Preface

The Numerical Electromagnetics Code version 2 (NEC-2) is a public domain computer program that eliminates most of the shortcomings inherent in the conventional design methods for medium-frequency (MF) broadcast directional antennas. It is most useful for the increasingly complex arrays, and it yields parameter settings that greatly aid the initial adjustment of a new array.

NEC-2 was developed at Lawrence Livermore National Laboratories by G. J. Burke and A. J. Poggio in 1981 as a general-purpose tool for the design and analysis of antennas in general. For the most part, its published applications deal only with antennas that have a single drive source such as those found on dipoles, yagis, rhombics, and so on. Broadcast applications, on the other hand, use multiple sources to individually drive the separate elements of a multi-element array, with each source having a unique magnitude and phase so as to create a particular radiation pattern.

NEC-2 is not necessarily user-friendly to the broadcaster. From the very beginning, it gives the broadcaster a swift kick in the shin when it calls for defined source voltages as the inputs to initiate a given antenna analysis, whereas broadcasters have conventionally started their design with a given set of field ratios. Then among other complications, NEC-2 uses the coordinate system common to mathematics, whereas broadcasters have traditionally used the geographical coordinate system. Then NEC-2 deals with peak values, not RMS, as does the broadcaster.

Thus, while NEC-2 is indeed a magnificent tool, it is not at all directed to broadcast use. As a result, when NEC-2 first made its appearance, it was not immediately accepted by the broadcast community and only a few broadcast engineers even attempted to use it.

In time, however, broadcast engineers with various levels of expertise studied the use of NEC-2 and each developed their own postprocessing software to read data from the standard output file and to use that data to perform the various tasks necessary for broadcast work. Such studies have been carried on rather independently, and somewhat privately, however, and some engineers even consider their work proprietary. Unfortunately, little of the work has been published to enjoy peer review or to serve as tutorials for those seeking entry into the profession.

In recent years, some engineers have written software to make the moment method programs more user-friendly to the broadcaster, and some of the more developed efforts have been packaged as commercial products for sale to the broadcast community. The consensus is, however, that the commercial products all use the same basic moment method engine and that they differ mainly in the user interface with, perhaps, some special features added.

Because there has been such a diversity in the development of the procedures for applying NEC-2 calculations to broadcast arrays, there may be multiple approaches to accomplish a given analysis, and each approach possesses merit. Therefore, as you study NEC-2 from this book and other sources, you might find additional and even contradicting methods of analysis; in that case, I encourage you to weigh the merits of each and to exercise your judgment as to the best use of available methods.

At the present time, there is no modern and comprehensive tutorial available to a person desiring to enter the field of MF directional antenna design, and the few remaining people who are knowledgeable of the science are becoming older and, unfortunately, fewer. This regrettable circumstance occurs at a time when we are on the brink of a resurgence of need as new forms of modulation make their way into the MF band. Digital modulation methods make it necessary to renew our interest in directional antenna systems as we study bandwidth requirements and learn new implementation schemes to accommodate the more sophisticated application. At the same time, 5000 existing directional antenna systems continue to demand maintenance, and the need for replacement grows as they come to their life's end at the rate of approximately 100 per year.

Basic NEC with Broadcast Applications was written to show how the features of NEC-2 can be accommodated in the design and analysis of MF broadcast directional antennas. I also envision this book as a depository where one may come to learn modern basics of the science; in so doing, the book will assist younger people who wish to enter the field. This book also serves as a useful desk reference that can remind the professional user of the many details encountered while using NEC-2 to analyze and design modern MF directional antenna arrays.

Finally, a somewhat lofty goal is that others more skilled in the science than I will see this book as an incentive to make their own contributions to publicly documenting the use of NEC for broadcast applications.