Browsing by Author "Schonken, Francois"

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    Open Access
    An open source array antenna toolbox implementation
    (2021) Jaffer, Abubaker; Schonken, Francois
    Around the early 1900's, the first transmission of radio waves by means of phased array antennas was demonstrated. Since then, the further development of phased array radars was largely driven from a military point of view, with operational phased array systems implemented as far back as the second World War. In recent times, phased array antenna systems have become much more prevalent, not only in a military context but also increasingly in the commercial space, with array antenna implementations used in satellite communication systems and direction finding systems around the world. With this in mind, the following dissertation presents the development, functionality and results of a phased array simulation toolbox developed in the open source programming language, Julia. Some of the main concepts demonstrated in this dissertation include various array antenna configurations, including linear, planar, circular, cylindrical, spherical and conical, each of which are customisable in terms of beam steering, number of array elements, inter-element spacings and in some cases, array tapering to name but a few. The aim behind the development of the array toolbox is to provide array antenna enthusiasts and students with a simple to use simulation package, enabling an investigation into the effect of various array antenna parameters on the output antenna pattern produced
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    Open Access
    Beamforming and scan pattern performance evaluation of rotating maritime multi-beam phased array surveillance radar
    (2022) van Heerden, Lourens; O'Hagan, Daniel; Schonken, Francois
    Modern naval ships face a wide spectrum of threats, from fast-moving sea-skimming missiles to slow-moving unmanned vehicles and boats. The rotating phased array naval surveillance radar provides 360° azimuth coverage at large elevation angles for early warning to initiate the appropriate action and countermeasures timeously. This dissertation aimed to provide a simulation model to test and evaluate the effect of different beamforming and scan patterns on the detection performance of different possible targets in the maritime environment per antenna rotation. MATLAB, with various Phased Array Toolbox objects, was used as the platform to create the simulation model. A wide variety of variables were adjusted to test the effect on detection performance per rotation against a specified target. It distinguished between air (fast) targets with a medium Pulse repetition frequency (PRF) waveform and surface (slow and low) targets with a low PRF waveform. Coherent and noncoherent (surface only) processing algorithms were used. Complex clutter data sets from the CSIR Fynmeet sea clutter measurement trial were adapted according to the Georgia Institute of Technology (GIT) model, clutter area model and radar range equation to be inserted into each pulse. The output was a detection table for the air and surface channel for the evaluated sector per rotation. In order to determine detection performance, multiple rotations and target scans with clutter data offsets were required. The simulation model provided insight into the effect of beamforming and scan patterns on the detection performance of both fast and slow-moving targets. It was found that with coherent fast target air surveillance and sufficient clutter suppression, a fast-moving target could be detected with a detection probability of 1 and a false alarm probability of 0. This could be achieved with a single or dual-axis stacked beam. In a single axis stacked beam, only non-coherent integration of low quantity, low PRF pulse bursts could be used for the surface channel as time resources were limited to a single beamwidth. In this case, cell averaging constant false alarm rate (CA-CFAR) detection provided better results in a homogeneous and small target signal-to-clutter ratio (SCR) environment (as was the case at far ranges). The detection probability was 1 and false alarm probability was 0.03. In a spiky and large target SCR environment (as was the case at near ranges), constant threshold detection performed better. The detection probability was 1 and false alarm probability was 0. High Doppler resolution coherent integration of a large number of low PRF pulses could be used in a dual axis beam for the surface channel. This type of detection provided results comparable to non-coherent detection. The detection probability was 1 and false alarm probability was 0.02 at far ranges, and 1 and 0.06 respectively at near ranges. When the target's Doppler frequency was within the clutter's Doppler spectrum, the target was not detectable.
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    Design of an HF transmit antenna for bistatic ionospheric soundings in Antarctica
    (2020) Macwilliam, Kathleen; Schonken, Francois; Kosch, Michael; Ward, Jonathan
    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.
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    Self-Interference Cancellation for Simultaneous Transmit and Receive (STAR) Applications
    (2019) Parker, Asif Ahmed; Schonken, Francois; O'Hagan, Daniel
    Co-channel interference between transmit and receive antennas means that simultaneous transmission and reception (STAR) of signals on the same frequency is an engineering challenge when co-locating the transmit and receive channels. Due to advancements in Radio Frequency (RF) receiver and antenna hardware, as well as electromagnetic computation software, this technology is becoming more and more realisable, with applications in the fields of radar and communications. For a STAR system to be effective, high isolation (in excess of 90 dB) between transmit and receive channels is required to avoid self-interference. A lack of isolation will result in a significant reduction in the receiver sensitivity and dynamic range, reducing its ability to adequately detect incoming signals. This study involves the design and analysis of a STAR demonstrator where the theoretical and practical viability of such a system is evaluated. High isolation is achieved through the use of a combination of passive suppression, as well as analogue and digital cancellation techniques. The design consists of three cancellation layers: passive suppression, which uses a transmit antenna array to increase the transmit-receive antenna isolation through null placement; analogue cancellation, which aims to reduce self-interference by subtracting a copy of the estimated interference signal from the received signal; and digital cancellation, which uses adaptive filtering in the digital domain to further suppress residual self-interference. The demonstrator is tested in a typical real-world environment to characterise the performance of the system. The measured isolation between transmit and receive antennas is 29.4 dB. Passive suppression increases this isolation to 51.5 dB when using a four element linear transmit array. Analogue cancellation provides up to 30 dB of additional isolation, with digital cancellation providing a further 20 dB of suppression. Together, as an integrated system, the demonstrator is capable of providing a combined 101.5 dB of self-interference suppression. This clearly demonstrates that a STAR system is viable through the use of a multi-layer cancellation scheme comprising of passive suppression, analogue cancellation and digital cancellation techniques.