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Print PageComputer Aided Antenna Design

By Joseph H. Reisert

Until recently, most engineers designed antennas by experience and a certain amount of "black magic". A basic understanding of impedance matching and the various matching methods to accomplish a good match over the desired frequency range were required. A good understanding of mechanical engineering principles was a plus as well as not minding tinkering, doing "cut-and-try", gluing and screwing parts together until the design met the desired specifications.

However, after the prospective antenna was designed and built, there always remained the question of how well the gain and radiation pattern held up over the desired bandwidth. If an antenna test range was available (a real plus), more testing time was spent trying to verify that all design criteria was met. Next, if all went well on the final testing, the prospective antenna would be transferred to the production department. Finally, the production antennas would be tested to verify that the design could be duplicated. Needless to say, this was a very expensive and time consuming procedure.

Those days are gone forever. We live in a competitive market where time and performance are critical. Guesswork and long design cycles are out of the question. New state-of-the-art (SOA) antennas are required that meet all the design criteria for sophisticated applications that may involve large quantities at competitive cost.

In the mid 1960's, main frame computers were finally large and fast enough to do antenna modeling. However, it still requires an astute antenna design engineer with a good analytic mathematical background to write the equations and computer programs to model antennas.

In the 1970's, the NEC (Numerical Electromagnetic Code) program was developed by the US government. This program was very accurate but difficult to use and required a large main frame computer. Furthermore, its use was restricted by the government.

This all changed in the mid 1980's when PCs (personal computers) became widely available. Soon there were several antenna modeling programs available. The primary one was MININEC, a smaller program based on NEC that runs fast on a PC. Also, a PC version of NEC became available.

As PCs increased in speed, so did the antenna modeling programs. At first, it was not unusual to wait 60 seconds or more for the analysis of an antenna. Nowadays, PCs with Pentium (RTM) processors, can model that same antenna in just seconds.

These powerful modeling programs allow the antenna designer to use keystrokes on a PC keyboard instead of getting their hands dirty from hours of cutting and testing the actual antenna. Furthermore, the performance goals can be tested before any actual antenna is constructed. You can now "cut-and try" an antenna design for maximum gain, front-to-back ratio, impedance, or a combination of the three by changing element lengths, diameters and spacings using only a keyboard. This significantly shortened the design to completion of the production antenna.

Modeling programs do not design an antenna per sé. They only "test" in software how the inputted antenna design should perform. Modeling programs leave it up to the ingenuity of the antenna designer to "tweak" or optimize the mechanical dimensions until the desired antenna performance is attained.

In the mid 1990's, optimization programs became available for PCs. These programs will perform the optimization for you. All you need to do is to input a reasonable design along with a properly weighted trade-off or Figure-of-Merit (FOM). The FOM weighs the relative importance of each antenna parameter such as gain, pattern and impedance over a set of input frequencies and out comes a design.

The above may give the impression that antenna design is no longer a "black art". While this may be true, it still requires a lot of knowledge and understanding of the interrelated parameters in an antenna design as well as the FOM. In fact, as the antenna designs of today get more complicated, so does the work of the antenna designer.

The antenna modeling and optimization programs available today do not completely compensate for all variables. The output data file must be properly interpreted by the user. Sometimes the tolerances of the element dimensions may be difficult to realize. Element correction factors may still be required to compensate for the mechanical mounting of the antenna elements. This is where the antenna designer comes in to play. He or she must be able to realize what designs can and can't be built as well as any compensations that are required.

In summary, Astron Wireless Technologies, Inc. engineers now have better design tools with which to work. Computer modeling and optimization software are a very big step forward in designing SOA antennas. In fact we utilize one of the most advanced software programs in the industry made by Ansoft. It is called the High Frequency Structure Similator (HFSS) program. Regardless of the software programs, the proper design requirements must be supplied by the user (Ref. 1.). This is where the professional antenna designer can step in and provide the optimum design based on your inputs.

Astron Wireless Technologies, Inc. and the author retain the rights to all intellectual property contained herein.
This information should be used as a guideline only to help you in the appropriate selection of an antenna.

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