The Importance of Mutual Coupling in the Analysis of Finite Arrays of Rectangular Apertures

Abstract

The paper describes the theoretical analysis of the radiation characteristics of a finite array of rectangular horn apertures including the effects of mutual coupling. Results are presented for a square array of 25 elements demonstrating the importance of the mutual coupling effect. The effect on the radiation characteristics of using different array lattice geometries and the ability of the theory to predict blind spot angles in finite arrays is also shown. Experimental results confnmning the predictions are presented. LNIRQDUCTlIOArrays are now widely used in the fields of radar and telecommunications antennas because of the considerable versatility in the control of the antenna radiation characteristics that they offer. However, the design of an array antenna is complicated by the mutual interactions that occur between the radiating elements. This effect is termed mutual coupling and neglecting it in the theoretical analysis of an array can lead to errors in predicting such parameters as the levels and distribution of radiated cross-polar power, the embedded element co-polar radiation patterns, and the embedded element input impedance.Many antennas use rectangular array elements because of the versatile stacking geometry and the improved aperture efficiency they offer. This is particularly true in the field of radar antennas. Only with the availability of accurate array analysis, based on mutual coupling effects, can realistic studies be made of some of the key radiation properties of radar arrays. For example the use of a full mutual coupling analysis would permit the accurate prediction of embedded element pattern nulls. It is important to know the position of these nulls since they lead to blind spots in radar coverage. It is clear that the recent advances in the area of adaptive antennas will only be fully exploitable in the near future if accurate radiation characteristics of the array can be predicted. For example, such analysis would permit the choice of optimum sub-array geometry permitting clusters of array elements to be combined to form the currenfly limited number of adaptive channels. In this paper we describe the analysis of the mutual coupling in finite arrays of rectangular apertures leading to an accurate model for the prediction of the radiation characteristics of these structures. THEORETICAL ANALYSISThe elementary theory of arrays separates the array factor and element factor in the expression for the array radiation pattern. However, mutual coupling between the elements causes the element factor to change and become dependent on the element spacing and its precise location within the array. The analysis of mutual coupling in a finite planar array of rectangular apertures embedded in an infinite ground plane follows on from the successful study performed at Queen Mary College for the case of circular apertures [1]. In practice the infinite ground plane approximation is not restrictive if the ground plane exists to half a wavelength past the outer active elements. This is because the pattern...