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

[摘要] ENGLISH ABSTRACT: This project comprises the design, analysis, and construction of a Rotman lens withreduced conjugate-port coupling. The Rotman lens is a beam-forming network, usedin wide-angle scanners to feed an antenna array. The scanning operation is based onoptics and is therefore frequency invariant, a desirable feature of the Rotman lenscompared 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 incidentonto 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 thearray. Ports are placed on these focal points to feed and receive signals. Bootlacelenses allow up to four focal points for linear arrays whereas the Rotman lens isdesigned for three foci. Scanners usually require to scan many beam-widths, thusports are employed along a focal arc which intersect these foci. Inter-focal ports donot 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 degreesof freedom at the designer's disposal, the e ect which these available parameters haveon 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 feedports 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 approximatesthe array contour as a reecting wall, we show that this is due to the reected energyfocusing 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 forve feed ports, elevenelements, and up to30scan angles. Therst is designed for minimum phaseerrors, and the second for the defocusing of the reected energy from the focalarc. Simulations show up to a 10dB reduction in conjugate-port coupling for thesecond lens with a negligible degradation in performance from the phase errors.Measurements show that the reected energy is spread between the feed ports asexpected, compared to the focusing at a single port of the traditional Rotman lens.

[发布机构] Stellenbosch University