Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: This work in this study covers a powerful technique to derive propagation and scattering information
in an expedient fashion. Expedient because time-domain (TD) data gathers a broad spectrum
in a single transmitted pulse. TD has been criticised because of a lack of dynamic range, which has
now been overcome by the direct sampling system, RATTY and RTA. This study focuses on the investigation
of a TD metrology system, to assist with the characterisation of MeerKAT systems. The
elementary components of the system include a fast-rising impulse generator that was coupled with
an impulse radiating antenna (IRA). The system was calibrated and tested before practical measurements
and preliminary testing in the Karoo were done.
For TDmetrology a larger bandwidth accelerates measurements without the loss of accuracy. The
pulse generator’s (PG’s) fundamental components are an avalanche transistor and a step recovery
diode (SRD), to sharpen the leading edge of the pulse. Improving the rise-time of a pulse increases its
bandwidth in the spectrum. The external circuitry around these components is pivotal and it determines
the shape, amplitude and rise-time of the pulse. In the course of the investigation, the general
circuitry around the PG was improved to obtain the best possible pulse for measurements inside a
reverberation chamber (RC) and for measurements in the Karoo. In light of this, a second and third
PG source were obtained. For measurements in the Karoo, a larger amplitude pulse was required to
increase the spectral content and this is essential for propagation measurements over distance and
the shielding effectiveness (SE) of structures. Stacking avalanche transistors allow larger amplitude
pulses and it improves the dynamic range of the spectrum. A PG incorporating stacked avalanche
transistors, was designed, built and measured to assist with RC and small-scale field measurements
in the Karoo. The third PG was bought for the practical measurements in the Karoo. The PG produces
kilovolt pulses with pico-second rise-times that extend the spectral range of the current PGs at our
disposal.
With these PGs, an antenna is required for the radiation of impulse-like transients. The IRA is a
high-gain large-bandwidth antenna. The IRA consists of a parabolic reflector, conical-plate transmission
lines that are terminated through resistors onto the dish, and a feeding balun. The IRA design
was 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 the
propagation attenuation over the Karoo soil and vegetation, with great promise. A broad spectrum
was measured over a few kilometres and compared to free-space loss. The SE of the berm covered
the same spectral bandwidth. In this measurement, scattering effects and knife-edge diffraction were
observed. / AFRIKAANSE OPSOMMING: Die werk in hierdie studie dek ’n kragtige tegniek wat gebruik kan word om die voortplanting en
die verstrooiingsinligting van elektromagnetiese golwe op ’n voordelige manier af te lei. Dit is voordelig,
want tydgebieddata versamel ’n wye spektrum in ’n enkele oordraagbare puls. Tydgebied is
in die verlede baie gekritiseer omdat dit ’n dinamiese reikwydte kortkom en dit is nou oorwin deur
die direkte steekproefnemingstelsel, RATTY en RTA. Hierdie studie fokus op die ondersoek van ’n
tydgebiedmetingssisteem en dit help met die karakterisering van MeerKAT sisteme. Die elementêre
komponente van die sisteem bestaan uit ’n vinnig-stygende impulsgenerator wat gekoppel is aan ’n
impulsuitstralende-antenna (IRA). Die sisteem is gekalibreer en getoets voordat praktiese metings en
toetse in die Karoo uitgevoer kon word.
Vir tydgebiedmetings versnel ’n groter bandwydte die metings sonder om die akkuraatheid daarvan
te beïnvloed. Die pulsgenerator se fundamentele komponente is ’n stortvloedtransistor en ’n
stap-herstel diode (SRD) wat die voorpunt van die puls verskerp. Die eskterne stroombaan rondom
hierdie komponente is noodsaaklik en dit bepaal die vorm, amplitude en die stygtyd van die puls.
Deur die loop van hierdie ondersoek is die algemene stroombaan rondom die puls verbeter, om die
beste moontlike puls vir metings binne in die weerkaatsingskamer en vir metings in die Karoo, te
verkry. Na aanleiding van dit is ’n tweede en derde pulsgenerator bron verkry. Vir die metings in
die Karoo is ’n puls met ’n groter amplitude vereis om die spektrale inhoud te vermeerder. Dit is
noodsaaklik vir elektromagnetiese golf voortplantingsmetings oor afstand asook die beskermings effektiwiteit
(SE) van die strukture. Stapel-stortvloed transistors skep pulse met groter amplitudes en
dit verbeter die dinamiese reikwydte van die spektrum. ’n Pulsgenerator wat gestapelde stortvloedtransistors
insluit is ontwerp, gebou en gemeet om te help met metings in die weerkaatsingskamer en
kleinskaal veldmetings in die Karoo. Die derde pulsgenerator is gekoop vir praktiese metings in die
Karoo. Die pulsgenerator vervaardig kilovolt pulse met pikosekond stygtye, wat die reikwydte van die
spektrum van ons huidige puls uitbrei.
Hierdie pulsgenerators vereis ’n antenna vir die uistraling van impulsagtige seine. Die IRA is ’n
hoë-wins, groot-bandwydte antenna. Die IRA bestaan uit ’n paraboliese weerkaatser, konieseplaat
transmissielyne wat deur weerstande op die skottel getermineer word, asook ’n voedings "balun".
Die IRA ontwerp is deeglik bespreek en ’n model is ontwerp, gemeet en verbeter. Die IRA is ook gesimuleer
met behulp van ’n rekenaarsagtewareprogram, FEKO. Voordat die tydgebiedsisteem benut kan word, moet dit gekalibreer word en karakteriseringsmetings
moet ook daarmee uitgevoer word. Die meetinstrumente wat in hierdie studie gebruik is,
is steekproefneming-ossilloskope en direkte steekproefneming-sisteme. Die tekortkominge van elke
instrument is ondersoek en bespreek. Die finale meting wat uitgevoer is, dra by tot die werk wat
geassosieer word met die SKA. Dit behels antennapatroonkalibrasie, voortplanting van elektromagnetiese
golwe oor afstand en die SE van ’n "berm"wat gebou is uit Karoo-grond. Hierdie sisteem is
gebruik om die voortplantings-verswakking oor die Karoo-grond en plantegroei te ondersoek en dit
lyk baie belowend. ’nWye spektrumis oor ’n paar kilometer gemeet en dit is met wrywinglose ruimte
vergelyk. Die SE van die "berm"het dieselfde spektrale bandwydte gedek. In hierdie meting is verstrooiingseffekte
en mespunt-diffraksie waargeneem.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/95963 |
| Date | 12 1900 |
| Creators | Matthysen, Nardus |
| Contributors | Reader, H. C., Wiid, P. G., Stellenbosch University. Faculty of Engineering. Department of Electrical and Electronic. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | xv, 133 p. : ill. |
| Rights | Stellenbosch University |
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