Prediction of mutual coupling between base station antenna arrays

Abstract

A simple approach is developed to compute mutual coupling (or isolation) between base-station antenna arrays of N dipoles. This approach utilizes a closed form expression for the self and mutual impedances for two slanted dipoles in free space. Two arrays of these dipoles are then assembled together. The elements of the 2N /spl times/ 2N Z matrix of the resulting system are the self impedance of individual dipoles as well as the mutual impedance between every two dipoles within each array and between the two arrays. The input impedance of each array and the mutual impedance between them may be easily derived by combining this Z matrix and the S matrices of two corporate feeds made of two-way power dividers. Alternatively, using a circuit theory approach and working with the Y matrix of the array, the individual antenna elements in each array may be connected together in parallel. This will reduce the 2N-port system into a two-port system whose input and mutual impedances can be easily calculated. The above approach cannot directly include the effect of a finite ground plane that usually utilized in the base-station arrays to provide higher gains and a one-sided beam. However, it was found that the mutual coupling between two arrays without a ground plane could be corrected to provide a reasonable estimate for the coupling between two arrays with a finite ground plane. The correction suggested here is based on the far field coupling predicted by the Friis transmission formula. The mutual coupling between the two arrays predicted by this method agrees reasonably well with the measured results for arrays in both broadside and colinear configurations.