Fiber-based structures are highly desirable for wearable electronics that are expected to be light-weight, long-lasting, flexible, and conformable Many fibrous structures have been manufactured by well-established lost-effective textile processing technologies, normally at ambient conditions The advancement of nanotechnology has made it feasible to build electronic devices directly on the surface or inside of single fibers, which have typical thickness of several to tens microns However, imparting electronic functions to porous, highly deformable and three-dimensional fiber assemblies and maintaining them during wear represent great challenges from both views of fundamental understanding and practical implementation This article attempts to critically review the current state-of-arts with respect to materials, fabrication techniques, and structural design of devices as well as applications of the fiber-based wearable electronic products In addition, this review elaborates the performance requirements of the fiber-based wearable electronic products, especially regarding the correlation among materials, fiber/textile structures and electronic as well as mechanical functionalities of fiber-based electronic devices Finally, discussions will be presented regarding to limitations of current materials, fabrication techniques, devices concerning manufacturability and performance as well as scientific understanding that must be improved prior to their wide adoption

Fiber‐Based Wearable Electronics: A Review of Materials, Fabrication, Devices, and Applications

▪ Abstract A large collective effort to study the variability of active galactic nuclei (AGN) over the past decade has led to a number of fundamental results on radio-quiet AGN and blazars. In radio-quiet AGN, the ultraviolet (UV) bump in low-luminosity objects is thermal emission from a dense medium, very probably an accretion disk, irradiated by the variable X-ray source. The validity of this model for high-luminosity radio-quiet AGN is unclear because the relevant UV and X-ray observations are lacking. The broad-line gas kinematics appears to be dominated by virialized motions in the gravity field of a black hole, whose mass can be derived from the observed motions. The “accretion disk plus wind” model explains most of the variability (and other) data and appears to be the most appropriate model at present. Future investigations are outlined. In blazars, rapid variability at the highest energies (gamma-rays) implies that the whole continuum is relativistically boosted along the line of sight. The gener...

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Variability of active galactic nuclei

This article surveys the alternative fade mitigation techniques for satellite communication systems operating at Ku, Ka and V frequency bands. The specific phenomena influencing the propagation of radiowaves on Earth-space links are also overviewed. Emphasis is placed on modeling, experimental work carried out in the past, and practical implementations related to each mitigation technique.

Satellite communications at KU, KA, and V bands: Propagation impairments and mitigation techniques

We report the results of a new analysis of 13 wide band BeppoSAX observations of the BL Lac object Mkn 421. The data from LECS, MECS and PDS, covering an energy interval from 0.1 to over 100 keV, have been used to study the spectral variability of this source. We show that the energy distributions in different luminosity states can be fitted very well by a log-parabolic law F(E) = K (E/E1) −(a+b Log(E/E1)) , which provides good estimates of the energy and flux of the synchrotron peak in the SED. In the first four short observations of 1997 Mkn 421 was characterized by a very stable spectral shape, with average values a = 2.25 and b = 0.45, independently of the source brightness and of the fact that the source luminosity was increasing or decreasing. In the observations of 1998 smaller values for both parameters, a � 2.07 and b � 0.34, were found and the peak energy in the SED was in the range 0.5-0.8 keV. The observations of May 1999 and April-May 2000 covered runs of a duration of several days and provided a very high number of events for all the instruments. The resulting spectral fits were then limited by some instrumental systematics. Also in these cases, the log-parabolic model gave a satisfactory description of the overall SED of Mkn 421. In particular, in the observations of spring 2000 the source was brighter than the other observations and showed a large change of the spectral curvature. Spectral parameters estimates gave a � 1. 8a ndb � 0.19 and the energy of the maximum in the SED moved to the range 1-5.5 keV. We give a possible interpretation of the log-parabolic spectral model in terms of particle acceleration mechanisms. An energy distribution of emitting particles with curvature close to the one observed can be explained by a simple model for statistical acceleration with the assumption that the probability for a particle to increase its energy is a decreasing function of the energy itself. A consequence of this mechanism is the existence of a linear relation between the spectral parameters a and b ,w ell confirmed by the estimated values of these two parameters for Mkn 421.

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Log-parabolic spectra and particle acceleration in the BL Lac object Mkn 421: Spectral analysis of the complete BeppoSAX wide band X-ray data set

From the material presented here, it is clear that the theory underlying the SNF approach is complex and involved to implement. However, it is also very elegant and provides one with many measurement options and powerful capabilities. The numerical implementation of the theory can be efficiently deployed through the use of the fast Fourier transform (FFT) enabling transforms of even electrically large antennas to be accomplished in a matter of a few seconds on a modern powerful digital computer. With the advent of commercially available SNF test systems, the user can exploit these techniques, largely unimpeded by the burden of the theory or the implementation thereof. The material presented here highlighted some of the fundamental concepts and limitations the user needs to be aware of in order to use these test systems with confidence.

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Spherical near-field antenna measurements

Study of low elevation angle scintillation modelling

In this paper, an antenna arrangement and a control system that decreases the losses caused by the user of the mobile terminal, is presented. Performance of the system is evaluated with the presence of user hand and index finger. Experimental results prove that two switchable antenna elements are able to increase the total efficiency up to 2.5 dB at 900 MHz.

http://ieeexplore.ieee.org/iel5/5497865/5504894/05505405.pdf

Control system for compensation of antenna mistuning and absorption caused by user's index finger

In this paper, two approaches for frequency reconfigurable planar inverted F-antennas (PIFAs) are proposed and experimentally studied. In the first PIFA, an electrically-controlled frequency shift is achieved with a tuning circuit designed for the grounded PCB beside the antenna, while in the second PIFA the tuning circuit is integrated directly into the antenna with no separate DC wiring. The measured return losses, radiation patterns and total efficiencies of the antennas are presented and briefly discussed.

Frequency reconfigurable planar inverted-F antennas for portable wireless devices

In the present paper, the applicability of various on-ground antenna type at 2.45 GHz for launching creeping waves is examined. The performance of three antenna configurations is compared with the discone having purely dominant normal polarization to a body surface. The results are set against an analytical path loss model for a wireless body area network. The type of the studied antennas on a ground is seen to have marginal impact on the achievable maximum path gain (|S21|) and each configuration maintains omnidirectional pattern in the tangential direction with the human body surface.

Performance of WBAN on-ground antenna type with relation to analytical path loss model

A common problem with wearable antennas is the influence of body tissues: they decrease the performance of an antenna due to detuning and electromagnetic absorption. From the wearable device point of view, a ground plane can mitigate the absorption although its difficult aspects on antenna operation are admitted in the field. For ultra wideband (UWB) radiators, impedance behaviour is conceded to be sensitive to the shape of an antenna. Therefore, taking a ground plane into account into a design might involve huge challenges for the proper matching as well for other parameters. The present paper considers the impact of a conductive layer in between skin tissue and broadband dipoles. The effect of the layer on antenna operation is considered by separating the proportion of body, absorption and mismatch losses. The conductive layer is observed to be useful when the absorption is required to decrease, since the layer reduces those losses up to 10 dB in the lowest frequencies. Nevertheless, the benefit is profitable only if caused mismatch can be compromised or an antenna is co-designed with a ground plane. This is because mismatch losses are increased simultaneously as demonstrated in this paper. The separation of the loss parameters for the co-design of a layer and antenna is the next step of this work for various configurations.

Erkki Salonen

Papers

Study of low elevation angle scintillation modelling

Control system for compensation of antenna mistuning and absorption caused by user's index finger

Frequency reconfigurable planar inverted-F antennas for portable wireless devices

Performance of WBAN on-ground antenna type with relation to analytical path loss model

Conductive layer under a wearable UWB antenna: Trade-off between absorption and mismatch losses