Near Vertical Incidence Skywave
Near Vertical Incidence Skywave (NVIS)
Near Vertical Incidence Skywave, or NVIS, is an antenna configuration method that enables local radio communications (0 - 500 miles) using low-band HF frequencies
NVIS becomes important when short range line-of-sight VHF and UHF communications are impaired
The NVIS concept is easy to grasp. It makes use of the well understood properties of low-band HF signal propagation - their ability to reflect off of the ionosphere with relatively little signal loss. Amateur Radio operators normally leverage this reflectivity for long distance (DX) contacts by radiating their signals at an angle so they skip across the ionosphere to stations hundreds or thousands of miles away.
But what if, in an emergency situation, our close-range line of sight communications infrastructure is down? Think a 2017 Hurricane Maria scenario. What happens when Amateur Radio, EMS and public service point-to-point and repeater infrastructure is not working and we need to establish communications with the next town, the next county or the next state?
Using your regular Amateur Radio HF transceiver and a horizontal antenna resonant on 40 or 80 meters, you can easily re-establish close-in communications with other stations. The idea is to simply set your antenna close to the ground (less than 1/4 wavelength for the frequency you are using) so that the Earth acts as a signal reflector, pushing your signals near vertically into the atmosphere. The ionosphere acts like a mirror, sending those signals back to earth at the same reflected angle. So if a signal from your NVIS antenna hits the ionosphere at 0 degrees of angle (straight up), it will be reflected back to Earth at 0 degrees (straight down).
But this is an imperfect world. Our reflective surfaces (the Earth and the ionosphere) are not perfectly reflective, and our antenna setups are never optimal. Because of this we know that only a small part of our signal will hit the ionosphere at 0 degrees. Most of the signal will hit the ionosphere at angles close to zero, and be reflected back at the same angles. This is the beauty of NVIS (remember, the NV in NVIS stands for Near Vertical). Your signal is 'rained down' over a relatively close area, even reaching hard to contact stations that are well shielded by terrain or urban canyons.
There's a lot of good information on NVIS available on the web, so rather than try to develop custom NVIS content for Fayette ARES we'll just link to the best sources available.
- NVIS Page on Wikipedia - not much detail, but a good general overview of the NVIS concept. Short and to the point
- NVIS Antenna Theory and Design - an excellent reference (PDF) put together for Texas MARS. Perhaps the most thorough discussion of NVIS theory and practice available on the web today. If you are serious about NVIS this is the document to read
- DXEngineering's NVIS publication - a PDF that provides both a good overview of NVIS theory and goes on to discuss how to construct an NVIS antenna (using DXEngineering components, of course)
Perhaps the best NVIS video on YouTube, from a US Army Signal Corps officer and Amateur Radio operator
Dave Casler, KE0OG, has put together one of well done training videos covering NVIS.
An equal mix of theory and good practical advice
Think constructing an effective NVIS antenna is hard or expensive? Tim Carter, W3ATB, shows how to make one using materials most of us already have at-hand