3-D Printed All-Dielectric GRIN Lens Antenna With an Integrated Feeder

In this paper we present the design, fabrication, and experimental verification of a new type of Graded-index (GRIN) lens antenna with an integrated feeder. The continuously varying refractive index distribution is chosen appropriately to offer the rays collimation at the lens aperture. It is practically implemented by varying the material density in a host medium, thus realizing a new type of all-dielectric high gain antenna, entirely using 3D printing. This solution can find application to high gain wireless communication and measurement systems. This GRIN lens antenna is printed in one monolithic process and does not require the feeder to be placed at a focal distance, thus complying with more strict space requirements. It accepts interchangeable feeds that can cover a wide frequency range. The directivity and gain are evaluated using near-field measurements in the Ku-band. A 40% measured aperture efficiency is achieved at 14GHz. The challenges and performance limitations that come with 3D printing, as compared to the design of idealized continuous distribution GRIN lenses are discussed

Double-Tuned Birdcage Radio Frequency Coil for 7 T MRI: Optimization, Construction and Workbench Validation

The aim of the present study is the optimization, construction, and workbench validation of a double-tuned 1H- 23Na volume radio frequency (RF) coil suitable for human head imaging at 7 T, based on the birdcage geometry. The birdcage-like design which is considered is the four-ring model, in which two standard birdcage-like structures with the same diameters are nested along the longitudinal axis. Simulations based on Maxwell’s equations are performed to evaluate the RF magnetic field homogeneity and the RF coil efficiency varying the coil geometrical parameters. The RF magnetic field homogeneity is evaluated both on the transverse (z = 0) and longitudinal (y = 0) planes without performing the impedance matching procedure, so that the RF coil symmetry is not perturbed by the matching network. The RF coil efficiency is instead dependent on the effective coil input RF power, and it is evaluated after matching the coil, so that the reflected power is minimized, assuming that the stimulation power is totally delivered to the RF coil. Considering the simulation results and the target application, the useful RF coil geometrical parameters are fixed. The four-ring model, which showed the best performances, has been built and tested on a workbench, using a cylindrical phantom filled with a 0.05 M saline solution as load. This provides the first example of a four-ring realization intended 1H- 23Na for human head imaging at 7 T

Effect of housing system on reproductive behaviour and on some endocrinological and seminal parameters of donkey stallions

Reproductive management of male donkeys employed for artificial breeding has been poorly studied. The aim of this study was to evaluate the effect of housing system, with the animals grouped together in a paddock or kept in individual boxes, on sexual behaviour, cortisol and testosterone concentration and seminal characteristics of adult male donkeys. The study included four Amiata donkey jacks (stallions) from which ejaculates, saliva and blood were collected during two distinct 3 weeks periods, one in the group and one in the box housing system. Time needed for semen collection was shorter when donkeys were kept in paddocks compared to when they were kept in single boxes (14:57 ± 07:27 and 20:52 ± 09:31 min, p < .05). Native semen characteristics were not influenced by housing system, while cooled preservation in an Equitainer(®) showed that sperm motility parameters were significantly higher during the paddock period compared to the box period. Salivary cortisol was influenced by housing system, both before and 60 min after ejaculation, being statistically higher when donkeys were housed in paddocks. On the contrary, overall and basal testosterone concentrations were significantly higher when animals were kept in boxes. In conclusion, in the present study, good quality semen could be successfully collected from donkeys irrespective of the housing system despite some differences in hormone concentrations

Notes on Profile Inversion and Closed Form Formulation of Compact GRIN Lenses

This note is an addendum to the paper Profile Inversion and Closed Form Formulation of Compact GRIN Lenses [1] and summarizes a few comments obtained after publication of [1]

Profile Inversion and Closed Form Formulation of Compact GRIN Lenses

New formulas for the design of cylindrical Graded-Index (GRIN) lens-Antennas with integrated feeder are presented. The possibility of integrating the feeder within the lens makes the system more compact, avoids complex mechanical design and alignment errors. The lens is characterized in analytical form by Geometrical Optics (GO). The expression of the radially varying refractive index is derived by applying the conservation of the momentum inside the lens and imposing parallel rays at the lens output interface. The mathematical procedure to retrieve the refractive index is based on the inversion of a truncated Abel transform. The Poynting vector at the lens aperture is derived by applying the conservation of energy in each elementary ray-Tube. By approximating the Poynting vector to a Gaussian function the total efficiency of the lens-Antenna is derived in a closed form, allowing for a quick lens design. The proposed formulation has been successfully validated by using ray-Tracing and a full-wave simulations. Finally, we present examples of practical design of GRIN lenses by using holes of different shapes in a dielectric ABS/Teflon host media

Radial GRIN Lenses based on the Solution of a Regularized Ray Congruence Equation

We introduce a novel formulation for flat radial GRadient INdex (GRIN) lenses allowing for the optimal lens design through closed form expressions. The validity of the proposed formulation covers a very large range of GRIN lens design parameters (focal distance, lens thickness and maximum refractive index). The formulation is based on the derivation of a new form of non-linear integral equation representing the equalization of all the optical ray-path lengths, denoted as Regularized Ray Congruence (RRC) equation, and on its closed form solution. An analytical form of aperture efficiency is given for standard feed patterns. The application of the formulas presented here allows for an instantaneous design for medium/high gain antennas with controllable total aperture efficiency till 80%. The accuracy of the formulation is tested by a full-wave analysis and compared with other formulations available in the literature. We found that the new formulation proposed here significantly reduces the phase error in a wide range of the lens parameters, thus allowing for a more efficient, accurate and flexible design for GRIN lenses. As a proof of concept, a thin GRIN lens antenna for operation in E-band (60-90 GHz) is designed, demonstrating a high aperture efficiency (63%) across a wide frequency range

Experimental setup to compare measurements and numerical simulations in magnetic resonance imaging RF dosimetry

Many of the parameters associated with the magnetic resonance imaging (MRI) coils are estimated by means of numerical simulations. Taking into account the unavoidable numerical approximations and imperfections of the models, an experimental validation of the theoretical results becomes essential. This paper describes two measuring setups which allow the comparison between measurements of electromagnetic fields and the same quantities computed numerically. The experimental activity highlighted some critical aspects of the numerical results that could bring to a wrong estimation of the parameters associated with the MRI coils. Results show the importance and feasibility of a dosimetry experimental setup suitable for MRI coils characterization

Ray-tracing in Dielectric Inhomogeneous Metalenses

A flexible and efficient algorithm, based on Geometrical Optics (GO) ray-tracing, is presented. The algorithm allows the analysis of graded index (GRIN) lenses, namely dielectric inhomogeneous lenses with radially varying refractive index. The source incident field in absence of the lens is supposed to be known by simulation or by measurements. Phase and amplitude distributions at the output interface of the lens are obtained by solving the ray-equation and the energy transport equations, respectively. Once the field distribution at the lens-antenna aperture has been achieved, the radiation pattern is derived by aperture type radiation integrals. The ray-tracing algorithm is validated by full-wave analysis, after predetermining the feed source, the lens dimensions and the refractive index profile. Moreover, beam scanning capability of GRIN lenes can be investigated with the aid of the presented algorithm. Results achieved by the raytracing algorithm and the full-wave analysis have shown to be in good agreement, demonstrating that the proposed algorithm is a valuable alternative to time-consuming full-wave simulations in the GRIN lens analysis

Double-Tuned Surface 1 H–23 Na Radio Frequency Coils at 7 T: Comparison of Three Decoupling Methods

Magnetic Resonance Imaging (MRI) and Spectroscopy (MRS) with nuclei other than protons (X-nuclei) often require the acquisition of proton signal for shimming and co-registration procedures. Double-Tuned Radio Frequency (DT-RF) coils improve these procedures, avoiding the need for movement and repositioning of the subject during the examination. The drawback of DT-RF coils is basically the coupling between the two resonant structures, which increases signal losses leading to a degradation of the final MR image. To improve MR signal quality acquired via DT-RF coils, a suitable decoupling strategy should be implemented. For this purpose, three DT-RF coil prototypes, which differed only in the decoupling method, were built and their performances were compared through workbench measurements. Each prototype consisted of two concentric loops. The inner and outer loops were tuned at sodium (≈ 79 MHz) and proton (≈ 300 MHz) Larmor frequency at 7 Tesla, respectively. Active and passive decoupling designs were compared measuring the Q factor and the S21 parameter for each prototype. Active decoupling was tested as an alternative to the standard passive decoupling with a trap circuit, in which a non-negligible amount of current flows at resonance, perturbing the magnetic field responsible for producing the MR image. Workbench measurements showed satisfactory Q factors and S21 for both active and passive decoupling cases. Thus, active decoupling could be a promising alternative to achieve better MR signal quality. Furthermore, for active decoupling, two circuit elements were examined: PIN diodes and micro-electromechanical system (MEMS) switches