What use is an F-call?

Much of what Amateur Radio is about relates to the planet we live on. I realise that you might think that it's about radios, antennas, contacts and logging, but the vast majority of our hobby relates directly to the Ionosphere, the Sun, the Earth and all manner of other natural influences.

One of these influences happens twice a day, Sunrise and Sunset. The transition from light to dark and from dark to light has a period during which there is twilight. This period in amateur radio is called the Grey Line, because if you were to map it on a map of the earth, there's a line that travels all the way around the globe where the separation between dark and light exists.

A funny phenomenon occurs along this line. Propagation is very efficient along its path.

You might recall that there are several layers of the Ionosphere. Closest to us is the D-layer, then up from that the E and F layer. You might also remember seeing satellites in the early evening, they are still in sunlight while you're in darkness. If you've flown a plane near dusk, it might have been light while you're up in the air, but dark by the time you land.

Light from the sun hits different layers in the Ionosphere at different times. The D-layer doesn't get hit when it's dusk, but the E and F layers still are, they're light, while the D-layer is dark.

This has the effect of allowing a 10m or 15m signal to travel through the D-layer without loosing much strength, but bouncing along happily following the E and F layer around the earth.

This means that if you're on-air during this time, you'll suddenly hear stations from all over the place. This phenomenon might last no more than 10 minutes, but it's absolutely magical if you are there to hear it, and it happens twice a day!

From a technical perspective, the evening grey-line works around 10m and 15m, the morning grey-line around 15m, 20m and sometimes on 40m and 80m.

So turn your radio on before dusk or dawn and sit around with a warm beverage having a listen - or a coldie if the temperature is better suited to DX with a beer.

I'm Onno VK6FLAB