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

Otung and Evans presented some results of slant path scintillation measurements from Sparsholt (UK) which indicate that tropospheric scintillation is dependent on polarisation angle. However, results from Kirkkonummi (Finland), Lessive (Belgium) and Darmstadt (Germany), which are presented, do not show any significant polarisation dependence. The theoretical basis given by Otung and Evans is studied further, showing that tropospheric scintilation is not expected to be significantly influenced by polarisation angle.

Polarisation independence of amplitude scintillation

The scattering Properties of Wideband Dual-Polarized Vivaldi Antenna for MIMO OTA test system was measured using three non-overlapping 2 GHz sub-bands within 1.35-7.25 GHz. The measurements were performed in two orientations with three different link distances (radius of 1.0 m, 1.5 m, and 2.0 m).

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Scattering Properties of Wideband Dual-Polarized Vivaldi Antenna for MIMO OTA

This work was supported by Finnish Funding Agency for Technology and Innovation, Nokia Devices Oulu, Pulse Finland, and Spanish Ministerio de Ciencia y Tecnologia under project TEC2007-66698-C04-03/TCM. M. Sonkki thanks Universidad Politecnica de Valencia and COST IC0603 ASSIST for financially supporting his research exchange in Valencia.

Wideband planar slotted radiating ground plane antenna for portable devices

In the present paper, the design of an artificially anisotropic substrate (AAS) with an aperture-coupled feeding structure is proposed for the generation of circular polarization (CP) at 28 GHz. The radiator configuration is constructed by assembling thin sheets with different relative permittivities as a stack for generating different effective permittivities for the wave components. It is demonstrated that by using sufficient high inclusion permittivity material together with material close to air, the AAS is observed to show AR < 2 dB with 25 dBi higher RHCP gain than LHCP. The proposed radiator configuration provides excellent polarization purity in addition to opportunity to be integrated close to other electrical RF components in transceiver to avoid strong wiring losses at mmWaves.

Artificially anisotropic substrate with aperture-coupled feeding for generating circularly-polarized antenna solutions at mmWaves

This paper presents a differentially driven planar monopole antenna for UWB (Ultra-Wideband) applications. The dimensions of the antenna structure are first studied to achieve wideband characteristics with good impedance matching. The goal is to find dimensions which satisfy the UWB demands but also to show that antenna size can be miniaturized by using differential feeding. The −10 dB impedance matching of the presented antenna structure starts from 1.3 GHz and simulations are performed up to 12 GHz. Normalized surface current distributions are presented and 3D radiation patterns are compared to a reference monopole. The antenna structure shows more stable and omni-directional radiation patterns over the studied frequency range compared to a same sized single-fed reference monopole. The antenna structure is finally scaled to the UWB bandwidth.

Erkki Salonen

Papers

Polarisation independence of amplitude scintillation

Scattering Properties of Wideband Dual-Polarized Vivaldi Antenna for MIMO OTA

Wideband planar slotted radiating ground plane antenna for portable devices

Artificially anisotropic substrate with aperture-coupled feeding for generating circularly-polarized antenna solutions at mmWaves

UWB planar monopole antenna with differential feed