NVIS
| NVIS | |
|---|---|
| Stands for | Near vertical incidence skywave |
| Coverage | Continuous, from nearby out to a few hundred miles |
| Typical bands | 40 m by day, 80 m at night, 60 m in between |
| Frequency limit | Below the F2-layer critical frequency |
| Antennas | |
| Type | Horizontal dipole or inverted V |
| Height | About 0.1–0.25 wavelength above ground |
| On 40 m | About 14–34 ft (4–10 m) |
| On 80 m | About 26–66 ft (8–20 m) |
| Uses | |
| Main uses | Regional emergency nets, military tactical links, portable operation |
NVIS, short for near vertical incidence skywave, is a way of using HF for dependable regional contacts. Instead of aiming the signal at the horizon for long-distance skip, a low horizontal antenna sends it almost straight up, and the ionosphere returns it almost straight down, covering everything from nearby to a few hundred miles away with no gaps. NVIS fills the distances that are hardest to reach any other way: too far for VHF and repeaters, too close for ordinary HF skip.
How it works
An ordinary HF signal leaves the antenna at a low angle and returns to Earth hundreds or thousands of miles away. Between the end of the ground wave and the first landing of the skywave lies the skip zone, where the station cannot be heard. NVIS removes the skip zone by sending energy at steep angles, so that it comes back down over the transmitter and the surrounding region.
A signal sent straight up is returned only if its frequency is below the critical frequency of the F2 layer, the highest frequency the layer reflects from directly overhead. Above it, the signal escapes into space. That limit decides the band:
- 40 meters (7 MHz) works by day when the critical frequency is high, more often in the active years of the solar cycle.
- 60 meters (5 MHz) is a useful middle choice in the morning and evening, and on days when 40 meters passes through the ionosphere.
- 80 meters (3.5–4 MHz) is the usual choice at night, although near solar minimum the critical frequency late at night can fall below even 80 meters.
The best band is generally the highest one that stays reliably below the critical frequency. D-layer absorption, which is heavier at lower frequencies, weakens daytime signals on 80 meters, and a near-vertical signal passes through the D layer twice. Near-real-time maps of the critical frequency, built from ionosonde measurements, take much of the guesswork out of the choice.
Antennas
The antenna does the work. A horizontal dipole about 0.1 to 0.25 wavelength above ground sends most of its energy upward. On 40 meters that is roughly 14 to 34 feet; on 80 meters, roughly 26 to 66 feet. An inverted V on a single mast works nearly as well and is the common field choice. Much lower antennas still work, but more of the power is lost in the ground. Some operators lay a wire a little longer than the dipole on the ground beneath it as a reflector, for a modest improvement.
A vertical makes a poor NVIS antenna, because it radiates very little straight up. Because a low horizontal antenna is relatively deaf to low-angle signals from far away, NVIS stations also tend to hear less noise and interference from distant stations, a real advantage on a crowded band.
Uses
The technique has military roots. Armies need reliable links across mountains and jungle, where VHF needs line of sight and HF ground wave fades quickly over rough terrain, and NVIS was studied and used extensively by US forces in Southeast Asia during the Vietnam War era. Tactical HF stations still tie vehicle whips back over the vehicle to turn them into makeshift NVIS antennas.
For hams, NVIS is the backbone of regional emergency communications. State and section nets, links between county emergency operations centers, and HF Winlink stations often rely on it when repeaters and the internet are down. Signals arrive from above, so mountains and valleys matter little, and a portable station with a battery and a dipole on a fiberglass mast can cover most of a state. The same setup suits Parks on the Air activations and Field Day stations working nearby states.