Dissertation (PhD)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: Though not so prominent as the wide band, high speed, mainstream development of data communication networks, cost and particular bandwidth limitations, still ensure extensive and continuing use of low-speed, half-duplex data link equipment. Most of these applications are radio based and aimed towards telemetry systems serving a wide range of utilities. Experience has shown that systems engineering for this type of installation, is seldom undertaken to a satisfactory analytical level. Investigation of published analyses of CSMA protocols in general, has indicated scope of extension of theoretical work to include system parameters for the type of protocol investigated in this dissertation. This dissertation describes the mathematical modeling of such a strategy by utilising a significantly modified, finite source, transition state-matrix approach derived from queueing theory. The contribution of the dissertation is to include system overhead parameters, such as backoff strategy, channel noise, equipment rise times, propagation- and retry delays, into the abovementioned model. The latter provides a relatively straightforward and readily applicable method for system analysis and performance prediction. A further contribution is the presentation of a software emulation with which different strategies could be simulated, allowing for adjustment of all design parameters. The simulation is intended for parallel and confimatory use with the theoretical model. A dual set of tools, theoretical and emulation based, is thus contributed to assist with the system design, performance prediction and protocol selection process. / AFRIKAANSE OPSOMMING: Alhoewel nie so prominent soos die wyeband, hoëspoed, hoofstroom ontwikkeling van datakommunikasie netwerke nie, verseker koste en spesifieke bandwydte beperkings nog die uitgebreide en voortdurende gebruik van laespoed half-dupleks data verbindingstoerusting. Meeste van die toepassings is radio gebaseer en gerig op telemetriestelsels wat deur 'n wye verskeidenheid diensverskaffers benut word. Stelselontwerp vir hierdie tipe installasies word selde op analitiese vlak benader. Ondersoek van gepubliseerde analises van kontensieprotokolle in die algemeen, het ruimte aangetoon vir die uitbreiding van bestaande teoretiese werk om stelselveranderlikes soos van toepassing op die tipe protokol in hierdie proefskrif ondersoek, in te sluit. Hierdie proefskrif beskryf die wiskundige modelering van sodanige strategie, deur gebruik te maak van 'n beduidend veranderde eindige bron, oorgangs-toestandmatriks benadering, afgelei van touteorie. Die bydrae van hierdie proefskrif is die insluiting van oorhoofse stelselveranderlikes, soos herhaal strategie, kanaalruis, toerusting stygtye, herhaal- en voortplantingsvertragings, in bogenoemde model. Laasgenoemde verskaf 'n relatief eenvoudige en maklik toepasbare metode vir stelselanalise en werkverrigtingvoorspelling. 'n Verder bydrae is die daarstelling van 'n sagteware simulasie waarmee verskillende strategieë nageboots kan word. Verstelling van alle ontwerpparameters word ondersteun. Die simulasie is bedoel vir parallelle en bevestigende gebruik tesame met die teoretiese model. 'n Dubbele, teoreties- en simulasie gebaseerde benadering, word dus aangebied vir gebruik by stelselontwerp, gedragsvoorspelling en optimale protokolseleksie.

Thesis (PhD)--University of Stellenbosch, 2003. / ENGLISH ABSTRACT: This thesis is concerned with the measurement of the complex permeability of thin round wires at radio frequencies. This is of interest as such wires are used in various applications, such as absorbing chaff. Iron and nickel alloys are also used for their good tensile properties but have an undesired electromagnetic effect which needs to be characterised. Although little work has been done in this field in recent decades it remains a relevant problem. In this thesis the advantages of accurate wide-band measurements performed by automatic network analysers are applied to the field. The measurement system is a closed coaxial transmission line with a short circuit termination. The centre conductor is the wire of interest. The surface impedance of the wire is related to complex permeability and is measured using low-loss transmission line approximations applied to half-wavelength resonances. The loss associated with complex permeability is separated from conductivity by a D.C. conductivity measurement. A full wave analysis of the coaxial mode was performed and compared to measured values. The maximum error of the propagation constant was found to be 31% at the highest frequencies and was primarily due to length uncertainties. By varying parameters expected error bands around the measured permeability were found. These bands are of the order 1 and demonstrate that the system is sufficiently robust. The measurement of the permeability of two non-magnetic wires was performed and a relative permeability of 1 was found, demonstrating the correct working of the system. A steel wire was measured and compared to measurements found in literature. The permeability dropped as frequency rose as was expected, and an acceptable comparison to other measurements was made as there is no verification standard. Thus a simple measurement system that takes advantage of calibrated automatic network analyser measurements has been developed and demonstrated to work with sufficient accuracy. / AFRIKAANSE OPSOMMING: In hierdie tesis word die meting van die komplekse permeabiliteit van dun ronde drade by radio frekwensies ondersoek. Hierdie drade word in verskeie toepassings gebruik, waaronder dié van absorberende materiale. Nikkel- en ysterallooie word ook vir hul goeie breekkrageienskappe gebruik. In laasgenoemde gevalle moet die ongewenste elektromagnetiese effekte wat voorkom, gekarakteriseer word. Hoewel baie min werk in onlangse dekades gedoen is, bly die meting van die komplekse permeabiliteit 'n relevante probleem. In hierdie tesis word die voordele van akkurate wyeband metings, soos geneem deur 'n outomatiese netwerk analiseerder, toegepas in dié veld. Die meetopstelling is 'n geslote koaksiale transmissielyn, kortgesluit aan een end. Die middel geleier is die draad van belang. Die oppervlak impedansie van die draad is verwant aan die komplekse permeabiliteit, en word gemeet deur die gebruik van lae verlies transmissielyn benaderings, soos toegepas op halfgolf resonante frekwensies. Die verlies wat met die komplekse permeabiliteit geassosieer word, word van die geleidingsvermoë onderskei deur 'n G.S. meting van die geleidingsvermoë. 'n Volgolf analise van die koaksiale mode is uitgevoer en met gemete waardes vergelyk. 'n Maksimum fout van 31% by die hoogste frekwensie is in die voortplantingskonstante gevind. Hierdie volg primêr uit onsekerhede in lengte. Deur die parameters te varieer kon 'n verwagte foutband rondom die gemete permeabiliteit gevind word. Hierdie bande is van die orde 1 waaruit volg dat die stelsel 'n genoegsame robuustheid toon. Die komplekse permeabiliteit van twee nie-magnetiese drade is gemeet en 'n relatiewe permeabiliteit van 1 is gevind. Hierdie bevestig die korrekte werking van die stelsel. 'n Staal draad is opgemeet en met gepubliseerde meetresultate vergelyk. Soos verwag, verminder die permeabiliteit met 'n verhoging in frekwensie. Hoewel geen verifiëringstandaard beskikbaar is nie, is 'n aanvaarbare vergelyking met ander metings gemaak. Die produk van die navorsing is 'n metingstelsel wat, met behulp van 'n gekalibreerde outomatiese netwerk analiseerder, aanvaarbare akkuraatheid in die meting van die komplekse permeabiliteit van dun ronde drade by radio frekwensies kan verkry.

Thesis (PhD)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: Analog-to-digital converters (ADCs) are an integral part of the interface between the analog and digital realms. This dissertation presents the design and simulation of a Complementary Output Switching-Logic (COSL) compatible, voltage state, switching logic comparator and a flash ADC for high speed applications with multi-GHz input bandwidth. Josephson technology and the COSL family of gates were utilized for this purpose. A detailed design for the switching logic comparator is first provided. The design is verified with simulations to obtain a functional comparator. The comparator is then optimized utilizing an optimization tool developed using the scripting facilities of WRSpice. Incorporated in this tool is a Monte Carlo capability to randomly vary the component values according to Gaussian distributions, and trimming facilities to be able to trim a non-functional comparator to restore functionality. The design component values are then optimized by maximizing the yield of a comparator. The optimized comparator is incorporated into the construction of a4-bit quantizer of an ADC. The output from the quantizer section yields a switching-logic Gray-code output. A Gray-to- Binary converter is designed with COSL gates to convert the Gray output from the quantizer into Binary code for further processing. The functionality, linearity, maximum input bandwidth and dynamic range of the 4-bit ADC is verified by simulation. A number of special input waveforms are used for this purpose. The performance of the comparator and the 4-bit ADC is also evaluated with thermal noise incorporated into simulation. Beat frequency simulations and Fourier spectra were also used in the evaluation of the ADC performance. A fully functional 4-bit ADC, with a maximum input bandwidth of 10 GHz for a clock speed of 20 GHz was achieved through simulations. Beat frequency simulations revealed that the comparators have an input bandwidth greater than 19 GHz with sufficient dynamic range for an ADC of greater than 6 bits of resolution. Due to the fact that the aperture time for the ADC is dependant on the rise time of the sampling pulse and not the width of the pulse, a much smaller aperture time is obtained which directly translates to higher input bandwidth. Finally, a layout of a 4-bit sampler circuit was done according to the Hypres manufacturing process to enable the high-speed testing of the comparator circuits. / AFRIKAANSE OPSOMMING: Analoog-na-Digitale Omsetters (ADOs) vorm 'n integrale deel van die koppelvlak tussen die analoog en digitale wêrelde. Hiedie proefskrif stel die ontwerp en simulasie van 'n Komplementêre Uittree Geskakelde Logika (COSL) aanpasbare, spanningstoestand, geskakelde logika vergelyker en ADO bekend. Hierdie ADO kan vir hoë spoed toepassings waar multi-GHz intree-bandwydte benodig word, aangewend word. Josephson tegnologie en die Komplementêre Uittree Geskakelde Logika (COSL) familie van hekke word vir hierdie doel gebruik. Die volledige ontwerp vir die geskakelde logika vergelyker word eerstens gegee. Die ontwerp word met behulp van simulasies bevestig om sodoende 'n ten volle funksionele vergelyker te verkry. Die vergelyker word verder geëptimeer deur middel van 'n proses wat met behulp van programmering in WRSpice ontwikkel is. Hierdie optimeringsproses sluit 'n Monte Carlo proses in wat die komponentwaardes van die vergelyker onwillekeurig volgens 'n Gaussiese verspreiding verander, sowel as 'n verstellingsmeganisme waarmee 'n nie-funksionerende vergelyker verstel kan word totdat dit weer ten volle funksioneer. Die komponentwaardes word dan geëptimeer vir maksimale opbrengs van 'n vergelyker. Die geëptimeerde vergelyker word gebruik in die konstruksie van 'n 4-bis kwantifiseerder vir 'n ADO. Die uittree van die 4-bis kwantifiseerder is in Gray kode. 'n Gray-na-Binêre kode omsetter word vir hierdie doelontwerp deur van COSL hekke gebruik te maak. Die volle ADO word voorts gesimuleer om die funksionalitet, lineariteit, maksimum intreebandwydte en dinamiese bereik te verifieer. 'n Verskeidenheid van intreeseine is vir hierdie doel gebruik. Die vergelyker en die 4-bis ADO is ook gesimuleer met termiese ruis om die effek daarvan te bepaal. Fourier spektra en ''verskilfrekwensie'' (Beat Frequency) simulasies word ook gebruik in die evaluering van die vergelyker en die ADO. Die korrekte werking van 'n 4-bis ADO met intreebandwydte van 10 GHz met 'n klokspoed van 20 GHz is deur simulasie bevestig. Verskilfrekwensie simulasies dui aan dat die vergelykers 'n intreebandwydte van groter as 19 GHz het, met voldoende dinamiese bereik vir 6 bis resolusie. Aangesien die vergelykers se venstertydperk bepaal word deur die stygende helling van die monsterpuls en nie deur die pulswydte nie, maak dit voorsiening vir 'n baie klein venstertydperk. 'n Klein venstertydperk is essensieel vir 'n hoë intreebandwydte. 'n Uitleg van 'n 4-bis vergelyker stadium is gedoen vir die Hypres vervaardigingsproses om die vergelyker teen hoë spoed te kan toets.

Thesis (PhD)--Stellenbosch University, 1994. / ENGLISH ABSTRACT: An artificial chiral medium can be made by embedding metal helices (chiral or handed structures) with random orientation inside au achiral hnst medium. The electromagnetic behaviour of such artificial chiral media can be explained by an additional (to p. and t') constitutive parameter, the chirality parameter e. Chiral media have certain special properties of which optical activity is the most prominent. Optical activity is the term used to describe the rotation of the polarization plane of a linearly polarized wave as it travels through a chiral medium. The chirality parameter is directly linked with this rotation. The reflection and the transmission coefficients from a chiral slab are well known in terms of the constitutive parameters and the thickness of the sample. In the thesis a set of .inversion equations are derived that can be used to determine the constitutive parameters (p., £, e) of a chiral medium from the measured S-parameters 811, S21x and S2l'jJ (Le. the reflection and co- and cross-polarized transmission coefficients respectively). An accuracy analysis of the measurement method is made. This is done by using the first order partial derivatives of the inversion equations. A sensitivity analysis is done on the inversion equations and the results are in an analytical form. This makes it possible to determine the contribution of each of the measurement errors to the total expected error. A statistical root mean square method is used to predict the expected error in the measured constitutive parameters. Several artificial chiral samples are measured and an accuracy analysis done on the measurements. The inaccuracy of the method of inversion for low loss samples that are multiples of half a wavelength thick is illustrated by example and explained by the sensitivity analysis. The free-space system (11-17 GHz) used to measure the S-parameters is described in detail. The radiation patterns of the focused lens antennas are measured and compared to those obtained from two theoretical models. A free-space calibration procedure is developed and the dispersion in the focal region of the antennas incorporated into it. The results from different combinations of calibration standards are compared and related to the idea of minimum sensitivity calibration. The possible use of an artificial chiral medium as a microwave absorber is also discussed. / AFRIKAANSE OPSOMMING: Kunsmatige kirale media kan gemaak word deur metaalhelikse lukraak te orienteer in 'n mekirale (bv. diEHektriese) medium. Die mikrogolfeienskappe van kirale media kan toegeskryf word aan die anti-simmetrie van die mikroskopiese insluitings. 'n Voorwerp word as kiraal beskryf indien dit nie deur translasie of rotasie op die spieelbeeld daarvan gepas kan word nie. Die menslike hand is 'n goeie voorbeeld van so 'n kirale struktuur. Die elektromagnetise eienskappe van 'n kunsmatige kirale medium kan beskryf word deuT 'n addisionele (tot j.t en E) samestellingsparameter, die kiraliteitsparameter ~. Kirale media het verskeie ongewone eienskappe waarvan opti~se aktiwiteit waarskynlik die prominentste is. Optiese aktiwiteit beskryf die verskynsel waar die polarisasierigting van 'n lineer-gepolariseerde golf geroteer word terwyl dit deur 'n kirale medium beweeg. 'n Direkte verband bestaan tussen die rotasie en die kiraliteitsparameter. Die weerkaats- en transmissiekotHfisiente van 'n kirale plaat is bekend in terme van die samestellingsparameters (j.t, f. en {) en die dikte van die plaat. In die tesis word 'n ste1 inversievergelykings afgelei wat gebruik kan word om die samestellingsparameters van 'n kirale medium te bepaal vanuit die gemete weerkaats- en transmissiekoeffisiente, Sl1, S'lIz en SUg. (S213/ is die kruis-gepolariseerde komponent van die transmissiekoeffisient wat 'n aanduiding van die rotasie gee.) ,n Analise word gedoen op die akkuraatheid waarmee metings uitgevoer kan word. Dit word gedoen deur gebruik te maak van die eerste orde parsHHe afgeleide van die inversievergelykings. 'n Sensitiwiteitsanalise van die inversievergelykings word gedoen en in 'n analitiese vurm aangebied. Die vorm maak dit moontlik om die bydrae van elke meetfout in S11, S2l:c en S21'!J tot die totale meetfout in p., € en ete bepaal. 'n Statistiese metode (wortel gemiddelde kwadraat) word gebruik om die verwagte foutwaardes in die gemete samestellingsparameters te bepaal. Verskeie kunsmatige kirale media word gemeet en 'n analise word gedoen op die akkuraatheid waarmee elk gemeet is. Die onakkuraatheid van die inversiemetode vir plate met lae verliese en wat veelvoude van 'nhalwe golflengte dik is, word getoon deur middel van 'n sensitiwiteitsanalise. Die vryeruimte meetstelsel (11-17 GHz) wat gebruik word om die weerkaats- en transmissiekoeffisH~ nte te meet, word breedvoerig bespreek. Die gemete stralingspatrone van die gefokusseerde lens antennes word vergelyk met teoreties bepaalde waardes. 'n Vryeruimte kalibrasietegniek word ontwikkel en die gemete dispersie in die fokusgebied word daarby ingesluit. Die resultate, soos verkry van verskillende kalibrasiestandaarde, word vergelyk en in verband gebring met die tegniek van kalibrasie-met-minimum-sensitiwiteit. Ten slotte word die moontlike gebruik van kunsmatige kirale media as mikrogolfabsorbeerders ondersoek.

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The triple layer pillbox antenna is an antenna topology well suited for use as a stackable "plank" element in radar applications. Its suitability arises from the folding which makes it compact, and from the layered nature which can be exploited for low cost manufacture. Existing designs of these "cheese" antennas, whether two or three layers, suff er from two drawbacks: a) The bend or fold between layers introduces undesirable reflections and b) Due to their parabolic sector geometry virtually no pattern control is available to the designer. This work addresses both of these shortcomings. A low reflection transition is realized by introducing simple compensating elements into the design which, with minimal manufacturing complexity, off er high performance over a broad frequency and incidence angle range. To cater for pattern control the concept of "dual shaped reflectors" is borrowed from the high performance dish antenna literature and implemented in the pillbox geometry. This shaping off ers limited but useful control of the aperture distribution and thus indirectly over the radiation pattern. To test these innovations three X-band antennas have been designed, built and measured. An initial unshaped geometry is used for the fi rst design to show the fold or bend performance. This antenna has a simulated and measured 2GHz usable bandwidth, with a reflection coeffi cient below -10dB and side-lobes below -27dB over a bandwidth in excess of 20%. Shaping is then added to show how either side-lobe levels can be lowered to below -32dB, or the gain enhanced by 2dB. The enhanced designs have been built and experimentally veri fied. / AFRIKAANSE OPSOMMING: Die drie-dubbel gevoude pildoos antenna is `n antenna topologie wat goed gepas is vir die gebruik as `n stapelbare "plank" element vir radar aanwending. Sy geskiktheid kom vanwee die vou van die struktuur wat dit meer kompak maak sowel as die konstruksie moontlik goedkoper maak. Huidige ontwerpe van die antenna strukture, hetsy twee of drie lae, ly aan twee tekortkominge: a) Die buiging of vou tussen die lae veroorsaak ongewensde weerkaatsings en b) Weens hul paraboliese meetkunde is feitlik geen beheer oor die stralings patroon beskikbaar nie. Hierdie werk spreek beide hierdie tekortkominge aan. `n Lae weerkaatsing by die oorgang is verkry deur eenvoudige kompenserende elemente by die ontwerp in te voeg wat, met minimale vervaardigings kompleksiteit, hoë werkverrigting lewer oor `n breë frekwensie en invals hoek spektrum. Om patroon beheer te voorsien is die konsep van "dubbele gevormende weerkaatsers" geleen vanaf die hoë verkverrigting skottel antenna literatuur en in die pildoos meetkunde ingestel. Hierdie vorming lewer beperkte dog nuttige beheer oor die openings verspreiding en dus indirek oor die stralings patroon. Om hierdie nuwighede te toets is drie X-band antennas ontwerp wat gebou en gemeet is. 'n Aanvanklike ongevormende meetkunde is gebruik vir die eerste ontwerp om die vou of buiging se werksverrigting te bewys. Hierdie antenna het `n gesimuleerde en gemete 2GHz bruikbare bandwydte met `n weerkaatsings kwosiënt onder -10dB en sylobbe van minder as -27dB oor `n bandwydte van meer as 20%. Vorming is dan bygevoeg om te bewys dat of verlaagde sylobbe van onder -32dB of verhoogde aanwins met 2dB verkrygbaar is. Die verbeterde ontwerpe is gebou en eksperimenteel bewys.

Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Electromagnetic shielding plays a significant role in the protection of electronic equipment. Its application is essential for mitigating radio-frequency interference for the Karoo Array Telescope (MeerKAT) project in the Karoo region of the Northern Cape. In this context, time-domain (TD) methodology for small enclosure shielding effectiveness (SE) is developed using a reverberated environment technique. Interest revolves around measurement time speed-up and an extended SE response which covers the under-moded condition of small enclosures. Recommended IEC standard 61000-4-21 [1] e ciency of 0.75, for log-periodic dipole array (LPDA) antenna, is also validated from a reverberation chamber (RC) characterisation of a printed circuit-board (PCB) LPDA e ciency. A built-in pulse generator and the Square Kilometre Array (SKA) receiver RATTY form the main elements of the TD metrology. For validation purpose, a reference coaxial airline is built. The cable is characterised with computational codes (CST and FEKO) and is also modelled with Vance and Kley's analytical expressions. The results are compared with TD transfer impedance (Zt) measurement within the RC. The study shows that the cable fixture within an RC shapes the cable under-test (CUT) Zt. The airline itself is also introducing an oscillating component within Zt. The resonance is proportional to the CUT length and it is visible within the measurement data and the simulations. It is not, however, taken into account by the theoretical models. The consequence of an incorrect antenna efficiency on RC applications is also addressed using a PCB LPDA antenna efficiency investigation. The unknown LPDA is simulated with CST for the study. The result is compared to an RC measurement validating the IEC 61000-4-21 standard efficiency recommendation of 0.75 [1]. This methodology characterised the unknown antenna parameter from a reference dipole antenna efficiency we investigated with FEKO. Simulated Wheeler-cap techniques permitted the reference antenna validation. We found that an inaccurate LPDA efficiency has little effect at higher frequency if the IEC efficiency is adopted. However, a difference of more than 7 dB can arise at low frequency if the real efficiency differs by more than 0.3 with respect to the IEC value. The study highlights the importance of a correct antenna efficiency for accurate RC applications. The nested-enclosure technique is regarded as the conventional method of investigating small enclosures SE [2]. The technique is in general time-consuming and works for a frequency range higher than three times the enclosure under test (EUT) lowest cut-o value. Our TD metrology covers a frequency band up to 1.4 GHz which coincides with our enclosure under-moded region. The SE characterisation is not well-documented within this particular region. The dissertation contributes to this field using a non-stirred nested-enclosure configuration. In contrast to the conventional use of the nested-enclosure methodology [2], the source is placed here within the EUT and the enclosure is treated as a normal radiator. The SE definition according to the IEEE standard in [2] is followed and the enclosure total transferred-power is computed from the port's reflection coeficient. The approach does not require a stirrer for the EUT. Our measurement shows an SE agreement between the modfied and the appropriate nested-enclosure technique from 390 MHz up to 4 GHz. The investigation is faster, but in addition the TD spectrum gives a more detailed SE response than the FD approach. / AFRIKAANSE OPSOMMING: Elektromagnetiese skerming speel n belangrike rol in die beskerming van elektroniese komponente. Die toepassing daarvan is noodsaaklik vir die versagting van radiofrekwensie steurings in die Karoo Array Telescope (MeerKAT) projek wat tans ontwikkel word in die Karoo omgewing van die Noord-Kaap. In hierdie konteks is daar op n tydsgebied metode vir klein-omhulsel beskermingse ektiwiteit (BE) gefokus, deur gebruik te maak van n weerkatingsomgewing tegniek. Die belangstelling handel om metingstye te verkort en n uitgebreide BE reaksie wat die lae-modus toestand van klein omhulsels dek. Aanbeveelde International Electrotechnical Commission (IEC) standaard 61000-4-21 [2] e ektiwiteit van 0.75, vir log-periodiese dipool-skikking (LPDA) antennas, is ook bekragtig deur n weerkaatsingskamer karakterisering van n gedrukte-stroombaanbord (PCB) LPDS antenna e ektiwiteit. n Ingeboude impulsgenerator en die Square kilometre Array (SKA) ontvanger, RATTY, vorm deel van die hoof elemente van die tydsgebiedmetings. Vir bekragtigings doeleindes is n koaksiale lugtransmissielyn gebou. Hierdie kabel is gekarakteriseer deur numeriese sagteware (CST en FEKO) en is ook gemoduleer met behulp van Vance en Kley se analitiese uitdrukkings. Die resultate is vergelyk met tydsgebied-oordragsimpedansie (Zt) metings, wat in die weerkaatsingskamer gedoen is. Die studie wys dat kabel posisie binne in die weerkaatsingskamer die Zt van die kabel-onder-toets vervorm. Die lugstreep voeg ook n ossillerende component by tot die gemete Zt. Die resonansie is eweredig aan die lengte van die kabel en is duidelik sigbaar binne die meting- en simulasiedata. Dit is egter nie in-berekening gebring in die analitiese modelle nie. Die gevolge van n verkeerde antenna e ektiwiteit in die weerkaatsingskamer toepassing is ook aangespreek deur ondersoek na die e ektiwiteit van n PCB LPDA. Die onbekende LPDA is gesimuleer met CST vir hierdie studie. Die resultate is vergelyk met n weerkaatsingskamer meting, wat die IEC 61000-4-21 standaard e ektiwiteit van 0.75 [1] bekragtig. Die metode karakteriseer die onbekende antenna veranderlike deur n verwysings dipool antenna e ektiwiteit wat ge-ondersoek is in FEKO. Gesimuleerde Wheeler-cap tegnieke het die veri kasie van die verwysings antenna resultate toegelaat. Daar is gevind dat n onakkurate LPDA e ektiwiteit n klein e ek op die ho er frekwensies het, as die IEC e ektiwiteit aangeneem is. Daar kan egter n verskil van 7dB voorkom by laer frekwensies, as die werklike e ektiwiteit met meer as 0.3 van die IEC waarde verskil. Hierdie studie lig die belangrikheid van n korrekte antenna e ektiwiteit uit vir akkurate weerkaatsingskamer toepassings. Die geneste-omhulsel tegniek word beskou as die konvensionele metode vir die bestudering van beskermingse ektiwiteit vir klein omhulsels. Die tegniek is gewoontlik tyd-rowend en werk net vir frekwensies wat drie maal ho er is as die af-sny waarde van die omhulselonder- toets. Ons tydgebiedmeting dek net n frekwensieband tot 1.4GHz, wat ooreenstem met ons omhulsel lae-modus gebied. Die beskermingse ektiwiteit karakterisering, in hierdie spesi eke veld, is nie goed gedokumenteer nie. Hierdie verhandeling dra by tot hierdie veld deur gebruik te maak van n onversteurde geneste-omhulsel kon gurasie. In teenstelling met die konvensionele gebruik van die geneste-omhulsel metode, is die bron geplaas binne die omhulsel-onder-toets en word dit gebruik as n gewone uitstraler. Die de nisie van beskermingse ektiwiteit volgens die IEEE standaard in [2], is gevolg en die totale oordragskrag van die omhulsel is bereken deur gebruik te maak van n poort se re eksie ko e si ent. Die benadering benodig nie n steurder vir die omhulsel-onder-toets nie. Ons metings se beskermingse ektiwiteit het ooreengestem met die veranderde - en die geskikte geneste-omhulsel tegniek, van 390MHz tot 4GHz in die spektrum. Hierdie ondersoek is vinniger en lewer n meer gedetailleerde beskermingse ektiwiteit reaksie as die frekwensiegebied benadering.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / This thesis focuses on the coaxial-microstrip T-resonator measurement technique to determine the aperture plane capacitance and radiation resistance of electrically short monopoles above a finite ground plane at Radio Frequencies. The determination of these impedance parameters is of interest as they are needed in the Thévenin equivalent circuit of the monopole to establish the relationship between the monopole, the electromagnetic field within which it is immersed, and its surrounding medium properties. Electromagnetically short monopoles are used in various applications such as in permittivity and biomedical measurement techniques and near-field metrology. The coaxial-microstrip device allows the measurement of transmission coefficient nulls when connected to the network analyser. By measuring the frequency shift in transmission null points and analysing the results, the capacitance of short monopoles of various lengths can be obtained. The radiation resistance of the monopole can also be determined by comparing the depth of transmission coefficient nulls points at various resonant frequencies. Monopoles ranging from flush-mounted to various sizes of protruding centre conductors, both bare and insulated, are investigated in the frequencies ranging from 45 MHz to 3 GHz. The experimental results compare well with FEKO numerical predictions at the ranges where this technique is applicable. The technique is however not applicable near and beyond the resonant frequency of the monopole where it is considered to be electrically long. Measurements to obtain monopole impedance parameters were conducted by connecting the device to the HP 8510C Network Analyser. To enhance accuracy, the Network Analyser was calibrated using a high quality calibration kit with precisely defined standards. The technique used was found to accurately yield capacitances in the range of 0.0211 to 1 pF and radiation resistances in the range of 0.9245 to 12.1 Ohms. These results were obtained for monopoles of length 0.0062 < h/l < 0.24. Results from literature, viz. W.R Scott, R.W.P. King and L.J. Cooper, show radiation resistance in the ranges: 2.5 to 35 Ohms (0.079 < h/l < 0.22), 1.26 to 74.08 Ohms (0.026 < h/l < 0.28) and 5.59 to 74.98 Ohms (0.1 < 0.28) for electrically short monopoles. The results of this work are compared with those in the literature and show variations of less than 6%. A technique capable of accurately measuring capacitance and radiation resistance of the monopole at various lengths of protruding centre conductor has been developed. The capacitance for a flush probe compares very well (agreement is less than 6%) when compared with those obtained by the previous researchers. The technique is capable of yielding accurate results radio frequencies in the challenging low radiation resistance range around 1 Ohm.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2009. / The nanosatellite is build-up of subsystems and payloads (defined as satellite nodes) connected together into the OBC using CAN bus as the main communication protocol. The flight software application is required to run within the eCos environment on the OBC to monitor and control satellite nodes. The ground station must generate commands and send them to the satellite in space. The application is developed to validate, schedule and dispatch the commands to the satellite nodes at appropriate times. Each node manager, in the flight software, is required to execute the response messages from its respective satellite node. The housekeeping and error recovery data files are defined to convey useful information about satellite status to the user and can be downloaded to the ground station. The flight software is developed using POSIX functions supported by eCos. Although it is not yet ready for real operation in space, the algorithm that can be used for full development is examined and approved.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2004. / The use of ground penetrating radar, and especially borehole radar in underground mines, is becoming increasingly attractive. This is due to the improvements in the user friendliness of this system. Borehole radar is able to sense accurately geological faults and structures hidden inside the rock body and this information is of value to geologists. In the quest to increase the accuracy of the radar, it was realized that the borehole trajectory and surrounding rock properties are needed. This thesis discusses a rock permittivity and loss sensor which is designed to be deployed in cylindrical hard rock boreholes with diameter of about 50 mm. The sensor consists of electrodes to produce capacitance that is dependant on the rock properties, and electronics to measure this dependency. The biggest problem foreseen is that the probe will not be in direct contact with the rock body. Cylindrical Electrodes were designed using numerical simulations and physical models. Sensitivity and noise attributes received attention. Electronic components were used to sense the small capacitance produced by the electrodes. The resulting signals are slow changing "DC• voltages from which an indication of the needed properties could be extracted. The system was integrated and tested in both laboratory and mine conditions. Test-result-b,ased improvements were introduced and led to satisfactory working of the probe. However, the deployment method will need attention before this probe can be used in field conditions.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / A real time attitude and heading reference system (AHRS) was successfully implemented for use on an airship. The AHRS was tested on board a small airship (blimp) with real data supplied from the inertial measurement unit and GPS receiver. The inertial measurement unit was built with lower grade sensors, resulting in significant reductions in component cost. To ensure accurate navigation results, the high rate inertial measurements were complemented with low rate GPS velocity and position updates in an extended Kalman filter configuration. A study was made of various Kalman filter configurations, especially the possibility of splitting a big Kalman filter into smaller Kalman filters. It was found that the best trade-off between accuracy and processing power was achieved by having two smaller Kalman filters running in sequence. The first extended Kalman filter estimates the attitude of the airship, while the second extended Kalman filter estimates the velocity and position of the airship. The two smaller Kalman filters were implemented on an onboard computer to provide real time estimates of the attitude, velocity and position of the airship.