Efficient radar backscatter suppression

Abstract

Researchers at Queen's University Belfast's Centre for Wireless Innovation (CWI) have combined expertise in on-body wireless sensors and frequency selective surfaces to solve a problem limiting the RF performance of medical sensors. Conventional frequency selective surfaces (FSSs) reflect or transmit microwave energy at particular resonant frequencies. However, there is a newer class of periodic structures, thin resistively loaded FSS absorbers, which absorb most of the incident power. This class of microwave absorber has the potential to significantly enhance the RF performance of all existing and future physical layer communication systems used in environments, hostile to propagation. Thin resistivity loaded FSS absorbers have planar, thin and lightweight metal-backed patterned screens, making them particularly suitable for suppressing wave refl ections from metal objects such as wind turbines as well as land, air or space-based vehicles. Normally, FSS arrays are simulated and measured in the ‘far field’ of the excitation antenna. This means that the separation distance is sufficiently large for the periodic absorber surface to be excited by plane waves. For all conventional RF applications this is a reasonable assumption, for example a radar system is generally positioned remotely and far from the structure that it interrogates. However, for healthcare monitoring applications, near field solutions are required as there is close contact between the absorber surface and excitation antenna. In this issue of Electronics Letters, Dr Gareth Conway and colleagues have evaluated the performance of this new class of FSS absorber when in close proximity to an excitation antenna. The thin resistivity loaded FSS consists of a 3 mm thick metal-backed patterned substrate with each unit cell composed of four hexagonal-shaped elements that are similar in size to the antenna. “So, effectively the excitation waves impinge on just one of the nested loops,” explains Conway. “Moreover, in the near field the amplitude and phase of the waves vary spatially across the unit cell. This is totally different to the conventional far field operation of these absorbers, where the wave ‘sees’ an infinite array of unit cells, and the amplitude and phase is the same across each cell.” The results reported show that no detuning is observed at the operating frequency of the antenna when the thin resistivity loaded FSS absorber is placed in very close proximity (λ/30 and λ/5), whereas at such distances a tissue phantom or metal sheet significantly reduces the radiation efficiency of the antenna.. This shows that the FSS structure greatly suppresses radar backscatter and is suitable for use in near field systems, such as wearable systems. Members of the team in the anechoic chamber at the university's Institute of Electronics, Communications and Information Technology. From the left: Dmitry Zelenchuk, Robert Cahill, Gareth Conway and Niamh McGuigan, who is wearing a test device incorporating the FSS. A close-up of a wearable medical monitoring device with the FSS absorber positioned between the device's antenna and the body-side of the device. The human body presents a notoriously difficult propagation environment that is very specific to the individual person. Thin resistivity loaded FSS surfaces decouple the environment surrounding the antenna, such that a tailored electromagnetic barrier is formed. This opens up a large number of new applications in near field environments that were previously “unimaginable”; making wireless performance predictable and reliable regardless of the platform on which the antenna is placed. This is in stark contrast to existing solutions, which cannot deliver this behaviour. In addition to enhancing the performance of on-body wireless sensors, this technology could meet the need to suppress RF energy scattering from satellite platforms, which host large antenna farms. By selective placement of thin resistivity loaded FSS structures on the surface of the space vehicle, radiation pattern degradation and antenna coupling will be reduced, both of which currently require costly solutions. The results obtained from the reported preliminary measurement campaign in this Letter, suggest that this approach has excellent potential for isolating mobile platforms such as the human body. “However, this is only the ‘tip of the iceberg’, researchers have been tackling the problem of electromagnetic scattering from platforms for years, and still continue to do so”, says Conway. “The step change in performance capability of field deployable antennas embedded into electromagnetic control surfaces will allow greater communication link margins, hence facilitating significantly higher quality of service for medical data transfer than is currently envisioned. However, at present, the processes necessary to optimise the design and manufacture of such FSS structures are not well understood. To develop this technology to an acceptable engineering standard, the research must explore the combined spaces between state-of-the-art artificial engineered electromagnetic surface design and new multifunctional additive manufacturing technology. We would be zealous to see the synthesis and demonstration of the operation of a new innovative surface which radically accelerates the application of wireless enabled mobile communication and healthcare sensor technology.”