What Is Grey Line Propagation?
Propagation · Answered
What Is Grey Line Propagation?
Short answer: the grey line is the band of twilight that circles the Earth — sunrise along one edge, sunset along the other. For a short window along it, the low HF bands open over long distances, because the layer that absorbs 40, 80, and 160 meters by day has faded while the layer that bends signals back to Earth is still in place.
Why it works
In daylight, the sun builds a low layer of the ionosphere, the D layer, that soaks up low-frequency signals before they can travel far. After dark it fades quickly. Higher up, the F layer — the one that bends HF signals back down — lasts much longer. Along the terminator, for a few minutes, you get the best of both: little absorption, and a reflecting layer still standing. Stations on opposite sides of the world, one at sunrise and one at sunset, can share that path.
How to work it
Pull up any grey-line map and find where your sunrise or sunset meets someone else's. Be on 40 or 80 meters about fifteen minutes before your local sunrise, and again around sunset, and tune slowly. On 160, the window is shorter and more precious. The stations you hear are riding the same twilight you are.
What the antenna has to do
Grey-line openings favor a low radiation angle and they reward aperture — more radiator on the low bands is more signal. John Devoldere, ON4UN, built his classic book on low-band DXing around exactly that. It is why a vertical is the natural low-band antenna, and why owners add height or a whip when the long nights come.
Go Deeper
The Twilight Band That Carries DX — the full Signal Lab
Why you can work Europe but not the next state
Why your antenna hates 80 meters
What height should I buy?
Why we named it Greyline: it is the hour that rewards a good vertical most. — Jon Kimball, KL2A · 435-200-4902