A. K. Skrivervik

École Polytechnique Fédérale de Lausanne, Lausanne, VD, Switzerland

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Publications (173)44.09 Total impact

  • 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: 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: 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: 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: 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
  • 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
  • B. Fuchs, A. 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 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
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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: 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
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    ABSTRACT: This letter describes a data telemetry biomedical experiment. An implant, consisting of a biometric data sensor, electronics, an antenna, and a biocompatible capsule, is described. All the elements were co-designed in order to maximize the transmission distance. The device was implanted in a pig for an in vivo experiment of temperature monitoring.
    IEEE Antennas and Wireless Propagation Letters 01/2012; · 1.67 Impact Factor
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    ABSTRACT: Nowadays there is an increasing need for radically miniaturized and low-power atomic frequency standards, for use in mobile and battery-powered applications. For the miniaturization of double-resonance (DR) Rubidium (87Rb) 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 87Rb) has been a long-standing issue. Here we present a newly developed miniaturized MWR, the μ-LGR, consisting of a loop-gap resonator based cavity with very compact dimensions (volume
    Sensors, 2012 IEEE; 01/2012
  • M. Koohestani, N. Pires, A.K. Skrivervik, A.A. Moreira
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    ABSTRACT: This paper reports a study of the influence on the fidelity factor due to loading an Ultra Wideband monopole with a pair of dielectric substrate pads. The fidelity factor was calculated, with and without loading, using the simulated transmitted and received pulses in three points in the azimuth plane. Results show that this dielectric loading technique does not cause significant pulse distortion. The fidelity factor is affected only by an average of 2.8% relative to the unloaded antennas in all situations considered. Two versions of the same antenna design were studied: coplanar- and microstrip-fed. It was found that the influence of feeding on the fidelity factor is also minimal.
    Antennas and Propagation Conference (LAPC), 2012 Loughborough; 01/2012
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    ABSTRACT: Slotted PIFA on the textile substrate suitable for body-centric applications in the UHF band is proposed. The influence of slot in the ground plane on the antenna parameters is theoretically investigated using simplified model of human body.
    Antennas and Propagation (EUCAP), 2012 6th European Conference on; 01/2012
  • B. Fuchs, A. Skrivervik, J.R. Mosig
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    ABSTRACT: An iterative procedure for the synthesis of sparse arrays radiating focused beampatterns is presented. The algorithm consists in solving a sequence of weighted l1 convex optimization problems. The method can thus be readily implemented and efficiently solved. In the optimization procedure, the objective is the minimization of the number of radiating elements and the constraints correspond to the pattern requirements. The method can be applied to arbitrary arrays which means that there is no restriction regarding the array geometry and individual element patterns to be synthesized. Numerical comparisons with examples found in the literature assess the efficiency of the proposed approach, whose computation time is several orders of magnitude below those of so-called global optimization algorithms.
    Antennas and Propagation (EUCAP), 2012 6th European Conference on; 01/2012
  • 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
  • M. Violetti, Qin Xu, O. Hochreutiner, A.K. Skrivervik
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    ABSTRACT: Real-time Blade Tip Timing (BTT) in rotating machinery enables condition-based maintenance and blade failure prevention. In this paper, we propose a new 6 GHz microwave sensing system suitable for BTT in gas and steam turbines. The system working principle is similar to the short range radar technique, using probes mounted in the turbine wall, in direct view of the passing blades. The front end of the system consists of a miniature open-ended coaxial resonator sealed by a ceramic cap. Its principle of operation is simple and robust, thus well-suited for the turbine harsh environment, while the radiated TEM mode allows for insensitivity to mounting orientation. The probe was validated through simulation and prototypes were successfully tested. Measurements on prototypes showed improved spatial resolution on the waveform compared to an existing 6 GHz patch antenna probe, and superior resolution and measurement range compared to Eddy current probes used for BTT.
    Microwave Conference Proceedings (APMC), 2012 Asia-Pacific; 01/2012
  • Benjamin Fuchs, Anja Skrivervik, Juan R. Mosig
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    ABSTRACT: An iterative procedure for the synthesis of uniform amplitude focused beam arrays is presented. Specifically, the goal is to optimize the locations of a fixed number of array elements with known excitations in order to synthesize narrow-beam low-sidelobe patterns. Any fixed elements excitations can be handled, although uniform amplitude and equiphase excitations are aimed because of their practical interest. Moreover, the method can be applied to both linear and planar arrays, and there is no restriction regarding the element patterns. On top of being easy to implement, the proposed procedure leads to solutions that either equal or outperform the ones found by previous approaches in various cases of interest.
    IEEE Antennas and Wireless Propagation Letters 01/2012; · 1.67 Impact Factor