Browsing by Author "Williams, Kevin"

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    An investigation into the computer-aided modelling of active microstrip patch arrays
    (Stellenbosch : Stellenbosch University, 1998) Williams, Kevin; Reader, H. C.; Davidson, D. B.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: Due to the complex nonlinear behaviour of active radiating antennas, very few formal design methods or comprehensive modelling procedures for these modules have been published. This thesis investigates the modelling of these structures using a commercial circuit simulation package, HP-EESof Communications Design Suite, in combination with the Finite-Difference Time-Domain (FDTD) method of computational electromagnetics. A five element active microstrip array composed of aperture-coupled patch antennas fed by individual oscillators is considered. The principle of injection-locking to provide phase control of the oscillators is studied. The frequency-domain harmonic-balance simulator, and a time-domain simulator using convolution to handle microstrip elements are used for the design of the oscillators. Techniques used for the simulation of free-running and injection-locked behaviour are reported, where it is shown that the use of conventional circuit simulation techniques is inadequate. The standard implementation of the harmonic-balance method is found to be unable to predict locking behaviour reliably, while the convolution simulator suffered from long-time instability. An approach based on the Van der Pol analysis is used to simplify the model of the oscillators, and allows the prediction of injection-locking behaviour using the circuit simulators, although this is limited in accuracy. This is achieved by splitting the oscillator into a one-port amplifier and a resonator. A function, extracted from loadpulling simulations, is used to characterise the nonlinear input conductance of the amplifier. The FDTD method is used to analyse the aperture coupled microstrip antennas used as radiators on the array, as well as the complete array. Various pertinent issues regarding the use of the FDTD method to model microstrip circuits, and in particular, aperture-coupled patch antennas are addressed. The Berenger Perfectly Matched Layer (PML) is compared to the well known Mur and dispersive boundary conditions. The implications of limiting the discretisation of components making up an aperture-coupled patch antenna on the accuracy of the computed impedance parameters are investigated. Such components include the microstrip transmission line and the slot. Comparisons to measurements show that the model of the complete array is accurate, even with a relatively coarse mesh.