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### Browsing by Author "De Villiers, Dirk I. L."

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- ItemAnalysis and design of conical transmission line power combiners(Stellenbosch : University of Stellenbosch, 2007-12) De Villiers, Dirk I. L.; Meyer, P.; Van der Walt, P. W.; University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
Show more This dissertation presents a technique for the design of N-way conical line power combiners, which offers significant advantages over existing axially symmetric combining techniques. A full analytical study of conical transmission lines is done, and evaluated results are presented. These include a proof of the cutoff frequency equation, and plots of the field patterns, of higher order modes which are unavailable in literature. A coaxial fed conical line combiner for 10 inputs is proposed, designed and evaluated. The design technique relies on the uniform transmission line characteristics of the conical lines to eliminate the need for complex full wave optimisation, typically needed in the design of the more commonly used radial line combiners. Circuit models are instead employed to achieve a wide matched bandwidth by using optimised stepped impedance coaxial lines to feed the combining structure. The prototype developed at X-band displays more than an octave bandwidth with a return loss of better than -14.5 dB. Using tapered line matching sections increases the power handling capability of the combiner by eliminating sharp edges, and allows for tolerance insensitive manufacture of the structure by widening conductor spacings. Such a 10-way prototype is developed at X-band which displays a -18.7 dB return loss bandwidth of 47% with very low losses. A study is done to determine the limitations on the design of general N-way combiners, and the results are incorporated into the design technique. The full process is demonstrated by the design and simulation of a 30-way combiner at Ku-band which displays a simulated -20 dB return loss bandwidth of 34%. The design technique is simple to execute and requires very little full wave analysis. Results obtained with the manufactured combiners are better than those of any previously published axially symmetric combiners.Show more - ItemCompact conical line power combiner design using circuit models(Institute of Electrical and Electronics Engineers, 2014) Beyers, Ryno D.; De Villiers, Dirk I. L.
Show more ENGLISH ABSTRACT: A simple equivalent circuit model with empirical equations describing the peripheral feeding ports of conical line power combiners is presented. The model allows the entire structure to be designed using transverse electromagnetic circuit theory without the need for any full-wave simulations. A summary of the model extraction process is given and the accuracy of the proposed model is confirmed by favorable comparisons with full-wave simulations. The circuit based design method is used to design a compact conical line combiner showing measured performance similar to the current state of the art combiners in this technology, while being significantly smaller.Show more - 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 - ItemFast parametric modeling of radio astronomy reflector antenna noise temperature(Institute of Electrical and Electronics Engineers, 2016) De Villiers, Dirk I. L.
Show more This communication presents an approximate method to rapidly estimate the antenna noise temperature of a reflector antenna system as a function of the geometric parameters describing the reflector dishes. The initial estimate is established by neglecting the main reflector from the calculation domain, thus speeding up the calculations significantly. The accuracy may be improved by aligning the estimated results with those from full model simulations, performed at a limited set of points scattered through the parameter space, by a linear regression correction on the residuals. Results of applications on a variety of reflector configurations for some commonly used parameter sets confirm the accuracy of the method to be better than 1%, with speed-ups of more than an order of magnitude typical.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 - ItemMultivariate adaptive sampling of parameterized antenna responses(Institute of Electrical and Electronics Engineers, 2017) Mutonkole, Ngoy; De Villiers, Dirk I. L.
Show more 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.Show more - ItemParametric 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
Show more 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.Show more - ItemPrediction of aperture efficiency ripple in clear aperture offset Gregorian antennas(Institute of Electrical and Electronics Engineers, 2013) De Villiers, Dirk I. L.
Show more ENGLISH ABSTRACT: Electrically small clear aperture dual offset reflector systems often exhibit a directivity ripple over frequency due to the interference of the diffracted field from the sub-reflector with the main beam field. This paper investigates the cause of the ripple, and presents a technique to predict the expected system directivity, including the ripple, using the feed radiation pattern augmented by an efficient simulation strategy in a clear aperture offset Gregorian system. The method allows for accurate prediction of the directivity ripple using a severely under-sampled set of simulation results. Predicted results are compared to several simulations, and agreement to better than 0.5 % is found for the majority of configurations using both analytical and full wave simulated feed patterns.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