Time domain metrology for MeerKAT systems

[摘要] ENGLISH ABSTRACT: This work in this study covers a powerful technique to derive propagation and scattering informationin an expedient fashion. Expedient because time-domain (TD) data gathers a broad spectrumin a single transmitted pulse. TD has been criticised because of a lack of dynamic range, which hasnow been overcome by the direct sampling system, RATTY and RTA. This study focuses on the investigationof a TD metrology system, to assist with the characterisation of MeerKAT systems. Theelementary components of the system include a fast-rising impulse generator that was coupled withan impulse radiating antenna (IRA). The system was calibrated and tested before practical measurementsand preliminary testing in the Karoo were done.For TDmetrology a larger bandwidth accelerates measurements without the loss of accuracy. Thepulse generator's (PG's) fundamental components are an avalanche transistor and a step recoverydiode (SRD), to sharpen the leading edge of the pulse. Improving the rise-time of a pulse increases itsbandwidth in the spectrum. The external circuitry around these components is pivotal and it determinesthe shape, amplitude and rise-time of the pulse. In the course of the investigation, the generalcircuitry around the PG was improved to obtain the best possible pulse for measurements inside areverberation chamber (RC) and for measurements in the Karoo. In light of this, a second and thirdPG source were obtained. For measurements in the Karoo, a larger amplitude pulse was required toincrease the spectral content and this is essential for propagation measurements over distance andthe shielding effectiveness (SE) of structures. Stacking avalanche transistors allow larger amplitudepulses and it improves the dynamic range of the spectrum. A PG incorporating stacked avalanchetransistors, was designed, built and measured to assist with RC and small-scale field measurementsin the Karoo. The third PG was bought for the practical measurements in the Karoo. The PG produceskilovolt pulses with pico-second rise-times that extend the spectral range of the current PGs at ourdisposal.With these PGs, an antenna is required for the radiation of impulse-like transients. The IRA is ahigh-gain large-bandwidth antenna. The IRA consists of a parabolic reflector, conical-plate transmissionlines that are terminated through resistors onto the dish, and a feeding balun. The IRA designwas thoroughly discussed and a first model for metrology was designed, measured and optimised.The IRA was also simulated with computation software code, FEKO.Before deployment of theTDsystem, calibration and characterisation measurements are required.The measuring devices used within this study were sampling oscilloscopes and direct sampling systems.The limitations of each device were explored and are discussed. The final measurements that were conducted contribute to work related to the SKA. This incorporated antenna pattern calibration,propagation over distance and the SE of a berm built from Karoo soil. The system investigated thepropagation attenuation over the Karoo soil and vegetation, with great promise. A broad spectrumwas measured over a few kilometres and compared to free-space loss. The SE of the berm coveredthe same spectral bandwidth. In this measurement, scattering effects and knife-edge diffraction wereobserved.

[发布机构] Stellenbosch University