2011 IR&D Annual Report

Modeling and Applications of RF and Optical Negative Index of Refraction Materials, 14-R8050

Principal Investigator
Diana Strickland

Inclusive Dates:  04/01/09 – 09/01/11

Background — Negative index of refraction materials (NIMs) are engineered materials that are designed to bend electromagnetic radiation "backwards" (in the direction opposite to the conventional Snell's Law). This is often accomplished by arranging and shaping metallic structures that are much smaller than a wavelength within a host dielectric. In this project, SwRI researchers used periodic arrays of printed copper structures on host dielectrics, either circuit board or air. NIM structures are useful in creating novel lenses, antennas and filters.

Figure 1.  A dipole antenna in direct contact with a novel ground plane. The impressive return loss in this low profile system would be impossible using a conventional ground plane.
Figure 1. A dipole antenna in direct contact with a novel ground plane. The impressive return loss in this low profile system would be impossible using a conventional ground plane.

This project focused on developing the capability to create the NIM models. To ensure a design cycle could be completed, researchers also built and tested two applications, a novel ground plane for antennas and a filter.

Approach — For several types of metal structures (split rings, mushroom structures, rod arrays and hole arrays), published data was used to validate computational models for entire periodic arrays, as well as the individual repeated units within arrays. Researchers also built and tested a dipole antenna with a NIM ground plane. Finally, working planar NIMS inspired filters were fabricated on conventional circuit board and on flexible substrates that validated the computational models.

Accomplishments — Researchers validated computational models of the listed structures, but also found analytic models to be useful in understanding complex behavior. Researchers developed analytic models where no conventional mixing rules apply. This work was presented as a poster, "Modeling Small Arrays of Spirals with Strong Interactions," at the Fourth International Congress on Advanced Electromagnetic Materials in Microwaves and Optics at Karlsruhe, Germany, and published in its peer-reviewed proceedings.

Researchers found that the NIM ground plane, pictured in Figure 1, could be used as copper "mushroom" structures printed on circuit board in complete contact with a dipole antenna and still achieve increased directivity and a strong impedance match. This means the antenna and ground plane system have an unusually low profile. The performance of the antenna was also found to be sensitive to position and orientation of the antenna with respect to the ground plane.

Figure 2. A distributed band pass filter using NIMs (negative index of refraction materials) design elements.  Left:  A filter circuit with three elements printed on circuit board.  Right: A magnified view of the three filter elements. This filter is about an eighth of a wavelength long, yet it performs well.
Figure 2. A distributed band pass filter using NIMs (negative index of refraction materials) design elements. Left: A filter circuit with three elements printed on circuit board. Right: A magnified view of the three filter elements. This filter is about an eighth of a wavelength long, yet it performs well.

A compact version of a printed NIM filter is shown in Figure 2. Reserachers fabricated and tested several types of these filters on both conventional circuit board and flexible substrates. The filter is not sensitive to temperature-like surface acoustic wave (SAW) filters. It has solid performance characteristics, yet it covers less than an eighth of a wavelength in its longest dimension. The filter may be arbitrarily small at the expense of losses.

This project explored methods of modeling NIMs and two applications. Most of the modeling conducted was computational by plan. However, researchers developed and published a new analytic model. The antenna ground planes and filters investigated performed well.

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Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 10 technical divisions.
04/15/14