A. K. Skrivervik

École Polytechnique Fédérale de Lausanne, Lausanne, Vaud, Switzerland

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Publications (176)57.95 Total impact

  • Jovanche Trajkovikj, Anja K. Skrivervik
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    ABSTRACT: 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 (W-BAN) 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 antenna indicate 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.
    IEEE Antennas and Wireless Propagation Letters 12/2014; PP(99):4. · 1.67 Impact Factor
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    ABSTRACT: 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.
    IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 11/2014; 61(11):1769. · 1.82 Impact Factor
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    ABSTRACT: This work presents a novel technology for the fabrication of soft and flexible antennas that can be used for Wireless Body Area Networks (W-BAN). By using standard lab facilities we are proposing a technique on how to build robust antennas intended to work in a harsh environment. A silicon based elastomer Polydimethylsiloxane (PDMS) has been used as a substrate material while copper meshes are used as a conductive material. The initial fluid state of the PDMS allows several degrees of freedom during the fabrication process, both from the electric and mechanic point of view. The dielectric properties of the substrate are adjusted by loading the PDMS with inclusions having a permittivity lower/higher than the PDMS istelf. From the mechanic point of view, liquid PDMS allows the shaping of the antennas in custom prepared moulds, and provides a complete encapsulation of the conductive parts inside the substrate. Moreover, the final antenna prototype is completely sealed and encpasulated inside the PDMS. Several antenna prototypes have been built, measured and characterized. The built antennas are flexible and resistant to different external influences like dust, water or humidity. As a examples of environmental tests, the antennas were exposed to washing, bending and real body (environment) measurements.
    International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting,, Memphis, USA; 07/2014
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    ABSTRACT: 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.
    IEEE Transactions on Antennas and Propagation 03/2014; 62(4). · 2.33 Impact Factor
  • M. Koohestani, A.A. Moreira, A.K. Skrivervik
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    ABSTRACT: A new compact coplanar-fed antenna suitable for polarization diversity in ultrawideband (UWB) applications is presented. The antenna consists of two identical monopoles that are printed on a low-loss substrate with 3 mm spacing and positioned perpendicular to each other. Both frequency- and time-domain results have been measured and presented to validate the design. Results show that the proposed antenna has not only ultra-wide bandwidth ( ${sim}115hbox{%}$ for port 1 and ${sim}107hbox{%}$ for port 2), but also good port isolation above 22 dB over the entire band of interest. Moreover, radiation patterns demonstrate good orthogonal polarization operation. Furthermore, the system fidelity factor is adequate for pulse transmission with averages of 85% and 75% for port 1 and 2, respectively. Finally, the envelope correlation coefficient $(rho_{e})$ has been calculated to evaluate the diversity performance. Results indicate that $rho_{e} leq {-}$20 dB across the ultra-wide bandwidth. These results show the suitability of the proposed antenna for future UWB diversity applications.
    IEEE Antennas and Wireless Propagation Letters 01/2014; 13:563-566. · 1.67 Impact Factor
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    ABSTRACT: Nowadays mobile and battery-powered applications push the need for radically miniaturized and low-power frequency standards that surpass the stability achievable with quartz oscillators. For the miniaturization of double-resonance rubidium ( (^{87}) Rb) atomic clocks, the size reduction of the microwave cavity or resonator (MWR) to well below the wavelength of the atomic transition (6.835 GHz for (^{87}) Rb) is of high interest. Here, we present a novel miniaturized MWR, the (mu ) -LGR, for use in a miniature DR atomic clock and designed to apply a well-defined microwave field to a microfabricated Rb cell that provides the reference signal for the clock. This (mu ) -LGR consists of a loop-gap resonator-based cavity with very compact dimensions (<0.9 cm (^{3}) ). The (mu ) -LGR meets the requirements of the application and its fabrication and assembly can be performed using repeatable and low-cost techniques. The concept of the proposed device was proven through simulations, and prototypes were successfully tested. Experimental spectroscopic evaluation shows that the (mu ) -LGR is well-suited for use in an atomic clock. In particular, a clock short-term stability of (7times 10^{-12} tau ^{-1/2}) was measured, which is better than for other clocks using microfabricated cells and competitive with stabilities of compact Rb clocks using conventional glass-blown cells.
    IEEE Sensors Journal 01/2014; 14(9):3193-3200. · 1.85 Impact Factor
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    ABSTRACT: This paper describes a new technology for the realization of soft, flexible, and robust antennas intended to operate in a harsh environment. One of the main characteristics of the proposed technology is the ease of fabricating antennas in a simple lab environment. The method is based on the use of a silicon-based elastomer, polydimethylsiloxane (PDMS), for the substrate material, and copper meshes or any kind of perforated metal plates as flexible conductive materials. The initial liquid state of the PDMS allows a lot of freedom in the fabrication process, both from the electrical and mechanical points of view. The electrical and mechanical properties of the substrate can be adjusted by loading the PDMS with inclusions having low or high permittivity and/or density, thus controlling the permittivity and the rigidity. Another good characteristic of the initial low viscosity of PDMS is the possibility of shaping the antennas and substrates in in-house fabricated moulds. Copper meshes or perforated metal structures enable good adhesion between the substrate and 288 IEEE Antennas and Propagation Magazine, Vol. 55, No. 5, October 2013 conductor, and improve the overall flexibility. Measured results showed good repeatability of the substrate samples, both with or without inclusions. To demonstrate the versatility of the proposed technique, several antenna prototypes were built and characterized. All the antennas were flexible and showed good radiation characteristics. The antennas were exposed to tests such as washing and bending, in order to assess their robustness to environmental changes.
    IEEE Antennas and Propagation Magazine 10/2013; 55(5):287-297. · 1.18 Impact Factor
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    ABSTRACT: 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 (|S11|) 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 |S11|, 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.
    IEEE Antennas and Wireless Propagation Letters 01/2013; · 1.67 Impact Factor
  • M. Koohestani, N. Pires, A.K. Skrivervik, A.A. Moreira
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    ABSTRACT: A previous study of a novel technique to design a band-reject UWB monopole antenna by using patch loading is extended. Frequency-domain measurements have confirmed that the antenna designed using this technique rejects the interfering signal at the two intended narrowband frequency ranges. Moreover, the antenna radiation pattern distortion is negligible except at the notched bands. A study of time-domain characteristics based on the antenna pulse response and system fidelity factor is presented. The time performance has been extracted from frequency-domain measurements using a standard method. The obtained time-domain results promise the suitability of the proposed technique to design band-reject antennas useful for future UWB system applications.
    Electronics Letters 01/2013; 49(6):385-386. · 1.04 Impact Factor
  • A.K. Skrivervik, J. Trajkovikj
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    ABSTRACT: There has been a huge increase in interest for wearable communication devices in the last ten years. Applications are manifold, ranging from rescue to fashion over medical devices and safety, leading to the definition of new standards for Body Area Networks (BAN). Wearable communication devices can be of different kind: On Body, Off Body or In Body, leading to different propagation channels and different antenna requirements. In the frame of this contribution, we will first briefly recall different scenarios and derive the related antenna requirements. We will then present the technology solution we selected and finally present some antenna designs were the emphasis was laid on flexibility and robustness.
    Applied Electromagnetics and Communications (ICECom), 2013 21st International Conference on; 01/2013
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    ABSTRACT: We present the short-term noise budget and metro-logical studies on the medium-to long-term stability of our Rb standard. Our clock exhibits a short-term stability of ∼ 1.4×10−13 τ−1/2. The dependance of microwave power shift on light intensity, and the possibility to nullify the microwave power shift is demonstrated. The perturbing vapor cell geometric effect on medium-to long-term time scales, limiting the stability around 6×10−14 level is identified. Further required improvements are discussed.
    European Frequency and Time Forum & International Frequency Control Symposium (EFTF/IFC), 2013 Joint; 01/2013
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    ABSTRACT: ABSTRACTA dielectric sandwich loading technique is proposed to enhance impedance matching bandwidth of an antenna when it is placed close to the human body. This technique mitigates the body influence by confining the reactive fields in the dielectric loads. This has no side effects on the antenna performance. Unloaded and loaded prototypes of a recently developed UWB antenna have been measured both in free space and near a human arm. Free space results confirm that loading the antenna neither detune |S11| nor decrease efficiency. It was found that in a body proximity scenario, loading the antenna increases the impedance matching by 22% with respect to the unloaded antenna. These results show the suitability of this technique to design antennas for WBAN applications. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2965–2967, 2013
    Microwave and Optical Technology Letters 01/2013; 55(12). · 0.59 Impact Factor
  • Benjamin Fuchs, Anja Skrivervik, J.R. Mosig
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    ABSTRACT: 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.
    IEEE Antennas and Wireless Propagation Letters 01/2013; 12:1049-1052. · 1.67 Impact Factor
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    ABSTRACT: 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.
    IEEE Antennas and Wireless Propagation Letters 01/2013; 12:1512-1515. · 1.67 Impact Factor
  • A.K. Skrivervik
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    ABSTRACT: A discussion on the main challenges designing efficient antennas for bio-implantable communication devices is presented, along with some of the main issues encountered in their characterization. Such devices are used in conjunction with health monitoring or health care systems. Implantable antennas are, by nature, electrically small, and difficulties linked to electrically small antenna design apply. But implants are also located in a lossy host body, which induces a major change of paradigm with classic Electrically Small Antennas (ESA), as the main design challenge for implantable antennas will be to reach an acceptable efficiency, and not a broad enough bandwidth. In this paper, we present first the main challenges to be met in designing implantable antennas, followed by suggestions for an efficient design procedure. Finally, the specific difficulties in characterizing implantable antennas are emphasized.
    Antennas and Propagation (EuCAP), 2013 7th European Conference on; 01/2013
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    ABSTRACT: A new low profile ultra-wideband (UWB) antenna for wearable applications is presented. Its structure is derived from the quadripod kettle antenna (QKA) [1], and is called tripod kettle antenna (TKA). The proposed antenna is very well suited for wearable applications due to its miniature size and low profile. Moreover, the antenna is completely encapsulated in a Polydimethylsiloxane (PDMS) substrate enabling a certain level of flexibility and a good robustness against different external influences (e.g. different weather conditions). The TKA is a broadband antenna with 70% coverage of the fractional UWB bandwidth (Federal Communications Commission, FCC). The antenna has a polarization perpendicular to the body surface with a quasi-omnidirectional radiation pattern in the plane orthogonal to the polarization. An antenna prototype has been built and characterized, and measured results are in a good compliance with the simulations.
    European Conference on Antennas and Propagation, EuCAP; 01/2013
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    ABSTRACT: We evaluate the potential of a new generation microwave cavity by double resonance spectroscopic studies. Both, the cell and the microwave cavity are microfabricated. This allows a substantial size reduction for the physics package, without compromising the RF field geometry and the optical quality of the cell windows. We demonstrate short term stability of 5 × 10−12τ−1/2 (1–100 s), and below 1 × 10−12 up to 103 s. Finally, we report on the AC stark shift effect, one of the main limiting factors for long term clock stabilities.
    European Frequency and Time Forum & International Frequency Control Symposium (EFTF/IFC), 2013 Joint; 01/2013
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    ABSTRACT: The design, realization, and characterization of a compact magnetron-type microwave cavity operating with a TE(011)-like mode are presented. The resonator works at the rubidium hyperfine ground-state frequency (i.e., 6.835 GHz) by accommodating a glass cell of 25 mm diameter containing rubidium vapor. Its design analysis demonstrates the limitation of the loop-gap resonator lumped model when targeting such a large cell, thus numerical optimization was done to obtain the required performances. Microwave characterization of the realized prototype confirmed the expected working behavior. Double-resonance and Zeeman spectroscopy performed with this cavity indicated an excellent microwave magnetic field homogeneity: the performance validation of the cavity was done by achieving an excellent short-term clock stability as low as 2.4 × 10(-13) τ(-1∕2). The achieved experimental results and the compact design make this resonator suitable for applications in portable atomic high-performance frequency standards for both terrestrial and space applications.
    The Review of scientific instruments 10/2012; 83(10):104706. · 1.52 Impact Factor
  • M. Violetti, A.K. Skrivervik, Qin Xu, M. Hafner
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    ABSTRACT: Real-time clearance control in turbine engines allows improving their efficiency and safety, by enabling prognostics and optimized condition-based maintenance in the turbine hot section. This paper presents a novel 24 GHz microwave sensing system for real-time blade tip clearance monitoring suitable for small-size turbines (aero-engines and aero-derivatives). The system principle of operation is similar to the short range radar technique and employs high temperature resistant circular waveguide resonator probes mounted into the shell of the turbine. Probes concept was proven through simulation and prototypes where built and tested, showing agreement of results. The system was validated in laboratory, where the harsh turbine environment was reproduced on dedicated test rigs. Finally, the system was tested on a real engine, showing its suitability for real-time blade tip measurement.
    Sensors, 2012 IEEE; 01/2012
  • Jovanche Trajkovikj, J.-F. Zürcher, A. K. Skrivervik
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    ABSTRACT: This work presents novel techniques for producing substrates for flexible antennas. The technique we propose is based on the use of an already existing and widely used substrate material Polydimethylsiloxane (PDMS), where the dielectric properties of the substrate are adjusted by loading the PDMS with low or high permittivity inclusions. The low adhesion characteristics of PDMS are overcome by immersing the conducting parts of the antenna inside the substrate, at the same time sealing the antenna against the influence of dust, or water. A patch antenna prototype is realized and characterized. The built antenna is soft and flexible and it shows good radiation characteristics in terms of input matching and total gain. Measurements are in a good compliance with the simulation results.
    LAPC; 01/2012

Publication Stats

614 Citations
57.95 Total Impact Points

Institutions

  • 1989–2013
    • École Polytechnique Fédérale de Lausanne
      • Electromagnetics and Acoustics Laboratory
      Lausanne, Vaud, Switzerland
  • 2007
    • IMST GMBH
      Kamp-Lintfort, North Rhine-Westphalia, Germany
  • 2002
    • University of California, Los Angeles
      • Department of Electrical Engineering
      Los Angeles, CA, United States
  • 1997
    • Eawag: Das Wasserforschungs-Institut des ETH-Bereichs
      Duebendorf, Zurich, Switzerland