Browsing by Author "Geschke, Riana"
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- ItemOpen AccessConcept Demonstrator for MeerKAT Operation from 14.5 to 20 GHz(2019) Mundia, Sitwala; Geschke, Riana; Malan, SiasIn this thesis, a proof of concept receiver system operating from 14.5 to 20 GHz for the MeerKAT Radio Telescope is presented. MeerKAT is a 64 element telescope antenna array consisting of offset-fed Gregorian reflector antennas with a 13.5 m main reflector and 3.8 m sub-reflector. Currently, the MeerKAT is planned to operate up to 14.5 GHz. However, the reflector surface accuracy of 0.6 mm RMS achieved for the MeerKAT potentially allows it to operate at much higher frequencies. The system design consists of a feed horn antenna and front-end down conversion receiver ready for integration with back-end digital signal processing. The antenna design was carried out using electromagnetic simulation software and system level simulation software was used for the front-end receiver. A single polarization wide-axially corrugated horn with low side-lobes and cross-polarization has been designed for the proof of concept with a predicted aperture efficiency of 60% including surface accuracy loss when illuminating the MeerKAT reflector. The measured results for the antenna show a return loss better than 15 dB in the operational band and boresight gain of 12 dB. The measured E- and H-plane cross-polarization for the antenna is lower than -40 dB. The measured edge taper at the halfsubtended angle of the sub-reflector is between -11.8 dB and -13.2 dB. The front-end receiver was designed to use a single down-conversion stage to a 4.5 GHz IF with an instantaneous bandwidth of 2.5 GHz to be bandpass sampled at 6 Giga-samples per second (GSPS). The receiver was designed using off-the-shelf connectorized modules and custom designed microstrip filters for image rejection and anti-aliasing. Laboratory measurements of the receiver show a maximum gain of 76 dB, 40 dB image rejection and 27 dB spurious free dynamic range (SFDR). The simulated noise figure of the system using the measured noise figure of the LNA is 1.74 dB. The measured gain flatness of the receiver is ±7 dB due to poor performance of one of the amplifier modules used in the system.
- ItemOpen AccessDesign and feasibility evaluation of low-cost 3D printing of Horn Antennas(2019) Gao, Ming; O'Hagan, Daniel; Geschke, RianaThis dissertation investigates advances in additive manufacturing (AM) technology to determine the feasibility of low-cost 3D printing of horn antennas. Relevant antenna theory and current 3D printing technologies are reviewed and a literature review is conducted looking specifically at microwave and RF devices that have been fabricated using 3D printing technologies. The literature indicates that the fabrication of antennas using AM and metallisation techniques is realisable. One of the objectives of this study has been to design, fabricate and test the performance of lowcost 3D printed antennas to determine their feasibility. To achieve this, a commercial X-band pyramidal horn has been replicated using the microwave simulation package FEKO. The X-band horn has been fabricated using an FDM-based 3D printer and metallised using conductive paint. Ku-band pyramidal and conical horns have also been designed and 3D printed using the same method and have been metallised using both conductive paint and electroplating. The fabricated horns have been measured and tested in an anechoic chamber with the measured results analysed. The fabricated X-band pyramidal horn achieved a gain of 9.2 dBi with an input reflection coefficient of −11.9 dB at a centre frequency of 10 GHz. This is in agreement with the measured gain and reflection coefficient of the X-band commercial horn. The Ku-band pyramidal horns that have been metallised using conductive paint and copper plating achieved gains of 17.5 dBi and 17.7 dBi respectively, measured at a centre frequency of 15 GHz. The input reflection coefficients for the painted Ku-band pyramidal horns are measured as −24.2 dB while the copper plated horns are measured as −23.3 dB. The second set of Ku-band conical horn antennas designed have also been metallised using conductive paint and copper plating. These two antennas achieved gains of 12.0 dBi and 16.6 dBi respectively at a centre frequency of 15 GHz. The input reflection coefficient for the painted Kuband conical horn is −15.2 dB while the plated version has a reflection coefficient of −18.3 dB. The total cost of fabricating and testing each antenna amounted to approximately ZAR 475 per antenna, an order of magnitude lower than the price of a traditional cast or milled antenna. The method of fabrication demonstrated in this report is relatively fast and inexpensive while producing favourable results. As such, this method is highly suited for rapid prototyping and development of more advanced antenna designs.
- ItemOpen AccessDesign and Implementation of an RF front end for the NeXtRAD radar system(2017) Stevens, Adrian Dale; Geschke, Riana; Downing, BarryThis dissertation presents the design of the RF front end for use on the NeXtRAD radar system. The system is intended for research purposes to investigate potential target detection benefits to be derived from a multistatic, dual-band (X- and L-band), polarimetric radar architecture, particularly within dense clutter environments such as the maritime environment. By examining the high-level system requirements and objectives, requirement specifications for the RF front end were derived and a suitable architecture, making use of commercial off-the-shelf components, proposed. This architecture was modified in order to meet cost constraints - subsequently offering reduced levels of functionality but suitable for an initial build. Using this modified RF front end architecture, design verification and system analysis was conducted, both analytically and with the aid of SystemVue, in order to predict both the front end and overall radar detection performance. Once the front end design was found to be satisfactory, it was built and tested in a laboratory environment. Test results revealed a general improvement in performance when compared with the design predictions, yielding peak transmitter power levels in excess of 61dBm at L-band, and 54dBm at X-band. Some non-conformances were also identified, but these were as a result of component problems and not system design. Since the front end could not yet be integrated into the radar, performance modelling was repeated using the final lab test results. This indicated a negligible improvement in receiver single-pulse signal-to-noise ratio, but confirmed that the system performed as predicted. Based on the lab test results, it was concluded that the 'as-built' front end design closely matched the design goals and would be suitable for eventual integration into the first revision of the NeXtRAD system. It was, however, recommended that a concerted effort be made to secure funding to implement the original front end architecture in order to achieve the full system functionality originally desired.
- ItemOpen AccessDesign and implementation of L and X-band filters for the NeXtRAD front end(2017) Gouveia, Dominique; Geschke, RianaMicrowave filters are required at the RF front end of a research radar called NeXtRAD to suppress out of band transmitted and received signals. NeXtRAD is a multistatic pulse-Doppler radar system, developed at the University of Cape Town (UCT) in collaboration with the University College London (UCL). It has been designed to operate in two frequency bands, designated as L and X-bands. NeXtRAD will be used as a research tool, for the purposes of measuring sea targets and detecting sea clutter. The measured data will be stored in a database, and it will be made freely available to the public for research purposes. A coaxial comb-line filter was designed, manufactured and measured at L-band. The narrow band measurement results showed that the filter was centred at 1300 MHz, with an equal ripple bandwidth of 210 MHz. The filter has a spurious-free window of 2050 MHz at -60 dB, with the first spurious approximately at 2.86 times the operating frequency. The return loss of the filter was 19.52 dB, and the insertion loss at mid-band was 0.14 dB. The measured filter agreed extremely well with the L-band specifications. The X-band iris coupled filter was also designed, manufactured and measured. The narrow band measurement results showed that the filter was centred at 8.5 GHz, with an equal ripple bandwidth of 121 MHz. The spurious-free window of the measured filter at -60 dB was 6.571 GHz, with the first spurious at 1.82 times the operating frequency. The insertion loss of the filter was measured to be 2 dB at mid-band and the return loss of the filter was measured to be 18.58 dB at mid-band. The filters are currently being used at the RF front end of the NeXtRAD system.
- ItemOpen AccessPlanar groove gap waveguides(2019) Oyedokun, Titus Oluwale; Geschke, Riana; Stander, TinusWith the increasing demand for wireless services and applications, the integration and coexistence of multi-standard and multi-band operations into a single device has led to intensive research in the design of tunable and reconfigurable planar devices. A planar medium to achieve this integration is the Substrate Integrated Waveguide (SIW). However, due to a lack of DC isolated planes of the structure, bridging wires or concentric etched rings are often used to enable active device biasing. This research presents a novel planar structure referred to as the Planar Groove Gap Waveguide (PGGWG). The new structure has similar modal characteristics to air-filled machined Groove Gap Waveguide (GGWG), but in a low-cost fabrication technology that is readily integrated with surface mount components. The structure provides two DC isolated conducting planes, while still providing a low loss planar transmission medium. Simulation results demonstrate the existence of a TE10 propagating mode within the artificially created bandgap. There is good agreement between de-embedded simulated and measured results over the usable bandwidth of the waveguide (28 to 40 GHz). A passband is measured having an average insertion loss of 1.2 dB and 0.5 dB insertion loss variation implemented on a substrate of relative permittivity r of 3.5, and loss tangent of 0.004. The broadband characterization of the transmission line loss and phase constant for PGGWG at Ka-band shows that PGGWG has comparable attenuation over the band of interest to SIW. The transmission line Q-factor is found to vary from 135 to 140 over the band of interest, which is comparable to SIW in the same medium. PGGWG is also found to have a phase constant of nearly double that of comparable SIW, which is a significant results for system miniaturization. The unloaded Q-factor of a 33.5GHz PGGWG rectangular cavity resonator is measured to be 209. This is found to be comparable to an SIW resonator on the same substrate and frequency band. This work further explores the DC isolation property of the PGGWG by presenting electrically tunable PGGWG resonant cavities. It is found that a simple biasing network can be applied to the cavity using a varactor diode to vary the resonant frequency of the cavity. This is done without bridging wire and concentric etched rings as a direct result of the DC isolation of the PGGWG. A tuning range of 4.5% is achieved in measurement. From the experiments conducted, it is concluded that PGGWG can be used as an alternative planar waveguide media. The PGGWG platform can be used in the design and implementation of RF front-end components at millimeter waves. Its DC isolated conducting planes also provide a simple way of biasing active components in frequency agile applications.
- ItemOpen AccessX-band digitization systems aspects and filters for MeerKAT radio astronomy receiver(2018) Malan, Jocias A; Geschke, RianaThe MeerKAT Radio Telescope is a 64 element antenna array under construction in South Africa. This array will be used to observe radiation from celestial sources at radio frequencies. Once completed, this radio telescope will be the largest and most sensitive radio telescope in the Southern hemisphere. MeerKAT is being designed to observe radio signals produced by celestial sources at UHF-, L-, S- and X-band frequencies. With the development of the X-band receivers for MeerKAT scheduled to start in the near future, research is required to determine an optimum receiver design for this frequency band. There are two architecture options available for digitization of this band namely direct digitization of the entire band or digitization of a portion of the frequency band using a heterodyne configuration. In this thesis, both these options are investigated to determine the feasibility of both these architectures. A key outcome of this investigation will be the derivation of the specifications of the anti-aliasing filter used in a direct digitization receiver as well as the image reject filter used in the heterodyne receiver. The design of a wideband microwave filter capable of meeting these specifications is also presented. To conclude this investigation, the impact of the designed filter on the performance of the system is presented.