4TAB Carolina Moxon NVIS Antenna
This antenna evolved from consideration of NVIS antenna types in
common usage and for which public domain documentation exists.
The concept of a wire beam antenna is not new; indeed, Les Moxon
developed a unique form of such an antenna which bears his name.
All of the public domain documentation and descriptions that I can
locate describe Moxon Rectangle antennas as either horizontally
polarized forms or vertically polarized forms, intended for orientation
toward the horizon.. Since I believe that my usage of this
antenna type is unique as an NVIS antenna, I assert that my flag is
firmly planted within this form and therefore, the NVIS form is the N4TAB Carolina Moxon NVIS Antenna.
The NVIS version is similar to other forms of the Moxon Rectangle
antenna in that it consists of two wire elements, a driven element and
a reflector element. This antenna differs,
however. The physical orientation is such that the
predominant lobe is directed in a plane that is, essentially,
orthogonal to the surface of the Earth. That is, it is angled
"straight up". The resulting form is an antenna intended to focus
its primary ray (incidence) in a vertical plane, resulting in an NVIS
The usefulness of such a development is manifold. Unlike most
NVIS antennas, this form provides "real" gain, that is, gain with
reference to a dipole. Additionally, it offers
significant loss to signals with low angles of arrival, thereby
favoring the high angle NVIS rays. This is equivalent to sidelobe
suppression in high performance antennas and discriminates between
desired signals (those of vertical impingement) and those arriving from
A major advantage of this antenna is that it exhibits a 50 Ohm
feedpoint impedance at its resonant frequency and may be fed directly
with conventional coaxial cable, obviating the need for a
"tuner". In addition, owing to the wire construction, this
antenna is simple to build and
The general form of a Moxon type antenna
is described in published
literature and is readily available from numerous sources, among which
is a particularly well known website at www.cebik.com, owned and
operated by Dr. L. B. Cebik W4RNL. At that site, some
Moxon-specific information is available at the page
Dr. Cebik mentions a particularly useful Moxon-specific software tool
for calculating the basic Moxon geometry dimensions is available at
. This tool can create basic
dimensions, but more important, can generate files that can be read by
Roy Lewallen's (W7EL) EZNEC modeling application (http://www.eznec.com). Data
generated by both of these programs is presented herein.
Meter N4TAB Moxon NVIS Antenna. This particular version
was originally developed for ARES/RACES/NTS use in NC and is intended
for use at ~3.9 mHz. which corresponds to the common frequencies of
nets operated by those entities.
First, we should determine the basic dimensions for this antenna at 3.9
mHz. Using the AC6LA application and entering data for a 3.9 mHz
antenna using a 14 AWG conductor, the following image, Figure 1
calculated data from this program.
A model suitable for examination by an antenna modeling program such as
EZNEC may be generated by selecting the option available. Note
that the options for horizontal or vertical polarization do not pertain
to an NVIS orientation - the "wires" must be rotated so that the driven
element is "up". The calculated VSWR plot for this
antenna is shown below as Figure 2. An EZNEC antenna model file for
antenna is available here.
A vertical slice of the calculated 3D polar pattern is shown below in
description. This antenna is constructed as a rectangle
with support required at the upper corners, the upper center and "tag
lines" at the lower corners to maintain the rectangular outline.
Since this antenna utilizes a coupled reflector, "ground losses" are
minimized and the antenna can be installed quite close to the earth,
without incurring excessive losses.
The N4TAB Carolina Moxon NVIS Antenna is easy to construct and
install. From the dimensions calculated and shown in Figure 1,
prepare the 2 lengths of wire needed for the driven element and the
reflector. The required lengths are the sum of the horizontal
sections and the side sections. You will need to add an
additional 2 feet of wire to each length to allow for attachment to the
separators at the side sections.
Cut the driven element at its center and attach a connector or coaxial
cable, as you prefer. I prefer a Budwig center
insulator/connector as it allows easy coaxial cable termination and it
also provides the required
center support for the antenna. The upper corners need to be
formed with an attachment point that does not move or slide after
installation. This can be accomplished in numerous ways, from a
simple pinched loop closed with a nylon tie-wrap, but I prefer a proper
insulator attached via a loop.
supports. I use a Budwig insulator with one terminating
hole enlarged to a quarter inch to accommodate two wires. The
general form is shown in Figure 4. Note that the hole shown at
the left has been enlarged.
The technique described can be applied to a
wide range of insulator types, Essentially, the corner is forced
through the insulator hole and formed into a loop. the loop is
then passed over the top of the insulator and tightened around the
bottom of the insulator. This is illustrated in Figure 5 and
The driven element's side sections and the reflector element's side
sections must be spaced correctly, if the antenna is to perform as
expected. These spacers/insulators can be as simple as suitable
lengths of PVC
tubing with holes drilled to fasten the wire side sections.
the pieces. Build the driven element and the reflector
according to the dimensions given. Attach the side sections to
the side insulators and twist the excess wire along its respective
element for strength. Lay the antenna on the earth in a fashion
that will permit systematic elevation to its final height. Attach the
coaxial cable and route it away from the antenna to the extent
possible. Ideally, the coax should depart the driven element at a
90 degree angle, but I have found little if any penalty from allowing
the coax to hang directly below the center of the antenna.
Attach suitable cords or ropes to the support points and raise the
antenna to its desired height. The corners of the reflector
should be tensioned such that the reflector is pulled tight and is
generally parallel to the driven element.
Enjoy the NVIS characteristics of this
antenna! If you find this useful or have any comments,
please let me know n4tab at earthlink dot net .