Ismael Vico Triviño
Microstrip antennas offer a broad set of advantages such as low profile, light weight, easy fabrication and low cost. As these are desirable or even critical for a broad range of applications, there has been a large interest for these antennas in the antenna community. In applications like drones or satellite communications, the especially stringent constraints on weight and size make microstrip antennas a good option, despite their intrinsic performance limitations such as their narrow bandwidth. The radiation pattern of these antennas is directive, which for some of these cases translates to an insufficient beamwidth if one uses only one antenna. In the literature, one can find an extensive analysis on gain and bandwidth enhancement of microstrip antennas, but the research on broadening their beam is not as popular. In this thesis, we focus on the beamwidth enhancement of microstrip antennas, and specifically on antennas for small drones and satellites.Most of the work in the thesis is dedicated to the antennas for a drone protected by a carbon fibre which enables it to fly indoors. It is used for industrial inspections, and to reach places that are dangerous or inaccessible to humans, where the reliability of the connection between the drone and the controller is critical. This is especially challenging indoors, due to the multipath effect. The antenna system should be light, compact, and provide a quasi-isotropic coverage. The number of antennas is limited to two, and therefore, the typical beamwidth of a patch of 70º is insufficient. They should also radiate circularly polarized patterns to minimise the effect of the cage, the potential cross polarisation losses, and the multipath effect. We analyse the limitation of the gain at low elevation angles for patch antennas, providing an overview and guidance on different techniques to improve the beamwidth. We illustrate some of these techniques with various designs with application for drones. The proposed antennas consist of a patch combined with several parasitic elements that radiate in end-fire. This research results in an antenna with a CP and broadened beam. We perform and describe a system validation of the antenna in a real scenario comparing with the original set of antennas in the drone. The test shows that the reliability of the system is improved compared to the original configuration of the antennas especially in NLOS. We extend the research on wide beam microstrip antennas, to the use of parasitic elements to enhance the beamwidth and achieve broad-beam and/or iso-flux patterns, with applications to pico-satellites. Using passive elements in an array-like structure allows to increase the low elevation gain and even achieve conical patterns. This last feature is very convenient for pico-satellites in Low Earth Orbit.With this thesis we contribute with an analysis on the different techniques to broaden the beamwidth of microstrip antennas. It tackles each different approach from a didactic perspective that allows to understand and identify the main aspects of each design, providing unified guidelines to the beamwidth broadening of patch antennas. It helps to establish compromises between size, weight, bandwidth and beamwidth for this type of antennas, which are dependent on the application. All the designs presented in this thesis keep a very low-profile, which is not commonly seen in the literature due to the intrinsic limitations on the beamwidth of patch antennas.
EPFL2023
