Agates in Paleocene/Eocene tuffs from El Picado/Los Indios, Cuba were investigated to characterize the mineral composition of the agates and to provide data for the reconstruction of agate forming processes. The volcanic host rocks are strongly altered and fractured and contain numerous fissures and veins mineralized by quartz and chalcedony. These features indicate secondary alteration and silicification processes during tectonic activities that may have also resulted in the formation of massive agates. Local accumulation of manganese oxides/hydroxides, as well as uranium (uranyl), in the agates confirm their contemporaneous supply with SiO2 and the origin of the silica-bearing solutions from the alteration processes. The mineral composition of the agates is characterized by abnormal high bulk contents of opal-CT (>6 wt%) and moganite (>16 wt%) besides alpha-quartz. The presence of these elevated amounts of “immature” silica phases emphasize that agate formation runs through several structural states of SiO2 with amorphous silica as the first solid phase. A remarkable feature of the agates is a heterogeneous distribution of moganite within the silica matrix revealed by micro-Raman mapping. The intensity ratio of the main symmetric stretching-bending vibrations (A1 modes) of alpha-quartz at 465 cm−1 and moganite at 502 cm−1, respectively, was used to depict the abundance of moganite in the silica matrix. The zoned distribution of moganite and variations in the microtexture and porosity of the agates indicate a multi-phase deposition of SiO2 under varying physico-chemical conditions and a discontinuous silica supply.
Millimeter-wave (mm-wave) technology is a viable candidate to address the growing data traffic in fifth-generation (5G) wireless communication and beyond. However, challenges related to free space propagation loss, atmospheric absorption, scattering, and non-line-of-sight propagation must be addressed to benefit from the promised bandwidth available in the mmwave regime. In this context, phased array technology is considered as vital to provide high-speed and seamless wireless solutions to the industry. A phased array can be defined as a multiple-antenna system that electronically controls the radiated electromagnetic beam. The official origin of the antenna array concept is attributed to Guglielmo Marconi. A repeated Morse code signal letter "S" from Poldhu, UK to St. John's in Canada was successfully demonstrated in December 1901, using a twoelement antenna array. In the early 1940s, Luis Walter Alvarez designed the first electronically scanning phased-array radar.Both scientists were awarded the Nobel Prize for their discovery. Thanks to their ability to shape or steer the radiated beam and the possibility to integrate such versatile solutions in the mm-wave regime, phased arrays have shown a growing interest from the industry [2]- [8]. While these systems have been considered for decades for large radar applications and communication links, their high cost limited their penetration for commercial applications. However, the development of new architectures, packaging and semiconductor technologies has drastically reduced the cost and complexity of phased arrays making them available to commercial markets for 5G wireless and satellite applications. In particular, 5G wireless communications is bringing this paradigm to the general public. This article presents an overview of the rise of mm-wave phased arrays in the industry including the principles and design considerations, with an emphasis on Over-The-Air (OTA) characterization. In Section II, the principle of operation and design considerations of phased array antennas are presented. A special attention is paid to phase-shifters developed in monolithic integrated technology required to control the phase profile of such arrays. The evolution toward wide bandwidth and wide scan array concepts is then proposed with connected arrays of slots and dipoles, tightly coupled dipole arrays, planar ultra-wideband modular arrays and continuous transverse stub arrays. In Section III, the OTA methodology to address the challenging task of phased arrays characterization is described [9]- [11]. Main issues encountered in mm-wave phased array measurements are identified. Dedicated OTA set-ups for standardized and specific antenna measurements are described including a discussion about measurement uncertainty. Thanks to its high potential for 5G applications and beyond, an abundant literature related to phased arrays is reported. In the last Section, the main mm-wave applications are briefly described and future trends discussed.
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