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The application of NEC programs
A program known as NEC-2 (Numerical Electromagnetics Code-2 ) was developed by Messrs. A. Poggio and G. Ruggles in the early 1980s. It was an excellent tool and it became available in the public domain, although it was not designed specifically for broadcast directional antenna design use. It formed the foundation for many variations that are used for broadcast directional antenna design. A book entitled Basic NEC with Broadcast Applications was written recently by J. L. Smith, PE. It takes the reader through the application of NEC-2 and shows working examples of directional antenna design. It also provides interesting examples, and extremely useful, easily available, essential design and operating information. Several typical and necessary pieces of information are easily extracted when using this program.
When making the required non-directional measurements on a directional antenna, the unused antennas in a multi-tower array are usually either grounded or left open depending on the tower height. Short towers were usually left floating and taller towers shorted to ground. This was generally reasonably satisfactory, although as the number of towers in an array increases, the effects of reradiated energy can cause bad distortion of the nondirectional pattern. This has sometimes become a time-consuming part of a proof. One of the advantages of using NEC-2 is what amounts to an invitation to model the effects on the non-directional pattern with the ability to nullify their effects. The recently approved FCC rules concerning directional antenna design and proof of performance depend vary greatly on the NEC products.
The program used by the FCC when dealing with AM directional antennas is the NEC 4-1 which was designed specifically for modeling and designing directional antennas. This program allows the engineer to model, and thus observe the effect of almost any design modification he wishes. Generally speaking, a change in a single tower's operating parameters can cause a significant change in the overall antenna pattern. The opportunity for instant and basically unlimited iteration makes this program an ideal tool for the consulting engineer.
The NEC programs offer a great improvement over the slide rule era when the sheer volume of manual operations involved tended to influence full and comprehensive searches for perfection.
E-mail Battison at email@example.com.
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