HF Near Vertical Incidence Skywave (NVIS)

NVIS propagation is used to fill in between the distance that is normally covered by ground wave and the first hop for low angle of radiation long distance communication. This typically falls between 50 miles and 600 miles.

NVIS is often compared to pointing a shower head straight up at the ceiling and having the water droplets fall everywhere. Counter to most HF antenna installations, NVIS requires a high angle of radiation. This is achieved by installing the antenna at a low height, usually no higher than 1/8 wavelength. Lower antenna heights result in only a slight reduction in received signals and it is not atypical for a portable installation to have the antenna installed just a few feet above the ground.

NVIS does not require a high transmit power level. Power levels of 25 watts are quite effective in maintaining good quality HF communications. 

NVIS requires using a frequency that is below the Critical Frequency, the highest frequency at which NVIS communications can operate at, but above the frequency at which the D-Layer absorption results in excessive attenuation, or the Lowest Usable Frequency (LUF). NVIS communications is not possible below the LUF or above the Critical Frequency. Use of frequencies below the LUF or above the Critical Frequency will result in a loss of NVIS communications.

For all practical purposes, NVIS communications at Idaho latitudes rarely are usable at frequencies above 13 MHz as the Critical Frequency rarely climbs above this level.

NVIS HF Propagation is in daily use by United States Military Forces and government emergency agencies at all levels of government on a daily basis. The reason is that it works very reliably.

NVIS Critical Role

NVIS is typically used for communication ranges of from 50 to 600 miles. When topographical features, such as mountainous terrain, are involved, the typical ground wave distance of 50 miles can be reduced to very short distances. Ground wave distances can also vary in distance where the take off angle of radiation goes over the top of the target station or where terrain obstructs the radio wave from reaching the target station. Ground wave may be reduced to even a few miles.

Between the D-Layer absorption frequency (LUF) and the Critical Frequency, reliable NVIS communications can be had and will fill in the distances between ground wave and long hop propagation. NVIS capabilities present the best option for communications that span between ground-wave and approximately 600 miles.

https://www.idahoares.info/tutorial_hf_nvis_band_selection.php

The AS-2259 Antenna:

https://practicalantennas.com/applications/nvis/as2259/