The utilization of technical devices which, at least in part, contain high-frequency components is now an integral part of everyday life. In future, these devices will become more compact and efficient and they will also offer a growing number of functions. Electromagnetic metamaterials offer developers a new degree of freedom in coping with the challenges arising from this trend.

Electromagnetic metamaterials are artificial materials that can be designed in such a way that they have effective, application-specific electromagnetic properties in a given frequency range. The large number of realizable properties also extends to those that do not normally exist in nature. These macroscopic material properties are created through microscopic, mostly periodic circuit structures and can be realized using standard and cost-effective PCB manufacturing methods. Some of the effects realized in this way are refractive indices with negative signs in so-called left-handed materials or frequency bands in which wave propagation cannot take place (bandgaps). This facilitates the miniaturization of high-frequency circuits and antennas. An object enclosed by a metamaterial cover can even be made invisible in certain frequency ranges (cloaking) through the clever selection of effective material properties.

The research activities at FHR focus on the utilization of these innovative materials in the improvement of key technological products. To achieve this, a theoretical basis must be established and efficient design methods developed. Moreover, application areas must be identified parallel to the technological implementation of innovative solutions.

Suppression of parasitic waves

To accommodate more functions in devices with steadily decreasing dimensions, the high-frequency components that are used must also be more compact or moved closer together. This automatically leads to increased parasitic coupling, which can have a negative effect on the function of the components. The parts of the electromagnetic fields responsible for this coupling can, however, be significantly suppressed through the utilization of metamaterials. The metamaterial structure most frequently used here is the so-called mushroom structure which prevents the propagation of electromagnetic waves in a given frequency range.

FHR has successfully used this structure for the decoupling of closely located antennas (illustration) and has also realized the wideband suppression of parasitic parallel-plate waves in multilayer high-frequency circuits (Fig. 1). The latter application is excellently suited for the reduction of simultaneous switching noise (SSN), also known as power/ground noise or ground bounce, in high-speed digital circuits on printed multilayer circuit boards. Artificial magnetic reflectors, which were used in the miniaturization of antennas and antenna arrays, were also realized. Another application can be found in the reduction of the susceptibility of high-precision GPS receivers to interference caused by multi-path propagation through the utilization of low-cost and light weight mushroom surfaces.

Within the framework of an EDA (European Defence Agency)-financed research project that aims to improve the properties of modern, electronically controlled antenna arrays, metamaterials will be used to reduce side and rear parasitic radiation.

Innovative RF transmission lines and antennas

The development of a compact and low-cost, high-frequency distribution and combination network for antenna arrays is also foreseen within the framework of the EDA project. The fact that metamaterial structures can be used to create RF transmission lines with vanishing or even negative phase velocities will be exploited here. A similar approach was already adopted by FHR in the development of a new leaky-wave antenna (Fig. 3). Leaky-wave antennas are interesting candidates for a low-cost approach to electronic beam scanning. The main radiation direction can be scanned within an angular range of up to 180° in one plane by way of frequency variations. Conventional leaky-wave antennas, however, show a loss of performance in the middle of this scanning range. This can be prevented through the utilization of metamaterials.