Design of an HF transmit antenna for bistatic ionospheric soundings in Antarctica

• Main Author: Macwilliam, Kathleen
• Other Authors: Schonken, Francois
• Format: Dissertation
• Language: English
• Published: Faculty of Engineering and the Built Environment 2020
• Subjects: Electrical Engineering
• Online Access: http://hdl.handle.net/11427/32406

Studying high-latitude travelling ionospheric disturbances (TIDs) is of importance be-cause they often correspond to space weather events which affect the earth's climate. The South African National Space Agency (SANSA) plans to install a low-powered high frequency (HF) transmitter at the South Pole for use in a bistatic ionospheric sounding system intended to detect such TIDs. The aim of this dissertation was to design a suitable transmitter antenna such that propagating skywave signals could successfully be received by the SANAE SuperDARN radar some 2090 km away. A transmitter beacon with an operating frequency of 12.57 MHz and a maximum 1 W power output has already been designed previously for the system. A highly directional antenna was required to reduce interference with another existing SuperDARN radar situated at the South Pole Observatory. A key goal was to transmit as little power as possible, with mainly narrowband antennas being taken into account. Additionally, a wide azimuth beamwidth was desired to allow for the possible illumination of other nearby Antarctic SuperDARN stations. The rest of the parameters were not defined explicitly and were established during the design process. More specifically, the antenna gain, elevation beamwidth and transmitter power required to achieve successful communication had to be determined. A thorough investigation of HF ionospheric propagation was undertaken, with the po-lar ionosphere and its impact on system functionality being of particular concern. Freely available propagation prediction tools were reviewed and ICEPAC was selected for use based on its high-latitude capabilities. It was discovered that the models used in both ICEPAC and the online Virginia Tech SuperDARN ray tracer ignore the presence of the extraordinary wave mode, the significance of which was discussed. The non-deviative radiowave absorption in the D and lower E layers of the ionosphere is one of the most notable contributors to total transmission loss. Consequently, manual calculations of it were done(for both extraordinary and ordinary wave modes) by using the magnetoionic Appleton-Hartree equations in conjunction with relevant ionospheric and geophysical models. These results were used to supplement the transmission losses estimated by ICEPAC to ensure that enough power is supplied to allow for both wave modes to reach the receiver. The properties of the lossy ice ground at the South Pole were researched in depth and a multi-layered substrate ground plane was modelled for use in FEKO simulations. Several antennas were investigated through an iterative design process and a three-element rectangular loop Yagi-Uda was chosen for final consideration. This was because it not only performed the best but was the most compact antenna and allows for easy transportation and construction with minimal equipment. Ultimately, based on the research presented in this dissertation, a final transmitter antenna has been designed which is believed will operate successfully for its intended purpose.