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A compact asymmetric coplanar strip (ACS)-fed monopole antenna is presented, which operates within the Bluetooth and UWB frequency bands with the capability to simultaneously reject the lower WLAN interfering band. The antenna consists of an inverted right triangle patch monopole loaded by open-ended L-shaped slits not only to produce the additional 2.4 GHz passband to the UWB design but also to achieve stopband characteristics around 5.2 GHz. The conceptual equivalent circuit model as well as characteristic mode analyses are carried out in the design evolution process. The proposed antenna has an overall size of only 20 mm x 10 mm, having the smallest area among the so far developed designs, which can be easily integrated within any wireless gadgets. A prototype is fabricated and measured to validate the design, demonstrating the predicted behavior fairly achieved by full-wave analysis. The antenna -10 dB operating bandwidth ranges from 2.38 to 2.42 GHz and from 3.35 to 11 GHz while rejecting from 4.69 to 5.2 GHz. Unlike the unwanted stopband, where the radiation characteristics are adequately deteriorated, the proposed antenna fairly provides stable omni-directional radiation patterns in the H-plane, and has an average efficiency (gain) of 87.3% (2.6 dBi) in the desired passband. As far as the antenna transient behavior is concerned, an adequate measured (simulated) system fidelity factor of 0.7 (0.68) is achieved for the transmission of impulse-type UWB signals in the face-to-face configuration.

To miniaturize the double-resonance (DR) rubidium (Rb) vapor-cell atomic clocks, a new type of micro-loop-gap microwave resonator (mu-LGR) is proposed for TE011-like mode where the magnetic field inside the cavity is homogeneous and oriented along its longitudinal axis over a large volume. It provides more design degrees of freedom by elaborating the printed pattern in the middle layer, while the mechanical strength of the cavity is strong. It also possesses a wider tuning range of the resonances in order to compensate the fabrication tolerances on such miniature precision devices. A theoretical analysis of the general mu-LGR without tuning is presented first, serving as the basic guideline to design the tunable mu-LGR. To demonstrate the wide tuning mechanism, an equivalent circuit model and different tuning schemes are discussed. The measured results show that the proposed tunable mu-LGR can operate at 6.835 GHz by properly adjusting the tuning screw position. Compared to the existing mu-LGRs, this new design can achieve a 40% volume reduction (572 mm(3), approaching the physical limit) and a comparable magnetic field quality, and enlarges the frequency tuning range to 260 MHz (twofold) in measurement. Thus, the proposed compact tunable mu-LGR has a high potential in miniature vapor-cell atomic frequency standards.

An ultralow-profile circularly polarized (CP) reflectarray (RA) antenna is presented in this article, based on the variable-rotation technique for beam collimation and scanning. Starting from a conventional split-loop element, a new wideband element is designed having a single conductor layer on a single thin dielectric. The RA element, based on two coupled microstrip loops, exhibits a dual-resonant CP behavior and a stable response over a wide range of incident angles and frequencies. The wideband response is explained through eigenmode and full-wave simulations in an infinite-array environment, by examining the current distributions on the coupled loops. A high-gain 900-element offset-fed RA antenna is designed for CubeSat intersatellite links at 24.6 GHz using the proposed element. A new element rotation procedure is used to determine the required rotation angles, while eliminating phase errors in the RA aperture. The antenna prototype exhibits a 1 dB gain bandwidth of 6.8% and an axial ratio smaller than 1 dB, with the array profile of only $0.042\lambda _{0}$ at 24.6 GHz. The maximum measured antenna gain is 31.5 dBi, which corresponds to a total aperture efficiency of 58%.

This letter presents the design and measurement of an antenna integrated into an agricultural sensor network node operating on the 2.4-GHz ISM band using the ZigBee protocol stack. The novel structure, coined bowtie-shaped folded dipole (BSFD) , has a differential 100- $\Omega$ input impedance matching a particular controller widely used in wireless sensor network (WSN) applications. This letter also presents a survey of existing WSN antennas to which the proposed design compares favorably. The challenges of testing a differential antenna using conventional single-ended equipment are addressed regarding antenna impedance and radiation pattern measurements. The differential antenna input impedance was confirmed with four different test setups using a conventional vector network analyzer. The radiation pattern of the proposed antenna was measured in-prototype resorting to a spectrum analyzer. The measured patterns show that the proposed antenna has a maximum received power over 4 dB compared to two early versions of the WSN node that used commercial off-the-shelf antenna solutions. The performance was also confirmed in a field test where a BSFD-equipped prototype achieved close to a 300-m range, tripling that typical for ZigBee applications.

This paper presents an experimental investigation of diversity gain influenced by polarization and spatial diversity techniques in ultrawideband (UWB) radio technology. To this aim, two different coplanar-fed UWB diversity antennas having the same size and employing identical monopoles were taken and both effective and apparent diversity gain were measured in a reverberation chamber. To extract the diversity gain, three commonly used approaches to combine diversity (i.e., selection, equal gain, and maximal ratio) have been considered. Results showed that >1 dB ( ∼26 %) improvement in diversity gain is obtained over most of the considered band for polarization diversity case as compared with spatial case, showing the usefulness of polarization diversity for future UWB diversity applications. © 2013 IEEE.

This letter presents a study on the specific absorption rate (SAR) evaluation of UHF wearable antennas. A planar inverted-F antenna (PIFA) intended to operate in wireless body area networks (WBANs) is taken as an example for the investigation. The presence of the ground plane introduces isolation between the body and the wearable antenna and decouples the antenna from the body, thus resulting in lower SAR values. The performed analysis of the proposed PIFA indicates increased near-field regions close to the shorting wall, which directly leads to increased SAR values. This can be avoided if alternative current paths are provided, e.g., extending the ground plane slightly. The dielectric materials used as substrates partially reduce the SAR by confining part of the fields inside the dielectric. All the suggested solutions are easily applicable for a practical implementation.

The Wireless Body Area Network (WBAN) has become an important part of the wireless communication world in the past decade leading to the need for efficient antennas for In-, On- and Off-Body links. The design of such antennas depends critically on the understanding of their interaction with the host body, and thus on the propagation of waves in inhomogeneous lossy media of finite dimensions. To this aim, we propose a simplified spherical multishell model of the lossy body, and study its interaction with elementary sources placed within or on the latter.

We present our studies on a compact high-performance continuous wave (CW) double-resonance (DR) rubidium frequency standard in view of future portable applications. Our clock exhibits a short-term stability of 1.4 × 10 -13 τ -1/2 , consistent with the short-term noise budget for an optimized DR signal. The metrological studies on the medium- to longterm stability of our Rb standard with measured stabilities are presented. The dependence of microwave power shift on light intensity, and the possibility to suppress the microwave power shift is demonstrated. The instabilities arising from the vapor cell geometric effect are evaluated, and are found to act on two different time scales (fast and slow stem effects). The resulting medium- to long-term stability limit is around 5.5 × 10 -14 . Further required improvements, particularly focusing on medium- to long-term clock performance, are discussed.

This paper proposes a novel, low-profile UWB antenna for wireless body area network (WBAN) applications. The antenna has a polarization perpendicular to the body-free-space interface, which is interesting in order to minimize the coupling into the body. Its structure comprises a modified mono-cone with a top-cross-plate and is coaxially fed through the ground plane. The higher frequency band |S11| performance is due to the mono-cone while the top-cross-plate is responsible for the lower frequency band. This plate also leads to a height reduction when compared to conventional mono-cone antennas. A comprehensive parametric study is done to provide design guidelines. Both frequency- and time-domain results have been measured and presented to validate the design. Results show that the antenna operates from 3.06 to beyond 12 GHz based on |S11| ≤ -10 dB, radiates omni-directionally in the H-plane, and has a radiation efficiency over 95%. The system-fidelity factor for UWB signals is adequate for pulse transmission. Finally, the influence of the human proximity on the antenna matching was tested. Results show that its impedance is nearly unchanged as compared to free-space.

The papers in this special section highlight current and emerging research in antennas for WBAN, as well as applications of this technology. This ssection brings together the latest work in antennas for in-, on-, and off-body communication, the WBAN channels, the measurement of wearable and implantable antennas, and also considers the important aspect of specific absorption rate (SAR) estimation and measurement.

A method to synthesize shaped beams with antenna arrays is presented. Specifically, the element excitations are determined such that the array radiates a power pattern that approximates a desired shape. To efficiently solve this problem, an iterative algorithm is proposed. Each step is reduced to a simple convex optimization problem. The method can be readily implemented and efficiently solved using freely accessible routines. Moreover, any shaped power pattern can be synthesized. Numerical comparisons to known optimal solutions show the validity of the approach.

This letter presents a new cavity-based approach for measuring the radiation efficiency of ultrawideband (UWB) antennas. The procedure is based on a generalized Wheeler cap method that is only practical for narrowband measurements, and so this letter introduces an extension to UWB using source-stirring. The number of required measurements needed is relatively low and independent of the frequency span, therefore making the technique convenient for UWB. A complete post-processing procedure to automatically identify and remove erroneous radiation efficiency predictions is presented. The new method was tested with a coplanar-fed UWB monopole and compared to existing UWB efficiency cavity-based measurement methods. The results show good agreement with simulations and compare favorably to other UWB-centric methods, confirming the proposed approach as a good alternative to measure UWB antenna radiation efficiency.

This letter studies the frequency- and time-domain performance of a recently developed printed coplanar-fed ultrawideband (UWB) monopole antenna aiming at predicting its behavior close to a human arm. The input reflection coefficient $(\vert{ S}_{11}\vert)$ and fidelity factor of the antenna were evaluated in free space and close to an arm. Simulations using three simplified arm models with different cross sections (flat, rectangular, and elliptical) were compared to measurements. All models include the relevant human tissue layers: skin, fat, muscle, and bone. It was found that an accurate model requires the inclusion of the tissues broadband dispersion characterization. Moreover, the skin layer has a major impact in $\vert{S}_{11}\vert$ , and a small effect on fidelity, while the models can be simplified by discarding the bone. Furthermore, the geometry of the models is less relevant than dispersion characterization. It has also been observed that using the simplified models with proper broadband tissues dispersion yields good performance predictions, and that the fidelity factor increases as the antenna gets closer to the arm.

A wireless sensor platform for use in ultralow-power wireless sensor networks (WSNs) is presented. The proposed round sensor (RoSe) node is physically robust, is resistant to humidity, and has a housing that can be in direct contact with food. These characteristics are achieved via injection molding technology which is investigated at both the component and the system levels. The results of injection molding tests on the node's electronic components are reported, while a housing-integrated overmolded antenna is conceived and realized. The radiator, operating in the subgigahertz industrial, scientific, and medical (ISM) band (ISM 868 or 915 MHz), has been characterized, and a good correspondence with theoretical predictions is observed. The proposed system enables industrialized mass production and results in a generic platform for wireless monitoring applications. In this paper, we present the RoSe node with a temperature sensor for use in food cold chain management, i.e., for use at temperatures down to -30 °C and in frequent contact with water due to condensation or cleaning. Its ultralow-power characteristics are demonstrated by the analysis of the charge consumption when the node performs specific tasks both in a test case and operating in a WSN. Long-term battery lifetime results while running a demo WSN are also presented.

This work presents the analysis of the influence of insulation on implanted antennas for biotelemetry applications in the Medical Device Radiocommunications Service band. Our goal is finding the insulation properties that facilitate power transmission, thus enhancing the communication between the implanted antenna and an external receiver. For this purpose, it has been found that a simplified model of human tissues based on spherical geometries excited by ideal sources (electric dipole, magnetic dipole and Huygens source) provides reasonable accuracy while remaining very tractable due to its analytical formulation. Our results show that a proper choice of the biocompatible internal insulation material can improve the radiation efficiency of the implanted antenna (up to six times for the investigated cases). External insulation facilitates the electromagnetic transition from the biological tissue to the outer free space, reducing the power absorbed by the human body. Summarizing, this work gives insights on the enhancement of power transmission, obtained with the use of both internal, biocompatible and external, flexible insulations. Therefore, it provides useful information for the design of implanted antennas.

In this paper, we present a simple log-periodic-dipole-array (LPDA) solution that allows us to achieve good ultrawideband (UWB) performances. The antenna has been manufactured and the measurements agree well with the theoretical predictions. The antenna presents an average gain of 8 dB and a return loss better than -10 dB over the band from 4.2 to 10.6 GHz. Both the measured antenna transfer function and the computed effect on pulse transmission show good performances in comparison with already known UWB antennas.

In this letter, we present a reconfigurable reflectarray cell operating at 12 GHz and fabricated in a monolithic MEMS process. A 5-bit digital control allows reconfiguration of the reflection phase over the full 360deg range, while alleviating the impact of MEMS and bias voltage tolerances on the device performances. The designed reflectarray cell exhibits low frequency phase error (large bandwidth) and excellent measured reflection loss (-0.3 dB) with regard to state-of-the-art. Close agreement between measurements and full-wave simulations is observed.

This paper presents the modeling and design of one-dimensional metamaterial phase shifters (MPSs) in monolithic-microwave integrated-circuit technology based on the composite right/left-handed transmission-line approach. From a theoretical point-of-view, we show that existing design expressions are restricted to phase shifters behaving as effective media, which is the case only if sufficiently large L and C loading elements can be designed in the given technology and at the frequency of interest. Therefore, we propose new design expressions, which are exact on the basis of the circuit model of a unit cell in the structure. We show that the new expressions must be used to design integrated MPSs and the theory is verified by means of measurements of one- and two-cell phase shifters exhibiting a 0/spl deg/ phase shift at 17.5GHz. The designed one-cell phase shifter provides a -10-dB input return-loss bandwidth larger than 10% with phase shifts of +29/spl deg/ and -30/spl deg/ at 16.5 and 18.5 GHz, respectively. The insertion loss is -1.3 dB at the center frequency and -1.7 dB at the limit of the bandwidth.

This paper presents the modeling and design methods of true-time distributed microelectromechanical systems transmission lines (DMTLs) making use of periodic structure theory. New contributions to the analysis and reduction of the microelectromechanical systems (MEMS) shunt capacitors parasitics are also introduced. A prototype of a digital DMTL has been fabricated and measured, showing very good agreement with the comprehensive circuit model used for the design. It is believed that the periodic structure approach presented could be useful for other microwave periodic MEMS devices

A novel and efficient method to determine Green's functions in photonic crystals (PCs), i.e., the phased-array method (PAM), is presented. The PAM is a combination of the plane-wave method and the array-scanning method, which is both more flexible and computationally faster than the eigenmodes expansion method. A complete derivation of the electric- and magnetic-field Green's functions associated, respectively, with an infinite electric and magnetic current line exciting a two-dimensional PC is given. Although the developments are presented only for a line source, the PAM can be extended to a dipole source. Thus, the PAM represents a promising method for the analysis of printed-circuit elements or antennas on PC materials. Numerical results for the Green's functions are shown for different positions of the source and a discussion about radiation patterns, asymptotic behaviors, and convergence characteristics is proposed.

PCS (personal communication system) devices have become an important part of everyday life. The pressure to design small, lightweight, and user-friendly mobile-communication devices has increased accordingly, creating the need for optimal antennas for mobile applications. In this paper, we present some basic rules about electrically small antennas, give clues and guidelines about efficient antenna miniaturization, and, finally, show some examples of miniature antennas developed in our laboratory for practical applications.

A simple but efficient analysis tool for printed antenna arrays is presented. It relies on a perturbation approach, which starts from the infinite periodic array assumption to ultimately take into account effects due to finite size, nonperiodicity, double-periodicity in a multilayer case, complex excitation with nonlinear phase shifts between elements, etc. The technique is an approximation, as are most of the array analysis tools. However, it gives good results for arrays presenting a relatively smooth variation of the antenna characteristics between adjacent cells. Results will be shown for several arrays, including nonperiodic and multilayered cases. The method is validated through the comparison with results obtained by a rigorous element-by-element method.

A method for the analysis of large phased arrays of microstrip patches is presented. It is based on an infinite array approach where the edge effects are taken into account through the convolution with a proper window function. In the first step, a rigorous Green's function corresponding to a finite array of elementary sources is derived. This Green's function is then used to analyze the finite phased array of microstrip patches. Results are shown for the active impedance and element patterns of several arrays, and compared with measurements or, in the case of small arrays, with results obtained by a rigorous element-by-element approach. It is shown that the method, even if developed for the analysis of large arrays, is able to handle small arrays. Indeed, the results obtained are good even for single patches. Although the method has been developed for the microstrip phased array case, the results are general and are valid for any phased array with a rectangular grid. >