A radially periodic 2-D leaky-wave (LW) antenna is studied for the generation of zeroth-order Bessel beams within a limited spatial region and over a wide-frequency range. The antenna design is wideband and based on an annular metal strip grating placed on the top of a grounded dielectric slab, supporting a cylindrical leaky wave (CLW) with a fast backward spatial harmonic. The focusing capabilities of the relevant LW aperture fields are investigated over the considered frequency range (15-21 GHz), in conjunction with the dispersion analysis of the optimized structure, which is developed by means of an efficient in-house method of moments code. Full-wave simulations using a commercial tool including a simple coaxial feeder are presented and discussed, demonstrating the desired wideband operation. The antenna design is validated by means of measurements performed on a manufactured prototype, considering different frequencies and components of the electric field within the nondiffracting range of the system. The proposed design represents an attractive simple and low-cost solution potentially able to generate arbitrary-order Bessel beams at microwaves as well as in the millimeter-wave and terahertz frequency regions.

In this study, the capability of GNSS Reflectometry in evaluating forest biomass from space has been investigated by using data coming from the TechDemoSat-1 (TDS-1) mission of SSTL and from the CyGNSS mission of NASA. The analysis has been first conducted using TDS-1 data on a local scale, by selecting five test areas located in different parts of the Earth's surface. The areas were chosen as examples of various forest coverages. The analysis has been extended by using CyGNSS to a global scale. The reflectivity has been confirmed as the most suitable parameter and its sensitivity to forest has been evaluated by a comparison with vegetation optical depth (VOD) from the SMAP L-band radiometer, with a pan-tropical aboveground biomass (AGB) map and with a tree height (H) global map derived from the GLAS laser altimeter on-board ICEsat. The analysis confirmed the decrease of reflectivity for increasing biomass, with correlation coefficients 0.31 ≤ R ≤ 0.54 depending on the parameter (VOD, AGB or H) and on the considered dataset. These correlations were not sufficient to retrieve the parameters by simple inversion. The retrieval has been therefore based on Artificial Neural Networks making it possible to add other inputs (e.g., incidence angle, SNR). Although not directly correlated to AGB, these inputs helped in improving the retrieval accuracy. The algorithm was tested on both the selected areas and globally, showing a promising ability to retrieve the target parameter, either AGB or H, with correlation coefficients R ⋍ 0.8.

This paper presents the design of a circularly polarized (CP) meshed patch antenna fully integrated within a solar panel for operation on CubeSats and other microsatellites. The structure has been designed to ensure optimal antenna performance at S-band as well as to minimize any shadowing effects that can reduce the received power at the solar cells. To generate CP, the antenna is driven by two orthogonal feed points, penetrating through a transparent borosilicate glass layer, as well as a silicon and PCB substrate. Simulated and measured performances, on both a preliminary FR4 design and a fully integrated prototype, demonstrate a good impedance bandwidth, satisfactorily axial ratio, as well as stable radiation patterns and minimum shadowing levels. The proposed antenna can be useful for communications between satellites as well as with the ground station, and since the structure is compact and completely integrated, the design can be an alternative approach to new-phased arrays on solar panels and other beam-steering systems.

A low-cost compact planar leaky-wave antenna (LWA) is proposed offering directive broadside radiation over a significantly wide bandwidth. The design is based on an annular metallic strip grating (MSG) configuration, placed on top of a dual-layer grounded dielectric substrate. This defines a new two-layer parallel-plate open waveguide, whose operational principles are accurately investigated. To assist in our antenna design, a method-of-moments dispersion analysis has been developed to characterize the relevant TM and TE modes of the perturbed guiding structure. By proper selection of the MSG for a fabricated prototype and its supporting dielectric layers as well as the practical TM antenna feed embedded in the bottom ground plane, far-field pencil-beam patterns are observed at broadside and over a wide frequency range, i.e., from 21.9 GHz to 23.9 GHz, defining a radiating percentage bandwidth of more than 8.5%. This can be explained by a dominantly excited TM mode, with low dispersion, employed to generate a two-sided far-field beam pattern which combines to produce a single beam at broadside over frequency. Some applications of this planar antenna include radar and satellite communications at microwave and millimeter-wave frequencies as well as future 5G communication devices and wireless power transmission systems.

In this communication, it is shown that a nondiffracting vortex beam (i.e., a higher order Bessel beam with azimuthal phase variation) can be generated in the near field by synthesizing an inward cylindrical traveling-wave distribution over a finite aperture antenna. A radial line slot array (RLSA) is then designed to prove the concept. The collimated vortex beam is excited in the proximity of the RLSA, within a region properly defined by the nondiffracting range of the generated beam. The radial dependence of the longitudinal electric field of the vortex-beam magnitude follows a first-order Bessel function, and its phase presents a linear azimuthal variation. Full-wave results validate the generation of the nondiffractive higher order Bessel beam within the radiative near field of the launcher.

The study of the freeze/thaw dynamic of high-latitude Earth surfaces is extremely important and informative for monitoring the carbon cycle, the climate change, and the security of infrastructures. Current methodologies mainly rely on the use of active and passive microwave sensors, whilst very few efforts have been devoted to the assessment of the potential of observations based on signals of opportunity. This work aims at assessing the performance of spaceborne Global Navigation Satellite System Reflectometry (GNSS-R) for high-spatial and high-temporal resolution monitoring of the Earth-surface freeze/thaw state. To this aim, reflectivity values derived from the TechDemoSat-1 (TDS-1) data have been collected and elaborated,and thus compared against the SMAP freeze/thaw fraction information. Shallow sub-surface soil temperature values recorded by a network of in situ stations have been considered as well. Even if an extensive and timeliness cross-availability of both types of experimental data is limited by the spatial coverage and density of TDS-1 observations, the proposed analysis clearly indicates a significant seasonal cycle in the calibrated reflectivity. This opens new perspectives for the bistatic L-band high-resolution satellite monitoring of the freeze/thaw state, as well as to support the development of next-generation of GNSS-R satellite missions designed to provide enhanced performances and improved temporal and spatial coverage over high latitude areas.