Analyzing modal decomposition data of finite antenna arrays

Abstract

Traditional antenna array theory is well suited for guiding the design of very large, uniform arrays. When an array is large enough one can often approximate that the array is of infinite extent, greatly simplifying the analysis of the structure and significantly reducing the cost of simulation. However, as array size decreases these approximations break down and the total radiating structure is subject to finite array effects. Such effects can perturb the expected radiation patterns and cause large variations in impedance across the array elements. These effects can be partially mitigated through conditioning certain elements of the array, or adding terminated “dummy” elements to the array. These methods often require many iterations of simulation and can become costly as the number of parameterized variables grows.

In order to better understand the finite array effects and reduce our dependency on parametric simulations, we study a modal decomposition of the array currents. In particular we use characteristic mode analysis (CMA) which produces an indexed set of “simpler” eigencurrents, and eigenvalues that dictate the energy storage properties of the modes. These modes are dependent entirely on the method of moments impedance matrix of the structure, and therefore are independent of the array feeding method. Whereas CM is often used in the study of single port, electrically small structures that are dominated by one or two modes, our template arrays are electrically large and made of multiple disjoint elements, with multiple feed points. This work explores and catalogs the types of characteristic mode results attained from two different classes of antenna arrays. We calculate and compare the accuracy of our modal summations and determine how matrix conditioning affects the modal decompositions of different arrays and different array elements. These results can help establish expected accuracy guidelines for this electrically large class of problems