Propagation phenomena

NVIS Propagation: Near Vertical Incidence Skywave Explained

NVIS (Near Vertical Incidence Skywave) is a propagation technique that sends a signal nearly straight up so it bounces off the F2 layer and lands within roughly 30 to 500 kilometers of the transmitter. NVIS exists to fill the skip zone, the donut-shaped dead zone where ground wave has died out and the first long-distance hop has not yet returned to earth. NVIS is the workhorse of regional nets, emergency communications, and any operating where you need to reach stations a few hundred kilometers away.

Why it matters for HF operating

Stations 100 to 500 km away are surprisingly hard to reach on HF without NVIS. They are too far for ground wave on most bands and too close for the first hop on a low takeoff angle. NVIS solves this with a high takeoff angle, often near 90 degrees, that puts the entire footprint of the first hop on top of you. On the right band at the right time of day, NVIS produces strong, fade-free signals over a wide regional area with modest power and a low antenna.

Key values to know

NVIS works when foF2 is above your operating frequency. The two bands that matter most are 80 meters and 40 meters:

Band NVIS daytime NVIS nighttime
80m (3.5 MHz) Open whenever foF2 > 3.5 MHz, typical Open most nights
40m (7 MHz) Open in midday with active solar conditions Closed most nights
  • Antenna. A horizontal dipole 0.1 to 0.25 wavelengths above ground produces a strong vertical lobe and is the classic NVIS antenna.
  • Power. NVIS is efficient. 50 to 100 watts produces strong regional coverage on 80 meters most nights.
  • Geomagnetic storms. NVIS is more resilient than long-haul DX during storms because the path is short and stays mid-latitude.

Common misconceptions

NVIS is not the same as ground wave. Ground wave follows the curvature of earth and dies off within 50 to 100 km on most HF bands. NVIS goes up and back down. The footprint of ground wave is a small disc around the antenna; the footprint of NVIS is a large disc that includes the small ground-wave disc. The two add together when conditions are favorable.

  • foF2: the critical frequency that gates NVIS bands
  • F2 layer: the reflective layer NVIS uses
  • Skip zone: the dead zone NVIS fills
  • Ground wave: the propagation mode NVIS complements
  • D layer: the daytime absorber that limits 40-meter NVIS

SkyWave’s NVIS sandwich on the Go screen shows foF2 and the D-layer floor against each amateur band, so you can see at a glance which bands are positioned for NVIS work. See it in the app →