The shared aperture antenna, an effective and versatile radar front-end design strategy based on deploying interleaved sub-arrays on a common aperture, is discussed. A number of instantiations of this concept, demonstrating multi-functionality and polarisation agility capabilities, is described in detail. Some limitations of this concept are singled out, and measures to counteract them are put forward.@en

A method for ensuring polarization agility in conjunction with beam steering in planar array antennas is proposed. It relies on interleaving two subarrays with orthogonal, linear polarizations that, together, can generate arbitrary polarization states: adjustable linear, elliptical, and left- or right-handed circular. The complexity of the resulting system is comparable with that of standard, fully populated array antennas consisting of identical, linearly polarized elements. By dynamically controlling the amplitude and the phase of the signals fed to the subarrays, a stable polarization state can be maintained during beam steering. The concept is validated by numerically investigating an architecture obtained by interleaving nonuniform subarrays designed by means of a deterministic placement strategy. The effects of the mutual coupling between the different radiating elements are modeled and discussed.@en

Application such as polarization diversity communications, frequency re-use and syntheti aperture radar (SAR) may benefit from the use of polarization-agile antennas, i.e. antennas whose polarization state can be dynamically changed over time (see [1] fir a comprehensive survey on this topic). Most polarization-agile antennas described [1] make use of doubly-polarized elementrary radiators in conjunction with active devices, such as PIN diodes, for varying the polarization properties of the radiated waves 9see[2] for a typical implemention of this conept). The present contribution advocates and alternative approach to obtaining polarization-agility. The strategy relies on interleaving (complementary) sparse sub-arrays each of which radiates waves with linear, orthogonal polarizations. In this manner, any polarization state: adjustable linear, right- and left-handed circular or elleptical can be synthesized at array level. The advantage of the proposed appraoach is that it makes use of simple elementary antennas radiating linearly-polarized waves, thus avoiding the complicated doubly-polarized elements abd the need fir PIN diodes for controlling the obtained polarization. The validity of this approach is demonstrated by analyzing two examples of inter-leaved array antennas that allow obtaining an arbitrarily rotated, linear polarization.@en

A post-processing procedure that yields accurate array-antenna radiation-pattern estimaions is described. By resorting to this strategy, measurements of large array antennas inside small anechoic chambers become feasible. The method employs an adequate phase-correction derived from an optical ray approach, under the assumption that teh elementary radiators themselves are electrically small and van be measured under far-field conditions inside the relevant anechoic chamber. The effectiveness of the proposed thecnique is demonstrated by examing the case of nonuniform linear array antenna.@en

A new family of structures launching the fundamental mode in rectangular waveguides is proposed. Individual or stacked cavity-backed patch antennas radiate into a rectangular waveguide, providing a good matching with a coaxial input over a wide frequency range. The configuration results in a very compact, colinear transition from a standard coaxial connector to a rectangular waveguide. The components have applications as general-purpose coaxial-to-rectangular waveguide transitions and are well suited for low-profile, phased-array antennas. Experimental results confirm the computional estiumations of the proposed components' performance.@en

A solution to desiging multifunction array antennas is discussed. Distinct system functions performed at separate frequency ranges are associated to sparse sub-arrays that are interleaved in a shared aperture configuration. The sparse architectures are designed using a combinatorial method that ensures an acceptable behaviorin the side-lobes region in conjunction with a narrow beamwidth. Experimental and numerical results are presented for a shared aperture antenna consisting of two interwoven sub-arrays that jointly cover a fractional bandwidth of 14% in the X-band.@en

A consistent strategy for the design of finite array antennas consisting of differently sized radiating elements is discussed. In view of increasing the total bandwidth of the antenna system, while maintaining a low mutual coupling between the elementary radiators, sparse subarrays, operating at adjacent frequency ranges, are interleaved on a common (shared) aperture. The sparse architectures are designed using a combinatorial method that ensures an acceptable behavior in the side-lobes region in conjunction with a narrow beamwidth. The effect of the mutual coupling between identical and differently sized elements is accurately evaluated and is accounted for in predicting the performance of each individual radiator. The concept is illustrated by designing a shared aperture antenna consisting of two interwoven subarrays that jointly cover a fractional bandwidth of 14% in the X-band. Cavity-backed, stacked-patches antennas with coaxial feeding are used as elementary radiators.@en

The design of large antenna arrays that are amenable to a cost-effective fabrication in a lithographic technology is demonstrated. For reducing the inter-element coupling, cavities consisting of metal-plated through-holes are employe. The required bandwidth is achieved by resorting to U-slot patch antennas. The genetic algorithm is used for assembling a 30 elements non-uniform, linear array antenna. Experimental results confirm the predicted behaviour of the antenna.@en

A grid-search type placement method for the assembling of large, non-uniform, linear arrays is presented. The required data is generated by means of an efficient computational engine that provides the locations of the elementary radiators for user-specified beam-widths and peak side-lobe levels. The scanning properties of the designed arrays are also investigated. An effective method for reducing the side-lobe level at large scanning angles is contrived.@en

For Accurate estimation of AOA of multiple sources an antenna array can be used. In this paper a comparison between the directions finding capability of three array geometries circular array, uniform- and non-uniform Y-shaped array are presented. the center frequency is 2.25 GHz with abandwidth of 100MHz. The antenna clement is the skirt monopole. The antenna array performance is severely affected by the Mutual Coupling (MC). The direction finding capabilitiesof the Music algorithm for three arrays in then analyzed using thedata with-and without MC. It is concluded that the circular arrayoutperformed the other two array configuations. The performance of the circular array is then evluated in noisy environment. A method to calibrate the algorithm for MC is presented. The performance of the algorithms is evaluaed for different number of sources, and NSR.@en

The design aspects concerning the sparse array antennas are discussed. Commonly, the beamwidth, the side-lobes level and the gain are the relevant features in antenna design procedures. These parameters are evaluated for different sparse topologies and generalised design relations are provided. The capabilities of sparse architectures are evaluated by comparision with uniform arrays for a similar level of performance.@en

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.@en

This contribution presents a full-wave analysis and the pertaining design strategies for cavity-backed and probe-fed circular microstrip patch antennas. The analysis procedure is based on the Mode Matching Method that was consitently employed fo charcterizing the feeding circuit, the matching network (identified with the microstrip patch) and the radiating rectangular aperture. The modes of the non-canonical waveguide section are calculated by applying the Boundary Intergral - Resonant Mode Expansion technique. The design approach is based on classical mathematical programming algorithms, and aims at optimizing either the scattering parameters of the antenna or the admittance of the matching structure.@en

A novel structure for launching the fundamental mode in rectangular waveguides is proposed. The geometry consists of a cavity-backed circular patch radiating into a rectangular waveguide. The matching properties of the proposed components are comparable with the ones of the classicallu used L-shaped loop end-launcher. The structure results in a very compact, colinear transition form a standard coaxial connector to a retangular waveguide. The component has applications as general purpose coaxial-to-waveguide transition and is well suited for low-profile phased-array antennas.@en

A combinatorial method for the design of spatially interlaced sparse array antennas is advocated. The cyclic difference sets are used infinding optimum positions of the elements in the sparse sub-arrays that share a common number of sub-array that can be interwoven on a common aperture. Radiation properties of these sparse configuration are investigated by numerical estimations of the antenna array factor at different frequencies.@en