Design of a dual-polarized dense dipole array for SKA mid-frequency aperture array

Gilmore, Jacki (2016-03)

Thesis (D.Phil)--Stellenbosch University, 2016.


ENGLISH ABSTRACT: This dissertation presents the design of a dual-polarized Dense Dipole Array, or “DDA”, as a candidate element for the SKA Mid-Frequency Aperture Array. The design consists of tightly coupled dipole elements above a ground plane and is fed differentially through a specially designed commonmode suppressing feed. Apart from the DDA, there are currently three design concepts under evaluation for the SKA Mid-Frequency aperture array as part of the SKA Advanced Instrumentation Program. The strength of the DDA is that it is a planar structure consisting only of a ground plane and an array element layer in addition to the feed structure, while the other geometries are either three layer or three dimensional structures, all of which complicates mass production of the array tiles. The design is an implementation of Wheeler’s current sheet array and based on work by Munk, where a combination of the capacitances between the tips of neighbouring elements and the close proximity of the elements to one another are exploited in order to increase the overall bandwidth of the array. In the first part of the dissertation, the design is restricted to the singlepolarized case and an extensive parameter study is done in order to gain a better understanding of the physics involved. The single-polarized design is then optimised using a commercial genetic algorithm optimiser and simulation results are obtained that indicate that a bandwidth of 3.8:1 is achievable with good impedance behaviour with scan angle. A scan loss of < 5 dB across all in-band frequencies was also demonstrated. The second part of the dissertation expands the single-polarized design to a dual-polarized design. Although difficulties were encountered with the optimisation of the dual-polarized design resulting in a perceived performance penalty from that of the single-polarized case, it is anticipated that performance similar to that of the single-polarized case will be achievable should an optimal design be identified. It has, however, been shown that the stability of the impedance with scan angle as well as the scan loss is still comparable to that of other MFAA front-end concepts. The third part of the dissertation presents a design of an antenna feed that suppresses common-mode resonances commonly encountered in connected antenna arrays. The design makes use of symmetrical wideband microstrip-slotline transitions to cancel out common-mode signals, while differential-mode signals will still propagate through. The design is verified using both simulations and measurements of a manufactured prototype. A wide bandwidth and a CMRR > 30 dB is achieved that exceeds the design specifications set out. Lastly, both the single-polarized as well as the dual-polarized designs are verified using manufactured prototypes. A major contribution of this dissertation is the manufacturing of a 1 m2 dual-polarized DDA prototype. The relatively flat embedded element pattern and good cross-polarization characteristics demonstrated with the large prototype is a crucial performance characteristic in in achieving cost-effective digital beam-forming.

AFRIKAANSE OPSOMMING: Hierdie proefskrif bied die ontwerp van ‘n dubbel-gepolariseerde digte dipool samestelling, of “DDA”, aan as ‘n kandidaat topologie vir die SKA Mid-Frekwensie Apertuur Samestelling. Die ontwerp bestaan uit nou gekoppelde dipool elemente bo ‘n grondvlak met ‘n spesiaal ontwerpte voer wat gemene-modus resonanse onderdruk. Buiten die DDA is daar tans drie ander ontwerpe onder oorweging vir die Mid-Frekwensie Apertuur Samestelling as deel van die SKA Gevorderde Instrumentasieprogram. Waar die ander ontwerpe of drie laag- of drie dimensionele strukture is wat massavervaardiging bemoeilik, is die DDA ‘n planêre struktuur. Die ontwerp is ‘n implementasie van Wheeler se konstante-stroomplaatsamestelling en is gebasseer op werk gedoen deur Munk, waar ‘n kombinasie van die kapasietansies tussen die punte van buurelemente en die nabyheid van elemente gebruik word om die bandwydte van die samestelling te verhoog. In die eerste deel van proefskrif word die ontwerp beperk tot die enkelgepolariseerde geval en ‘n uitgebreide parameterstudie is voltooi om ‘n beter verstaan te ontwikkel van die onderliggende fisika van die ontwerp. Die enkel-gepolariseerde ontwerp word dat ge-optimeer met ‘n kommersiële optimeerder en simulasieresulte word bekom. Die simulasieresultate dui aan dat ‘n bandwydte van 3.8:1 haalbaar is met goeie impedansiegedrag oor ‘n wye skanderingshoek. ‘n < 5 dB skanderingsverlies oor die volle frekwensiebereik word ook gedemonstreer. Die tweede deel van die proefskrif brei die ontwerp uit na ‘n dubbelgepolariseerde ontwerp. Alhoewel probleme ondervind is met die optimering van die dubbel-gepolariseerde ontwerp wat gelei het tot ‘n skynbare prestasieverlies, word daar verwag dat indien ‘n optimale ontwerp ontwikkel kan word, die dubbel-gepolariseerde ontwerp soortgelyk aan die enkel-gepolariseerde ontwerp sal kan presteer. Daar is egter getoon dat die impedansiestabiliteit met skanderingshoek sowel as die skanderingsverlies steeds vergelykbaar is met die van ander MFAA topologië. Die derde deel van die proefskrif handel oor die ontwerp van ‘n antennavoer wat gemene-modus resonanse - wat algemeen voorkom in verbinde antenna samestellings - onderdruk. Die ontwerp maak gebruik van simmetriese, wyeband mikrostrook-slootlyn oorgange om gemene-modus seine uit te kanselleer terwyl differensiële-modus seine toegelaat word om steeds deur te propageer. Die ontwerp word geverifieër deur van beide simulasies sowel as metings van ‘n vervaardigde prototipe gebruik te maak. ‘n Wye bandwydte en ‘n CMRR > 30 dB word getoon wat die ontwerpspesifikasies wat gestel is oorskry. Laastens word beide die enkel-gepolariseerde ontwerp sowel as die dubbel-gepolariseerde ontwerp geverifieër deur gebruik te maak van vervaardigde prototipes. A groot bydrae van hierdie proefskrif is die vervaardiging van ‘n 1 m2 dubbel-gepolariseerde DDA prototipe. Die relatiewe plat ingebedde stralingspatroon en goeie kruispolarisasie prestasie van die groot prototipe is ‘n deurslaggewende prestasiekarakteristiek om koste-effektiewe digitale bondelvorming te vermag.

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