Saturday, March 21, 2009
Antenna Modeling & Analysis
Some powerful software tools are available for modeling and analyzing many forms of antennas that are popular among hams. These can be used to guide the designs of home-brew antennas and to gain insight into measurements made on actual antennas that you are using. Models can include antenna elements, detailed in 3 dimensions, transmission lines, matching / loading components, sources, ground surfaces (with various characteristics) and surrounding objects. Program outputs include: impedance, SWR, radiation patterns, current distributions, front/back ratios, impedance matching. Currently available versions of the program can sweep frequency inputs or component values (e.g. antenna height, wire size, loading coil inductance) to produce plots depicting the variation of results with the swept parameters.
The granddaddy of antenna analysis programs is the Numerical Electomagnetics Code (NEC) family of programs originally developed at Lawrence Livermore Labs in 1981, on a contract to the U.S. Navy.
NEC2 was released to the public. Browsing the NEC2 User's Manual reveals the punch-card FORTRAN genesis of this program, and the terminology continues in current versions of the program that hams can run today on their personal computers. Historical documentation and information links are collected at the NEC2 web site.
Among the links at the NEC2 web site is one to the EZNEC versions of the program developed by W7EL. A free demo version of this program is available and it is a very good place to start to learn what this program can do. The free version is full-featured, only limiting the complexity of antennas (20 segments max) that can be modeled. Paid versions of the program permit much greater complexity of antenna systems.
Another implementation is the 4NEC2 program developed by Arie Voors. You can get an appreciation of this program's capabilities from the large collection of screenshots. It is a free program and, like EZNEC, includes a good number of pre-built antenna models, many developed by W4RNL. These are good for getting familiar with the program and as starting points for developing models specific to your needs.