What is the Greyline?
The greyline (also spelled "grey line" or "gray line") is the narrow band on the Earth's surface where day transitions to night — the terminator. At sunrise and sunset, the ionosphere is in a transitional state that creates a unique propagation path along the terminator itself.
Signals traveling along the greyline experience less D-layer absorption (because the D-layer is only partially formed) while still having F-layer support for long-distance propagation. This creates a brief window — typically 20-40 minutes — where signals on 160m and 80m can travel extraordinary distances with relatively low power.
Why It Works
- 1. D-layer is weak. During full daylight, the D-layer absorbs low-frequency signals (160m, 80m). At the terminator, the D-layer is forming (sunrise) or decaying (sunset), so absorption is minimal.
- 2. F-layer is still active. The F-layer, which reflects HF signals back to Earth, doesn't disappear instantly at sunset. It persists for hours. So you get F-layer reflection without D-layer absorption.
- 3. The path follows the terminator. Signals don't just go straight between two points — they follow the greyline path where conditions are favorable. This means you can work stations that are also experiencing sunrise or sunset, even if they're on the opposite side of the globe.
Best Bands for Greyline
160m The premier greyline band. Most 160m DX is worked on the greyline. QRP contacts across oceans are possible.
80m Excellent greyline band. Wider window than 160m due to less D-layer sensitivity.
40m Benefits from greyline but less dramatically. The band is often open anyway.
20m+ Higher bands don't benefit significantly — they rely on F-layer propagation which isn't terminator-dependent.
How to Use It
- 1. Check the greyline map. Our Propagation Dashboard shows the day/night terminator in real-time. Look for DX targets that are experiencing sunrise or sunset at the same time as you.
- 2. Know your sunrise/sunset. Check the sunrise/sunset table on the propagation page for common DX targets.
- 3. Be on frequency BEFORE the window opens. The greyline window is brief. Be tuned and ready 15 minutes before your sunrise or sunset.
- 4. Listen on both long path and short path. Greyline signals can arrive from unexpected directions. Rotate your beam (if you have one) or use an omnidirectional antenna.
- 5. Use CW or FT8. The greyline window is short and signals can be weak. CW and FT8 punch through better than SSB.