Search

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%.

International audience; The wireless power transfer (WPT) efficiency to implanted bioelectronic devices is constrained by several frequency-dependent physical mechanisms. Recent works have developed several mathematical formulations to understand these mechanisms and predict the optimal operating conditions. However, the existing approaches rely on simplified body models, which are unable to capture important aspects of WPT. Therefore, this article proposes the efficiency analysis approach in anatomical models that can provide insightful information on achieving the optimum operation conditions. First, this approach is validated with a theoretical spherical wave expansion (SWE) analysis, and the results for a simplified spherical model and a human pectoral model are compared. The results show that although a magnetic receiver outperforms an electric one for near-field operation and both the sources could be equally use in the far-field range, it is in the mid-field that the maximum efficiency is achieved with an optimum frequency between 1 and 5 GHz depending on the implantation depth. The receiver orientation is another factor that affects the efficiency, with a maximum difference between the best and worst case scenarios around five times for the electric source and over 13 times for the magnetic one. This approach is used to analyze the case of a deep-implanted pacemaker wirelessly powered by an on-body transmitter and subjected to stochastic misalignments. We evaluate the efficiency and exposure, and we demonstrate how a buffered transmitter can be tailored to achieve maximum powering efficiency. Finally, design guidelines that lead to optimal implantable WPT systems are established from the results obtained with the proposed approach.

We present a prospective experimental study on double-resonance Ramsey spectroscopy in a micro fabricated Rb vapor cell, in view of future miniature atomic clocks with improved stability performance. Ramsey signals with a narrow central fringe width of 1.5 kHz combined with a competitive contrast of > 5 % are observed, resulting in a clock short-term stability of approximate to 1x10(-11) tau(-1/2). The employed pulsed optical pumping (POP) scheme bears promise for reduced light-shift effects and thus for improved clock stability on medium- to long-term timescales.

International audience; The efficiency of an on-body wireless power transfer system for implant powering is defined by how the electromagnetic energy interacts with the lossy, heterogeneous, and dispersive body tissues. The objective of this study is to discuss the methodology and evaluate the theoretical bounds for the frequency-dependent electromagnetic energy transfer efficiency. We propose a simplified model that uses a finite tissue-equivalent phantom enclosing an implantable receiver surrounded by a medium that represents a transmitter matched to the wave impedance of the body. This model is used to study different cases and evaluate the wireless power transfer efficiency as a function of the operating frequency and implantation depth. The obtained results can be used as a guideline to choose the design parameters and constraints of the on-body power source and gauge its performance against the predicted maximum achievable efficiency.

International audience; A recent study proposed a technique to mitigate the far-field emission and susceptibility of printed circuit boards (PCBs) with the use of a suspended metal loop. This article provides, with the help of the small-loop theory, a methodology to analytically predict and further estimate the electromagnetic field (EMF) distribution and level at very large distances from the emitting PCB (as is the case for CISPR radiated emission measurements up to 10 m test distances), where full-wave simulations and experiments are very challenging (if not impossible as a practical matter). The suitability of the methodology was assessed for various radiator size and frequency as well as the loop radius and configuration with respect to the emitting PCB. As proof-of-concept, full-wave simulations and measurements have been performed. The root-mean-squared error (RMSE) was considered as the measure of the accuracy between the analytical and full-wave numerical results. It has been demonstrated that, whatever the loop radius and/or position with respect to the radiating integrated circuit (IC), the proposed methodology can approximate the far-field EMFs with low RMSE values. Moreover, as far as the emitting IC is concerned, independently of its size and radiating frequency (except near the loop resonance), a good precision was achieved with the analytical model compared to the full-wave numerical analysis. Furthermore, the approach was found to be effective not only on-axis but also off-axis of the suspended conductor loop. Apart from the required computational power for full-wave simulations, thanks to the proposed methodology, it is possible to significantly reduce the huge difference in computational time for numerical ( ≫ 30 min, depending on the distance) and analytical ( Misplaced & few seconds, independently of distance) analyses

Design of medical sensor implants is very challenging due to numerous restrictions in terms of size and biological acceptability. In addition, large absorption and reflection of electromagnetic waves in body tissues significantly degrades the communication link capabilities. The usual assumption in calculating the link budget is that the permittivity and conductivity of the body tissues are known. However, biological tissues show considerable diversity in structure and consequently in dielectric properties. The first aim of this article is to determine how much the uncertainty of the body tissues’ constitutive parameters affects the prediction of the achievable radiation efficiency of the implant. The second aim is to understand the loss mechanism and thus which tissue permittivity and/or conductivity has the dominant influence on losses. The analysis is done by considering a spherical multilayer human phantom and applying spherical wave decomposition proposed in a previous article. The stochastic collocation method is used to estimate the uncertainty of power density outside the human body, and sensitivity on the uncertainty of each tissue parameter is performed using the analysis of variance (ANOVA) approach. Several test cases were considered and the results clearly indicate which constitutive parameters have dominant effect on uncertainty.

International audience; Fundamental in-body limitations on achievable radiation efficiency could provide decision-making assistance to engineers working on antennas for implantable bioelectronics. In this study, proof-of-concept conformal microstrip antennas are proposed based on these theoretical foundations. In particular, maximizing the effective aperture and loading the antenna with materials having the permittivity higher than that of surrounding tissues is a promising solution for increasing the radiation efficiency. The operating frequencies are tuned to operate within the optimal range for deep-body implantation: 434, 868, and 1400 MHz. The achieved radiation efficiencies at these frequencies are 0.4%, 2.2%, and 1.2%, respectively, when simulated in a f 100 - mm spherical phantom with muscle-equivalent electromagnetic properties. The radiation performance at each frequency is compared to the fundamental limitations and closely approach them. Prototypes are characterized for the experimental validation. © 2020 IEEE.

In this paper, a wideband four port 2&ndash, 6 GHz antenna is proposed. One-, two-, and four-port antennas are implemented and characterized between 2 and 6 GHz. The isolation between the ports is improved by connecting and optimizing the ground plane sections. The results show that the antennas&rsquo, reflection coefficients are better than 10 dB in the frequency band. The measured isolation between the ports is greater than 15 dB (between 2.3 and 6 GHz) and 10 dB in the whole band for two- and four-port antennas, respectively, however, it is more than 20 dB around 2.4 and 5&ndash, 6 GHz for both antennas. The calculated correlation coefficient between ports is below &minus, 30 dB (&gt, 2.14 GHz) and &minus, 15 dB for the two- and four-port antennas, respectively. The measured gain and efficiency scale are 3.1&ndash, 6.75 dBi and 62&ndash, 98%, respectively. To the best of our knowledge, an antenna both being wideband from 2 to 6 GHz and having independent four ports is only addressed in this work. The four-port antenna can be used for MIMO systems or smartphones operating on many wireless systems simultaneously such as 3G/4G/5G Sub-6 GHz and WLAN including the next generation WiFi7 with full-duplex operation.

International audience; Fundamental limits on radiation performance of implantable antennas serve as the design quality gauge, facilitate the choice of the antenna type, and provide simple design rules to maximize the radiation performance. This study obtains the limits using two formulations 1) theoretical spherical-wave expansion using elementary magnetic and electric dipoles and 2) realistic full-wave 2D-axisymmetric models of TM10 and TE10 mode capsule antennas. Using both formulations, the optimal radiation conditions are investigated, the effects of antenna dimensions and its implantation depth are quantified. The results also demonstrate that an electric antenna operating close to the optimal frequency could achieve higher efficiency than a magnetic one. The latter, however, is more efficient below the optimal frequency range. © 2020 EurAAP.

Retlectarray (RA) and transmitarray (TA) antennas that use the element-rotation technique require the radiation from the feeding antenna to be circularly polarized (CP). A CP axially corrugated horn antenna is developed as a feeding element for CubeSat RA and TA antennas. The CP operation is enabled using a septum polarizer. The all-metal geometry is attractive for space applications and allows the horn to be 3D printed in aluminum in a single piece. A prototype of the feed chain was fabricated using the Direct Metal Laser Sintering (DMLS) technique. The results of 3D simulations and VNA/far-field measurements of the feed chain arc presented in this paper.

In this paper a subarray fed by an analog beamforming network for 5G picocell applications is proposed. Design requirements are presented taking into account frequency band operation, bandwidth, radiation pattern shape and both, antenna element and transmission line technologies. The synthesis and implementation of the analog beamforming network are described, and the final subarray architecture is provided. The proposed antenna, verified by simulation, operates at 26 GHz with a bandwidth larger than 25%. The subarray gain is higher than 16 dB with cross-polarization better than -28 dB. Its radiation pattern in elevation fulfills the desired csc2shape with side lobe level below -15 dB. The performance of the proposed antenna meets the requirements of the defined application.

The design of a specific antenna for a cranial implant is used to illustrate design and simulation issues linked to implantable antennas. After a brief introduction, we will review the requirements for the antenna, and go through the design process, discussing the tools used and the issues encountered. The final design will then be presented and discussed

Fundamental radiation limitations of in-body or implantable antennas should give us an estimate on the feasibility of some desired system. It was shown that near-field effects and distance to the body - free space boundary are critical aspects, while the shape doesn’t play a major role in the achievable power. Through the frequency analysis of these limitations in this paper we provide another perspective and show how rigorous and approximate approach to near-field and reflection losses manifest in the results. This is demonstrated on a spherical body example which contains a small spherical implant which can be placed at different positions within the body in order to simulate different implant depths. The results reveal that reasonably accurate description of in-body loss mechanisms can be achieved using our previously defined fundamental radiation limitations for power density, however, depending on the frequency different loss aspects must be treated with special care.

International audience; Fundamental bounds on achievable radiation efficiency serve as the design quality gauge, facilitate the choice of the antenna type considering the available dimensions, and provide simple rules to check the feasibility of a given design. This study quantifies the effect of body-implanted capsule dimensions and materials on achievable radiation efficiency. We also show that a dielectric-loaded electric antenna operating close to the optimal frequency can radiate more efficiently than a magnetic one. The latter, however, is more efficient when electrically small. © 2019 IEEE.

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.

The authors present the realisation and characterisation of an additively manufactured (AM) microwave resonator cavity for double-resonance (DR) vapour-cell atomic clocks. The design of the compact microwave cavity is based on the loop-gap resonator approach, previously demonstrated for conventionally-machined aluminium components. In the present study, the resonator is fabricated by AM using a metal-coated polymer. A resonance frequency at the desired 6.835 GHz rubidium atomic frequency is obtained. When employed in an atomic clock setup, the AM cavity enables a DR signal of

International audience; Wireless implantable bioelectronics and accurate in vivo characterization of path loss require efficient and robust in-body antennas. Loading electric antennas with the effective permittivities higher than that of surrounding tissues are a promising solution. In this letter, proof-of-concept conformal microstrip antennas are proposed. The antennas are optimized for a standard impedance of 50 \Omega and operate in all high-water-content tissues. Integral ground plane shields the antenna from inner circuitry. The radiation efficiencies are 0.4%, 2.2%, and 1.2% for the (434, 868, and 1400) MHz designs, respectively, when computed in a \varnothing100 mm spherical phantom with muscle-equivalent properties. The efficiencies are compared to the fundamental limitations and closely approach them. Specific absorption rates are evaluated, and the corresponding maximum input power levels are established. Prototypes are fabricated and characterized to validate the design.

International audience; This paper reports a novel methodology to design broadband orthomode transducers with two arbitrarily spaced ports carrying orthogonal linear polarizations. The component is based on a combination of two Bøifot junctions through a series of power dividers that act as an embedded first level of the beamforming network of a radio frequency system. By controlling the separation between the ports, this original component can be used in different applications such as antenna arrays for satellite connectivity-on-the-move (where the spacing between ports must be as small as possible) or multibeam sparse arrays (where the spacing between ports may change along the array lattice). In order to verify the proposed design methodology, two identical prototypes have been produced using additive manufacturing. A complete set of measurements (standalone and back-to-back) have been carried out producing satisfactory results that are in very good agreement with the simulations.

We propose an autonomous battery-less wireless sensor node that combines on a single printed circuit board an ultra-wideband (UWB) transmitter and its printed antenna, together with a piezoelectric cantilever and a solar cell array to harvest vibrations and light energy, respectively. The co-design of the solar cell array with the printed UWB antenna allows a prototype size of only 85x35 mm2, i.e., less than 65% of a credit card size. Low-cost is achieved by using inexpensive FR4 dual-layer substrate, standard-ceramic capacitors, and low-cost harvesters. The vibrational energy scavenger is fabricated at the wafer scale based on commercially available bulk polycrystalline Lead Zirconate Titanate (PZT), and the solar cells are fabricated by depositing amorphous-Si on 0.5 mm thick glass substrate. The cold-startup time of the demonstrator is about 42 min under indoor-ambient light conditions, and about 34 min under 700 mg vibrations at a frequency of 100 Hz. Once started, the sensor requires only 12.6 µW to allow a transmission rate of one temperature sensor readout every 34 s, thanks to the UWB transmitter that consumes only 206 pJ per pulse and a custom protocol with a reduced overhead.

In this paper, two THz reflectarray surfaces have been designed and fabricated in order to deflect a plane wave with any polarization and with a specific incident angle to three different specific directions each at distinct three frequencies of 0.7, 1.0 and 1.5 THz. The surface is composed of an array of 100 $\times$ 100 cells, each comprised of gold crosses and parasitic dipoles printed on thin grounded high resistivity silicon. Finite-element method (FEM) simulations are in line with the measurement results obtained using THz time-domain spectroscopy (THz TDS) showing the intended deflections for the two fabricated samples each with an arbitrary frequency-vs-deflection angle relationship. In addition, the use of silicon as the substrate paves the way for the integration of reconfigurable technologies which enhances the reflectarray versatility.

We demonstrate operation of a Ramey-mode Rb vapor-cell atomic clock based on a microwave cavity realized by additive manufacturing (3D-printing). The cavity design is based on a loop-gap approach and its critical electrode structure is realized in one monolithic piece by stereolithography of a polymer which simplifies the assembly of the cavity. The microwave magnetic field in the cavity shows excellent uniformity and high homogeneity across the Rb cell, which results in high-contrast Ramsey fringes observed on the clock transition. A measured clock stability of 2.2∗10−13 τ−1/2 demonstrates the feasibility of the approach. We discuss aging studies performed on 3D-printed test samples that are of relevance for long-term operation of the clock.

In this paper, a 41 x 42.2 mm2 ultra-wide band (UWB) diversity antenna is designed to operate in the frequency band 5.45- 7.45 GHz with the isolation better than 15dB. To achieve omnidirectional coverage pattern diversity is applied. The proposed design is suitable for UWB applications requiring reliable radio link and coverage.

This study reports an experimental study of the system fidelity factor (SFF) of ultra-wideband wireless body area network antennas in an on-body environment. This study also investigates the influence in the SFF because of the antenna polarisation with respect to the body. For this purpose, two antennas having different polarisations are compared: a mono-cone antenna with the polarisation perpendicular to the wearer and a printed monopole with the polarisation either parallel or perpendicular to the wearer. A phantom made of pork meat was considered as a model for the wearer's body, and five different communication scenarios involving each pair of antennas were considered. On-body experimental results show that the time-domain behaviour of the transmission is better for two different antenna types having the same polarisation, but, more importantly, that the SFF (and thus the time-domain fidelity of the signal) is better for a link having a polarisation perpendicular to the body.

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.

The modulated scattering technique is based on the use of reconfigurable electromagnetic scatterers, structures able to scatter and modulate an impinging electromagnetic field in function of a control signal. The modulated scattering technique is used in a wide range of frequencies up to millimeter waves for various applications, such as field mapping of circuits or antennas, radio-frequency identification devices and imaging applications. However, its implementation in the terahertz domain remains challenging. Here, we describe the design and experimental demonstration of the modulated scattering technique at terahertz frequencies. We characterize a modulated scatterer consisting in a bowtie antenna loaded with a vanadium dioxide switch, actuated using a continuous current. The modulated scatterer behavior is demonstrated using a time domain terahertz spectroscopy setup and shows significant signal strength well above 0.5 THz, which makes this device a promising candidate for the development of fast and energy-efficient THz communication devices and imaging systems. Moreover, our experiments allowed us to verify the operation of a single micro-meter sized VO2 switch at terahertz frequencies, thanks to the coupling provided by the antenna.

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.

We have designed and built an optically isolated millimeter-wave detection system to prevent interference from a nearby, powerful, 2.45 GHz microwave source in millimeter-wave propagation experiments in the TORoroidal Plasma EXperiment (TORPEX). A series of tests demonstrates excellent system noise immunity and the ability to observe effects that cannot be resolved in a setup using a bare Schottky diode detector.