Antenna Project
NVIS: Regional HF Without the Skip Zone
A compact 40/80 m antenna built to put signals straight up and back down, covering the region a VHF repeater can't reach and normal HF skip jumps over.
What NVIS is
Near Vertical Incidence Skywave sends HF energy almost straight up. When the frequency is below the ionosphere's critical frequency (foF2), the signal reflects back down over a wide circle around the station, roughly 0 to 300+ miles, with no skip zone in the middle.
Short range, drops off fast over terrain. Good across town, poor beyond that.
Low takeoff angles land hundreds of miles away and leave a dead zone close in.
High takeoff angles fill in that close-in zone with steady, even coverage.
Picking the band: foF2
NVIS only works below foF2, the highest frequency the F2 layer will reflect straight back down. It rises during the day and with the solar cycle, and falls at night.
| Conditions | Typical band |
|---|---|
| Daytime, good conditions | 40 m (7 MHz) |
| Evening / transition | 60 m (5 MHz) |
| Night, low solar activity | 80 m (3.5–4 MHz) |
Check a current ionosonde or foF2 map before picking a band. Operating a little below foF2 gives the most reliable path.
The antenna: a loaded inverted V
The goal was 40 and 80 m NVIS from a single, quick-to-deploy support.
- Support: one 15 ft fiberglass mast.
- 40 m: full-size inverted V legs.
- 80 m: center-loaded legs crossed with the 40 m pair, so both bands share the same feed and mast.
- Why low: at 15 ft the antenna is well under a quarter wave on both bands, which pushes the pattern straight up, exactly what NVIS wants.
Model vs. measurement
The design was modeled in EZNEC first, then checked with an antenna analyzer after it was built.
The coil costs well under 1 dB, a small price for shortening the 80 m legs enough to fit the same mast. The model's predicted coverage rings show the even, close-in footprint NVIS is known for.
On the air: A/B against a vertical
To see the difference in practice, I ran FT8 on 40 m and compared reports from the inverted V against a 6-BTV vertical. The vertical's low-angle pattern favors distant stations; the inverted V favors the regional ring the vertical tends to miss.
Takeaway: for regional HF, a low horizontal antenna beats a vertical close in. For DX, the vertical wins. Having both covers the whole range.
Try it yourself
- Keep the antenna low, around 0.1–0.25 wavelength.
- Pick your band from current foF2, not habit.
- Model first. It's free and tells you what to expect before you cut wire.
- Compare against another antenna on FT8 or WSPR to see the difference with real data.