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Synthesis of tapered resistive ground plane for a microstrip antenna

Microstrip antennas have been employed in airborne and spacecraft systems because of their low profile and conformal nature. Recently, they are being used for other commercial applications such as GPS. Most of the antenna design schemes assume that the microstrip elements are mounted on an infinitely large ground plane, therefore, the diffraction from the edges of the ground plane is usually neglected. However, there are applications where the effect of the edges can become very significant because these edges distort the far-field pattern and also the phase center of the antenna. Attaching resistive cards to the edges of a scatterer is a well known technique for reducing the edge diffracted fields, and was successfully applied to reduce the echo width of a semi-infinite PEC half plane. In this paper, we introduce simple two dimensional (2-D) E- and H-plane models for the microstrip antenna, and study the effect of attaching resistive strips to both ends of a metal ground plane on the radiation pattern as well as the phase center. Note that in general, the edges have a more significant impact in the E-plane pattern. Our main goal is to find an optimum resistive profile which yields the desired radiation pattern and phase center. A nonlinear optimisation algorithm is used where the cost function, which depends on the desired antenna pattern, is minimised to determine the resistive profile which gives the best results. Different types of resistive profiles (piece-wise constant, linear, exponential, quadratic) have been optimized by using two gradient type non-linear optimization techniques, namely, conjugate gradient and Fletcher-Powell algorithms. Numerical and experimental results for the radiation patterns in the E-plane as well as the H-plane are found to be in good agreement.