Viability of UAV-based antenna pattern measurements
Date
2020-03
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
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
AFRIKAANSE OPSOMMING: Geen opsomming
Description
Thesis (MEng)--Stellenbosch University, 2020.
Keywords
Drone aircraft, Radio telescopes -- Calibration, Unmanned aerial vehicles (UAV), Antenna radiation patterns -- Measurement, UCTD, Square Kilometre Array (SKA) -- Calibration