Shared aperture array antennas composed of differently sized elements arranged in sparse sub-arrays

Abstract

A novel solution for conceiving wide band (multi-band) array antennas is presented. The solution is based on the concept of interleaving sparse, sub-arrays operating at separate frequencies. Sparse array antennas offer two major advantages, namely: they have non-uniformly distributed elements, with possibly large distances between elements (this providing the necessary space for interleaving different sub-arrays), and they do not require the amplitude tapering of the elements for controlling the array radiation pattern.

In the first part of the thesis, the theoretical formulation of the electromagnetic problem is presented. Variants of the Mode Matching Method (MMM) have been developed for the full wave investigation of the elementary radiators and of the array configurations. The second part of the thesis is dedicated to the analysis and the design of elementary antennas, sparse array antennas and shared apertures consisting of interleaved sparse sub-arrays. Moreover, the impact of the technological aspects on the antenna performance are discussed and the measurements validating the simulated results are presented.

In the formulation of the numerical electromagnetic analysis method a special attention is given to the problem of computing the modes of uniform waveguide segments with arbitrary cross-section by means of the Boundary Integral - Resonant Mode Expansion (BI-RME). Numerous mathematical aspects related to the calculation of the BI-RME modes are presented. The problem of selecting the modes in the BI-RME method is extended and an analytical solution is proposed for the optimum rotation angle that should be used in the case of processing quasi-degenerate modes. It is also demonstrated that the BI-RME modes are quasi-orthogonal and that the orthogonality of these modes can be improved by increasing the number of canonical modes used in the series representation of the Green's function. The software implementations of the MMM/ BI-RME analysis is accurately investigated by solving a large number of test cases. Furthermore, the problem of evaluating the mutual coupling effect in arbitrary arrays consisting of differently sized, rectangular apertures is addressed. The analysis is done by means of the MMM, as well. The software implementation of this MMM variant is validated by studying different test configurations and observing specific aspects related to the electromagnetic field and to the scattering parameters of the aperture antennas. A solution for the fast evaluation of the mutual coupling effect is proposed. This fast technique is based on a polynomial interpolation of the coupling admittances between the modes used for representing the field on the aperture antennas. The conducted numerical tests point to the conclusion that the modal techniques are the most appropriated full wave methods for the analysis of waveguide (like) antennas.

The developed modal analysis tools are subsequently used in the design of elementary antennas. A number of time efficient design procedures are presented in connection to two antenna topologies: the dielectric filled waveguide antennas with an air gap matching circuit and the cavity-backed, stacked-patch and probe fed antennas. In relation to the cavity-backed, stacked-patch antennas it is noted that the systematical design methodology adopted in this work has provided antenna models with bandwidths of around 10 %. The numerical simulations are validated by comparisons with measurements performed on physical models. Technological aspects are meticulously investigated and some practical solutions are proposed in the context of simplifying the manufacturing process and reducing the fabrication cost.

The study of sparse array antenna is initiated by an exhaustive investigation of the design methods available in the literature. Particular aspects of the sparse array configurations are investigated by numerical and experimental studies. These studies concentrated on two relevant aspects, namely, the estimation of the matching properties of the individual radiators functioning in non-periodic, finite array environments, and the evaluation of the radiation properties of the sparse array antenna, while also accounting for the effect of the mutual coupling. The sparse techniques are also used for generating synthetic aperture radar imagines from ground penetrating radar measurements, this solution reducing significantly the required volume of acquisition data and the computing times.
Another important finding is that the mutual coupling is not correlated in sparse array configuration and, therefore, the coupling effect has a reduced influence (compared to uniform arrays) on the performance of individual radiators while electronically scanning the antenna beam. This remark can be useful in designing array antennas characterized by wide angle impedance matching (WAIM) properties.

For implementing the concept of shared aperture based on sparse array antennas one has to deal with two design stages: one of these is dedicated to the design of different sparse sub-arrays operating in isolated mode and the other one focuses on the interleaving procedure that will allow for deploying the sparse architectures on a common aperture. Contrary to the thinning techniques, the topic of interleaving sparse sub-arrays is seldom encountered in the literature. The main goal of this design step is to preserve the sparsity
properties of the sub-arrays (especially in the sense of avoiding the generation of high side lobes), while preventing the overlapping of elementary radiators. Three techniques are proposed in this thesis for solving the problem of interleaving sparse configurations. The novel shared aperture concept is experimentally demonstrated by manufacturing and measuring an antenna consisting of two interleaved sparse sub-arrays. One of these sub-arrays operates in the frequency range from 8 GHz to 8.5 GHz (relative bandwidth of 7%) and the other one is properly matched to the feeding line in the band from 8.5 GHz to 9.5 GHz (relative bandwidth of 10%), the total bandwidth of the shared aperture being around 17%. The measured radiation patterns of the shared aperture antenna are in a good agreement with the calculated ones obtained by means of the full wave MMM software routines developed during this Ph.D. research.