One of the most fascinating propagational irregularities is the effect of the grey line on HF radio comms.
Ham, CB and 11m radio ops, as well as shortwave listeners, can optimize long distance comms to various areas of the world by monitoring this enigmatic phenomenon as it moves around the globe and passes over their QTH.
What is it?
For those who don’t know, the grey line (also referred to as the terminator or twilight zone) is a shifting line that separates the illuminated day side and the dark night side of a planetary body.
Another way of explaining it is that the grey line is a band, naked to the human eye, around the Earth that splits daylight from darkness. Here, the grey line region is the part of the world that is neither in darkness nor in daylight.
Many DA-RC members will concur that prop along the grey line is very efficient. In decent conditions, radio waves can travel along the terminator to antipodal points. One major reason for this is that the D layer, which absorbs HF signals, disappears rapidly on the sunset side of the grey line, and it has not yet built upon the sunrise side. This process is known as skywave propagation.
Because two grey-line stripes move constantly around the earth, the propagational alterations are brief though — usually only about 30 minutes to an hour or so in length. This is your window of opportunity! At this time, there is almost no signal attenuation, but the MUF is still very high, so long-distance skip is still possible. However, when the sun quits illuminating the E and F layers, the MUF can drop dramatically … sometimes with only a few minutes of warning, sometimes between heartbeats. So when you establish a contact, get the QSL info fast!
Most DA-RC members actively scour the bands at sunrise or sunset, taking advantage of these times when the ionosphere is highly efficient, often pulling in some amazing signals when the hours before them have produced little.
Working the grey line, for example, was definitely the most profitable of times for the 135DA0 DXpedition team in the Solomon Islands when Tom, Shane and Darren worked most stations from Europe at this time.
Of course, grey-line prop is probably of far less appeal to those who use the radio bands in conjunction with their occupation. If you are one of these guys, chances are that grey-line propagation will be either a curiosity or a nuisance, as more stations that could cause interference to your signal become audible.
For DX Hunters and DXpeditioners though, the grey line represents a terrific twice daily opening to work DX.
Grey Line Resource
To monitor the grey line in your DXCC and cash in on some awesome DX opportunities in the evenings and mornings, please check out the world map at http://dx.qsl.net/propagation/greyline.html . This map shows the current position of the grey line terminator and refreshes every 5 minutes.
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