Low-profile and end-fire antennas for airborne applications
Date of Issue2017-05-25
School of Electrical and Electronic Engineering
Electromagnetic Effects Research Laboratory
Recent years have witnessed a growing demand for low-profile and end-fire antennas, which can be flush-mounted for aircraft, missile, and unmanned aerial vehicle (UAV) applications. However, there are a lot of limitations associated with traditional end-fire antennas. For example, the performance of log-periodic, Yagi-Uda and Vivaldi antennas is very sensitive to the conducting ground-plane on which these antennas are mounted. H-plane horn antennas can hardly achieve wide bandwidth and maintain low profile simultaneously. In addition, although helical antennas are excellent candidates to achieve end-fire radiation of circular polarization, they are non-planar structures, which are unsuitable to be mounted on moving platforms in consideration of aerodynamic characteristics. In this thesis, we aim to solve these limitations and propose novel solutions to realize low-profile end-fire antennas with various characteristics such as wide bandwidth, high gain, circular polarization, etc., which are potentially very useful to be conformal and flush-mounted for airborne applications. First, the propagation characteristics of surface waves propagating on a grounded dielectric slab are theoretically analyzed. The slab is assumed to be anisotropic as a general discussion. Next, a wideband and low-profile surface wave antenna based on grounded ceramic slab is proposed. A surface wave launcher is designed by employing a probe-fed parallel plate waveguide with a modified parabolic reflecting wall and a metallic post, which are critical for effectively transforming the cylindrical wave to a unidirectional plane wave within a broad frequency range. The ceramic slab is smoothly tapered to transform the guided surface wave into radiated space wave over a wide bandwidth. The proposed antenna operates over a wide frequency range of 6.1-18 GHz, and has the advantages of wideband, low profile, and high radiation efficiency. In addition, an H-plane horn antenna exhibiting an ultra-wide bandwidth from 3.4-18 GHz with an antenna thickness of only 5.508 mm is presented. A wideband coaxial-to-waveguide transition is proposed by employing a tapered ridge and metallic posts in order to enhance the bandwidth and to effectively suppress higher-order modes in the waveguide. An ellipse-shaped copper taper is extended along the horn aperture in order to obtain a smooth transition from horn aperture to free space. A pair of metallic posts is also employed in the H-plane horn to improve the impedance matching. Both planar and conformal versions of the proposed antenna are investigated. The conformal structure is fully embedded in a large cylindrical platform and it is extended to an antenna array configuration. Furthermore, we propose planar and conformal cavity-backed supergain slot antennas for end-fire radiation. In the proposed structures, slots with different lengths are etched out at proper positions of the cavity wall to generate multiple resonances as well as to decrease the Q value of the cavity, which broadens the bandwidth effectively. The required magnitude and phase of the excited electric field for achieving supergain are obtained by adjusting the inter-element spacing and the position of each slot. Furthermore, off-centered microstrip feedlines are utilized to improve the impedance matching and to excite the required cavity mode. Prototypes of back-to-back structure, conformal structure mounted on a cylindrical platform, as well as conformal array configuration with six separate antenna elements embedded in a conical platform are fabricated and tested. Measured results are in good agreement with simulated ones. They have the advantages such as compact cavity size, enhanced bandwidth, high gain, and high radiation efficiency. Besides designing linearly polarized end-fire antennas, a planar helical antenna with rectangular cross-section is also presented for achieving end-fire radiation of circular polarization. The helix is formed using printed strips with straight-edge connections implemented by plated via-holes. The currents flowing on the strips and along via-holes of the helix contribute to the horizontal and vertical polarizations, respectively. Besides, the current on the ground plane is utilized to weaken the strong amplitude of the horizontal electric field generated by the one on the strips. Thus, a good circular polarization can be achieved. Furthermore, a tapered helix and conducting side-walls are employed to broaden the axial ratio bandwidth as well as to improve the end-fire radiation pattern. The designed antenna operates at the center frequency of 10 GHz, which can achieve wide impedance and axial ratio bandwidths of 54% and 34%, respectively. The antenna thickness is only 0.11λ0 at the center frequency. This thesis is finally concluded by making a few recommendations for future investigation. With further research on the suggested topics, it is expected that the present limitations may be overcome, possible solutions may be provided, and more novel structures utilizing the new concepts may be proposed.
DRNTU::Engineering::Electrical and electronic engineering::Antennas, wave guides, microwaves, radar, radio