Wide-angle scanning wide-band phased array antennas

University dissertation from Stockholm : KTH

Abstract: This thesis considers problems related to the design and the analysis of wide-angle scanning phased arrays. The goals of the thesis are the design and analysis of antenna elements suitable for wide-angle scanning array antennas, and the study of scan blindness effects and edge effects for this type of antennas. Wide-angle scanning arrays are useful in radar applications, and the designs considered in the thesis are intended for an airborne radar antenna.After a study of the wide-angle scanning limits of three candidate elements, the tapered-slot was chosen for the proposed application.A tapered-slot antenna element was designed by using the infinitive array approach and the resulting element is capable of scanning out to 60° from broadside in all scan planes for a bandwidth of 2.5:1 and an active reflection coefficient less than -10 dB. This design was implemented on an experimental antenna consisting of 256 elements.The predicted performance of the antenna was then verified by measuring the coupling coefficients and the embedded element patterns, and the measurements agreed well with the numerical predictions.Since the radar antenna is intended for applications where stealth is important, an absorbing layer is positioned on top of the ground plane to reduce the radar cross section for the antenna's cross-polarization.This absorbing layer attenuates guided waves that otherwise lead to scan blindness, but does not adversely affect the antenna performance for the desired scan directions and frequencies.The highest frequency limit of the tapered-slot element is set by scan blindnesses. One of these scan blindnesses is found to be unique to tapered-slot elements positioned in triangular grids. This scan blindness is studied in detail and a scan blindness condition is presented and evaluated.The evaluation of the experimental antenna shows that edge effects reduce the H-plane performance of the central elements.These edge effects are further studied and characterized, by comparing the scattering parameters for finite-by-infinite arrays and infinite arrays.In this way it is possible to divide the edge effects into two categories: those caused by finite excitation, and those caused by perturbed currents due to the geometry of the edge. A finite difference time domain code with time shift boundaries is used to compute the active reflection coefficients needed to compute the scattering parameters, but this code cannot directly compute the active reflection coefficient for all the required phase shifts.Hence, an additional method is presented that makes it possible to compute arbitrary phase shifts between the elements using any numerical code with limited scan directions.