Modeling 160-Meter Vertical Arrays

L. B. Cebik, W4RNL

Nowhere else on the amateur allocations are vertical antennas and arrays more widely used than on 160 meters. The advent of antenna modeling software has made the design and analysis of such arrays far easier than earlier pencil-and-paper and cut-and-try methods. In fact, the ease with which one can use modern software tools may also be the root of some problems that beset such array work. One difficulty is a lack of appreciation of what these tools have to offer and where they meet their limits. In some cases, the software offers us more than we know how to use wisely, because we are not fully aware of what the different options really mean. The ground quality options offered to us by modeling software is a case in point.

A second group of problems surrounds the availability of shortcuts, again, mostly in the area of ground concerns. NEC-2 offers us a highly accurate Sommerfeld-Norton ground calculating system, but this most commonly used form of NEC does not permit the modeling of buried radial systems. A work-around is the use of radials placed very close to the ground--within about 0.001 wavelength. A more tempting shortcut offered on some implementations of NEC-2 is the MININEC ground. With this system, we simply omit the radials and let the vertical monopole elements touch the ground. Very often we use these alternatives to a buried-radial system without knowing much about their accuracy relative to a more complete model. That lack of knowledge has led to numerous inadequate and possibly misleading models.

NEC-4 has the ability to handle buried wires and hence permits the complete modeling of a radial system. The models correlate quite well to reality and to standard calculations used by such engineering efforts in fields such as the AM broadcast industry. The question at hand is how well the short cut and substitute models measure up to full models of arrays using vertical antenna elements.

Let's be a bit more specific in what each part of the series will cover. In this first part, we shall examine some baseline data on 1/4 wavelength verticals using various types of modeled ground systems available to us within versions of NEC. In the second part, we shall seek a more comprehensive view and appreciation of the relative effects of soil conductivity and permittivity (relative dielectric constant) on the performance of our baseline antenna model. Since the project will simultaneously involve some problems associated with using the MININEC (no-radial) ground system and with the construction of models of radial systems, we shall tackle both problems in Part 3. The 4th episode will be devoted to a potpourri of models of some common vertically polarized antennas we typically use on 160 meters, as we seek some guidelines for the most adequate modeling possible. In the final installment, we shall look at the suggested use of inner and outer ground qualities to simulate a radial system.

This series of items, each which has appeared in The National Contest Journal in 2000-2001, will address these and related questions of significance to modeling 160-meter vertical arrays. Each episode in the 5-part collection will appear at this site after it has appeared in print. I hope the notes are useful to you.

1. Some Baseline Data (Nov/Dec, 2000, pp. 19-24)

2. Appreciating Conductivity and Permittivity (Mar/Apr, 2001, pp. 4-9)

3. Complex Radial Systems and Limitations of the MININEC (No-Radial) Ground (May/Jun, 2001, pp. 3-8)

4. A Potpourri of 160-Meter Vertical Antennas and Modeling Issues (Jul/Aug, 2001, pp. 4-9)

5. The Use of Multiple Ground Qualities in Lieu of Radials (Sep/Oct, 2001, pp. 4-9)

Updated 08-01-01. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.

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