Computer-Aided Antenna Modelling
Electromagnetic waves are in a sense a bit mysterious: You cannot see them, not smell nor touch them. Nevertheless, the propagation of electromagnetic waves is greatly influenced by surrounding objects and media. The performance of an antenna, and in particular its radiation pattern, is therefore difficult to visualise, interpret and measure in an objective way.
Additionally, there are (almost) no closed-form formulas for designing antennas. Antennas are usually designed in an iterative trial and error process, often comparable with Darwinian organic evolution, in which certain aspects of partially successful designs are combined in the hope of producing a more successful design. Antenna synthesis software based on this principle employ, what is called, a genetic algorithm (GA). There is little need to say that the alternative to this, which is: repetitive building, installing and measuring antenna design variations is very time- and material consuming.
Both problems are to a great extent overcome with computer-aided antenna modelling. As a bonus, you will be eager to try out more design variations, often resulting in better performing antenna designs. The reiterative modelling of antenna variations will furthermore provide you with insights that are difficult to gain in a different way.
The software codes I have used for HF antenna modelling are:
Antenna Optimizer (AO) v6.5 by Brian Beezley, ex-K6STI
for antenna synthesis.
(Due to software piracy issues the program is no longer available and, for the very same reason, Brian has retired from the hobby!)
- EZNEC a commercial antenna analysis program based on NEC-2 (or -4) by Roy Lewallen, W7EL
4nec2 is especially unique in the sense that it is the only freely available amateur antenna modelling software offering antenna synthesis capabilities.
ON4AA with W7EL,
author of EZNEC
Both EZNEC and 4nec2 are also truly unique in employing the best professional code available, NEC, for the modelling of wire or aluminium tubing antennas above «real» ground in both the near- and far-field of the antenna. Apart from antenna synthesis capabilities, 4nec2 adds to this some extras in the form of surface patches, support for inclusion of the surface-wave component in the far-field results and last but not least, antenna pattern export to VOACAP, the best propagation prediction engine on the face of this planet.
Other non-NEC-2(4)-based codes like MININEC etc. model the ground only as a perfect conductor right underneath the antenna, in the near-field. This simplification leads to only very approximative values for feed point impedance and resonant length.
Apart from those mentioned here, there are many more codes around for a huge variety of different antenna configurations and purposes. If you want to learn about more modelling codes written by professional antenna engineers, you can nose around on the site of the Applied Computational Electromagnetics Society (ACES).
Most important of all when using antenna modelling software, is knowing how to use any particular software code. The modelling and segmentation of structures for which the code was not designed, will certainly produce erroneous results. To quote one of my former professors:
«Bullshit in = Bullshit out!»
(Now you know the kind of people that lectured me!)
Modelling simple antenna configurations in a real-world setting is not as difficult as it sounds. I learned the tricks of the trade, by focusedly reading the EZNEC manual, paying attention to geometrical limits and potential pitfalls. Similar 4nec2 manuals are available from the 4nec2 website. However, both programs are based on NEC. Once you are accustomed to one of these programs, getting your grips around the other happens rather intuitively.
With the advent of the web era, there was also the antenna modelling website of antenna-guru L. B. Cebik, W4RNL (SK). It was the most complete, useful, scientific and knowledgeable amateur radio web site on the subject of antenna modelling. For any serious ham or antenna professional it truly was must-read material. However, where Cebik's site used to be an open and free access place to learn, an unscrupulous entrepreneur vulturously converted the site to a closed commercial site within a month after Cebik's passing. This was most probably not what Cebik had in mind when he ceded his copyright to his hosting provider... It was Cebik's vain attempt to perpetuate his knowledge. Nevertheless, Cebik's site inspired many hams ---including myself--- to set up a little web site of their own, forever doomed to be overshadowed by Cebik's monumental achievement.