Gray Line Propagation

What It Is

The gray line is the band of twilight that circles the Earth, dividing the half in daylight from the half in darkness. At any moment, somewhere along it the sun is rising and somewhere else it is setting. You pass through it twice a day, at your own sunrise and sunset.

Scientists call the dividing line the terminator. Hams say gray line because the twilight zone around it is where the interesting radio happens.

Why It Helps Your Signal

In daylight, the sun builds up the D layer, the lowest layer of the ionosphere. The D layer does not bounce signals back to Earth; it soaks them up, and it soaks up the lower frequencies most. That is why 160, 80 and 40 meters are short-range bands at midday.

After sunset the D layer fades within minutes to an hour, while the higher F layer, the one that bounces signals over long distances, stays charged for hours. Before sunrise the same thing happens in reverse: the F layer is still there from the night before, and the D layer has not built up yet.

So along the gray line, signals get the reflecting layer without the absorbing one. A path that runs along the twilight band, with both stations in or near it, can carry low-band signals remarkably far with very little power.

When to Listen

Start about half an hour before your local sunrise or sunset and keep going until about half an hour after. The openings are often short, sometimes only a few minutes to a particular place, so be on the band and ready before they start.

The best chances come when the other station is also at its sunrise or sunset. On a winter morning in the eastern US, for example, the gray line can link you with stations whose evening it is on the far side of the world.

It works best on 160, 80 and 40 meters, and often helps 30 and 20 meters as well. Winter, with its long nights and quiet static, is the prime season for the low bands.

Why It Changes Through the Year

The gray line only runs straight north and south around the March and September equinoxes. The rest of the year it is tilted, by up to about 23 degrees at the June and December solstices, because the Earth’s axis is tilted. So the parts of the world that share your sunrise or sunset change from month to month, and a path that is wide open in December may not line up at all in June.

A gray line map, which shows the twilight band live on a world map, makes this easy to see at a glance: find your location on the line, and look along it to see who is sharing your twilight. OneShack has one: see the Gray Line Map under Tools.

Using It on the Air

Know your sunrise and sunset times for today; any weather app will tell you. Tune across 40 and 80 meters in the half hour around them and listen for stations that are usually out of reach. On FT8, watch the far-away callsigns that appear on your waterfall for a few minutes and then vanish.

If you have a beam or a directional antenna, the path along the gray line is often not the direct heading you would expect. It can even be the long way around the world.