EXPERIMENTAL AND THEORETICAL INVESTIGATIONS ON REACTIVELY LOADED WAVEGUIDE-FED SHORT-BACKFIRE ARRAYS

Abstract

A new class of antenna arrays, namely, the reactively loaded waveguide-fed short-backfire array has been proposed and studied. The structure basically consists of an active element (driven waveguide) and a nunber of symmetrically located react ively loaded parasitic elements (short-circuited waveguides) in a ground plane, witii an appropriately spaced rectangular subreflector in front of the apertures. By varying the reactive loads, a directive beam can be steered in space. The proposed array is attractive from tiie point of view of compactness and simplicity of feed arrangement, associated witii high gain and low side-lobe level and may be useful in several applications >< where limited scanning is desired. For the purpose of analysis, the electromagnetic excita tion of an arbitrarily oriented conducting plate by a waveguidebacked aperture in an infinite plane has first been considered. The problem is formulated with the help of equivalence theorem in conjunction with the boundary conditions on the aperture and the plate which leads to two coupled integro-differential equations. These equations are first reduced to a set of matrix equations by using the method of moments and then solved numeri cally on a digital computer. A particular case of this problem leads to the analysis of a waveguide-fed short-backfire antenna (SBFA) which is not available in the literature, so far. The analysis has furtiier been extended to include the case of a reactively loaded waveguide-fed short-backfire array.

The reactive loading equations for gain are nonlinear equations which normally cannot be solved analytically. Therefore, nunerical techniques (optimisation algorithm) have been applied to determine the reactive loads for maximum gain in a specified direction. Experiments have especially been conducted to test the validity of analyses and the results indicate a good agreement t between the numerical and experimental results. Finally, the results of a detailed experimental study on reactively loaded waveguide arrays in general and reactively loaded SBF arrays in particular, have been presented. Two basic configurations have been studied, namely, the E-plane arrays in which the waveguides are stacked in E-plane and the H-plane arrays in which tiie waveguides are stacked in H-plane. It is found that the most attractive configuration from the application point of view is the H-plane reactively loaded SBF array, with a half-wavelength rim around the edge of the ground plane. For example, a 7-element array is capable of scanning between + 15 with a 3-dB beamwidth of 9°, power gain of 13-5 to 16 dB, and a peak side-lobe level of 10 to 25 dB.