Antenna design traditionally involves a trial-and-error process consisting of building a series of prototypes and testing their performance while iterating to an optimized design. More recently, antenna designers have begun to simulate antennas as software prototypes, making it possible to analyze alternative designs in a fraction of the time required by physical prototyping. But normally this approach still follows the iterative process that was previously used in physical prototyping: model the design, simulate its performance, make changes to the model in an effort to improve the design, then start the process over again by simulating the new design. A few companies have adopted a new approach, in which a wide range of design parameters are evaluated in a single analysis run with the goal of exploring the entire design space and selecting the optimized design without need for the normal iterative process. As will be shown, this method was used to design the feed network of a WiMAX array and helped achieve full frequency coverage over the band of interest.

The past decade has witnessed the introduction of many new wireless technologies, including Bluetooth, WLAN, 2.5G and 3G cellular telephony, radiofrequency identification (RFID), ultrawideband (UWB) communications, and more. Each new technology requires innovations in antenna design to achieve their full potential; often multiple wireless technologies are combined within a single system, creating further complications. A modern personal computer (PC) may have one or more WiFi, Bluetooth, and cellular antennas in close proximity. In addition to normal antenna design issues, this creates a new range of complex concerns caused by coupling between the antennas.

The traditional approach to designing the antenna involves a trial-and-error process consisting of building a series of prototypes and testing their performance levels while iterating to an optimized design. The biggest problem with this approach is that it typically takes about a month to design, build, and test each prototype. A considerable number of iterations may be required to meet design requirements and more are generally needed to optimize the design. Another problem with this approach is that it is usually impossible to achieve the final installed environment on the bench. It usually becomes necessary to perform additional rounds of design iterations late in the design cycle. This sometimes means that the product launch may have be held up for antenna development, with the potential for losing a substantial amount of revenues or even, in a worst-case scenario, missing the market window that the product was intended to address.

What follows is an example of a newer antenna design approach in which the initial concept design is modeled and simulated and then key design parameters are replaced by variables. The user defines ranges for each variable and the simulation engine creates models and performance predictions for every possible combination of variables. The time required to optimize the design is substantially reduced because rather than individually creating each design the user need only define the design space of interest and pick the best design from the alternatives created by the parametric simulation process.

The purpose of this project was to design a WiMAX antenna array to cover the band from 3.4 to 3.65 GHz. The wavelength is (2.998 x 10⁸)/(3.4 10⁹) = 8.818 mm. The design strategy-is to have a central feed with equal length distribution to each patch so that the elements radiate in phase. The network is fed at its center with a 50-Ω coaxial probe, connected to the center of a 100-Ω line. Each end of the line ends in a quarter-wave transformer that transforms the 100-Ω impedance to a segment that splits into two lines, each feeding a patch antenna element.

The first basic step in the design process is to calculate the edge impedance of the patch and match it back through the transformer to the 50-Ω line through the feed network. This will be done using a formula-based transmission-line calculator, but could also be performed with trace impedance formulas from basic microwave theory. Another constraint is that the four radiating patches must be sufficiently separated to avoid interfering with each other.

The thickness of the substrate is 1.6 mm and the substrate material is selected for a relative dielectric constant (ε_t) of 3.58. The next step is calculating the edge impedance of the patches using approximation formulas. A thin halfwavelength patch has a corrected side length of:

L = 0.49[λ/(ε_t)^0.5] = 22.18 mm

All trace impedances must be matched to the coaxial probe feed so there is no need to do an insert feed at the element. By selecting a 25-mm-wide patch, the approximate edge impedance can be calculated from the length (L) and width (W) as:

Z_edge = 90[ε_t/(ε_t - 1)](L/W)² = 100 Ω

A simple RF calculator was used to calculate the width of a 100-Ω feed on the desired substrate:

W₁₀₀ = 0.852 mm

With the edge impedance known, the other impedances and microstrip widths can now be calculated. Two 100-Ω patches will be connected to the feed point from above and the other two patches connected to the feed point from below. Each connection trace segment must have the impedance (Z):

Z=100/2 = 50 Ω

The 50-Ω microstrip width is:

W₅₀=3.497 mm

In addition, a quarter-wave transformer will be used to connect the 50Ω segments at each point of the 100-Ω line:

Z_t = (100 x 50)^0.5= 70.07Ω

W₇₀ = 1.96 mm

L_t = 11.9 mm

Figure 1 shows the resulting feed network.

The next step is to evaluate the performance of this initial design. Rather than taking the time that would be required to build a prototype, the antenna will be simulated as a software prototype using MicroStripes software from Flomerics (www.microstripes.com). This software package uses the Transmission Line Matrix (TLM) method for solving Maxwell's equations in the time domain. MicroStripes solves for all frequencies of interest in a single calculation and therefore captures the full broadband response of the system in one simulation cycle. The TLM method creates a matrix of equivalent transmission lines and solves for voltage and current on these lines directly. This approach uses less memory and central-processing-unit (CPU) time than solving for electric (E) and magnetic (H) fields on a conventional computational grid.