PhD Thesis Defence
- Friday, 17 January 2020
- Aula Senaatszaal
3D Elements for Phased-Array Systems: Analysis and Design.Cristina Yepes
In recent years, radar systems and satellite communications require phased array antennas that are capable of incorporating frequency and angular selectivity while maintaining a low profile. Active phased array antennas must comply with stringent requirements in terms of sensitivity to interference caused by other nearby radiating systems, especially in complex platforms, where a multitude of sensors and radiating systems need to co-exist. In such environments, antennas working at different frequencies can interfere with each other and the implementation of frequency filtering functions with a good out-of-band rejection are needed. Moreover, the interference between different systems can be mitigated by reducing the radiation in the direct path between them. For this purpose, angular filtering functions for pattern shaping can be beneficial. However, standard planar printed circuit board technology puts constraints on the possible antenna elements that can be realized to achieve frequency and angular selectivity. Thus, using new methods such as additive manufacturing technology and 3D printing can provide more degrees of freedom to fabricate complex geometries with a desired operation.
For these reasons, this work focuses on studying new solutions for frequency selectivity with rejection of higher order harmonics, developing a spectral method of moment to study antennas to achieve angular shaping and finding the guidelines needed to design such antennas, and testing additive manufacturing technology to find its suitability at high frequencies for phased array antennas. A bandpass miniaturize-element frequency selective surface with harmonic rejection properties has been designed and manufactured. The design is based on an equivalent circuit model, taking a 3-pole Chebyshev bandpass filter as a starting point, where the inter-layer interaction is only described with a single transmission line representing the fundamental Floquet wave. The prototype consists of five metallic layers, interdigitated patches and grids, separated by dielectric slabs and exhibiting good stability over a wide conical incidence range. A practical case to estimate the effects of placing the FSS in the proximity of a wide-scanning wideband connected array of dipoles has been performed. The performance of the array combined with the FSS has been experimentally characterized, defining the optimal distance between FSS and array to avoid the propagation of surface waves between both structures, showing a good response within the FSS bandwidth and a good frequency rejection outside of this bandwidth.
A spectral method of moments for tilted elements in free space and in the presence of a backing reflector for infinite and finite arrays has been derived. Such method allows to study dipole and stacked dipole elements and find the guidelines needed to design, in a future, a phased array antenna with an undesired angular range where suppression of gain is intended. The parametric study concluded that the main parameters that shapes the pattern are the inter-element distance between elements and the tilt angle of the elements. The requirements to achieve an asymmetric radiation pattern are a directive element and an inter-elements distance higher than half wavelength, while the tilt of the elements allows to shape the gain levels in the suppressed angular region. To validate this study a linear arrays consisting of tilted dipoles loaded with artificial dielectric layers has been fabricated. The prototype shows a good comparison with simulations and measurements.
A simple design of a dipole antenna has been derived and fabricated using Stereolithography process as an additive manufacturing method. The polymer and metal paste used in the process have been characterized and results have been discussed. A good agreement between simulations and measurements has been achieved after including the geometric deviations found in the fabricated antenna. The fabrication process for high frequencies appears to be prone to systematic errors and the challenges related to the use of additive manufacturing technology for high frequency RF antennas and components has been discussed.