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Diffraction of antenna radiation patterns by local obstacles Cheung, Hong D.


Radiowave blockage by buildings is a problem common to mobile radio and cellular telephone systems in high-density urban areas. Plane wave incidence is an approximation which simplifies analysis of the scattering problem, but this is not an accurate assumptions if the obstacle is near the antenna. If the transmitting antenna has an omnidirectional pattern and the obstacle is large in wavelengths the scattered field can be calculated by the techniques of high frequency diffraction theory such as the geometrical theory of diffraction. If the antenna pattern is directive and the obstacle is local, the omnidirectional source solution can be converted to a beam source solution by the complex source point (CSP) method. Antenna patterns with sidelobes can be synthesized from arrays of CSP beams, each with appropriate amplitude, phase and direction. Their individual scattering from apertures and buildings may be calculated and the result summed for the total field. The complex source point method is used here to produce the basis elements of an array of linearly and directionally equispaced two-dimensional CSP beams and compared with Gaussian beam results obtained by others earlier. For efficiency a limited number of significant beams and beam directions is required. Approximately twice as many beams as the aperture width in wavelengths, with all beam directions normal to the aperture, is found to be sufficient here for simple uniform and cosinusoidal distributions in apertures of moderate size at ranges outside the evanescent field zone of the aperture. Now the exact solution for the far field of a line source, or here a beam source in the presence of a conducting half plane, is used as our basis element, to give the solution for antenna pattern diffraction by a local half plane. In this work a beam source in the presence of a conducting half plane, is used as our basis element, to give the solution for antenna pattern diffraction by a local half plane. Antenna pattern diffraction by an aperture near a wide slit is presented as simply a superposition of the solutions for two coplanar half planes with separated parallel edges. Antenna pattern distortion by various other local obstacles, such as conducting wedges and rectangular cylinders, can be obtained similarly. Numerical results, including both extended apertures and a single beam source, represented by arrays of complex source point beams and by a single CSP beam are given. From scattering patterns the latter are shown to accurately represent in some cases aperture distributions such as cosine-squared and with less accuracy uniform aperture distributions. The exact series solution and the moment method solution are used to verify the accuracy of the solution for conducting wedges and a rectangular cylinder of moderate size. Two dimensional scattering by conducting circular cylinders of small to moderate size in wavelengths, in the presence of local directive sources, is also rigorously analyzed. Calculated scattering cross sections show that for E-polarized local sources the cross section first decreases and then increases with increasing directivity whereas for H-polarized sources the reverse occurs.

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