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Greyline propagation (also spelled "grayline" in American English) is an enhanced form of HF propagation that occurs along the Earth's terminator — the boundary between the sunlit and dark halves of the planet. During the brief periods of dawn and dusk, this twilight zone creates a natural "waveguide" that can carry HF signals over remarkable distances with surprisingly strong signal levels, often opening paths that are difficult or impossible at other times of day.
For DX enthusiasts, the greyline is one of the most valuable propagation tools available. Learning when and how to exploit it can unlock contacts to parts of the world that seem unreachable under normal conditions.
The greyline is the band of twilight that circles the Earth at all times, separating day from night. Because the Earth's axis is tilted relative to its orbit around the Sun, the greyline is not a simple north-south line — it's a great circle that shifts its angle with the seasons. At the equinoxes, it runs roughly north-south. In the Northern Hemisphere's summer, it tilts so that high northern latitudes remain sunlit while high southern latitudes are in darkness, and vice versa in winter.
The greyline is not a sharp line but a zone, roughly 200–400 km (125–250 miles) wide, where the Sun is near the horizon. Within this zone, the ionosphere is in a transitional state: the D layer (which absorbs HF signals) is either forming (at sunrise) or decaying (at sunset), while the F layer still has enough ionization to refract signals.
The greyline creates favourable propagation conditions through a combination of effects:
Reduced D-layer absorption: The D layer is the main absorber of HF signals on the lower bands (160 m, 80 m, 40 m). During full daylight, the D layer is strong and absorbs much of the signal before it reaches the reflecting F layer. At twilight, the D layer is thin or absent — either not yet formed (dawn) or already decaying (dusk). This dramatically reduces absorption losses.
F-layer still active: While the D layer fades quickly at sunset, the F layer persists much longer due to slow recombination rates at its higher altitude. At dawn, the F layer begins to strengthen before the D layer has fully formed. This window — F layer present but D layer weak — is what makes greyline propagation possible.
Waveguide effect: Signals traveling along the greyline path stay within the twilight zone for extended distances. Because the D layer is weak along the entire path, the signal experiences less total absorption than it would on a path that crosses into full daylight. Some propagation researchers describe this as a "duct" or waveguide effect along the terminator.
Simultaneous twilight at both ends: Greyline propagation is strongest when both stations are simultaneously experiencing dawn or dusk. This means the D layer is weak at both the transmitting and receiving locations, as well as along the path between them.
The greyline circles the Earth continuously, so at any given moment, some locations are at dawn and others are at dusk. The key insight is that locations on exactly opposite sides of the greyline — one at dawn and one at dusk — are the most likely to experience greyline propagation between them.
To find greyline paths from your location:
Several propagation tools display the greyline on a world map in real time. Some of the most popular include:
These maps let you see at a glance which DX locations are along the greyline from your QTH (location) at any given time.
Because the greyline angle changes with the seasons, the DX paths it enhances also change:
Equinoxes (March and September): The greyline runs nearly north-south. This creates opportunities for enhanced propagation between stations at similar latitudes on opposite sides of the globe, as well as north-south paths. Many operators report their best greyline results during equinox periods.
Northern Hemisphere summer: The greyline tilts, running from northwest to southeast. This can open unusual paths, such as from Europe to the South Pacific or from North America to the Indian Ocean.
Northern Hemisphere winter: The greyline tilts the opposite way, from northeast to southwest. Paths from North America to southern Africa and from Europe to Oceania may benefit.
Greyline propagation is most significant on the lower HF bands where D-layer absorption is the dominant limitation:
160 metres (1.8 MHz): The greyline effect is most dramatic here. 160 m is almost unusable for DX during daylight due to extreme D-layer absorption, but during greyline periods, brief and sometimes spectacular DX openings occur. Working rare DX on 160 m during the greyline is one of the great achievements in amateur radio.
80 metres (3.5 MHz): Also strongly enhanced by greyline conditions. DX paths that are marginal at best during full daylight or full darkness can produce strong signals during the greyline window.
40 metres (7 MHz): The greyline effect is noticeable but less dramatic than on 80 and 160, because 40 m suffers less from D-layer absorption in the first place. Still, greyline periods often produce the day's best DX on this band.
20 metres and above (14+ MHz): The greyline effect is less significant because these bands are not limited primarily by D-layer absorption. However, some operators report enhanced signals on 20 m at greyline, possibly due to the transitional ionospheric conditions focusing energy along the terminator.
The greyline window is short — typically 15 to 45 minutes around sunrise and sunset. You need to be ready before it arrives:
On 160 m and 80 m, listen for DX stations calling CQ around the popular DX windows:
On 40 m, the DX window around 7.000–7.025 MHz (CW) and 7.160–7.200 MHz (SSB, Region 2) is where greyline-enhanced signals are most often spotted.
Because greyline openings are brief, efficiency matters:
Greyline propagation is sometimes confused with long-path propagation, but they are different phenomena. Long-path signals travel the "long way around" the globe (more than 20,000 km), while greyline enhancement occurs along the terminator regardless of path length.
However, the two can combine: a long-path signal that follows the greyline for much of its journey can be remarkably enhanced, producing contacts that seem almost impossible given the distance involved.