Optimized Manipulation of the Network Characteristic Modes for Wideband Small Antenna Matching

Abstract

Multiport antenna systems can be regarded as $N$ -port networks, which can be analyzed using the theory of network characteristic modes (NCMs). Due to the profound physical insights provided by an NCM, a desired antenna performance can be achieved by properly manipulating the modes through reactive loading at specified ports. This paper presents a new matching technique for an $N$ -port internally loaded small antenna. By combining the NCM with the differential evolution algorithm, optimal reactive load values can be calculated. These loads can manipulate the NCMs to match the antenna in a desired bandwidth. Unlike other matching techniques based on the NCM, the desired bandwidth is achieved through internal reactive loading without the need for an input matching network. Two examples of electrically small monopole antennas are studies. In both examples, we study the possibility to match the antennas in the GSM 900 band, and we further investigate the possibility to achieve wider bandwidths by varying the number of ports and the loading topologies based on the interpretation of the characteristic modes. The loaded antennas are wideband with a relatively stable radiation pattern and an efficiency higher than 90%. To confirm the theoretical results, a prototype is fabricated and measured.