The use of India ink in tissue-simulating phantoms

The optical properties of India ink, an absorber often used in preparation of tissue simulating phantoms, have been investigated at visible and near infrared wavelengths. The extinction coefficient has been obtained from measurements of collimated transmittance and from spectrophotometric measurements, the absorption coefficient from multidistance measurements of fluence rate in a diffusive infinite medium with small concentrations of added ink. Measurements have been carried out on samples of India ink from five different brands, and for some brands also from different batches. As also reported in previously published papers the results we have obtained showed large inter-brand and inter-batch variations for both the absorption and the extinction coefficient. On the contrary, our results showed small variations for the ratio between the absorption and the extinction coefficient. The albedo is therefore similar for all samples: The values averaged over all samples investigated were 0.161, 0.115, and 0.115 at λ = 632.8, 751, and 833 nm respectively, with maximum deviations of 0.044, 0.019, and 0.035. These results indicate that, using the values we have obtained for the albedo, it should be possible to obtain with uncertainty smaller than about 4% the absorption coefficient of a sample of unknown ink from simple measurements of extinction coefficient. A similar accuracy is not easily obtained with the complicated procedures necessary for measurements of absorption coefficient

Preliminary Analysis of the Effects of the Ground Plane on the Element Patterns of SKA1-Low

Each station of the SKA1-Low radio telescope is composed by 256 dual-polarized log-periodic antennas deployed over a metallic ground plane with 42 m diameter. This station is usually modelled in EM simulators by considering an infinite ground plane, which drastically reduces the computational time. This contribution shows that a finite ground plane can bring to quite significant differences in some embedded element patterns with respect to the infinite ground plane case. Furthermore, we show the impact on the antenna pattern of different dielectric media surrounding the finite ground plane. For instance, at 50 MHz the maximum of the antenna gain decreases by 5% for the terrain with 10% moisture level

Impact of mutual coupling between SKALA4.1 antennas to the spectral smoothness response

One of the advantages of arrays with aperiodic distributed elements is their ability to mitigate the detrimental mutual coupling effects on the radiation pattern. However, we show that the mutual coupling inside a random array can still generate undesired structures in the frequency response although the single antenna features a spectral smooth response. For small subsets (a couple of SKALA4.1 antennas and a 16-element array) of a low-frequency instrument station of the Square Kilometre Array, the combination of large mutual coupling and antenna geometry creates systematic distortions in the element frequency responses. This phenomenon compromises the station spectral smoothness response versus frequency. However, we demonstrate that it is possible to partially mitigate these frequency structures by reconfiguring the antenna distribution based on exclusion zones

Mutual Coupling Analysis for a SKA1-LOW Station

The modelling of the antenna patterns represents one of the main challenges for the instrumental calibration of radio telescopes composed by antennas randomly distributed. In this work, the electromagnetic characterization of a single station of the low-frequency instrument of the Square Kilometre Array (SKA) radio telescope operating from 50 to 350 MHz is reported. The station is assumed to be composed by Log Periodic antennas. The effects of mutual coupling on the complex embedded element patterns and on the array beam are investigated by means of a full-wave electromagnetic analysis. The accuracy of a simplified, mutual coupling-free approach is presented as well

Using Embedded Element Patterns to Improve Aperture Array Calibration

Several existing and planned low-frequency (<; 350 MHz) radio astronomical facilities exploit subarrays or stations consisting of receiving elements in an irregular configuration. Calibration of the RF path of each receiving element is crucial for accurate beamforming with these stations. Currently used station calibration methods usually assume that the embedded element patterns (EEPs) of the receiving elements within a station are identical. In this contribution, we show that ignoring the inter-element EEP variations causes systematic errors in the calibration results using the stations of the low-frequency receiving systems of the Square Kilometre Array (SKA) and the Low Frequency Array (LOFAR) as examples. We show that the magnitude of these errors increases with increasing EEP variations. We also discuss the challenges faced by SKA and LOFAR to mitigate these errors by exploiting a priori knowledge of the EEPs

Absorption and scattering properties of carbon nanohorn-based nanofluids for direct sunlight absorbers

In the present work, we investigated the scattering and spectrally resolved absorption properties of nanofluids consisting in aqueous and glycol suspensions of single-wall carbon nanohorns. The characteristics of these nanofluids were evaluated in view of their use as sunlight absorber fluids in a solar device. The observed nanoparticle-induced differences in optical properties appeared promising, leading to a considerably higher sunlight absorption with respect to the pure base fluids. Scattered light was found to be not more than about 5% with respect to the total attenuation of light. Both these effects, together with the possible chemical functionalization of carbon nanohorns, make this new kind of nanofluids very interesting for increasing the overall efficiency of the sunlight exploiting device

Forward solvers for photon migration in the presence of highly and totally absorbing objects embedded inside diffusive media

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Comparison between Measured and Simulated Antenna Patterns for a LOFAR LBA array

A UAV-based system has been employed for a measurement campaign on a station of the radio telescope LOFAR to characterize the individual Low Band Antenna patterns. The experimental set-up has been then simulated with a full-wave software and numerical embedded element patterns have been compared to the measured results. A statistical analysis of the differences between the two data sets has been finally carried out to estimate the accuracy of the electromagnetic model

Computational electromagnetics for the SKA-Low prototype station AAVS2

We summarize the activities conducted since 2019 in the numerical electromagnetic analysis of one prototype station of the SKA-Low telescope. Working closely with the SKA Observatory, two teams based in Australia and Italy have collaborated effectively in modeling and analyzing AAVS2, which is the most recent prototype of an SKA-Low station installed in Western Australia. A comprehensive overview of the main electromagnetic parameters at element and array level obtained with two different commercial solvers is presented. Results for scattering parameters, individual element patterns, and station beams are shown; all these fully incorporate mutual coupling effects. Sensitivity of the station is addressed, as the cross-polarization performance. Finally, we also address some lessons learned and their impact on the project

Electromagnetic modelling of the SKA-LOW AAVS1.5 prototype

The numerical modelling of the Square Kilometre Array (SKA) Aperture Array Verification System Version 1.5 is discussed. The role of this SKA prototype is placed within the perspective of the overall SKA project. The dual-polarized log-periodic SKALA4.1 antenna elements comprising a station are briefly described, along with some considerations regarding the station. Computational simulation aspects are considered, and preliminary results shown. An appraisal of the implications for station-level calibration concludes the paper