N4TAB Carolina Moxon NVIS Antenna

INTRODUCTION.   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 G4XN 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 pattern.

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 non-vertical sources.

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 install.

Precedence.  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 http://www.cebik.com/moxpage.html.

Dr. Cebik mentions a particularly useful Moxon-specific software tool for calculating the basic Moxon geometry dimensions is available at http://www.qsl.net/ac6la/moxgen.html .  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.

The 75 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 illustrates the calculated data from this program.

Figure 1

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 this antenna  is available here.

Figure 2

A vertical slice of the calculated 3D polar pattern is shown below in Figure 3.

Figure 3

Mechanical 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.

Construction.  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.

Corner 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.

Figure 4

Forming the corner.  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 Figure 6.

Figure 5

Figure 6

Side insulators.  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.

Assembling 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 .