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An introduction to gray-line DXing

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Understanding the practical aspects of this exciting form of long-distance propagation can help you make the most of your DXing hours.

The transitions from darkness to daylight and daylight to darkness cause drastic changes in the ionosphere, affecting all high-frequency (HF) propagation. All-band DXers need to understand these changes to improve their chances of long-distance communication, especially over long or difficult paths.

Several factors affect HF propagation: path, frequency, solar activity and time. Of all of these factors, frequency and time are the only ones over which we have much control. Understanding the sun's effects helps DXers select the best frequency for a given time, or perhaps more importantly, the best time for a given frequency, for each selected path and level of sunspot activity.

Time is a complex factor in propagation, encompassing time in the 11-year sunspot cycle; time of year (season and tilt of the earth); and time of day (daylight, darkness or twilight).

From October through March, the north pole is in partial to total darkness, permitting over-the-pole, northern-hemisphere low- and high-frequency propagation between opposite sides of the earth. This is also true for the south pole and the southern hemisphere from March through October. Summertime propagation is totally different from the rest of the year, with HF openings and closings occurring much later in the day. The polar paths are primarily restricted to 40 through 15 meters in the summer, while transequatorial paths predominate on the low bands and on 10 meters.

Generally, the low bands (1.8 through 10 MHz) exhibit best long-distance propagation from just before sunset, through darkness, to just after sunrise. The high bands (14 through 28 MHz) tend to open near sunrise, stay open throughout the day, and close after darkness. These openings and closings depend, of course, on solar acti vity, frequency and season. The ionosphere's behavior at sunrise and sunset is highly dependent on frequency. Next we'll examine this behavior in detail for the northern-hemisphere winter months of October through March.

The sunrise/sunset line, also known as the terminator, gray line or twilight zone, is a circle about the earth whose position depends on the time of year (season) and time of day. Maximum usable frequencies (MUFs) are highest on the illuminated face of the earth. So is the absorption caused by the effects of the energized D region of the ionosphere. Propagation losses are lowest near the maximum usable frequency.

Maximum usable frequencies drop on the dark side of the earth, to a minimum value of approximately one third the daytime MUF, just before sunrise. Absorption also quickly subsides as the D region is no longer illuminated by the sun.

Propagation along the gray line is especially interesting. MUFs are rising rapidly on the sunrise side and are still high on the sunset side of the earth. The D region has not yet been energized on the sunrise side, and is rapidly dissipating on the sunset side, resulting in low absorption. The net result: For a period ranging from a few minutes at low and high frequencies (160 and 10 meters), to one or two hours at intermediate frequencies (20 meters), with suitable ionospheric conditions, stations in the twilight zone can communicate with stations at any other location within the twilight zone on any HF band!

To better understand these effects, let's examine what happens on a band-by-band basis as the sun begins to rise in the northern hemisphere (eastern US) during the winter months. Before sunrise, while it's still totally dark, the MUF is at its lowest level of the day. This is usually the least productive time of day unless 20 meters is already open to Europe, which happens when sunspot activity is high and geomagnetic conditions are quiet. As the first rays of light approach overhead, the MUF begins to rise in the direction of the sun (southeast) and the high bands will open first in that direction before swinging northward toward Europe.

As the sun continues to rise, it illuminates the higher altitudes first and the lower altitudes later, forming a westward-moving wedge of light that cuts into the darkness. This sloping wedge has an especially beneficial effect on low-frequency propagation to the west. It acts as a reflector, similar to a parabolic reflector, focusing energy (signals coming from the west) along the terminator. This focusing effect is seen as a sudden (and often large) increase in signal strength from stations to the west that continues to build, peaks, then fades back down as the D-region absorption increases with illumination. The lower the frequency, the earlier the peak and the shorter the duration.

The 160-meter band has the shortest sunrise and sunset peaks of all the amateur bands. These may vary from only a few minutes to 20 or 30 minutes maximum. On October 25, 1987, at 1148 UTC, I received the surprise of my life as a voice rose from the noise to S5 or S6 in less than a minute and was completely gone five minutes later. The call was from VU4GDG in the Andaman Islands! (See Fig 1.)

Fig 1
Fig 1-October 25, 1987, 1148 UTC, was a magical time for N4KG: VU4GDG, in the Andaman Islands, briefly peaked at S5 to S6 on 160 meters! Thanks to the gray-line enhancement. This graphic shows the daylight, darkness and twilight areas at that time. Figs 1 and 2 were generated using the Geoclock computer programs.

The 160-meter band is also the most unpredictable of all the amateur bands. There may be no peak whatsoever at either sunset or sunrise, yet tremendous propagation may arise a few hours after sunset and drop out after an hour or two. I have also heard signals from Europe and Africa peak and disappear a full hour before their sunrise. As I said, 160 meters is the least predictable of the amateur bands.

The 80-meter band exhibits a consistent pattern of signal enhancement (from the west) beginning with first light, building to a maximum at official sunrise (90° solar zenith angle) which may last from 10 to 20 minutes, and slowly dropping from D region absorption as the sun rises. If you're using a simple antenna, 30 minutes past sunrise usually marks the end of the opening.

Long-path openings even occur on 80 meters, with shorter peaks than for signals from the west, typically exhibiting a pronounced peak for 5 to 7 minutes and a total duration of as much as 15 to 20 minutes.' I have observed long-path openings to Asia as far west as the sunset terminator. On January 31, 1989, UJ8JMM heard my long-path signals for 3 minutes starting at 1232 UTC. My sunrise and his sunset were coincident at 1245 UTC on that date (see Fig 2).

Fig 2
Fig 2-When sunrise and sunset coincide for two stations along the gray line, the ionosphere sometimes propagates signals between these stations on all HF bands at once. On January 31, 1989, UJ8JMM and N4KG hooked up over the long path on 80 meters.

Such long-path openings are predominantly observed during high sunspot activity and quiet conditions. Some of my better 80-meter morning long-path DX includes HS, UA9 (CQ Zones 17 and 18), UI, UJ, UL, UM, VU, 4S7 and 8Q7.

The 40-meter band is the premier low-frequency band, with long-path openings to central Asia from October through March that are more reliable than 20 meters! Contrary to some published propagation material, 40-meter propagation does not peak to the west before sunrise but about 30 minutes after sunrise. In fact, propagation just before sunrise can be quite poor. Peak propagation spans about 30 minutes and drops off slowly for an hour or more. Long-path fans benefit from a convenient beacon located in Kazakh (UL7) that transmits the letter v at 10 WPM at 7002 kHz. This signal is also audible short path in the evenings around 0200 to 0300 UTC.

For polar paths, the 10-meter band is the most difficult of the high-frequency bands, requiring quite a high MUF and quiet solar conditions. In the eastern US, for example, this band will open to the southeast from just before to just after sunrise, depending on solar flux levels, and swing northward into Europe and Western Asia in another 15 to 20 minutes. From the eastern half of the US, the best times to work central Asia on this band are usually within the first two hours following sunrise. In the spring and fall, there is often a long-path (southeast) opening to the Far East and the South China Sea region in the same time period.

In the evenings, as the sun begins to set, the MUF begins to drop and the D region begins to dissipate, reducing absorption on all frequencies. During high sunspot activity, the higher bands can remain open to the west for 2 or 3 hours past sunset.

In the late afternoon, the low bands begin to open in a direction across the approaching terminator (northeast in the northern hemisphere's winter) beginning with 40 meters as much as 2 hours before sunset. As sunset approaches, signals from the southeast become more and more audible. From just before sunset until total darkness, signals will peak along the terminator (southeast) on all bands from 160 through 20 meters. Long-path propagation to Southeast Asia is especially good on 20 and 40 meters during this period, and occasionally on 80 and even 160 meters.

The Northeastern states have perfect gray-line alignment with JA, DU, 9M6, 9M8, V85 and YB in midwinter. The Southeastern and Midwestern states have perfect gray-line alignment with HL, BY1, BY4, XU, XV, XW, HS, 9V1, 9M2 and YB in the same period.

On 80 meters, DU, HS, JA, V85, VK9Y, XW, YB, 9M2, 9M6 and 9V1 are known to have been worked on the sunset long path from the Southeastern US. A lucky few have even worked 9M2AX on 160-meter long path.

Several problems combine to make low-band, long-path contacts difficult and rare. Low activity, brief openings, atmospheric noise, E-region MUF, absorption and station capabilities are other reasons. Yet with the right timing, persistence can be rewarded with rare and exotic DX! Although these openings do not always occur on a daily basis, they do always occur in the time periods indicated.

Enjoy your DX chasing!


  1. Bob Brown, NM7M, has done extensive research on long-path propagation and written a book, Long-Path Propagation, based on this research. You can obtain a copy of this book for $10 postpaid directly from Bob at 504 Channel View Dr, Anacortes, WA 98221.
  2. S. Ford, ed, The ARRL Operating Manual, fourth edition (Newington: ARRL, 1991), pp 17-71 through 17-142.


D Region-The ionospheric layer closest to the earth, at about 45 miles. This region is primarily responsible for low-frequency absorption. Absorption in this region is highest during daylight hours.

E region-The lowest ionospheric layer that supports HF propagation, this region usually affects only the uppermost HF bands.

F Region-The ionospheric layer that supports long-haul DXing by refracting HF signals. During daylight, this region separates into two distinct layers-F1 and F2-of which the upper (F2) layer is primarily responsible for long-distance upper-HF propagation.

Gray line-Also known as the terminator and the twilight zone, this is the transition region between daylight and darkness. Stations within this zone experience enhanced propagation across the MF and HF range.

Long path-The longer arc of the great circle path between two stations. Particularly when both stations are in darkness and the long path spans the nighttime hemisphere, the long path may propagate signals when the short path won't.

MUF-Maximum usable frequency; the highest frequency that a given radio path will support.

N4KG, Tom Russell.