NEC2

NEC2 stands for Numerical Electromagnetics Code (version 2) and is a free antenna modeling package. NEC2 lets you build an antenna model in 3D, and then analyzes the antenna's electromagnetic response. It was developed more than ten years ago and has been compiled to run on many different computer systems. NEC2 is particularly effective for analyzing wire-grid models, but also has some surface patch modeling capability.

The antenna design is described in a text file, and then the model is built using this text description. An antenna described in NEC2 is given in two parts: its structure and a sequence of controls. The structure is simply a numerical description of where the different parts of the antenna are located, and how the wires are connected up. The controls tell NEC where the RF source is connected. Once these are defined, the transmitting antenna is then modeled. Because of the reciprocity theorem the transmitting gain pattern is the same as the receiving one, so modeling the transmission characteristics is sufficient to understand the antenna's behaviour completely.

A frequency or range of frequencies of the RF signal must be specified. The next important element is the character of the ground. The conductivity of the earth varies from place to place, but in many cases it plays a vital role in determining the antenna gain pattern.

To run NEC2 on Linux, install the NEC2 package from the URL below. To launch it, type nec2 and enter the input and output filenames. It is also worth installing the xnecview package for structure verification and radiation pattern plotting. If all went well you should have a file containing the output. This can be broken up into various sections, but for a quick idea of what it represents a gain pattern can be plotted using xnecview. You should see the expected pattern, horizontally omnidirectional, with a peak at the optimum angle of takeoff. Windows and Mac versions are also available.

The advantage of NEC2 is that we can get an idea of how the antenna works before building it, and how we can modify the design in order to get the maximum gain. It is a complex tool and requires some research to learn how to use it effectively, but it is an invaluable tool for antenna designers.

NEC2 is available from Ray Anderson's "Unofficial NEC Archives" at

www.si-list.org

Online documentation can be obtained from the "Unofficial NEC Home Page" at www.nittany-scientific.com .