Characteristic Modes Analysis of a Near-Field Polarization-Conversion Metasurface for the Design of a Wideband Circularly Polarized X-Band Antenna

A metasurface (MS) based on loop elements operating in the near field of a linearly-polarized microstrip antenna is employed to realize a circularly polarized radiated field. The properties of the loop unit cell are highlighted with the help of the Characteristic Mode Analysis that is employed for investigating the achievable linear to circular polarization conversion bandwidth and providing the guidelines for the design of the final antenna. A finite structure comprising 4×4 unit cells is tailored for achieving a circularly polarized far field within the whole X-band adopted for satellite communications (7.25 GHz-7.75 GHz, 7.9 GHz-8.4 GHz). A simple but effective single-port excitation scheme is adopted, and the overall performance are assessed by measurements on the fabricated prototype. The good agreement between simulated and measured results confirms the reliability of the proposed approach as well as the meaningful insight provided by Characteristic Modes Theory

Efficient Excitation of Characteristic Modes for Radiation Pattern Control by Using a Novel Balanced Inductive Coupling Element

A design strategy exploiting the Characteristic Modes Analysis (CMA) is described for improving the radiation efficiency of a mounted-on-platform radiator. To this aim, a novel Balanced Inductive Exciter (BIE) is introduced to improve the modal excitation purity of some Characteristic Modes (CMs). In fact, even if the optimal position of the exciters on the hosting platform is determined by using the Characteristic Modes Theory (CMT), the excitation purity of each mode plays a fundamental role in the radiation efficiency of the radiating system. In particular, achieving a good level of excitation purity strongly reduces the reactive power (Preac) stored in the near field zone and hence maximizes the amount of the radiated power (Pr). To better highlight the benefits offered by the presented approach, a set of BIEs is applied on a platform to obtain a fully-reconfigurable radiation pattern. The evaluation of the Preac, Pr and Equivalent Isotropically Radiated Power (EIRP) provided by the BIE reveals the importance of a pure modal excitation. To assess the reliability of the proposed BIEs some prototypes have been manufactured and tested

Analysis of the Performance Enhancement of MIMO Systems Employing Circular Polarization

The advantages of adopting circular polarization in multiple-input-multiple-output (MIMO) systems are illustrated for both line-of-sight (LOS) and multipath propagation. More in detail, an analysis of the MIMO performance attainable by employing orthogonal circularly polarized (CP) radiators with respect to orthogonal linearly polarized (LP) ones, has been addressed. At first, an accurate analysis is presented aimed at the evaluation of the channel matrix by comprehensively including also the effects of the antenna in LOS condition. In particular, the channel matrix has been calculated as a function of the antenna parameters and orientation, demonstrating that CP radiators are capable of obtaining better average values of the matrix eigenvalues with respect to LP ones. The analysis is therefore completed by evaluating the characteristics of a CP MIMO system operating in indoor environment representing this latter a more challenging condition where multipath propagation occurs. In this latter case, some meaningful numerical experiments have been performed by using a reliable ray-tracing solver, followed by a measurements campaign conducted in a real environment for validation purposes. Measurements, which are in good agreement with simulations, confirm the benefits of adopting circular polarization in MIMO systems with respect to LP

An Inkjet Printed Chipless RFID Sensor for Wireless Humidity Monitoring

A novel chipless RFID humidity sensor based on a finite Artificial Impedance Surface (AIS) is presented. The unit cell of the AIS is composed of three concentric loops thus obtaining three deep and high Q nulls in the electromagnetic response of the tag. The wireless sensor is fabricated using low-cost inkjet printing technology on a thin sheet of commercial coated paper. The patterned surface is placed on a metal backed cardboard layer. The relative humidity information is encoded in the frequency shift of the resonance peaks. Varying the relative humidity level from 50% to 90%, the frequency shift has proven to be up to 270MHz. The position of the resonance peaks has been correlated to the relative humidity level of the environment on the basis of a high number of measurements performed in a climatic chamber, specifically designed for RF measurements of the sensor. A very low error probability of the proposed sensor is demonstrated when the device is used with a 10% RH humidity level discrimination

Indoor channel characterization for future 5G applications

The shortage of frequency band below 6 GHz available for communications and data transfer has recently fostered the interest toward the millimeter wave (mmW) spectrum. In fact, mmW carrier frequencies allow for larger bandwidth allocations thus higher data transfer rates. It is therefore useful to evaluate the channel propagation properties of mmW within an indoor environment. In particular, the statistical parameters such as path loss exponent and shadowing have been examined by using a reliable numerical solver based on a ray-tracing (RT) technique. The results for both line-of-sight (LOS) and non-line-of-sight (NLOS) conditions at 28 GHz and 72 GHz are reported for the case of an office environment

Benchmark problem definition and cross-validation for characteristic mode solvers

In October 2016, the Special Interest Group on Theory of Characteristic Modes (TCM) initiated a coordinated effort to perform benchmarking work for characteristic mode (CM) analysis. The primary purpose is to help improve the reliability and capability of existing CM solvers and to provide the means for validating future tools. Significant progress has already been made in this joint activity. In particular, this paper describes several benchmark problems that were defined and analyzes some results from the cross-validations of different CM solvers using these problems. The results show that despite differences in the implementation details, good agreement is observed in the calculated eigenvalues and eigencurrents across the solvers. Finally, it is concluded that future work should focus on understanding the impact of common parameters and output settings to further reduce variability in the results

ADATTAMENTO DI ANTENNE COMPATTE MEDIANTE L'USO DI RETI NON-FOSTER

Le antenne elettricamente compatte presentano un elevato valore del fattore di qualità Q pertanto sono caratterizzate da una elevata reattanza X_a e una piccola resistenza di radiazione R_r; ciò significa che esse sono a banda stretta e quindi è opportuno inserire delle reti di adattamento (MN) tra il generatore e l’antenna per modificarne l’impedenza d’ingresso Z_a e aumentare il trasferimento della potenza. Con le tradizionali reti d’adattamento passive non è possibile adattare l’antenna su bande molto grandi per colpa del prodotto “guadagno-banda” derivato da Bode [5] e Fano [6] tra un generatore resistivo e un carico passivo. Tuttavia è possibile ottenere un’adattamento su bande più grandi utilizzando delle reti d’adattamento attive che utilizzano degli elementi “Non-Foster” che sono in grado di superare questa limitazione. In questa tesi mostreremo i principali vantaggi offerti dalle Non-Foster matching network, rispetto alle tradizionali matching-network a elementi passivi, e la loro applicazione ad un'antenna di tipo monopolo di lunghezza l=3.5 m collocata su un piano di massa

CHARACTERISTIC MODES ANALYSIS FOR THE DESIGN OF SMART ANTENNAS ON COMPLEX PLATFORMS

The goal of this thesis is to prove the breakthrough on pattern control capability offered by the Characteristic Modes Theory (CMT). In particular, useful design guidelines are proposed to achieve pattern reconfigurable antennas by exploiting all the potentialities of orthogonal current modes supported by different platforms. The organization of this thesis is shown in the navigation map of Figure 1. Chapter 1 is mainly focused on the mathematical formulation of the Characteristic Modes (CMs) and the feeding structure to efficiently stimulate a desired CM over the investigated structure. Moreover, a preliminary examination of the CMT on a Perfect Electric Conductor (PEC) circular disk is carried out to illustrate the CMs analysis and introduce the preliminary concepts useful to obtain an efficient radiator. In Chapter 2, a rectangular conductive plane with the dimension of a portable device has been exploited to achieve a null-steering antenna feature in the principal plane by a suitable excitation of two current modes over the investigated plate. To this aim, the asymmetric excitation paradigm has been introduced to alter the Modal Weighting Coefficient (MWC) of different orthogonal CMs over the object. As a proof of concept, a prototype of the antenna has been realized by resorting to a discrete phase shifter employing several pin diodes. Thanks to the shifter, it is possible to alter the phase difference between two capacitive exciters and thus scan the null of the pattern in the principal plane. In the same Chapter 2, a novel design strategy for realizing a three-dimensional (3-D) null-scanning radiator by applying the CMT was described in detail. An accurate feeding strategy to stimulate the desired current modes over the plate has been selected thanks to the CMT. By manipulating both the amplitude and phase difference among the exciters above the rectangular plate, an optimal current distribution can be generated on the PEC plate, which is able to guarantee a pattern null in any desired direction (,). The following Chapter 3 is devoted to the Characteristic Modes Analysis (CMA) of a three-dimensional platform. In particular, the multiple CMs excitation technique is addressed by showing its capability of pattern control. Specifically, the CMA of a rectangular box is performed and afterward some orthogonal current modes have been properly combined to satisfy the imposed far field requirements. As proof of concept, both null-steering and beam-steering capability was achieved by accurately stimulating four CMs over the investigated rectangular box. Afterwards, an innovative Balanced Inductive Exciters (BIEs) composed of two symmetric half loops is introduced to strongly improve the radiation efficiency of mounted-on-platform radiators. This radiation efficiency improvement is obtained by increasing the modal excitation purity. The following Chapter 4 is devoted to Multiple-Input-Multiple-Output (MIMO) antenna systems. More in detail, the performance enhancement due to the adoption of orthogonal Circularly Polarized (CP) radiators instead of orthogonal Linearly Polarized (LP) ones is addressed in this chapter for MIMO systems. The results of this analysis proved that CP radiators are capable of obtaining greater eigenvalues, as a function of the MIMO antenna orientation, than LP ones. For this reason, the achievable channel capacity of the CP radiators outperforms the LP ones when the MIMO antennas are not perfectly aligned. Moreover, CP MIMO allows obtaining better performance from a statistical point of view provided that the 3 dB axial ratio of radiators is guaranteed for angular sectors greater than 40 degrees. Finally, the channel matrix in LOS environment (HLOS) has been evaluated with a rigorous mathematical approach whereas, in the multipath propagation scenario, the MIMO performance have been assessed by both numerical simulations and measurements campaign

Characteristic Modes for the design of compact radiators on complex platforms

The placement of a communication system on a three-dimensional platform always poses a series of problems that the antenna designer has to solve or mitigate for guarantee the desired functional operability. Among the several challenges, the need of compact and conformal radiators is one of the hardest tasks to accomplish, especially if working on objects whose dimensions are comparable to the wavelength of interest. The solution to this problem can be pursued by using the Characteristic Mode Analysis