Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Worldwide, more large antenna arrays are being deployed in areas of science
previously dominated by other antenna geometries. Applications for large
arrays include Radar, Satellite Communications and Radio Astronomy. Even
though the use of large arrays solve some of the difficulties posed by more
traditional antennas, new challenges are also faced. One of these challenges
is the problem of noise coupling, and how the overall system performance is
affected by it. The Focal Plane Array (FPA), which is a new example of a
large antenna array, is currently being researched at a number of institutions
worldwide for use in Radio Astronomy. As a result, FPA’s were used as an
example element to demonstrate the practical importance of this research.
In this study, the effect of mutual coupling on the noise performance of
FPA’s was illustrated. This was done by calculating the mutual coupling between
the elements of the array, and then calculating the noise power received
by each element as a result of the mutual coupling. Next, the Active Noise
Figure and Active Noise Temperature were calculated. These parameters were
introduced to visualise the effect of mutual coupling on the overall noise performance
of the array.
Since FPA’s are by definition large, conventional brute-force analysis techniques
are very resource intensive. Solving the coupling terms using these
methods therefore requires the use of computer clusters even during the design
phase of the antenna, which is very expensive. A method was therefore developed
to calculate the coupling terms of a large array using Periodic Boundary
Conditions. The method uses infinite array analysis, which resulted in an
improvement in memory usage in orders of magnitude. This improvement
comfortably places the memory requirements for the analysis of large arrays
within the range of current personal computers. The results also displayed a
reasonable amount of accuracy for use during the design phase of an array. The additional noise power on each element as a result of mutual coupling
were also calculated. This was achieved by developing an equivalent circuit
diagram that represents the system in terms of the noise and transmission
parameters of the LNA of each receiver channel, and the coupling terms of the
antenna array. Lastly, the active noise temperature and active noise figure are
calculated. The theory was implemented by means of a script with a graphical
user interface, to provide easy-to-use access to the theory. A quick reference
table of estimated noise coupling penalty versus first term coupling and LNA
noise temperature was also compiled.
The results of an example calculation showed a significant amount of noise
coupling in an 8×8 Vivaldi array. The noise coupling resulted in an increase in
system noise temperature, Tsys, in the order of 9% of the LNA noise temperature,
TLNA. According to the SKA Tsys budget, this results in an approximate
Tsys increase of 1.3 Kelvin. In the context of Radio Astronomy, this additional
source of noise cannot be ignored, as it can greatly affect the usebility of the
telescope for certain areas of research. / AFRIKAANSE OPSOMMING: Groot antennaskikkings word deesdae al hoe meer ingespan in plek van ander
tradisionele antennamodelle. Toepassings vir groot antennaskikkings sluit
Radar, Satellietkommunikasie en Radioastronomie in. Alhoewel die gebruik
van groot antennaskikkings baie van die probleme wat deur ander tradisionele
antennamodelle veroorsaak word oplos, word nuwe uitdagings terselfdertyd
geskep. Een van hierdie nuwe uitdagins is ruiskoppelling en hoe dit die ruisgedrag
van die stelsel as ’n geheel affekteer. ’n Beeldvlakskikking (FPA), is ’n
opwindende nuwe voorbeeld van ’n groot antennaskikking en die moontlikheid
vir die gebruik daarvan in radioastronomie word tans wêreldwyd nagevors.
Om hierdie rede is die FPA gekies as voorbeeldelement om die bruikbaarheid
van hierdie navorsing in die praktyk te beklemtoon.
In hierdie studie word die effek van wedersydse koppelling op die ruisgedrag
van FPA’s geïllustreer. Dit word gedoen deur eers die wedersydse koppelling
tussen die elemente van die antennaskikking te bereken en dan die ruisdrywing
wat deur elke element ontvang word as gevolg van wedersydse koppelling.
Daarna word die Aktiewe Ruistal en die Aktiewe Ruistemperatuur bereken.
Hierdie nuwe parameters word bekendgestel om die gevolge van wedersydse
koppelling op die ruisgedrag van die stelsel as ’n geheel te visualiseer.
Omdat FPA’s per definisie groot is, vereis die analise daarvan deur middel
van konvensionele metodes baie rekenaar hulpbronne. Hierdie metodes
vereis dus die gebruik van rekenaarbondels of superrekenaars selfs gedurende
die ontwerpfase van die antenna, wat baie duur en onprakties is. Daar is dus ’n
metode ontwikkel wat gebruik maak van periodiese randvoorwaardes om groot
antennaskikkings te analiseer. Die metode benader ’n groot antennaskikking
as ’n eindig-opgewekte oneindige skikking van antennas. As gevolg hiervan,
word die geheueverbruik met ordegroottes verbeter. Hierdie verbetering plaas
dus die analise van groot antennaskikkings binne die vermoëns van huidige persoonlike rekenaars. Die resultate wys ook ’n aanvaarbare graad van akkuraatheid
vir gebruik gedurende die ontwerpfase van die skikking.
Die bykomende ruisdrwying op elke element as gevolg van wedersydse koppelling
is ook bereken. Om dit te vermag, is daar ’n ekwivalente stroombaandiagram
ontwikkel wat die gekoppelde stelsel in terme van die ruis- en transmissieparameters
van die laeruisversterker (LNA) aan elke ontvangerkanaal en
die koppelterme van die antenna skikking voorstel. Laastens word die aktiewe
ruistal en die aktiewe ruistermperatuur ook bereken. Die teorie is geïmplimenteer
deur gebruik te maak van ’n grafiesegebruikerskoppelvlak (GUI). Die GUI
verskaf aan die gebruiker maklike toegang tot die teorie wat onwikkel is in hierdie
navorsing. Daar is ook ’n snelnaslaantabel geskep met benaderde waardes
van ruiskoppelling vir ’n verskeidenheid waardes van LNA ruistemperature en
eerste element koppelling.
Die resultate van ’n 8×8 Vivaldiskikking voorbeeld, het ’n beduidende hoeveelheid
ruiskoppelling getoon. Die ruiskoppelling het ’n maksimum toename
in stelsel ruistemperatuur, Tsys, van ongeveer 9% van die LNA ruistemperatuur
tot gevolg gehad. Volgens die huidige Tsys begroting van die SKA, kom
dit neer op ’n Tsys toename van byna 1.3 Kelvin. In die konteks van die radioastronomie,
kan hierdie toename in ruistemperatuur nie geïgnoreer word
nie aangesien dit die bruikbaarheid van die teleskoop vir sekere velde van navorsing
nadelig kan beïnvloed.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/4277 |
| Date | 03 1900 |
| Creators | Van der Merwe, Jacki |
| Contributors | Palmer, K. D., University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 93 p. : ill. |
| Rights | University of Stellenbosch |
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