NVIS vs. Skip Zone
Why you can sometimes talk to Europe but not the next town on HF, and how NVIS fills that gap for regional and emergency communication.
The skip zone
On HF, a signal that leaves your antenna at a low angle travels a long way before the ionosphere bends it back down. Between the end of your ground wave (a few kilometres) and where the sky wave first lands, there’s a gap. Stations in that gap can’t hear you, even though stations much farther away can. That gap is the skip zone, and it gets bigger as frequency goes up.
NVIS: shooting straight up
Near Vertical Incidence Skywave (NVIS) does the opposite: it sends the signal almost straight up, so it comes back down like an umbrella, covering everything from 0 to about 400 km with no skip zone. It’s how the military, emergency services and hams cover a province without repeaters.
What makes NVIS work
- The right frequency: it has to be below the critical frequency (foF2), the highest frequency the ionosphere will return straight down. In practice that means 80 m and 60 m most of the time, and 40 m by day when conditions are good.
- A low antenna: a dipole only 2 to 6 m off the ground, about a tenth to a quarter wavelength, sends most of its energy upward. This is the one time a low antenna is the better antenna.
- Daytime vs. night: the critical frequency drops at night, so 40 m that worked at noon may skip right over nearby stations after dark. Move to 80 m.
When to use it
- Regional nets covering southern Alberta.
- Emergency communications when repeaters and cell towers are down.
- Reaching places in coulees or behind hills that VHF can’t.
- Portable operation, where a low dipole is easy to put up.
Where to go next
Last updated October 2026.