Cabling and interfaces for Karoo Array telescopes : modelling and metrology

[摘要] ENGLISH ABSTRACT: The Karoo Array Telescope (KAT) will be used by South Africa in its bid to host theinternational Square Kilometre Array (SKA). As the SKA will have orders of magnitude greatersensitivity than existing radio telescopes, it will also be concomitantly more sensitive to radiofrequency interference (RFI). The influence of RFI on a differential mode (DM) KATconductive system is an unavoidable phenomenon. In this context, the conductive or galvanicsystem can, in its most basic form, consist of a source, a load, and connecting conductors. It canalso, in the case of the KAT-7 seven-dish interferometer, consist of each telescope, its functionalcabling, and the main correlator connecting the telescopes together. However, additionalconnections between the system and the environment exist. These might be an intentionalconnection made to the earthing layout, or unintentional connections due to parasiticcapacitances and inductive connections. As a result of this, additional conductive systems arecreated which carry common mode (CM) currents. Interference present in such CM paths entersthe DM system through the transfer impedance (Zt) between them. To reduce or eliminate thisinterference in the DM system, Zt has to be minimised.The use of an earthed parallel conductor (EPC as commonly referred to) in the form of a cabletray is considered to be one of the principal methods to reduce Zt. The properties of cable trays asEPCs at wavelengths which are greater than the tray length are well documented. One mainfocus in this dissertation is on cable tray mid-span and end connections. They are not welldescribed in the literature over the wide range of frequencies that is expected for KATdevelopments. The influence of the most common connections on the measured and computed Ztof the cable tray is determined. Computer Simulation Technology's Microwave Studio (CSTMWS) is employed to validate the measured results and also to enable visualisation of the fieldsand currents. Recommendations for the best connection to use for any cable tray installation isgiven. The overall shielding ability of optimally-connected cable trays has been evaluated usinga physical and computational model. In both cases the induced voltage on a victim conductor,and far-field gain functions for varying angles of incidence onto the model, are determined. Theresults also show that for certain scenarios, most of the coupling to the victim conductor, takesplace inside the end enclosure and not the cable tray. In general, properly-connected cable traysdo provide protection to their enclosed conductors, even at frequencies were the wavelength ismuch shorter than the width of the tray. The second main focus arises from an on-site radio frequency (RF) current audit undertaken ontwo of the seven KAT-7 telescopes. Shielding measures, such as interface barriers at the floorand roof of the lower telescope pedestal, are studied. The investigation is facilitated by thedevelopment of an accurate physical and computational scale model of the dish. Direct currentinjection and plane wave illumination methods are used to excite the system. The measured CMcurrent distributions are compared and comments made regarding the validity of themeasurement procedure. The CM currents, measured around the outside of the lower pedestalshow higher levels when a direct current path to ground is established, as opposed to when noclear path exists. This finding suggests at least two methods of preventing CM interferenceentering or leaving the pedestal: harden the floor and roof barriers, or manage current pathsoutside the telescope. Related to this, CM currents measured either side of the telescope interfacebarriers, are used to determine the level of shielding the interface provides. When compared tothe common definition of shielding effectiveness, the current measurement provides moreconservative shielding estimates.The research in this dissertation has influenced, and will continue to influence, the layout ofgalvanic systems for the present KAT-7 structures and the anticipated developments toMeerKAT.

[发布机构] Stellenbosch University