|Institution:||University of Hawaii – Manoa|
|Keywords:||compact HF; coastal radar; electrically small antennas|
|Full text PDF:||http://hdl.handle.net/10125/101053|
Ph.D. University of Hawaii at Manoa 2014. This dissertation presents new concepts and design approaches for the development and optimization of electrically small antennas (ESA) suitable for high frequency (HF) radio communications and coastal surface wave radar applications. For many ESA applications, the primary characteristics of interest (and limiting factors) are lowest self-resonant frequency achieved, input impedance, radiation resistance, and maximum bandwidth achieved. The trade-offs between these characteristics must be balanced when reducing antenna size in order to retain acceptable performance. The concept of 'inner toploading' is introduced and utilized in traditional and new designs to reduce antenna ka and resonant frequencies without increasing physical size. Two different design approaches for implementing the new concept were pursued and results presented. The first design approach investigated toroidal and helical designs, including combinations of toroidal helical antennas, helical meandering line antennas, and additional designs incorporating toploading and folding to improve performance. The other approach investigated fractal-based designs in two and three dimensions to improve performance, reduce size, and lower resonant frequency. The performance characteristics of fractal geometries were analyzed and compared with non-fractal designs of similar height, total wire length, and ka. Inner toploading was also applied in the two design approaches and shown to reduce antenna Q by up to a factor of 4 with a corresponding increase in input resistance by up to a factor of 10, when properly applied. When folded arms were applied to various designs, Q was further decreased by a factor of 2 with a corresponding increase in input resistance proportional to the number of arms. Genetic algorithms were developed for optimizing antenna designs and used in custom programs, including a new cost function for better comparison of ESA performance. Antenna performance was modeled, analyzed, and optimized using set performance criteria. Several unique antenna designs were simulated and experimentally tested in field measurements. Experimentation was conducted using full-size prototypes with performance measured using vector network analyzers and HF transceivers. Experimental performance measurements were reproduced in simulation models with a high degree of correlation. Successful two-way radio communications were established with amateur radio stations around the world using prototype antennas.