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- ItemConstant radiation characteristics for log-periodic dipole array antennas(Institute of Electrical and Electronics Engineers, 2014) Lehmensiek, Robert; De Villiers, Dirk I. L.
Show more Given the correct terminal voltages on each element of a log-periodic dipole array constant radiation characteristics can be achieved for an omni-directional radiation pattern. In this communication it is shown that this can only be achieved through optimization, correcting a previously reported oversimplification which resulted in an analytical approach being tractable. This oversimplification assumed that vanishing terminal currents on a dipole’s terminal/feed element implies no radiation from that dipole. This is clearly non-physical because, zeroing the terminal current on a dipole’s feed element is the same as disconnecting its two arms, while having no radiation from that dipole implies that in effect both arms be entirely removed. We show that the effect of currents induced in the disconnected arms of a dipole cannot in general be ignored.Show more - ItemEfficient adaptive sampling applied to multivariate, multiple output rational interpolation models, with applications in electromagnetics-based device modelling(Stellenbosch : Stellenbosch University, 2001-12) Lehmensiek, Robert; Meyer, P.; University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic Engineering; University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.; Jacobs, Ivan
Show more ENGLISH ABSTRACT: A robust and efficient adaptive sampling algorithm for multivariate, multiple output rational interpolation models, based on convergents of Thiele-type branched continued fractions, is presented. A variation of the standard branched continued fraction method is proposed that uses approximation to establish a non-rectangular grid of support points. Starting with a low order interpolant, the technique systematically increases the order by optimally choosing new support points in the areas of highest error, until the desired accuracy is achieved. In this way, accurate surrogate models are established by a small number of support points, without assuming any a priori knowledge of the microwave structure under study. The technique is illustrated and evaluated on several passive microwave structures, however it is general enough to be applied to many modelling problems.Show more - ItemMaximum likelihood estimation versus least-squares estimation in semi-continuous hidden Markov modelling(Stellenbosch : Stellenbosch University, 1995) Lehmensiek, Robert; Stellenbosch University. Faculty of . Dept. of .
Show more - ItemModeling of the radiation pattern of a pair of thin circular-arc dipoles over an infinite ground plane(Institute of Electrical and Electronics Engineers, 2013) De Villiers, Dirk I. L.; Lehmensiek, Robert
Show more This communication presents a model of the radiation pattern of two circular-arc dipoles above an infinite perfectly electrically conducting ground plane. A Fourier expansion is used to describe the currents on the wires, and thin circular loop antenna theory is used to derive the radiation patterns. The results are compared to Method of Moments simulations, and it is shown that higher order cosinusoidal harmonics are required in the current model to achieve good correlation between the radiation pattern models and the full wave simulations.Show more - ItemRapid calculation of antenna noise temperature in offset Gregorian reflector systems(Institute of Electrical and Electronics Engineers, 2015) De Villiers, Dirk I. L.; Lehmensiek, Robert
Show more Antenna noise temperature calculations of reflector systems is often a slow process and makes direct optimization of the sensitivity of these systems a difficult and time consuming task. This paper presents an improvement to a recently proposed method to speed up these calculations by several orders of magnitude for large dish systems. The accuracy of the improved method is tested for several types of offset Gregorian systems, and errors are shown to be in the order of a few percent. Comparisons of several layers of simplification to a standard brightness temperature model are also presented to aid the designer in the choice of model complexity to use.Show more