Understanding Link Dynamics in Wireless Sensor Networks with Dynamically Steerable Directional Antennas

Abstract. By radiating the power in the direction of choice, electronicallyswitched directional (ESD) antennas can reduce network contention and avoid packet loss. There exists some ESD antennas for wireless sensor networks, but so far researchers have mainly evaluated their directionality. There are no studies regarding the link dynamics of ESD antennas, in particular not for indoor deployments and other scenarios where nodes are not necessarily in line of sight. Our long-term experiments confirm that previous findings that have demonstrated the dependence of angleof-arrival on channel frequency also hold for directional transmissions with ESD antennas. This is important for the design of protocols for wireless sensor networks with ESD antennas: the best antenna direction, i.e., the direction that leads to the highest packet reception rate and signal strength at the receiver, is not stable but varies over time and with the selected IEEE 802.15.4 channel. As this requires protocols to incorporate some form of adaptation, we present an intentionally simple and yet efficient mechanism for selecting the best antenna direction at run-time with an energy overhead below 2 % compared to standard omni-directional transmissions.

A Delft family portrait (1638) by Jan Daemen Cool

Since 1935 the Royal Museum of Fine Arts in Antwerp has held an unusually animated family portrait featuring a couple and their fijive children, including twins, which dates from 1638. The recent restoration of the painting has led to technical and art-historical research, with new insights into the history of the creation of this painting. This has resulted in the discovery of the identity of the sitters. The painting was originally probably twenty centimetres wider on the right-hand side, but is otherwise still in good condition. On the basis of the provenance information, marriage and death certifijicates and notarial archives, it was possible to discover the family’s identity. The painting shows Cornelis van der Heijde (1601/2-1638), a Roman Catholic doctor from Delft, his wife Ariaentgen de Buijser (1610-1677) and their daughters – including twins – Aechken (d. 1652), Pieternelle (d. 1661), Digna (d. 1672), Anna (d. 1681) and an unknown child who died in 1639 (probably Adriaen or Ariaentgen). The painting was made only a few months after the birth of the twins, probably in the autumn of 1638. Cornelis van der Heijde died in November 1638. It is not clear whether his portrait was painted while he was still alive or made after an earlier portrait. Rudi Ekkart convincingly attributed this family portrait to the Rotterdam painter Jan Daemen Cool (ca. 1589-1660). The form of the inscriptions and the characteristic rendering of the hands, eyes and mouths all support this attribution. It is true that Cool was trained in Delft by Michiel van Mierevelt, but until now we have known of no activities by him in that city after 1614. As far as we know, this is the only portrait that Cool made for a Catholic client. It forms an interesting pendant to the portrait of the likewise Catholic Van der Dussen family by Hendrick Cornelisz van Vliet of 1640 (Prinsenhof, Delft). In 1660 one of the portrayed sons, Cornelis, would go on to marry Digna van der Heijde, one of the daughters in the discussed family portrait by Jan Daemen Cool from 1638

A cross-cultural study of endurance fitness in girls and boys.

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