I guess if you have a tower, you're not so interested in NVIS <grin>
Interesting paper (behind IEEE paywall unfortunately) in this month's
Antennas and Propagation Magazine. The authors have a series of papers
over the past few years of interest, looking at various performance factors.
P. J. Coetzee and W. P. du Plessis, "Performance Limiters of
Near-Vertical-Incidence Skywave Propagation: A Scientific Approach," in
IEEE Antennas and Propagation Magazine, vol. 62, no. 3, pp. 39-44, June
2020, doi: 10.1109/MAP.2019.2943313.
Abstract:
Near-vertical-incidence skywave (NVIS) propagation is defined as
providing continuous coverage from nearly 0 km (just beyond the line of
sight) to a couple hundred kilometers from the transmitter with no skip
or dead zones. NVIS communications are especially effective during
disaster-relief operations when infrastructure is severely damaged. The
ability to accurately determine the performance limiters of NVIS
propagation can help in the planning of high-frequency (HF) (3-30 MHz)
emergency communication links. In the literature, widely varying radial
distances (from as few as 50 to up to 160 or even 320 km) for the
coverage attainable by NVIS propagation have been reported. It is very
difficult to plan an NVIS link for homeland security or disaster relief
when the published guidelines vary to such a degree. In this study, a
scientific approach was utilized to determine the NVIS performance
limiters for varying solar conditions, times of day, and geophysical
locations.
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