Viability of UAV-based antenna pattern measurements
Thesis (MEng)--Stellenbosch University, 2020.
ENGLISH ABSTRACT: Unmanned aerial vehicle (UAV) based field measurements have been proposed as a possible solution to provide calibration data for large ground based radio telescope arrays, such as the Mid Frequency Aperture Array (MFAA) planned for the Square Kilometre Array (SKA) project. As such, we investigate the viability of performing antenna radiation pattern measurements in the frequency range 450–1450 MHz utilising a quad-copter equipped with a test source in the form of two orthogonal transmitting dipole antennas. The vehicle is fitted with the necessary flight controllers to enable autonomous navigation and uses a differential GPS (DGPS) module featuring real-time kinematics (RTK) to improve on the positional accuracy obtained from conventional GPS systems. Given the proposed size of the MFAA, the far-field region of the array, or it’s various sub-arrays, may exist at distances where measurement via UAVs becomes infeasible. Therefore, we go on to consider measurements performed in the near-field, from which a suitable near- to far-field transformation algorithm can be used in order to determine the far-field radiation pattern. The effect of random positional errors associated with DGPS on two different near- to far-field transformations, namely the planar plane wave expansion (PPWE) and the fast irregular antenna field transformation algorithm (FIAFTA), are investigated. The study shows that FIAFTA greatly outperforms the PPWE with regard to resilience to probe positioning errors. We find that the PPWE breaks down rapidly even for positional errors on the order of /50, whereas FIAFTA is seen to produce reasonable results up to error levels of /20. Considering a positional inaccuracy of 5 cm, typically associated with DGPS/RTK systems, we find that FIAFTA can produce satisfactory results across the whole frequency band of interest. However, in order to achieve these results, it was necessary to significantly increase the number of measurement samples from that necessitated by the minimum sampling requirements of the algorithm. Additional practical issues are also considered, such as an investigation into how to distribute a reference signal through the system. This is necessary in order to measure the phase response of the system under test, which is required in near- to far-field transformation. Given the nature of UAV measurements, this reference signal must be provided in a detached fashion, which we accomplish by incorporating a second antenna into the measurement process. With the receiving characteristics of this reference antenna well-known, we are able to extract the phase of the measured response at the test antenna, allowing for its far-field pattern to be predicted. While this method works well in general, we find that one must be careful in setting up the measurement configuration, a sentiment which is reinforced by results obtained from a practical near-field measurement attempting to extract the phase as described.
AFRIKAANSE OPSOMMING: Geen opsomming