Browsing by Author "Mutonkole, Ngoy"

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    Modelling of Antenna Responses
    (Stellenbosch : Stellenbosch University, 2016-12) Mutonkole, Ngoy; De Villiers, Dirk I. L.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: This dissertation presents surrogate modelling techniques for the radiation patterns and scattering parameters of antennas. The models are developed in the context of calibration of re ector antennas as well as antenna design activities such as design space exploration, optimisation and sensitivity analysis. On the calibration front, a method is proposed to recover the radiation pattern, resulting from some physical deformation of an o set Gregorian re ector antenna, over a wide frequency bandwidth by taking a few directional measurements at a single frequency. The proposed technique combines characteristic basis function patterns (CBFPs) with the linear algebraic notion of subspace projection and is shown to achieve pattern recovery with better than -40 dB accuracy over a bandwidth of up to a decade. Concerning surrogate models for antenna design, models based on the parametrisation of CBFPs are presented at three levels: (i) CBFPs are parametrised on a xed grid of EM simulated samples to yield multivariate models of the full radiation pattern. The associated S-parameters are modelled through an indirect parametrisation of the poles and residues obtained by tting S-parameter data with rational functions. (ii) A univariate adaptive sampling algorithm is devised to model the frequency dependence of the full radiation pattern by judiciously selecting the frequency samples at which the antenna is simulated. The proposed algorithm is guaranteed to converge to an accurate model in a modest number of iterations, thereby improving the e ciency of frequency domain antenna simulations. (iii) A multivariate adaptive sampling algorithm is devised to model the full radiation pattern as well as S-parameters as a function of multiple design variables (including frequency). The proposed adaptive sampling techniques have the additional feature of approximation error control. The proposed surrogate modelling techniques can be used to improve the calibration e ciency of re ector antennas (since fewer measurements are required for wideband systems), as well as to improve the design work flow of antennas by reducing the computational cost of the associated design activities.
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    Multivariate adaptive sampling of parameterized antenna responses
    (Institute of Electrical and Electronics Engineers, 2017) Mutonkole, Ngoy; De Villiers, Dirk I. L.
    ENGLISH ABSTRACT: We present a robust method to adaptively construct parameterized models of the full radiation patterns of antennas and the associated S-parameters. The method sequentially selects points (geometric parameters of the antenna and frequency) such that an accurate model is obtained over a constrained multivariate parameter space. The algorithm consists of a balance between exploration and exploitation of the parameter space, resulting in a near optimal coverage of the design space, with some emphasis being placed in regions of the parameter space where the patterns or S-parameters vary rapidly. In addition, the technique is equipped with a measure of absolute error control. The proposed method is validated through pertinent numerical examples.
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    Parametric modeling of radiation patterns and scattering parameters of antennas
    (Institute of Electrical and Electronics Engineers, 2016) Mutonkole, Ngoy; Samuel, Elizabeth Rita; De Villiers, Dirk I. L.; Dhaene, Tom
    This paper describes a data-driven method to model the radiation patterns (over a large angular region) and scattering parameters of antennas as a function of the geometry of the antenna. The radiation pattern model consists of a linear combination of characteristic basis function patterns (CBFPs), where the expansion coefficients of the CBFPs are functions of geometrical features of the antenna. Scattering parameters are modeled by means of parameterized state-space matrices. The obtained models are quick to evaluate and are thus suitable for design activities where multiple simulations are required. The proposed method is validated through illustrative examples.
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    Study of a wideband sinuous feed for reflector antenna applications
    (Stellenbosch : Stellenbosch University, 2013-12) Mutonkole, Ngoy; De Villiers, D. I. L.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: This thesis presents a thorough study of the printed sinuous antenna and its characterisation as a feed for re ector antenna applications. Two di erent techniques are used in this study, namely a parametric study and an e cient surrogate based optimisation strategy. A planar sinuous antenna over a re ecting ground plane, with no absorber lining, is designed following a parameter study from which e ective design guidelines are derived. The designed prototype displays a bandwidth ratio of more than 3 : 1 from 1:9􀀀6:2 GHz, at a measured return loss of 10 dB, representing a signi cant improvement over the octave band previously achieved with a similar antenna. An optimisation based approach is followed in formally investigating a conical sinuous antenna over a re ecting ground plane. An e cient surrogate based optimisation strategy, in which the antenna's response is approximated by a Kriging model, is used. The search for optimal design parameters as well as improvements in the accuracy of the Kriging model is accomplished by using expected improvement as the in ll sampling criterion. The antenna is optimised for return loss, aperture e ciency for a prime-focus paraboloid re ector as well as cross-polarisation and results from the optimisation are used to derive e ective design guidelines and performance limitations. The investigations are conducted for the 2 􀀀 6 GHz band and the obtained results can be easily applied for designs with wider bandwidths. Simulation results reveal improved return loss, aperture e ciency and cross-polarisation performances compared to what has previously been reported for this antenna.