↑ What is Greyline propagation?

By Doron Tal, 4X4XM

The "grey line" refers to the Earth's twilight zone, a boundary between day and night, and is used in radio communication to describe specific propagation conditions.

Greyline illustration
The height of the F and D layers
is exaggerated in comparison to Earth dimensions.
  An example of a greyline mapref.

Greyline map

The twilight zone is a "sweet spot" for HF radio transmission, allowing signals to travel further with less attenuation due to the reflective layers E and F, while the absorbing layer (D) is inactive.

Greyline propagation can be particularly advantageous for long-distance communication, as it allows radio waves to travel further than they might otherwise. However, it is a relatively short-lived phenomenon, lasting only a few hours each day as the Earth rotates and the grey line moves. In addition, the specific conditions of the ionosphere can affect the behavior of radio waves during greyline propagation, and not all frequencies may work equally well during this time.

Amateur radio operators often take advantage of greyline propagation to make long-distance contacts with other operators around the world. Some operators use specialized software or tools to help identify when the grey line is passing over their location, and to determine the best frequencies and modes of propagation to use during this time. Overall, greyline propagation is an interesting and useful phenomenon that can provide unique opportunities for long-distance radio communication.

Back to the main page: why is radio propagation better along the greyline, as well as in-depth tutorial about the propagation of sky waves. See also an index for HF Radio Propagarion.


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The Understanding HF Propagation Project provides radio amateurs with a detailed overview and tutorials on several aspects of HF propagation.

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