Book Description

The HF/VHF frequency bands (3-300 MHz) are used by various long-range wireless communication systems in both military and commercial applications. Due to the large wavelengths in these bands, the antennas used in such applications are often electrically-small. While wide bandwidth is desired for high data rates, the electrically-small antennas (ESAs) tend to have very small bandwidth since there is a trade-off between small antenna size and wide bandwidth. For each ESA, the upper bound of its bandwidth can be calculated. Despite the fact that fundamental limitations restrain the performance of small antennas, the growing need of compact and broadband wireless devices for communication and sensor systems has tremendously stimulated the demand for small antennas with performance levels approaching, or even exceeding, these limitations. To address this need, I pursue novel bandwidth enhancement and miniaturization techniques for small antennas. In this dissertation, three parallel approaches that I took to investigate bandwidth enhancement and miniaturization techniques for small antennas are presented. The first method employs a novel loading structure to allow antennas to achieve compact and miniaturized dimensions while maintaining a wide bandwidth. The second method involves utilizing the presence of metallic objects that are in the vicinity of the ESAs-more specifically, the platforms (e.g. vehicles, airplanes) on which the ESAs are mounted. In this method, the platforms are considered as the main radiators, and the ESAs act mainly as coupling elements. The third method is to design highly-efficient active non-Foster matching circuits to bypass the gain-bandwidth limitations of the ESAs and achieve wide impedance bandwidth. All three methods have been experimentally validated.