ILASH is a lens design tool based on Geometrical Optics and Physical Optics, integrated with a Genetic Algorithm optimization tool. ILASH is especially tailored to address shaped integrated double-shell dielectric lenses, although it may be used for the analysis and design of canonical lenses.
The ILASH tool comprises:
- The kernel code, which implements the methods described ahead for the design, analysis and optimization of high dielectric constant circular symmetric integrated lens antennas;
- The user interface, which provides the user with a simple way to interact with the kernel, to generate and manipulate lens design data and to export and import results.
The ILASH kernel is based on DLL libraries, compiled from code originally developed in Fortran 77, C++ and Matlab. The user interface is developed in MS Visual Basic environment and uses Open GL library for the efficient manipulation of graphical objects.
The general lens geometry addressed in ILASH project consists on an axial-symmetric double-shell dielectric lens, shaped in the elevation coordinate, fed by an array of planar feed elements (imaging lens) or a single element (broadband lens) imprinted directly on the lens base.
Block diagram of ILASH design tool
The lens design and optimization process is divided into three main blocks:
Lens Synthesis - two paths for lens design are presented:
- Closed form shaped lens solutions given input parameters and design targets (direct synthesis method, based on closed form Geometrical Optics formulation);
- Iterative optimization loop combining parametric lens surface definition with lens analysis method (iterative synthesis method). It combines the lens analysis tools with an optimization algorithm that can successively generate and evaluate meaningful populations of test lenses and iterate the procedure to maximize a given figure of merit.
Lens Analysis - Evaluate accurately the lens performance given its shape and material characteristic;
Optimization - Lens results, obtained with lens analysis tools, are compared with initial specifications. Usually a cost function is defined which combines the different error functions with different weights to produce a “figure of merit” – a single parameter that represents the degree of approximation to the desired solution. When this “cost” is above a specified threshold an iterative scheme is started that successively modifies the previous lens geometry and feeds it again, performs the respective analysis and again evaluates the “cost”.
Reflector analysis is also included in ILASH, using the lens radiation pattern as the feed. All the involved losses can be calculated, starting from the power delivered by the lens feed and including the lens dissipation losses, internal reflection losses and reflector illumination spill over losses. Reflector is restricted to parabolic profile, with two alternative feed (lens) positions:
The ILASH user interface that links with the Kernel is organized into five type of functions:
- Design functions refer to the synthesis of the lens surfaces, subject to predefined design conditions and lens output specifications;
- Analysis functions allow the user to calculate the actual lens output performance and to diagnose potential sources of discrepancy between simulation model and subsequent experimental results of fabricated prototypes, allowing correcting the design accordingly;
- Auxiliary tools offer miscellaneous functions that range from lens performance quantitative evaluators to specific files and formats manipulators;
- Plot tools provide x-y and 3D graphics of the relevant output data;
- List provides an updated list of the currently performed tasks in the project, the corresponding data files, and the main performance indicators.
Design functions in the user interface
- Analytical lenses comprise
- Canonical lenses like the elliptical, spherical or user defined;
- Shaped lenses, selected from a list of seven possible categories, according to target output specification (power template, scanning condition, phase center or combination).
- Genetic Algorithm optimized lenses, comprise
- Hybrid method, where the optimization is applied to shaping parameters of analytical shaped lenses, designed according the categories in the previous case;
- The test lens is defined by splines
Analysis functions in the user interface
ILASH includes five analysis tools that can be used to evaluate the performance of single or double shell lenses:
- Ray Tracing can perform a simple ray tracing in the lens (for forward rays only) or, additionally, it can also estimate geometrically the lens phase center by finding the intersection of the back-prolongation of the lens output rays.
- Fresnel Coefficients provides a visual representation of the amplitude and phase of the parallel and perpendicular polarization reflection and transmission coefficients as well as the wave transmissivity at each point of the lens surface. This tool is useful to qualitatively apprehend not only the extension of the regions of total internal reflection (this could be done also with ray tracing) but importantly also the extension of the regions where the incidence angle approaches the limit angle.
- Surface Currents tool provides a visual representation (3D) of the currents distribution over the lens surfacegnitude and phase
- The Near Field and Far Field tools allow calculating the electric fields over a target surface containing the observation points.
Auxiliary tools in the user interface
- The Efficiency tool calculates the feed radiated power and efficiency, as well as the radiation pattern directivity and gain
- Gaussissity is a measure of the coupling efficiency of an antenna to a Gaussian- beam. This indicator can be evaluated by considering as reference a pre-defined Gaussian beam with fixed Gaussian width or it can use a Gaussian beam with optimized Gaussian width for best Gaussissity result;
- Phase Centre position calculation is based on the calculated radiation pattern. In one case, the phase centre is calculated directly from the phase pattern versus cos(theta), where theta is the observation angle. A dedicated graphical tool is used, based on the fact that cos(theta) derivative becomes linear near the main beam maximum and its slope bares a simple relation with the antenna phase centre position. Alternatively the phase centre can be calculated by combining the lens with a parabolic reflector. An automatic tool adjusts the lens position at the parabola focus until the overall gain is maximized;