Understanding what NVIS is

NVIS is a special mode of propagation, acronym of Near Vertical Incidence Skywave, which is used for communication at short distances (less than 600 km), that are not covered by ground waves. NVIS signals are transmitted directly upwards and returned downwards by the ionosphere, with effective frequencies ranging from 2 to 8 MHz.

NVIS from F layer

Unlike long-distance communication methods, NVIS requires the use of a vertical radiation profile antenna.

1. Dipole

A dipole at 0.1-0.25 wavelengths is an efficient NVIS antenna, maximizing vertical and nearly vertical radiation while minimizing lower-angle radiation.

The signal to noise ratio improves at very low dipole heights, due to a greater reduction in background noise and interference from distant regions.

Very low dipoles have feedpoint impedances that are 50 ohms or less. To prevent a high SWR, care should be taken to match the antenna.

Low dipoles have another benefit in that they are simple to construct.

2. Inverted Vee

The inverted vee is a good NVIS antenna that may even be simpler to support. A dipole suspended from a height just below the inverted vee's apex will perform almost as well as an inverted vee if the apex angle is kept at least at 120 degrees. If the included angle is sharper than about 120 degrees, the inverted vee won't function for NVIS because there won't be enough vertical radiation.

An inverted vee is frequently easier to erect than a dipole because only one support is required above ground for the center of the antenna, which must support the weight of a center insulator and feedline.

3. Modification: - Adding a wire reflector

A wire placed beneath a dipole (or inverted vee) that serves as a reflector and is about 5% longer than the primary radiating element can improve its high angle radiation. When the antenna is too low to allow for that, a wire laid on the ground beneath the antenna may still be effective. The ideal height for such a wire refector is about 0.15 wavelengths below the main radiating element. However, the soil in the area affects how effective a wire reflector is.

A knife switch at the center point of the wire reflector can be used to eliminate its effect. This technique is useful for using a dipole for NVIS and longer distances, too. When the switch is closed, the vertical gain will increase, and the noise levels may drop.

Watch the near real-time NVIS propagation charts showing the current critical frequency.

The highest frequency that the ionosphere will vertically reflect is FoF2. This critical frequency is measured by numerous stations around the globe and used to create maps like the one below, which shows foF2 in near real-time. The following NVIS map was designed for radio amateurs:

The colored regions of this map, which are rebounded by Iso-Frequency contours, illustrate the Critical Frequency that is expected to bounce off of the ionosphere at near vertical angle. The ham bands (160, 80, 60, 40, 30 ,20m) are designated by iso-frequency contours: 1.8, 3.5, 5.3, 7, 10.1, and 14 Mhz.

foF2, as measured by ionosondes, is the raw data that powers the site.
Colored discs indicate the location of stations. A number inside each disc represents the Critical frequency, f_oF2.

1. NVIS covers the skip zone.
2. Reliable communications; No need for infrastructure.
3. Pure NVIS propagation is mostly fading-free.
4. Simple and low antena would perform very well. One person or a small group of people can quickly and easily erect a good NVIS antenna.
5. NVIS can be effective in valleys and low areas.
6. Low losses in the D layer, due to the shorter path to and from the reflecting E and F layers.
7. Better signal-to-noise ratio compared to ground waves.
8. Need lower power levels due to better signal-to-noise ratio and low path loss.

1. The propagation of NVIS may be poor if at least one station lacks an NVIS-optimized antenna.
2. Limited bandwidth because only the lower HF bands from the LUF upto 10 MHz may be suitable for NVIS.
3. Due to the variation in day and night propagation, a minimum of two different bands must be used to ensure reliable 24-hour communications.

NVIS was first used by the German Army during the late 1930s as a solution to the Skip Zone. The Allies used NVIS in the invasion of Normandy. During the Vietnam War, the US military found NVIS as a solution for communication over rugged highlands terrain. Today, NVIS communication is practiced and used by military organizations and their affiliates, as well as various members of the preparedness community, to provide reliable, fast, and secure communications with lower probability of DF location.

1. Understanding NVIS  ^{Rohde Schwarz}
2. HF NVIS  ^{Military HF Radio}
3. NVIS ^Wikipedia
4. NVIS Propagation: Near Vertical Incidence Skywave ^{Electronics-Notes}
5. Near-Vertical Incidence Sky-Wave Propagation 36 pages Presentation for radio hams ^{Gerald Schuler, DU1GS / DL3KGS}
6. Near Vertical Incidence Skywave (NVIS) ^{by W8BYH, Fayette ARES}
7. Near Vertical incidence Skywave Propagation NVIS Antennas  80, 60, 40m bands ^{KB9VBR Antennas}
8. NVIS Overview  ^{David Casler, KE0OG}
9. Ham Radio NVIS for Regional Communications  ^{Radio Prepper}
10. NVIS - Near Vertical Incidence Skywave ^{_{What is it? advantages; antennas; links} Jim Glover, KX0U (ex WB5UDE)}
11. Near Vertical Incidence Skywave (NVIS) ^{Ham Radio School, W0STU}
12. NVIS Explained - part 1 ^NCSCOUT
    NVIS Explained - part 2
    NVIS Explained - part 3
    NVIS EXPLAINED citing the above 3-parts publication ^AmRRON
13. NVIS Antennas ^{Dale Hunt, WB6BYU}

Extended Research papers

14. Radio communication via Near Vertical Incidence Skywave propagation: an overview ^{Telecommun Syst (2017) DOI, Ben A. Witvliet, Rosa Ma Alsina-Pagès}
15. Analysis of the Ordinary and Extraordinary Ionospheric Modes for NVIS Digital Communications Channels ^{Sensors (Basel)}
16. NVIS HF signal propagation in ionosphere using calculus of variations ^{Geodesy and Geodynamics, Umut Sezen, Feza Arikan, Orhan Arikan}