What is NVIS?

NVIS, or Near Vertical Incidence Skywave, refers to a radio propagation mode which involves the use of antennas with a very high radiation angle, approaching or reaching 90 degrees (straight up), along with selection of an appropriate frequency below the critical frequency, to establish reliable communications over a radius of 0-200 miles or so, give or take 100 miles. Although not all radio amateurs have heard the term NVIS, many have used that mode when making nearby contacts on 160 meters or 80 meters at night, or 80 meters or 40 meters during the day. They may have thought of these nearby contacts as necessarily involving the use of groundwave propagation, but many such contacts involve no groundwave signal at all, or, if the groundwave signal is involved, it may hinder, instead of help. Deliberate exploitation of NVIS is best achieved using antenna installations which achieve some balance between minimizing groundwave (low takeoff angle) radiation, and maximizing near vertical incidence skywave (very high takeoff angle) radiation.

As hams, we often faithfully follow the advice: get your antenna up as high as you can get it! We do this, and other things (like choosing antennas that have a low angle of radiation) in order to maximize the distance over which we can communicate. An antenna with a particularly high angle of radiation is often somewhat disparagingly referred to as a “cloudwarmer”, the implication being that if the signal isn’t radiated at a low enough angle, it’s being wasted. For NVIS, we ignore all this traditional advice, and select instead techniques which will maximize not our DX, but our ability to reliably communicate with other stations within a radius of 0-300 miles.

What are the advantages and disadvantages of NVIS?

Among the many advantages of NVIS are:

  • NVIS covers the area which is normally in the skip zone, that is, which is normally too far away to receive groundwave signals, but not yet far enough away to receive skywaves reflected from the ionosphere.NVIS requires no infrastructure such as repeaters or satellites.
  • Two stations employing NVIS techniques can establish reliable communications without the support of any third party.
  • Pure NVIS propagation is relatively free from fading.
  • Antennas optimized for NVIS are usually low. Simple dipoles work very well.
  • A good NVIS antenna can be erected easily, in a short amount of time, by a small team (or just one person).
  • Low areas and valleys are no problem for NVIS propagation.
  • The path to and from the ionosphere is short and direct, resulting in lower path losses due to factors such as absorption by the D layer.
  • NVIS techniques can dramatically reduce noise and interference, resulting in an improved signal/noise ratio.
  • With its improved signal/noise ratio and low path loss, NVIS works well with low power.

A complete and informational article on NVIS can be located HERE.