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An accurate determination of a spacecraft’s radio frequency electromagnetic field environment during launch and flight is critical for mission success. Typical fairing structures consist of a parabolic nose and a cylindrical core with diameters of 1 to 5 meters resulting in electrically large dimensions for typical operational sources at S, C and X band where the free space wavelength varies from 0.15 m to 0.03 m. These electrically large size and complex structures at present have internal fairing electromagnetic field evaluation that is limited to general approximation methods and some test data. Though many of today’s computational electromagnetic tools can model increasingly complex and large structures, they still have many limitations when used for field determination in electrically large cavities. In this dissertation, a series of test anchored, full wave computational electromagnetic models along with a novel application of the equivalent material property technique are presented to address the electrical, geometrical, and boundary constraints for electromagnetic field determination in composite fairing cavity structures and fairings with acoustic blanketing layers. Both external and internal excitations for these fairing configurations are examined for continuous wave and transient sources. A novel modification of the Nicholson Ross Weir technique is successfully applied to both blanketed aluminum and composite fairing structures and a significant improvement in computational efficiency over the multilayered model approach is obtained. The advantages and disadvantages of using commercially available tools by incorporating Multilevel Fast Multipole Method (MLFMM) and higher order method of moments (HO MoM) to extend their application of MoM to electrically large objects is examined for each continuous wave transmission case. The results obtained with these models are ii compared with those obtained using approximation techniques based on the Q factor, commonly utilized in the industry, and a significant improvement is seen in a prediction of the fields in these large cavity structures. A statistical distribution of data points within the fairing cavity is examined to study the nature of the fairing cavity field distribution and the effect of the presence of a spacecraft load on these fields is also discussed. In addition, a model with external application of Green’s function is examined to address the shielding effectiveness of honeycomb panels in a fairing cavity. Accurate data for lightning induced effects within a fairing structure is not available and hence in this dissertation, a transmission line matrix method model is used to examine induced lightning effects inside a graphite composite fairing structure. The simulated results are compared with test data and show good agreement.

Yes / When modeling objects that are small compared with the wavelength, e.g., biological cells at radio frequencies, the standard finite-difference time-domain (FDTD) method requires extremely small time-step sizes, which may lead to excessive computation times. The problem can be overcome by implementing a quasi-static approximate version of FDTD based on transferring the working frequency to a higher frequency and scaling back to the frequency of interest after the field has been computed. An approach to modeling and analysis of biological cells, incorporating a generic lumped-element membrane model, is presented here. Since the external medium of the biological cell is lossy material, a modified Berenger absorbing boundary condition is used to truncate the computation grid. Linear assemblages of cells are investigated and then Floquet periodic boundary conditions are imposed to imitate the effect of periodic replication of the assemblages. Thus, the analysis of a large structure of cells is made more computationally efficient than the modeling of the entire structure. The total fields of the simulated structures are shown to give reasonable and stable results at 900,1800, and 2450 MHz. This method will facilitate deeper investigation of the phenomena in the interaction between electromagnetic fields and biological systems.

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Contexte. Au cours du siècle, les expositions aux champs électromagnétiques se sont multipliées avec l'électricité et les moyens de télécommunications. En 2002, les champs électromagnétiques d'extrêmement basse fréquence (CEM-EBF) ont été classés comme possiblement cancérogène pour l'homme, suivis des radiofréquences en 2011. A ce jour, ce rôle cancérigène reste controversé. L'association entre les tumeurs cérébrales (TC) et l'exposition aux champs électromagnétiques a été étudiée dans une étude cas-témoins, CERENAT. Méthodes. Les sujets de plus de 16 ans, résidant dans quatre départements français, avec un diagnostic de TC posé en 2004-2006 ont été inclus, ainsi que 2 témoins appariés par cas. Le calendrier professionnel détaillé et l'utilisation du téléphone portable (TP) ont été recueillis dans un questionnaire standardisé lors d'un entretien en face-à-face afin d'estimer l'exposition aux CEM-EBF (par l'application d'une matrice emploi-exposition), et aux radiofréquences. Résultats. L'étude a inclus 596 cas et 1192 témoins. Aucune association n'a été observée entre l'exposition aux CEM-EBF et les gliomes ou les méningiomes. Pour les neurinomes, le risque augmentait avec l'exposition, non significativement, atteignant un rapport de cotes (RC)=2,7 [0,8-9,0] pour une exposition moyenne sur la vie ≥ 0,2μT. Par ailleurs, l'usage régulier du TP (O/N) n'était pas associé aux TC (RC=1,1 [0,8-1,4]). Cependant, une association significative était observée pour les gliomes chez les plus grands utilisateurs pour une durée cumulée des appels ≥ 896 heures (RC=2,3 [1,4-3,8]). Les risques étaient plus élevés pour les tumeurs temporales, l'utilisation professionnelle et l'utilisation urbaine du téléphone. Conclusion. Nos résultats vont dans le sens des études antérieures, montrant une association entre les tumeurs cérébrales et l'utilisation importante du TP; et l'absence d'association avec l'exposition aux CEM-EBF. Cependant, le lien entre CEM-EBF et neurinomes reste à explorer, de même que le suivi à plus long terme des effets du TP. / Context. During the century, exposure to electromagnetic fields have increased with electricity and telecommunications facilities. In 2002, extremely low frequency electromagnetic fields (ELF-EMF) have been classified as possibly carcinogenic to humans, followed by radiofrequencies in 2011. To date, this carcinogenic role remains controversial. We investigated the association between brain tumors and exposure to electromagnetic fields in a case-control study, CERENAT. Methods. Subjects over 16 years, living in four French areas, with a diagnosis of brain tumor in 2004-2006 were included, with 2 matched controls per case. The detailed occupational history and mobile telephone use were collected in a standardized questionnaire during a face- to-face interview to estimate exposure to ELF-EMF (using job-exposure matrix) and to radiofrequencies. Results. The study included 596 cases and 1192 controls. No association was observed between exposure to ELF-EMF and gliomas or meningiomas. For neuromas, the risk increased with exposure, not significantly, reaching an odds ratio (OR)=2.7 [0.8 - 9.0] for a life-long mean exposure ≥ 0.2 μT. Moreover, regular use of mobile phone (Y/ N) was not associated with brain tumor (OR=1.1 [0.8 - 1.4]). However, a significant association was observed for gliomas in the heaviest users when considering a life-long cumulative duration of calls ≥ 896 hours (OR=2.3 [1.4 - 3.8]). Risks were higher for temporal tumors, occupational or urban mobile phone use. Conclusion. Our results are consistent with previous studies showing an association between brain tumors and the extensive use of MP, and no association with exposure to ELF-EMF. However, the association between ELF-EMF and neuromas remains to be explored, as well as the follow of mobile phone effects in the long term.

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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.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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