Probing the Mysteries of NVIS
Research in South Africa is focusing on the mechanisms
behind Near Vertical Incidence Skywave, or NVIS, propagation. NVIS is most
useful for short-range communications on the lower HF bands. According to
Newsline, two South African stations 51 kilometers (32 miles) apart, ZS6KN and
ZS6KTS, tracked their contacts over a period of several months in 2014. On one
day in June, signals were good from 0500 to 1630 local time, after which they
disappeared (ground wave signals would have remained more or less consistent).
The pattern repeated in July but the signals were considerably weaker in August
and September, late winter and early spring in the Southern Hemisphere. The
findings also showed changes consistent with changes in sunrise and sunset,
which suggests that NVIS may be influenced by changes in the D-layer of the
ionosphere, which is energized only during daylight hours. There has not been
enough data collected to make any meaningful conclusions, according to the
report, which notes that the two hams are continuing their research.
