The design and analysis of a rotman lens with reduced conjugate-port coupling

Ibbotson, Alex (2012-12)

Thesis (MScEng)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: This project comprises the design, analysis, and construction of a Rotman lens with reduced conjugate-port coupling. The Rotman lens is a beam-forming network, used in wide-angle scanners to feed an antenna array. The scanning operation is based on optics and is therefore frequency invariant, a desirable feature of the Rotman lens compared to other beam-forming networks which employ phase shifters. The antenna array is connected to the lens's array ports via transmission lines. These array ports lie on the array contour which is designed so that a signal incident onto the antenna array will propagate into the lens and focus at a particular point. The position of this focal point depends on the signals direction of arrival at the array. Ports are placed on these focal points to feed and receive signals. Bootlace lenses allow up to four focal points for linear arrays whereas the Rotman lens is designed for three foci. Scanners usually require to scan many beam-widths, thus ports are employed along a focal arc which intersect these foci. Inter-focal ports do not focus perfectly and result in phase errors distributed across the array aperture. The derivation of three and four foci lenses is provided. There are several degrees of freedom at the designer's disposal, the e ect which these available parameters have on the lens geometry and phase errors is investigated. The waveguide implementa- tion of these lenses is examined, in which we use vertically polarised horns as feed ports and coaxial probes as array ports. Some designs of Rotman lenses published in literature show excess mutual cou- pling between symmetrically opposed feed ports. Using a model which approximates the array contour as a re ecting wall, we show that this is due to the re ected energy focusing on conjugate ports. It is identi ed that Rotman lenses designed for mini- mum phase errors will have near maximum conjugate-port coupling. Two Rotman lenses have been designed at 3; 5GHz for ve feed ports, eleven elements, and up to 30 scan angles. The rst is designed for minimum phase errors, and the second for the defocusing of the re ected energy from the focal arc. Simulations show up to a 10dB reduction in conjugate-port coupling for the second lens with a negligible degradation in performance from the phase errors. Measurements show that the re ected energy is spread between the feed ports as expected, compared to the focusing at a single port of the traditional Rotman lens.

AFRIKAANSE OPSOMMING: Hierdie projek behels die ontwerp, analise en konstruksie van 'n Rotmanlens met verminderde simmetries oorstaande poort koppeling. Die Rotmanlens is 'n patroon- vormingsnetwerk wat gebruik word as samestellingvoer in wye hoek skandeerders. 'n Gewensde eienskap van die Rotmanlens bo faseskuif voernetwerke is dat die skan- deerwerking gebasseer is op optika en dus frekwensie onafhanklik is. Die antennasamestelling word deur transmissielyne verbind aan die antennapoorte van die lens. Hierdie poorte l^e op die samestellingkontoer wat ontwerp is om te verseker dat seine wat inval op die antennasamestelling sal voortplant in die lens en fokus op 'n spesi eke punt. Die posisie van die fokuspunt hang af van die invalshoek van die sein op die samestelling. Poorte word op hierdie fokuspunte geplaas om te seine te ontvang en te lanseer. Hierdie tipe lense kan tot vier fokuspunte h^e , waar die Rotman lens spesi ek vir drie fokuspunte ontwerp word. Skandeerders moet normaalweg etlike bundelwydtes wyd kan skandeer, en daarom word poorte geplaas op 'n straal wat die fokuspunte onderskep. Poorte wat weg van die fokus geplaas word toon 'n e ens uit fokus gedrag, wat vertaal na fasefoute in die samestelling stralingsvlak. Die a eiding van drie en vier fokus lense word verskaf. Daar is verskeie grade van vryheid tot die beskikking van die ontwerper, en die e ek wat die beskikbare parameters op die geometrie van die lens en die fase foute het word ondersoek. Die gol eier implimetering van hierdie lense word beskryf, waar vertikaal gepolariseerde horings as voerpoorte, en ko-aksiale lyne as antennapoorte gebruik word. Sommige Rotman lens ontwerpe in die literatuur toon beduidende koppeling tussen die simmetries oorstaande voerpoorte. Deur van 'n model gebruik te maak wat die samestellingkontoer as 'n weerkaatsende muur benader toon ons aan dat die koppeling geskied as gevolg van die weerkaatsde energie wat fokus in die simme- tries oorstaande poorte. Dit word identi seer dat Rotman lense wat vir minimum fasefoute ontwerp word bykans maksimum koppeling tussen simmetries oorstaande poorte tot gevolg het. Twee Rotman lense is ontwerp by 3; 5GHz vir vyf voerpoorte, elf antenna ele- mente en skandeer hoeke van 30. Die eerste is ontwerp vir minimum fasefoute en die tweede vir uit fokus weerkaatsde energie vanuit die fokus straal. Simulasies toon tot 10 dB vermindering in koppeling tussen simmetries oorstaande poorte vir die tweede lens met weglaatbare werkverrigting verswakking as gevolg van fasefoute. Metings toon dat die weerkaatsde energie, soos verwag, versprei word tussen die voerpoorte vergeleke met energie gefokus in 'n enkele poort soos by die tradisionele Rotman lens.

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