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 |  |  Relational Databases and Knowledge BasesA database management system (DBMS), or simply database system, is characterized by the data model it suports . The first DBMSs, designed in the 1960s, were based on hierarchical or network models and have been viewed as extensions of file systems in which interfile links are provided through pointers . The data manipulation languages of these... |  |  Electrically Small, Superdirective, and Superconducting AntennasHarold Wheeler, in his classic 1947 papers, created the fi eld of electrically small antennas. This fi eld has long been important for frequencies below roughly 1 GHz, where a half-wavelength is about 6 inches. These electrically small antennas have characteristics in common that limit performance: low radiation resistance, high reactance, low effi... |
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