Characterization and Design Requirements for Antennas in the Near-field and the Random-LOS Propagation Environment

Doktorsavhandling, 2016

The choice of antenna for a particular application depends on the requirements imposed by that application. These can be, the application's field region (i.e., near-field or far-field), polarization, radiation pattern, radiation efficiency, system throughput, etc. During an antenna's design process, different characteristics and characterization methods have to be taken into account according to the intended application. The present thesis focuses on two different aspects of antenna characterization and design: The first is the characteristics and design criteria for antennas in near-field applications dealing with lossy materials. The second focus is on the Over-The-Air (OTA) measurements of wireless devices and the Random-Line-Of-Sight (Random-LOS) environment. The characterization, design criteria, and fundamental limitations of antennas in the near-field, especially in lossy media, have not been addressed as thoroughly as in the far-field. We introduce 3dB near-field beam radius (r_3dB) as a measure to characterize the near-field focus of antenna apertures in the presence of loss in the medium. r_3dB is then used to estimate the optimal size of uniform apertures that maximizes the scattered power from foreign objects in near-field sensing applications in lossy media. In addition, we present a generic numerical method for determining the optimal aperture field distribution that maximizes the near-field power transfer through lossy media, given the aperture size. Maximizing the power transfer is shown to result in increased sensitivity to the presence foreign objects in lossy media. The method is applied to both homogeneous and planar multi-layered media and is applicable to any non-homogeneous media, given that the Green's function of the emanated fields from the basis functions are known for the medium. OTA testing of wireless systems is performed in an emulated propagation environment with the objective to evaluate the system performance in a reliable, repeatable and controllable way. A complete understanding of the propagation channel and choosing the most representative model is still a challenge, let alone emulating it in an OTA measurement setup. Instead, we can gain valuable insight into the performance of a wireless system by investigating it in two limiting cases, i.e., the Rich Isotropic Multipath (RIMP) and Line-of-Sight (LOS) environments. LOS is especially relevant in vehicular wireless communications in suburban and rural environments, and in 5G systems where mm-wave radio links, small cells, and advanced beam forming are foreseen. However, such links undergo slow fading due to the random orientations of the user terminals or vehicles, which leads to a Random-LOS propagation environment. We first investigate the polarization-MIMO antennas and the requirements on their design to ensure good performance in the Random-LOS environment. We define two polarization-deficiencies in the radiation pattern of dual-polarized antennas and show that in the Random-LOS environment these two are the main contributing parameters to sub-optimal performance of polarization-MIMO systems in terms of MIMO efficiency. The characterization method is used to study the performance of a novel dual-polarized MIMO antenna for use in micro base-stations. Also, a measurement setup and characterization method for OTA Random-LOS measurement of automotive wireless systems is proposed. The test setup is characterized in terms of the performance of an ideal probe antenna and an ideal receiver in the test zone of the measurement chamber. Finally, we provide a "back-of-the-envelope" investigation of the prevalence of RIMP or Random-LOS propagation in scattering environment as a function of frequency from 500 MHz to 100 GHz.