NVIS — Near Vertical Incidence Skywave for EmComm
Near Vertical Incidence Skywave (NVIS) is an HF propagation technique that fills the dead zone between ground wave (0–80km) and conventional skywave skip (400km+). By directing radio energy almost straight up at the ionosphere, NVIS achieves reliable communication within roughly 50–500 kilometres — precisely the regional coverage needed for state and regional emergency communications. On 40 and 80 metres with the right antenna, NVIS works consistently and requires no infrastructure other than the radios themselves.
The propagation mechanism
Standard HF skywave sends signals at low angles, reflecting off the ionosphere hundreds of kilometres away and creating a skip zone close to the transmitter. NVIS eliminates this by radiating at very high angles — close to 90 degrees straight up. The signal hits the F-layer ionosphere almost directly overhead and comes back down within a relatively small radius. This covers the 50–500km range that VHF cannot reach and conventional HF skips over.
For NVIS on 80m (3.5–4 MHz) the ionospheric critical frequency is virtually always satisfied. For 40m (7 MHz) it is satisfied most of the time during daylight hours. At night on 40m, conventional skip often takes over, making 80m the more reliable nighttime NVIS band.
Why NVIS matters for EmComm
In a regional disaster — major flooding, large-scale power outage — VHF repeaters may be down, internet unavailable, and cell networks overwhelmed. HF conventional skywave is too long-range for intra-state coordination. NVIS on 40 and 80 metres fills this gap, providing reliable radio-to-radio communication across an entire state or multi-county region using nothing more than simple wire antennas and a 100W HF transceiver.
NVIS is the propagation mode behind most state-level ARES nets. When your state ARES net checks in on 80m, they are using NVIS to reach stations across hundreds of kilometres simultaneously. It is a core HF EmComm skill.
Antenna height and design
The key to NVIS is keeping the antenna low — 0.05 to 0.15 wavelengths above ground. For 80m, this is roughly 4–12 metres. For 40m, 2–6 metres. A horizontal dipole at this height peaks radiation nearly straight up, which is exactly what NVIS requires. Raising the antenna higher shifts the radiation angle down and reduces NVIS effectiveness — the opposite of what you want for DX.
Practical NVIS antennas include a horizontal dipole for 40 or 80m hung between two supports at 3–5 metre height, an inverted-V dipole with the apex at 6–8 metres, or an EFHW deployed horizontally at low height. A 40m or 80m dipole can be deployed from a go-kit in 15 minutes using fibreglass mast sections and paracord.
Frequency selection
80 metres (3.5–4.0 MHz) is the most reliable NVIS band and works day and night. The 75m phone portion (3.8–4.0 MHz) is where most voice NVIS EmComm traffic happens, including many state ARES nets. 40 metres (7.0–7.3 MHz) provides NVIS during daylight hours with somewhat longer maximum range than 80m.
For a regional activation, start on 40m during daylight and switch to 80m at night. Having both bands available in your go-kit is the right approach for operations spanning multiple hours or days.
What power do I need for NVIS?
100W is the standard for EmComm NVIS operations and provides reliable regional communications. QRP levels (5–10W) can work in good conditions but reduce reliability margins. For a go-kit intended for real EmComm deployment, 100W capability is the target.
Can NVIS work in mountainous terrain?
NVIS is actually better suited to mountainous terrain than VHF/UHF or low-angle HF, because the signal goes up and comes back down — terrain features between stations are largely irrelevant. A station in a deep mountain valley can communicate via NVIS with stations hundreds of kilometres away as long as both have clear sky overhead.
How do I know if NVIS is working on a given day?
The easiest check is to listen to a regional net or beacon on 40m or 80m. If you can hear stations 200–400km away clearly, NVIS is working. In practice, 80m NVIS is reliable enough that experienced operators simply use it and adjust if issues arise.
What is the best NVIS antenna for a portable go-kit?
A horizontal dipole for 40m or 80m deployed at 3–5 metres height is the simplest and most reliable NVIS antenna. An EFHW deployed horizontally at the same height is also excellent and easier to deploy with a single support point. Either can be set up from a go-kit in under 20 minutes using lightweight fibreglass mast sections.