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Emanuel Merulla, antenna design engineer for MegaWave Corp., Boylston, MA, has applied a new absorbing boundary technique in the Flomerics MicroStripes 3D electromagnetic (EM) simulation solution, to accelerate the design of low-profile and zero-profile antennas installed in the Earth. The new technique significantly reduces the computer memory requirements and cuts simulation times from days to an hour or two.

“We have already designed more than 50 antennas for various branches of the military using the new earth boundary technique,” Merulla said. “The reduced size of the model combined with the high speed of MicroStripes makes it possible to deliver better performing antennas to our customers in less time.”

MicroStripes uses the Transmission Line Matrix (TLM) method for solving Maxwell's equations in the time-domain. The antenna is driven by an impulse and the subsequent propagation of fields in space and time can be visualized in 3D. Fourier transformation is used to convert the impulse response into the frequency domain, providing results over a broad spectrum.

The US military is placing a strong emphasis on improving the performance of easy-to-conceal antennas that are either used on or near the ground or buried. Engineers designing these antennas face a major challenge in that ground waves, which attach to the surface of the Earth, play a major role in their performance. In order to accurately model ground waves using electromagnetic simulation, the Earth normally needs to be modeled to at least three times the skin depth of the wave, to allow space and time for the ground wave to be differentiated from the fields that propagate into the earth that are ultimately dissipated in the dielectric losses. This generally increases the size of the model to point that it takes days to solve, slowing down the design process and limiting the number of alternative designs that can be evaluated.

EM Engineers at Flomerics worked closely with Merulla to develop a new approach that enables the Earth to be truncated to a much smaller size with minimal impact on simulation accuracy. The challenge was to create a boundary condition that absorbs the field propagating into the Earth, without disturbing the ground waves that contribute to the overall antenna radiation pattern. This was achieved by inserting an absorbing surface into the Earth and matching its impedance to the fields.

Merulla consulted technical literature and used probes to measure the complex dielectric constant and magnetic permeability of the Earth. The parameters of the absorbing boundary condition were optimized to match the Earth so the boundary condition, like the real Earth, absorbs essentially all of the electromagnetic energy that reaches it. Merulla first used this approach to design a simple inverted cone antenna, which is similar to a wideband monopole, primarily to validate the technique. The company built a prototype of the antenna and the physical testing results matched the simulation within 10 percent. “The simulation results of every antenna we have modeled using this method have closely matched the physical prototypes once they were built,” Merulla concluded.