Metasurfaces and Blinking Jamming: Convergent Study, Comparative Analysis, and Challenges.

Blinking jamming is an active self-screening technique performed by at least two aircraft to tackle monopulse radars and all complexity related thereto. Nowadays, the technique can be performed with digital radiofrequency memories (DRFMs), which are cumbersome, complex, expensive, need a dedicated compartment and antenna, and introduce spurs in the signals. In this paper, we propose an alternative to the implementation of blinking jamming with DRFMs, namely with reconfigurable metasurfaces. By covering the aircraft parts that most contribute to the radar cross-section (RCS), reconfigurable metasurfaces can interchangeably absorb or amplify impinging waves, making the aircraft 'blink' from the radar perspective. To validate the feasibility, simulations accounting for realistic phenomena are conducted. It is seen that, if the aircraft RCS can be varied in a ratio of 10:1, either with absorptive or power-amplifying metasurfaces, a performance similar to that of the DRFM is achieved. Furthermore, a ratio of 2:1 is sufficient to make the radar antenna system movements exceed the angular range of the formation. We also anticipate our work to be a starting point for completely new ways of countering radars, e.g., with countless small drones performing passive or active stand-off blinking jamming.

Taming Fabry-Pérot resonances in a dual-metasurface multiband antenna with beam steering in one of the bands.

Metasurfaces are artificial materials that can provide properties not readily available in nature for the interaction with acoustic, elastodynamic, or electromagnetic waves. In Electromagnetics, metasurfaces allow particular functionalities to antennas, which in turn lately have been increasingly pushed to a multiband operation. To fully exploit metasurfaces' capabilities, the use of a metasurface reflector and a metasurface superstrate surrounding a radiating element in multiband antennas is interesting. However, such topology generally creates multiple reflections inside the formed cavity, known as Fabry-Pérot resonances. Here we show that one should tame this phenomenon to use two parallel metasurfaces surrounding a planar radiating element. We present the conditions to obtain directive, multiband antennas under such circumstances. The concepts are validated with a compact device for 5G/4G/Wi-Fi 2.4/5/6E performing a beam steering in the 5G without disturbing the radiation patterns of the other bands. This device demonstrates that the functionalities of two metasurfaces may be exploited in a single design if the presented conditions are respected. We also anticipate our work to be a starting point for other studies in the wave domain. For example, compact, multiband, beam-steerable microphones or sonar transducers with two parallel metasurfaces could be investigated in the future.

Subcranial Approach: A Straightforward Approach to the Anterior Fossa Floor: 2-Dimensional Operative Video.

INDICATIONS CORRIDORS AND LIMITS OF EXPOSURE: Subcranial approach is a versatile keyhole approach mainly indicated to the tumors of the anterior fossa and midline invasive tumors from the nasal cavity and paranasal sinus. 1-5 Endoscopic endonasal video assistance enhances the exposure and the safety of the approach. ANATOMIC ESSENTIALS NEED FOR PREOPERATIVE PLANNING AND ASSESSMENT: Preoperative images depicted the size of the frontal sinuses and the relations of the lesions with frontal, ethmoid, and orbits. ESSENTIAL STEPS OF THE PROCEDURE: After performing a bifrontal scalp flap and a pericranial flap, periorbits are dissected from the superior and medial orbital rims. After frontonasal osteotomy, we drill the posterior wall of the frontal sinus. PITFALLS/AVOIDANCE OF COMPLICATIONS: Avoiding disruption of the cantal ligaments prevents telecantus. Watertight closure of the dura with temporal fascia graft and reinforcement with the pericranial flap makes cerebrospinal fluid leak occurrence very rare. VARIANTS AND INDICATIONS FOR THEIR USE: Subcranial approach is our preferred approach for the tumors of the anterior fossa floor and the midline paranasal invasive tumors. This keyhole approach through a relatively small frontoethmoidal craniotomy combines the advantages of the regular craniotomies but with no manipulation of the frontal lobes. An advantageous inferior angle of view similar of the pure endonasal endoscopical approach is achieved, but better than in the pure endoscopical approaches, the subcranial approach allows a direct wider exposure and a superior watertight reconstruction of the anterior fossa floor.The patients consented to the procedure and to the publication of his/her image.

A Low-Profile, Triple-Band, and Wideband Antenna Using Dual-Band AMC.

When a wideband antenna is backed by an artificial magnetic conductor (AMC) reflector, the bandwidth is reduced. With the optimization of the shape of the AMC it is possible to exhibit multiband behavior, but the problem becomes complex if the bands are also intended to be wide. In this study, a methodology that exploits both the expected in-band and out-of-band behaviors of a dual-band AMC was used to design a low-profile, triple-band, and wideband directive antenna. The methodology was validated with a prototype suitable for the European standards of 4G/5G and Wi-Fi 2.4/5/6E, operating within the following bands: 2.4-2.7 GHz, 3.4-3.8 GHz, and 5.17-6.45 GHz. The measured results showed respective peak values of 8.0, 9.1, and 10.5 dBi for the broadside realized gain, front-to-back ratios larger than 19 dB, cross-polarized levels lower than -18 dB, and stable half-power beamwidths within each band. Furthermore, 3 dB gain bandwidths of 34.4%, 19.7%, and 31.0% were also measured.

A customisable pipeline for the semi-automated discovery of online activists and social campaigns on Twitter.

Substantial research is available on detecting influencers on social media platforms. In contrast, comparatively few studies exists on the role of online activists, defined informally as users who actively participate in socially-minded online campaigns. Automatically discovering activists who can potentially be approached by organisations that promote social campaigns is important, but not easy, as they are typically active only locally, and, unlike influencers, they are not central to large social media networks. We make the hypothesis that such interesting users can be found on Twitter within temporally and spatially localised contexts. We define these as small but topical fragments of the network, containing interactions about social events or campaigns with a significant online footprint. To explore this hypothesis, we have designed an iterative discovery pipeline consisting of two alternating phases of user discovery and context discovery. Multiple iterations of the pipeline result in a growing dataset of user profiles for activists, as well as growing set of online social contexts. This mode of exploration differs significantly from prior techniques that focus on influencers, and presents unique challenges because of the weak online signal available to detect activists. The paper describes the design and implementation of the pipeline as a customisable software framework, where user-defined operational definitions of online activism can be explored. We present an empirical evaluation on two extensive case studies, one concerning healthcare-related campaigns in the UK during 2018, the other related to online activism in Italy during the COVID-19 pandemic.