Radiocommunications Agency

Abstract Recently, stationary gamma‐ray spectrometry (GRS) detection was introduced in the field of hydrology for continuous soil moisture estimation at a unique spatial scale of tens‐of‐meters. This method offers promise for applications such as early warning systems for emerging droughts and floods, ecosystem health evaluation, and optimizing agricultural practices. Initial research has primarily focused on K‐40 concentrations measured at agricultural fields, with limited validation against independent soil moisture measurements. The objective of this study is to perform a comprehensive evaluation of the GRS soil moisture method. We estimate soil moisture with three conversion functions using the apparent K‐40, Th‐232, and Cs‐137 concentrations from three field sites. The performance is assessed using three independent soil moisture products of different spatial scale and temporal resolution. We obtain GRS soil moisture estimates with high accuracy (average rmse‐value = 0.036 [‐]). Across all sites, the most accurate soil moisture estimates, rmse‐value = 0.026 [‐], were found for the K‐40 concentrations using the modified Baldoncini equation, here named the Becker equation. This equation introduces an extra fitting parameter, that allows to adjust the slope of the conversion function. A high accuracy was also found for Th‐232 and Cs‐137 estimated soil moisture time series using the Becker equation, with rmse‐values of 0.033 [‐] and 0.040 [‐], respectively. An important finding was the strong correlation between cosmic‐ray neutron intensities and GRS nuclide concentrations across all sites. This is promising as the two methods rely on completely different physics, and thereby co‐validate each other.

The design, realization, and measurement results of a high-accuracy multiyear 3.5 GHz trans-horizon radio propagation measurement system are discussed, with both emphasis on the results and implemented technical measures to enhance the accuracy and overall reliability of the measurements. The propagation measurements have been performed on two different paths of 253 and 234 km length, using two transmitters and one receiver in the period September 2013 till November 2016. One of the paths travels over wetland; the other path can be considered as a land path. On each path, an additional transmitter is placed at 107 km (in the 253 km path) and 84 km (in the 234 km path) from the receiver. With this arrangement, the correlation between two nonaligned paths of comparable length, and two aligned paths of dissimilar length, was studied. The measurements show that for the land path, the predicted ITU-R P.452-16 cumulative distribution function (CDF) typically shows 5 dB higher path loss than the actual measured CDF for the region of interest; anomalous propagation. This means that the measured signal is on average weaker than predicted (a higher path loss). For the wetland path, the actual CDF is very close to the predicted CDF. Also, the measurements reveal that typically 30% of the anomalous propagation occurrences are correlated with other paths.

Separate excitation of the characteristic waves in the ionosphere results in two orthogonal propagation channels on the same frequency, which may be used in diversity and multiple-input, multiple-output (MIMO) systems. In this article, a method to measure the isolation between these paths is proposed and demonstrated in a near vertical incidence skywave (NVIS) experiment at a frequency of 7 MHz over a 105-km distance. Characteristic wave isolation exceeding 25 dB is measured during Happy Hour: the interval when the propagation path just opens or closes and only the extraordinary wave propagates.

The controlled crystallization of enantiomers of an organic compound (a cyclic phosphoric acid derivative) on templated micro-patterned functionalised surfaces is demonstrated. Areas where a complementary chiral thiol has been located were effective heterogeneous nucleation centres when a solution of the compound is evaporated slowly. Various organic solvents were employed, which present a challenge with respect to other examples when water is used. The solvent and the crystallization method have an important influence on the crystal growth of these compounds. When chloroform was employed, well-defined crystals grow away from the surface, whereas crystals grow in the plane from solutions in isopropanol. In both cases, nucleation is confined to the polar patterned regions of the surface, and for isopropanol growth is largely limited within the pattern, which shows the importance of surface chemistry for nucleation and growth. The apparent dependence on the enantiomer used in the latter case could imply stereo-differentiation as a result of short-range interactions (the templating monolayer is disordered, even at the nanometre scale). The size of the pattern of chiral monolayer also determines the outcome of the crystallization; 5 μm dots are most effective. Despite the low surface tension of the samples (relative to the high surface tension of water), differential solvation of the polar and hydrophobic layers of the solvents allows crystallization in the polar regions of the monolayer, therefore the polarity of the regions in which heterogeneous nucleation takes place is indeed very important. Despite the complex nature of the crystallization process, these results are an important step towards to the use of patterned surfaces for heterogeneous selective nucleation of enantiomers.

Near Vertical Incidence Skywave propagation can provide High Frequency (HF: 3-30 MHz) radio coverage in an area of 300 by 300 km without the need for any intermediate infrastructure. It is therefore very well suited for disaster relief communication. However, electromagnetic waves refracted in the ionosphere are prone to fading caused by multipath propagation and by polarization change in the ionosphere. Historically, different diversity reception systems have been devised to reduce this fading. By contrast, modern HF Multiple Input Multiple Output (MIMO) techniques exploit the multipath aspects of the ionosphere to achieve an increased data throughput or a reduction of the required signal-to-noise ratio. The effectiveness of diversity reception and HF MIMO increases with decreasing correlation between the received signal streams. Electromagnetic waves propagating through the ionosphere consist of two characteristic wave components with circular polarization of opposite sense, the ordinary and extraordinary waves, each following a different path through the ionosphere. Adaptation of the polarization of the receive antennas to these characteristic waves provides two signal streams with high isolation. This is demonstrated with an experiment where both the ordinary and extraordinary wave are received separately. An isolation of at least 13 dB is shown.

STM brings to light chirality aspects of the self-assembly of a functionalized helicene at the interface between a liquid and the solid substrates, gold and graphite. This reveals conditions for conglomerate formation.