NVIS (Near-Vertical Incident Skywave)

What is NVIS?

Near-Vertical (meaning somewhere around straight up) Incident (an event, something that happens) Skywave (the reflecting of a signal, usually RF, off the ionosphere). Think of it as pointing your antenna straight up and having the signal "shower" down all around you.

NVIS - compared to what?

Think of the two modes of antenna orientation with which we are most familiar: vertical and horizontal.

Vertical - line of sight, groundwave propagation

Horizontal - long-distance, skip

'Splain yourself!

If you can talk about 50 miles on a vertical antenna predictably, and on the horizontal plane you can pretty much count on shooting over most stations that are closer than 500 miles away, NVIS is a method that you can use to fill the gap (skip zone). NVIS allows you to have more reliable communications with stations within a 100-200 mile range.

Great! How do I do it?

You do this by directing your signal straight up. Rather than pointing your beam directly overhead, you can do it the easy way. Cut a dipole to the desired frequency, and install it less than 1/4 wave above ground. Last September at the Route 66 event in Chandler Park, N5FEM, Steve, installed one at about 5 feet and did very well with it. By installing it closer to the ground, you are using the ground as a reflector. In a sense, it becomes an upward pointed beam.


Putting One On Top of a Hill

Well...yes you can...but...with valleys generally containing more moisture than hilltops, often-times you get a better reflection in a valley. And, after all, you're directing your signal upward, so a valley may be more advantageous, especially if you're trying to avoid detection (military applications).


What Bands?

Generally speaking, 160M, 80/75M, 60M, & 40M is where you can use this system most effectively. As we have learned, the higher frequencies will tend to penetrate the ionosphere more, and reflect off the ionosphere less.


Why bother?

In an emergency or if you have friends within the dreaded skip zone (200 miles), this allows us to have fairly predictable communications. Skipping on the horizontal plane is not that predictable, and where it is predictable, may not be that useful. On the vertical plane, line-of-sight may not cut it. You often need more. NVIS fills the gap between the two.

You can read more about this at: athensarc.org/nvis.asp