NVIS Propagation: Complete Guide to Near Vertical Incidence Skywave for Ham Radio

Near Vertical Incidence Skywave (NVIS) is an ionospheric skip operating technique that directs the strongest signals from a station vertically, or upward, rather than toward the horizon. Signals propagating nearly vertically approach the ionosphere with steep incidence angles and may be bent back to earth with similarly small angles. The operational result is skip communications effective within a radius of a few hundred miles.

Definition and Basic Principles of Near Vertical Incidence Skywave

NVIS, or Near Vertical Incidence Skywave, is a high-frequency radio technique where you send signals almost straight up into the sky. The ionosphere bends these signals back down, so you can talk over a few hundred kilometers without needing repeaters or satellites. NVIS propagation requires a high angle or near vertical signal to be transmitted towards the ionosphere.

Near vertical incidence skywave, or NVIS, is a skywave radio-wave propagation path that provides usable signals in the medium distances range — usually 0–650 km (0–400 miles). The radio waves travel near-vertically upwards into the ionosphere, where they are refracted back down and can be received within a circular region up to 650 km (400 miles) from the transmitter.

With NVIS, you send radio waves almost straight up—usually at angles above 75° from the horizon. The signal hits the ionosphere, bounces back, and lands over a wide area around you. NVIS generally requires takeoff angles of 70 degrees or higher.

The Physics Behind NVIS Ionospheric Reflection

Ionospheric radio systems may send radio waves nearly vertically upwards, to be refracted in the ionosphere and returned to earth. This phenomenon is called 'Near Vertical Incidence Skywave' (NVIS) propagation. The refraction in the ionosphere depends on the electron density in the ionosphere.

This must be at a frequency that is below the critical frequency, i.e. the maximum frequency at which a vertically incident signal is "reflected" by the ionosphere. Typically it is just below the critical frequency for the ionospheric layer or region that is to be used. You need to keep the operating frequency below the ionospheric critical frequency. If you go too high, your signal just shoots through the ionosphere and disappears.

If the frequency is too high (that is, above the critical frequency of the ionospheric F layer), refraction is insufficient to return the signal to earth and if it is too low, absorption in the ionospheric D layer may reduce the signal strength.

The ionosphere is bi-refractive. Appleton and Builder showed that radio waves entering the ionosphere, under the influence of the Earth's magnetic field, are split in two circularly polarized characteristic waves in opposite rotational directions, the ordinary and the extraordinary wave.

Differences Between NVIS and Conventional Skywave Propagation

It fills the gap between line of sight and the longer distance skip type communications that are normally used at HF. The NVIS technique can help to bridge the communications gap between the local range of VHF/UHF repeater or simplex communications and the longer distance skip propagation of low-to-the-horizon HF signals.

Conventional HF skywave propagation uses low-angle radiation to achieve long-distance communication, with signals bouncing off the ionosphere at shallow angles. The HF bands of 10-meters (28 MHz) to 30-meters (10 MHz) are often effectively refracted back to earth's surface when directed toward the horizon where incidence angles into the ionosphere are closer to the horizontal, and this propagation geometry provides long skip distances with single skips up to 2500 miles.

In contrast, A typical HF antenna pattern transmits most of its energy at an angle of 30o or less to achieve long distance communications. In contrast, the pattern for a NVIS antenna is shown on the right. Most of its energy is transmitted straight up.

It's a lifesaver for short-to-medium range communication, especially when mountains, dense forests, or other obstacles kill your line-of-sight. Agencies and volunteers use NVIS to keep consistent coverage over disaster zones. This avoids the "skip zone" problem you get with other HF modes, so field units and command centers can stay in touch—even in remote or cut-off spots.

Critical Frequency and Maximum Usable Frequency Concepts

The critical frequency varies according to ionisation density in the relevant ionospheric layer or region which in itself is dependent upon the radiation received from the Sun. Accordingly it is dependent upon the sunspot cycle, time of day, season and a variety of other factors. Driven by the radiation of the sun, the electron density follows a diurnal cycle, the seasons and the 11-year solar cycle.

For NVIS propagation, the frequency of the radio waves must be smaller than the maximum plasma frequency of the ionosphere, for mid-latitudes typically between 3 and 10 MHz. To do so, the operating channels must be below the Critical Frequency, the highest frequency where signals radiated straight up will be returned to Earth by the ionosphere. Above that frequency, signals pass off into space, even though they may be reflected back when striking the ionosphere at flatter angles.

NVIS Frequency Bands and Propagation Characteristics

Optimal Frequency Ranges for NVIS (80m and 40m Bands)

The bending effect of the ionosphere is greater for lower frequencies. The ionosphere's bending effect is sufficient, even at steep "near vertical" angles of incidence, to bend back to earth the lower HF frequencies, particularly the 40-meter band frequencies.

Therefore lower amateur radio frequencies such as 40 and 80 meters are ideal for NVIS use. NVIS is the most effective for the low bands on the HF spectrum, such as 40, 60, and 80 meters.

The most reliable frequencies for NVIS communications are between 1.8 MHz and 8 MHz. Above 8 MHz, the probability of success begins to decrease, dropping to near zero at 30 MHz. NVIS communication uses frequencies between approximately 3 and 10 MHz.

However, the ionosphere usually does not have sufficient bending strength to return these upper HF band frequencies to earth with the steep take-off angles necessary for the NVIS technique. This is why the 2-meter band (144 – 148 MHz) and higher frequencies are almost never received via skip propagation.

Day vs Night NVIS Propagation Patterns

Military NVIS communications mostly take place on 2–4 MHz at night, and 5–7 MHz during daylight. Common bands used in amateur radio at mid-latitudes are 3.5 MHz at night and 7 MHz during daylight, with experimental use of 5 MHz (60 m) frequencies.

During winter nights at the bottom of the sunspot cycle, the 1.8 MHz band may be required. The ionospheric D layer, which absorbs HF signals, is stronger during daylight hours, requiring higher frequencies for effective NVIS propagation. At night, when D layer absorption decreases, lower frequencies become more viable.

60 meters fills an important gap between 80 and 40 meters and is exceptionally effective for NVIS (Near Vertical Incidence Skywave) propagation. During disasters, this allows dependable regional communication across several hundred miles, even in mountainous terrain or heavily damaged areas.

Solar Cycle Effects on NVIS Performance

The solar cycle significantly impacts NVIS propagation effectiveness. During solar maximum periods, the ionosphere becomes more densely ionized, raising the critical frequencies and allowing higher NVIS frequencies to be effective. Conversely, during solar minimum periods, lower frequencies are required for reliable NVIS communication.

Solar activity also affects the stability of NVIS signals. During geomagnetic disturbances, NVIS communications may experience increased fading and reduced reliability. Aurora activity, while disrupting high-latitude communication paths, can sometimes enhance NVIS propagation in certain regions.

Seasonal and Geographical Variations

Usable frequencies are dictated by local ionospheric conditions, which have a strong systematic dependence on geographical location. Mid-latitude regions generally experience more predictable NVIS propagation patterns compared to equatorial or polar regions.

Winter months typically favor lower NVIS frequencies due to reduced solar radiation and ionospheric density. Summer conditions often require higher frequencies for effective NVIS communication. The transition periods of spring and fall can provide excellent NVIS conditions across multiple frequency bands.

Geographic factors such as magnetic latitude, proximity to the geomagnetic equator, and local terrain features all influence NVIS propagation characteristics. It is heavily used for local and regional communications, including in mountainous and jungle regions where other forms of radio communications are impossible. It is extensively used for emergency operations and for modern day military radio communications in adverse terrain.

NVIS Antenna Design and Configuration

Low Horizontal Dipole Antennas for NVIS

NVIS antennas are usually horizontally polarized and set up low (about 0.1–0.25 wavelengths off the ground), so most of the energy goes up. An NVIS antenna is simply a horizontally polarized at a height ranging from 1/20th to 1/4 wavelength above the ground.

The horizontal dipole is the most common and effective antenna for NVIS operation. The simplest NVIS antenna is a 1/2 wave dipole with its peak at about 15 feet and each leg at about 7 feet. A 40 meter NVIS antenna would be about 66 feet long, or 33 feet for each leg.

Dipoles only exhibit directionality once they reach 1/2 wavelengths above ground. However, NVIS antennas are located from 1/4 to 1/10 wavelength above ground. Vertical RF energy radiated at a low enough frequency is reflected back to earth at all angles. The effect is similar to taking your garden hose with a fog nozzle and pointing it straight upwards. The water coming back down gives you an omni-directional pattern without dead spots. It's a continuous circular radiation pattern coming back down.

Optimal Antenna Height Above Ground (0.05 to 0.25 Wavelengths)

The optimum height for NVIS antennas is something over 1/8th wavelength, or about 30-35 feet on 75 or 80 meters. The OPTIMUM NVIS antenna height for 80 through 40 meters is about 30-feet!!

The optimum height for nvis, considering ground losses versus elevation pattern, is about 40-55 feet. Notice that the peak of the gain curves are very broad, somewhere between 0.13 wavelengths (10m, 35 feet) and 0.25 wavelengths (20m, 65 feet) depending on soil characteristics. However, for any of the soil conditions, a dipole at 0.07 wavelengths (6m or 20 feet), as shown by the red arrow, is within about 3 dB of the peak, and, especially for portable operation, this may be more practical height.

Signal level decreases rapidly as height is lower than about .05 wavelength, or approximately 14-feet. At .06 wavelengths high (16 feet on 80M, 8 ft on 40M) field strength is down 3dB. This is about 50% TX signal reduction.

At .04 wavelength (10 feet high on 80M or 5 ft on 40M) field strength is down 5dB. This is about 2/3 reduction in TX signal level! Please, let's not give silly advice like 5-foot high antennas are good ideas for emergency communications or NVIS operation. Very low antennas produce very low signal levels at any distance when compared to antennas of modest height.

My recommendation for a maximum height for an NVIS antenna is about 0.3 wavelengths. So, for 80m this would be 24 m (80 feet: conveniently, in Imperial units the maximum height in feet is equal to the wavelength in meters.) At this height, overhead gain is down about 1 dB from the maximum.