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The radio propagation

Working with the Gray line (V)

Working in the HF or UHF bands, you can take advantage of the "gray line" of the terminator, this twilight zone that separates the day from the night to work on distances longer to the ones reached using normal propagation means. The gray line area along the sunrise and sunset zones produces indeed significant improvements of the signals reception. This happens because in the area of sunset for example the D-layer, which absorbs HF signals, disappears faster on the sunset side than the highest layers that remains strongly ionized. Better, in that area HF signals have also less attenuation compared to the direct path.

At left, map of the gray line when the twilight is reaching western Europe and Brazil, and dawn is leaving Japan and New Zealand. Along this gray line the D-layer, and soon the F1-layer disappear rapidly on the sunset side, and are not built upon on the sunrise side yet. This phenomenon offers an excellent opportunity for DXing. Click on the image for a real-time and detailled update from Fourmilab (displaying city lights plunged into darkness as well) or connect to PA3CQR website to download his gray line software. At right, a Fritzel Beam at twilight waiting for DXing at Elvange, LX, on March 30, 2004 by 18:45 UTC. Document T.Lombry.

The gray line usually runs in north-south direction, but due to the inclination of the Earth on its orbital plane, it varies up to 23 either side of this twilight zone. At the equinoxes (21 March and 21 September) the gray line runs exactly north and south, both poles receiving the same amount of sunligh. At summer in the northern hemisphere the gray line is tilted at 23 to the north and 6 months later, at the beginning of the winter it is titled 23 to the south. 

This tilt has consequences on the area that you can work using this type of propagation. Indeed as soon as the gray line is tilted over the equator, for a specific day you can no more work the same countries at sunset that the ones you worked at sunrise. The next maps explain better than any text thie phenomenon.

Tilt of the Earth terminator on December 21, at winter solstice. Due to the tilt of the Earth orbit that reaches 23, at sunrise the area that you can reach via the gray line is different from the one that you can work at sunset. Maps created with DX4WIN. 5Z is reachable at sunset but not at sunrise.

At winter for example, on December 21, the Earth shows a maximum tilt of 23 over its orbital plane, and thus the gray line follows the same tilt over the equator.

At sunrise in the northern hemisphere, at 08:00 UTC, the tilt is 23 over the equator, looking eastward, allowing European amateurs to work stations located in the western coast of Africa and South America. Using the long path you can also work amateurs from New Zealand, Australia or Indonesia. But at sunset the situation changed. At 17:00 UTC  the tilt of the gray line is now 113 over the equator of simplier, 23 to the west. This means that you can no more work Western Africa but rather its eastern side, like Kenya or Madagascar. You can reach with ease Far East countries like Japan or Korea by the short path, this time plunged into darkness.

On this gray line we can take advantage of this phenomenon during one hour at sunrise or sunset and till more when your path cross the equatorial region where the electron density is higher. In such conditions when the gray line crosses Europe in the eve for example, it is very easy to hear or to contact amateurs in Argentina, and even in New Zealand, Australia and Japan via the long path; these stations arrive really loud and clear even using your transceiver bare foot.


In this mode both hams have to bear their antenna to a shared area in the E-layer of the ionosphere near 90 km aloft, where their respective signals are scattered forward. This VHF traffic requires efficient antennas and is open permanently, at all hours of the day and night. According hams specialized in this traffic, no elevation is request for antennas if the distant station is located over 2000 km away. Howerver, for shorter distance, around 1200 km, an elevation of about 5 is required. 

This type of traffic is the most efficient between 50 and about 70 MHz and takes also advantage of the ionization trail left by meteors that contribute to scatter radio signals. But contrary to the Meteor Scatter, here the signals received are continuous and weaks. Thefeore most hams work with linear amplifiers over 500 W and beams offering a gain over 10 dBd.

Backscatter traffic

Usually two nearby stations cannot hear each another due to the skip or silent zone that surrounds them. Therefore on HF bands you generally cannot hear stations in a range of about 200 km around your QTH on the 10m band and you can get better results on the lower bands where the skip distance is shorter or does not exist. But under special ionospheric conditions these two same nearby stations might be able to communicate thanks to the back or sidescatter.

If you work near the MUF limit for a given path and time, after the first skip via the E or F-layer, your signal is reflected to the ground. A part of these emissions is reflected back in an area shared by both emitters. Signals are very modulated and highly recognizable, sounding like expressed in a huge cathedral or similar to a hollow sound with no evidence of fading. 

This effect contributes to DX activities where contacts over 5000 km are possible especially during the periods of low solar activities when the normal ionospheric propagation is deeply affected.

Auroras as airways

Auroras are the favorite propagation medium for amateurs of VHF traffic. When the geomagnetic field is perturbated after a violent solar eruption, which effect is an increasing of the atmospheric ionisation, not only the F2-layer but more deep ionospheric layers like E and D are invaded by charged particles. 

GM4JJJ's antennas during an auroral event.

From the state of F2-layer depends the propagation between wavelengths of 100 - 10 m with an excellent reflection of signals in the 80 m band, favorable to DX in winter. Sometimes auroras are so powerful (solar K-index over 5) that they can also reflect the entire UHF band from 1 m to 10 cm.

Among the side effects, due to their fast evolution, propagation is often sensible to fading (QSB) and phase shifting, giving to the modulation a highly recognizable profile that sounds like "bubbling" or "underwater" modulation in the 10 m band. Signals reflected by aurora are also subject to fluttering with a rapid change of their frequency from about 100 hz to 2000 Hz. During these events it is thus preferable to work in more reliable modes like CW or digital when DXing.

However, these drawbacks show also advantages. In HF, when amateurs point their high gain antenna at high latitudes in presence of auroral activity, they experience enhanced propagation to other locations. Indeed, the auroral oval acts like a reflective wall and bends sky waves along paths that are off-azimuth over tremendous distances. In these occasions the MUF is well above 30 MHz and the propagation suddently opesn to unexpected DX entities.

In VHF, due to the erratic behaviour of these curtains and others bright rays that evolve according fluctuations of the geomagnetic field, this activity requires to stack several directive antennas, like we do working "Tropo" (see below) to get a good gain. This way radio signals can bounce on the plasma clouds and go down to the ground to another location in order to establish a contact.

If theoretically all latitudes can profit from this mode of propagation, northern latitudes from mid-USA to Alaska and Scandinavian countries are favored due to their position in relation to the geomagnetic pole. 

Remark. If by chance you have the opportunity to visit Canada, Alaska or Finland during the next paroxysm of the solar activity (around 2010) do not hesitate to drive some dozen kilometers away from city lights. There, in the wild rise your nose up and observe this beautiful stage directed by dame Nature. At these occasions auroras occurs every day ! But back to Earth !

To check : Aurora Forecasts

To read : La belle aurore ! with stunning images 

Meteor scatter

Have you ever try to work in HF with "shooting stars" ?... If you erect a yagi above the horizontal plane in order to point in the direction near the radiant of meteors showers - Perseids, Leonids, etc - each time a meteor will strike the upper atmosphere above or at a few hundred km from your location, while burning in penetrating the denser layers of the atmosphere you can use its ionization trace as a reflector to reach other hams working a similar way. 

List of meteors showers

The Leonid 1997 shower seen from space. Each white dot or line is a meteor striking the upper atmosphere. Doc SETI Institute colorised by the author.

To work a MS station, both hams have to be placed symmetrically with the meteor trajectory and the density of the ionosphere must reach a high value in order that the ionized trail is able to reflect shortwaves in place of absorbing them.

Most of these radio waves are scattered at the E-level and can be used on frequencies from 6 m to 60 cm but also in the HF band of 10 m. 

As meteors enter the atmosphere at very high speeds, most between 60 and 72 km/s, the scatter of radio waves last only a few seconds. In very exceptional circumstances the trace last a few minuts while the smoke has already persisted over one hour ! 

You will record the best the meteors during the first hours of the morning until dawn because the eastern side of the Earth captures much more meteors during its rotation than the opposite side where meteors have to catch up with Earth on its orbit. Therefore in the morning the meteors are more numerous, their are more bolids and their trail is also brighter than before midnight.

The ionized clouds moves at a speed of about 72 km/h or 0.02 km/s, thousand times slower than a fast meteor ! 

Echo of a strong Geminid (MP3 of 534 KB) recorded on Dec 15, 2003 at 1140 UTC by the author using a Navspasur radar tuned on 217 MHz. Recording made via Internet.

To work a station in a few seconds delay requests special equipments, often directive aerials, and preprogrammed procedures. This activity is mainly practiced in APRS and VHF packet radio where signals are categorized either as "ping" or "burst" depending their last and strength. Meteors Scatter allows of course SSB communications. Click on this file to listen an MS communication via a Leonid worked by F6CRP in 2000 (.MP3 file of 63 KB). Other recordings are available on this page.

For more information about MS and other EME activity I suggest you to subscribe to the european magazine DUBUS or take a look at the following websites : Meteor Scatter.Net and Jordanian Astronomical Society. Refer also to the Meteor-Scatter page for more detail about this exciting activity.

If you are only interested in counting the number of meteors falling from the sky during a shower, Pierre Terrier intends to help you in building a monitoring system based on FM signals called the "Radiometeor". In addition he also provides an analysis software called HROFFT2RMOB.

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Tropospheric, satellite and other EME traffic

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