Calibration of large Rogowski coils

Combrink, Stephanus Lucas (2016-03)

Thesis (MEng)--Stellenbosch University, 2016

Thesis

ENGLISH ABSTRACT: The Square Kilometre Array aims to be the world’s largest radio telescope. South Africa will host the mid to high frequency division of the SKA and the core site will be populated with 64 offset Gregorian radio telescopes for the first phase. To achieve a resolution 50 times higher than the Hubble Space Telescope, radio frequency interference (RFI) must be kept as low as possible. Past and present studies are committed to keep the introduced and already present RFI as low as possible. Our study stems from a previous study on RFI originating from lightning (induced) currents. The study examined KAT-7, the precursor to MeerKAT, extensively and from it valuable recommendations were made. This study also gave rise to the creation of a Rogowski coil (RC) for current measurements. The RC is physically large to be able to fit around antenna struts. However, calibration was not considered for that study. Since current measurements on antenna struts are still of interest, it gives rise to our investigation into the calibration of the large RC for accurate measurements. The study begins by analysing a frequency domain (FD) method commonly used by current probe manufacturers. Good agreement between a theoretical model, simulation and measurement is obtained, but without proper de-embedding, the method is only accurate at frequencies up to 100 MHz. To extend the frequency range, a time domain (TD) technique is applied where the signals are analysed in the TD and unwanted responses are gated out, providing accurate calibration to much higher frequency. Both approaches are verified with calibration data and measurements of two commercial current probes. To improve RC calibration accuracy, an attempt is made to reduce the main mismatch reflection between the measurement device and calibration fixture. A high permittivity dielectric is investigated which, unfortunately, produced inconclusive results. Using a larger diameter conductor with a matching profile resulted in a decrease in reflection magnitude as expected. However, the matching profile introduced sub-reflections. To avoid uncertainty due to sub-reflections, the large RC was calibrated without the matching profile. To achieve a good calibration over all frequencies, a hybridised method is applied which uses the results from the frequency ranges where each method provides the most accurate calibration results, low frequency from the FD method and the higher frequency from the TD method.

AFRIKAANSE OPSOMMING: Die Vierkante-Kilometer Reeks (VKR) gaan die wêreld se grootse radio teleskoop wees, waarvan Suid-Afrika die middel tot hoë frekwensie afdeling van die VKR huisves. Vir die eerste fase sal die kern area bevolk word met 64 Gregorian-afset radio teleskope. Om ’n resolusie van 50 keer hoër as die Hubble ruimte teleskoop te behaal moet die radio frekwensie steurings (RFS) so laag as moontlik behou word. Dit word verseker deur vorige en voortgaande studies. ’n Spesifieke vorige studie het gekyk na RFS van weerlig (geïnduseerde) strome op KAT-7, die voorloper tot MeerKAT. Hieruit het waardevolle voorstelle gekom asook ’n Rogowski winding (RW) stroom meetinstrument. Die RW het ’n groot gemene omtrek sodat dit om groot antenna stutte kan pas. Die RW is egter nie gekalibreer nie, wat gedien het as motiveerder vir ons projek. Die studie begin met die analise van die frekwensiedomein (FD) metode, alom gebruik deur stroom meetinstrument vervaardigers. Goeie resultate is verkry wat korreleer oor teorie, simulasie en meting. Die metode is onakkuraat bo 100 MHz omdat dit die refleksies en resonansies nie verwyder van die gemete response nie. ’n Tyddomein (TD) analise tegniek word toegepas waar die ongewenste response verwyder word. Beide metodes word geverifieer met kommersiële meetinstrumente. Om RW kalibrasie te verbeter word impedansie aanpassing van die kalibrasie bakkie ondersoek. ’n Dielektriese medium met ’n hoë permittiwiteit word ondersoek asook ’n aanpassing profiel vir ’n vergrote diameter geleier. Die hoë permittiwiteit ondersoek het onbevredigende resultate gelewer. Die gebruik van ’n vergrote diameter geleier het gelei tot ’n verkleining van die eerste refleksie, maar as gevolg van die profiel wat gebruik is, verskyn daar sub-refleksies. Om onsekerheid te vermy was die RWgekalibreer sonder die aanpassings profiel. Om ’n goeie kalibrasie oor die hele frekwensie band te verseker word ’n hibriede metode toegepas. In hierdie metode word die FD en TD metodes se resultate gebruik waaar dit die mees akuraatste is; laer frekwensies van die FD metode en die hoër frekwensies van die TD metode wat gekombineer word vir ’n algehele kalibrasie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98550
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