Systematic analysis and design of multimode antennas based on symmetry properties of characteristic modes

Abstract

This thesis deals with the systematic analysis and design of multimode antennas based on characteristic modes. A multimode antenna is a single physical antenna element with several independent antenna ports. The ports are intended to excite mutually orthogonal radiation patterns in order to provide pattern and polarization diversity. Therefore, the use of multimode antennas is a space-efficient alternative for multiple-input multiple-output (MIMO) systems compared to conventional antenna arrays with spatially distributed antenna elements.

A systematic analysis and design of multimode antennas is enabled by means of the theory of characteristic modes. This is due to the fact that the characteristic modes of an arbitrary antenna object possess advantageous orthogonality properties. In particular, the modal radiation patterns are orthogonal to each other. Therefore, the ports of a multimode antenna should excite mutually exclusive sets of characteristic modes. This way, perfectly uncorrelated antenna ports are realized by exploiting the diversity potential of the characteristic modes.

In order to selectively excite a certain set of characteristic modes, their characteristic surface current densities must be orthogonal to those of all other modes. This orthogonality property, however, is not guaranteed by the theory of characteristic modes. It is found, though, that the orthogonality of the characteristic surface current densities is governed by the symmetry of the antenna. This is due to the fundamental fact that the characteristic surface current densities are basis functions of the irreducible representations of the symmetry group of an antenna. Characteristic surface current densities belonging to different irreducible representations or belonging to different rows of a multi-dimensional irreducible representation are orthogonal to each other. The mutually orthogonal sets of characteristic surface current densities are thus found by assigning the characteristic modes to the irreducible representations of the symmetry group of a given antenna, which can be done automatically by means of the projection operator method. Consequently, the number of mutually orthogonal sets of characteristic surface current densities is governed by the finite number and dimensions of the irreducible representations and thus limited.

These mutually exclusive sets of characteristic modes can be excited separately by antenna ports that fulfill the symmetry requirements of the irreducible representations. This means that a single antenna port consists of several feed points placed symmetrically on the antenna element. The input signals of the antenna ports are distributed to the feed points by means of a feed network. The optimal port configurations are governed solely by the symmetry of an antenna and are thus independent of the actual antenna shape and size. In other words, the optimal port configurations are known a priori and there is an upper bound for realizing orthogonal antenna ports. These optimal port configurations can be constructed automatically by means of the projection operator method.

Further a priori knowledge is gained by exploiting relationships between different symmetry groups. Symmetry groups may be isomorphic or may be decomposed as direct-product groups, allowing to reuse or build upon the analysis of simpler symmetry groups. Additionally, related symmetry groups can be collected into families. The characteristic modes of the corresponding antenna geometries have similar properties in terms of both eigenvalues and characteristic surface current densities. Moreover, these properties can be estimated by means of a modal analysis of a generalized antenna geometry with an infinite symmetry group. These relationships are exploited in order to compare potentially suitable antenna geometries and estimate the minimum antenna size for realizing a desired number of orthogonal antenna ports.

Based on this generalized modal analysis and the a priori knowledge gained from the symmetry analysis, a compact six-port multimode antenna based on a square geometry is designed. The feed points of the optimal port configurations are replaced by excitation slots in order to flexibly perform impedance matching. A feed network which distributes the port signals to the excitation slots with the correct amplitude and phase relations as required by the irreducible representations is realized in multilayer technology. Following a modular design approach, the antenna element and the feed network are first optimized separately and then assembled. The simulation and measurement results show that the six antenna ports are practically uncorrelated, offering the desired pattern and polarization diversity. With these results, the fabricated prototype demonstrates the practical feasibility and relevance of the presented design concepts.