Creating and sharing knowledge for telecommunications
 

Overview

     

Scope

The main objective of ILASH project was to develop and deliver to ESA-ESTEC a validated software tool for the design, analysis and optimization of high permittivity circular symmetric integrated lens antennas (ILA) with optimized coupling to incoming broadband on-axis beam, or to narrowband off-axis beams in scanning applications – the ILASH tool. Although single material lenses can be handled with this tool, it is especially tailored for the design of mm-wave or sub-mm-wave double-shell lenses with inegrated feeds that enable the radiation pattern compliance of two simultaneous conditions selected from a predefined list. The tool has a windows-based user-friendly interface with advanced OPEN GL graphical functionalities that manages the kernel for the lens design, GA optimization and performance characterization. The experimental validation of the tool is based on scaled lens prototypes operating between 40 GHz and 65 GHz. Several lens prototypes were fabricated and susscessfully tested for different applications, including stand-alone scanning, integration with reflectors in quasi-optical scanning systems. ultra-wideband, etc. Besies the lens, new ultra-wide band printed feeds with integral mixer were developed and tested.

 

Objectives / requirements

One of the main challenges in this project stems from the requirement for 100% lens operation bandwidth, to match the large bandwidths of emerging integrated quasi-optical receivers. This requirement applies to the on-axis case (broadband lens) problem. The other objective of the project was to propose new designs for imaging lenses: a lens that is fed at its base by multiple off-axis planar narrowband feeds, to be used with off-axis incoming beams. Bandwidth is not the main issue in this case, but rather the degradation of the beam shape as it deviates from the lens axis. Besides the amplitude template, also a condition for radiation pattern phase may be indirectly imposed through the phase centre specification, at least for applications where the lens is used at the focus of a reflector.

 

Motivation

Current (narrow-band) designs employ hemispherical, hyper-hemispherical, extended-hemispherical or elliptical lenses fed in most cases by slot dipoles printed at the lens base to obtain high coupling to the incoming Gaussian beam. High values of the dielectric constant, in the order of 12, are used to force the otherwise bidirectional radiation pattern of the feed to radiate predominantly into the lens body. This choice of high permittivity has the drawback of important internal reflections at the high contrast dielectric/air interface. The usual lambda/4 matching layer approach has been used to minimize reflections. However this narrowband approach is not compatible with the above 100% bandwidth specification. A new approach is required to deal with the problem of efficient power transmission across the lens/air interface and efficient Gaussian beam coupling over the specified band. The approach with ILASH project is to explore the use of multiple-shell lenses with integrated feeds and optimized surface shaping in the elevation coordinate to reasonably solve the above stated efficiency problems. Classical designs of dielectric lenses refer in most cases to phase correction problems to produce collimated beams. Nevertheless, aperture taper control was simultaneously required in some applications, so some examples of amplitude plus phase correction design formulations are available in the literature, mostly based on the Geometrical Optics (GO). All these examples refer to single material lenses with the respective focal arc, and consequently the feed position in air, far from the lens body.

The need for highly shaped constant flux beams for emerging millimetre wave wireless communications created new lens designs, where the output beam is required to match a stringent amplitude template. These concepts are extended in the present work for double-shell lenses with on and off-axis feeds. A strong emphasis is directed also to global optimization of this type of lenses. The work spans from the development of new design and analysis methods to the design, fabrication and experimental tests of lens prototypes.

 

Achievements

The double-shell concept was fully explored and proved to be viable both for the ultra wide band specifications and for scanning applications. Further to dedicated tapered waveguide feeds for precise evaluation of the lens performance, a new compact ultra wideband integrated feed was developed based on crossed exponential slots (XETS) which includes a mixer and the IF retrieving circuit. This integrated feed enabled to test the double-shell lenses in the same configuration of a practical implementation. The experimental tests were quite successful.

A dedicated powerful tool – ILASH – was developed for the design and optimization of single or double-shell lenses, and for complete evaluation of its performance. It includes GO tools for quick design of these types of lenses subject to a number of different target specifications, GO/PO tools for the computation of the lens equivalent surface currents and radiation pattern, and a Genetic Algorithm optimization tool for advanced lens design. The ILASH tool also enables to evaluate the combined performance of a lens with a parabolic reflector. The windows based user friendly interface enables the user to easily manage the different available tools and to manipulate input and output data.

Seventeen lens antenna prototypes were fabricated for validation of the IALSH tool against experimental data. In all cases the agreement between measured and ILASH simulated amplitude radiation patterns can be considered quite good.

The conclusion of the study is that the double-shell lens concept is valid and very flexible. One of its main challenges, the ideal 100% bandwidth lens, was reasonably approached. The use of higher permittivity materials with lower loss should enable to improve the lenses performance. A good design depends critically on very accurate knowledge of the permittivity of the materials and of course on the fabrication accuracy. Its was proved double-shell lens fabrication is perfectly possible and reliable using regular Lab facilities, so its manufacture can be mastered in professional workshops even when its size is reduced by a factor of 10-20.