Optimal Near-field Antenna Apertures in Lossy Media

Licentiatavhandling, 2014

Antennas are traditionally designed for far-field applications such as wireless communications, radar, etc., where the antenna systems interact with other systems and objects which are located in the far-field zone of the antennas. For this reason, the characterization of antennas in the far-field is well-defined and measurable by well-developed measurement techniques both in line-of-sight and multipath environments. However, with the recent increase in applications for near-field microwave systems such as in detection and sensing, wireless power transfer or near-field communication, there is a need for the study of design criteria of antennas tailored for near-field applications. This thesis addresses some aspects of this topic. In order to assess the antenna's ability to transfer power to a desired target in the near-field, penetration ability is introduced and investigated besides the directivity which is a far-field characteristic. It can be demonstrated that these near-field and far-field characteristics are not explicitly correlated and that the penetration ability is dependent on the depth of interest in the near-field as well as the loss in the medium. The axial pattern cannot be defined and used in lossy media since in the presence of loss, the field intensity is affected by a monotonous attenuation with distance from the antenna. The 3dB near-field beam radius is introduced as a measure to characterize the antenna's focusing behavior in lossy media in the near-field and it is further used to find an optimal size of the uniform field apertures for the near-field detection of foreign objects in lossy media. In the final part, array signal processing techniques are applied to the transmitting aperture field modes in order to determine the optimal aperture distribution that maximizes the power transmission through lossy media. The optimal apertures which are determined by this method are applicable in many near-field systems, such as the detection of foreign objects in lossy matters (e.g. food contamination detectors), wireless charging of batteries of human body implanted devices, and for near-field communication systems. Keywords: near-field antennas, maximum penetration, near-field focusing, optimal near-field aperture, near-field power transfer, array signal processing.