Frequency-Reconfigurable Low-Profile Omnidirectional Antennas

Abstract

The high demand of today’s wireless technologies has resulted in increasing research efforts dedicated to modern antenna designs. A ”smart” antenna with abilities to tune its performance properties into a new environment can significantly increase communications reliability and decrease systems costs. Therefore, reconfigurable features have become a new standard of antenna designs, particularly when stringent performance indicators are required to keep up with increasing system demands. Owing to the radiation characteristics of antennas, an omnidirectional pattern is one that is widely sought after by antenna designers. Due to their uniform coverage, omnidirectional antennas are an ideal choice for numerous indoor or outdoor implementations. In this context, this thesis investigates substrate-integrated, low-profile, and reconfigurable design solutions for omnidirectional antennas. Firstly, the thesis discusses the development of low-profile monopoles made of shorted patches, where the main objective of this work is to find via-less alternatives for the antenna shortings. Two alternative strategies, namely quarter-wave stubs and complementary split ring resonators, are deployed in substrate-integrated monopoles and are compared with the classical shorting pins in terms of performances. Secondly, the thesis focuses on the investigation of frequency-tunable antennas. The stub-loaded monopole from the first part of the thesis is further developed to create reconfigurable antennas. This work aims to provide multi-band reconfigurable devices with independent tunability between the operating frequencies. In this part, a novel method of designing reconfigurable lowprofile monopoles is proposed based on independent magnetic current loops sharing the same thin aperture. Lastly, a circularly-polarized frequency-reconfigurable omnidirectional antenna is demonstrated as the final contribution of the thesis. The antenna operation principle is based on a combination of magnetic current sources, electric current sources, and phase compensation lines between them. Varactor-loaded slots are added to the structure to enable frequency reconfigurability. Moreover, a description of the antenna feeding aspects to maintain the circular polarization in real conditions is presented. Overall, this thesis provides different designs of high performance low-profile omnidirectional antennas. The results suggest that all the antenna designs are promising for numerous wireless applications. The benefits include simple antenna geometry, ease of fabrication, and low-profile. Importantly, the proposed design principles can be extended to other types of reconfigurable antennas.