Artikelrobot
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Near Vertical Incidence Skywave (NVIS) involves the use of a frequency just below the critical frequency along with antennas that have a very high radiation angle to establish reliable communications over a radius of about 300 km
http://www.southgatearc.org/news/october2005/nvis_antenna.htm
http://www.southgatearc.org/news/october2005/nvis_antenna.htm