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he goal of NVIS antennas is to allow local highly reliable communication for pleasure and emergency communication.  The German and U.S. forces used it in WWII and the D-DAY Victory was largely because of local, reliable HF communications day and night. It was also used in Viet Nam with success and is used by Russian forces routinely.  It does not require or use space communications.

Most antennas are intended and designed for DX work and therefore have low angle take off to maximize the horizontal distance traveled before being reflected back to earth.  This allows communication out to 1000 to 3000 miles (or more with multiple bounces).   NVIS is successful because the antenna radiates straight up (+/-45 degrees from vertical) and is reflected back at the same angle the signal left the antenna.  This provides an area of coverage of 600 to 800 miles across and in all directions.  Even though there is ground adsorption loss the signals are strong because of the short distance traveled through the Troposphere and D layer (day time only for D layer). Reflection occurs at the F1/F2 layers (Daytime) between approximately 250 and 450 Km up and 300-350 Km as a
single layer at night.

How it looks at the top of the pole:  a corner of Bert's 80-meter antenna.

NVIS antennas are low to the ground (.05 to .25 wavelengths at the operating frequency) avoiding ground wave interference (multi-path) at the receiving station.  They are horizontally polarized and operate from 2 to 10 to 12 MHz.  This usually requires two to three antennas.  Frequency selection is based on time of day and is SFI (Solar Flux Index) dependent. With high SFI numbers 40m daytime and 80m night is the  choice. If the SFI is low, then 80m daytime and 160m night is the choice. 

Dipoles, long wires with radials at the feed point, inverted Ls with radials at the feed point and horizontal loops work well as long as the height vs. wavelength rule is observed.  My favorite fixed antenna is a full wave 80m loop horizontal at 15 to 25 feet.  Mine is at 14 foot 6 inches.  No verticals are allowed because of the low angle of radiation.  A side benefit is low noise and has to be heard to be believed.

NVIS is truly a case of hearing is believing and has to be experienced to be appreciated.  On 3993.5 KHz I'm usually S9 to 20 over S9 state wide and region wide with S0 to S3 noise level locally.  I hear all counties if they're NVIS and most of the ones that are not.  TRY IT , YOU'LL LIKE IT!!

-WA7AXO