Research and Development of Low-Profile, Small Footprint Antennas for VHF-UHF Range Applications
Efficient, but low-profile and small-footprint antennas for VHF-UHF range applications remains an ongoing work. VHF range spans approximately 54 - 88 MHz while UHF roughly ranges from 174 - 890 MHz. The inverse relationship between the physical length and resonant frequency of an antenna, which is a measure of its operating frequency range, is well known. A direct correlation between an antenna's physical length and radiation efficiency has also been established. Therefore, a combination of these constraints complicates the design of low-frequency antennas that have small physical size but with enough radiation resistance to be an efficient radiator when connected to a source having a comparable resistance. Given the frequency bands above, their corresponding wavelengths will be: 3.4-5.5 m (VHF) and 0.3-1.7 m (UHF). The length of an antenna operating at these wavelengths would need to be electrically-small i.e. a fraction of wavelength given size constraints for applications such as defense or commercial mobile communication equipment. As a consequence, the radiation resistance of the antenna, which is a function of its radiation efficiency, is greatly reduced. In other words, the input impedance or radiation impedance (assuming negligible ohmic losses in the antenna structure) features a small resistive component and a large capacitive component, causing reflections of most of the incident power to the antenna. Highly-reactive antennas are not desired for most transmitters and receivers. Therefore, the radiation resistance of an antenna must be increased by increasing its electrical length while simultaneously maintaining a low profile and footprint. This aim can be achieved by configuring the antenna to excite a resonance at, or very close to a desired operating frequency. An approach that I will explore in this dissertation is to exploit the broadband characteristics of meander-line and helical (or "spiral") antennas typically applied in the microwave frequency range to the UHF-VHF range. I will also propose novel antenna geometries that combine spiral and meander-line properties and analyze their performance. These antennas offer significant size reductions; for example, a bowtie meander dipole antenna studied yielded a height reduction of 55% at 64 MHz relative to a half-wave dipole antenna of the same resonant frequency. In addition, I will present a set of equations developed for predicting the fundamental resonant frequency and radiation resistance of meander-line antennas.
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