1,721,094 research outputs found
Characteristic modes analysis of mutually coupled log-periodic dipole antennas
No full textCharacteristic Modes Analysis (CMA) is a widely used method with recent progress in multi-antenna systems. We employ this method to characterize the mutual coupling phenomenon between two SKALA4.1 antennas, the low-frequency array elements of the future radiotelescope Square Kilometer Array (SKA-Low). The CMA accuracy is first validated at the lowest frequency range of interest with respect to a standard Method of Moments (MoM) solution by decomposing the single antenna into its characteristic modes. We then examine critical frequencies of a two-antenna system in modal decomposition, and characterize those responsible for the radiated electric field spurious spectral features owing to the mutual coupling. We connect these modes to first-order coupling of single antenna CMA modes, by using the eigenvalue data of both single- and two-antenna simulations
Spectral smoothness of ground plane backed log-periodic dipole antennas for radioastronomical applications
Full text linkThe spectral smoothness properties of the low-frequency array of the square kilometer array (SKA), namely, SKA-Low, are an important issue for its scientific objectives to be attainable. A large array of 256 log-periodic dipole antennas, installed on top of a 42 m circular ground plane, will work as an SKA-Low station in the frequency range of 50-350 MHz. In this article, the ground plane-induced effects are examined in terms of antenna beam spectral characteristics, while different antenna placements are considered. Results are produced both at the isolated antenna and at the array level in the band 50-100 MHz, by employing an approximate method for the speeding-up of array simulations. We attempt to distinguish the ground plane effect from that of mutual coupling among antennas, which appears to be more severe at specific frequencies, using 2 figures of merit. The discrete Fourier transform (DFT) components of gain pattern ratios identify the fundamental spatial components of the ripple, while the envelope correlation coefficient quantifies the penalty to considering an infinite ground plane
Suppresion of Log‐Periodic Dipole Antenna Spurious Radiation by Lumped Element Loading for Radioastronomical Application
Full text linkLog-Periodic Dipole Antennas have been known to provide good performance over large fractional bandwidths. However, optimized designs of log-periodic antennas, in particular for radioastronomy and remote sensing, need to tackle the spurious radiation of secondary dipole harmonics that can compromise the gain smoothness and therefore the ability to detect natural spectral features emitted by the probed sources. A new, low-cost method applicable also at a post-design stage is proposed in this paper, which relies on loading some of the dipoles with appropriate lumped circuit elements in order to avoid this unwanted feature while preserving the desired performance characteristics. Application is found on the log-periodic dipole antenna selected for the low-frequency instrument of the SKA (Square Kilometer Array), and general guidelines are described for other designs. An experimental test using a transmitting VHF antenna and the antenna under test with new RLC loaded dipoles confirms our solution
UAV-based measurement of sharp spectral resonances in mutually coupled SKA-low elements
Full text linkStrong mutual coupling between antennas can have detrimental effects in very sensitive radio telescopes based on aperture arrays. For the low-frequency instrument of the square kilometer array, narrowband perturbations in the radiation patterns of the inner elements have been predicted through electromagnetic simulations. This phenomenon deteriorates the spectral smoothness properties and disturbs the station calibration at very specific frequencies. In this letter, we present a new unmanned aerial vehicle (UAV)-based methodology to experimentally characterize such narrowband phenomena with a high-frequency resolution by means of independently-sweeping transmitting and receiving equipment. This solution does not add complexity to the system (e.g., by tethering the UAV). Results and predictions are in good agreement confirming both the presence of spurious resonances and the validity of the simulation approach
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
UAV-based antenna measurements: Scan strategies
Full text linkIn the recent years, the authors developed a system for the characterization of the radiation pattern of VHF and UHF antennas by means of a test-source mounted on a micro Unmanned Aerial Vehicle (UAV). So far, the adopted scan strategies typically consisted of two orthogonal straight paths at constant height from ground to obtain E- or H-plane cuts. In this paper, more complex scan strategies are presented to perform two-dimensional pattern measurements over planar and curved surfaces
Measurement of the LOFAR-HBA beam patterns using an unmanned aerial vehicle in the near field
Full text linkAn unmanned aerial vehicle (UAV) is exploited to characterize in situ the high-band
antennas (HBAs) of the low-frequency array (LOFAR) CS302 station located in Exloo, The
Netherlands. The size of an HBA array is about 30 m. The Fraunhofer distance (a few kilometers)
is not reachable in the frequency band (120 to 240 MHz) within the flight regulation limits.
Therefore, far-field patterns cannot be directly measured. The UAV, equipped with an radio frequency
synthesizer and a dipole antenna, flies in the near-field region of the considered array.
Measurement of three different frequencies (124, 150, and 180 MHz) is efficiently made during
the same UAV flight. The near-field focusing method is exploited to validate the far-field pattern
of the array under test within an angular range around the beam axis. Such a technique avoids
both the time consuming λ∕2 sampling of the aperture field and the further application of computationally
heavy near-field to far-field transformations. The array beam is well reconstructed in
the main lobe and first sidelobes within a 2D scan plane sampled with a radial raster. A further
postprocessing technique is proposed and validated on a subarray of HBAs. It suggests efficient
ways for the future characterization of regular aperture arrays for SKA-MID Phase 2
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
VGOS Wideband Reception and Emerging Competitor Occupations of the VLBI Spectrum
Full text linkThe VGOS wideband receivers cover a spectrum from 2 to 14 GHz. In this range, many frequencies are allocated to other services. VGOS provides up to four 1 GHz wide sub-bands, which can be tuned to frequencies where detrimental radio frequency interference is absent. The increasing demand of commercial users of radio spectrum and related on-going telecommunication projects are threatening the VGOS observation plans. The examples of a compatibility study for 5G concerning the German Wettzell site and the global availability of Starlink/OneWeb illustrate the impact on VGOS and the need of regulation by spectrum authorities. This article contains a brief introduction how spectrum management is organized and what needs to be done on the national level to achieve protection for VGOS sites
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