Making Waves
Steve White G3ZVW details a mode of propagation on the High Frequency bands that is often overlooked.
Steve White G3ZVW details a mode of propagation on the High Frequency bands that is often overlooked.
The E Layer of the ionosphere lies at about 80-110km above the Earth’s surface. Like other layers in the ionosphere, during the hours of daylight, its intensity is enhanced by radiation from the Sun. A daytime and night-time illustration of the layers of the ionosphere can be seen in Fig. 1. The basic differences between day and night are that at night:
1. the D Region dissipates,
2. the E Layer weakens, and
3. the F layers combine, weaken and move higher.
If you look at a daily chart on the Propquest website (URL below) you’ll see that the E Layer starts to get going at around sunrise and fades at around sunset. This happens every day. A really simplified illustration of its Critical Frequency is shown in Fig. 2. The illustration shown is from the summer, when the days are long and the Sun is high in the sky, but of course when it is winter and the days are short and the Sun low in the sky the enhancement is less (and present for less time). It’s a different story for the F Layer(s) of the ionosphere, because to a greater or lesser degree they are present 24 hours per day.
In the Summertime
For long distance communication at VHF, it is well known that during the period from about April to August (in the Northern Hemisphere) small patches of the E Layer can become sufficiently highly ionised to support propagation into the VHF part of the radio spectrum. The enhancement is thought to be caused by wind shear, when winds in the ionosphere blow past one another. Lots of distant stations are worked on 50MHz by so-called Sporadic E propagation in the summer months. This type of propagation had distinct characteristics.
1. It can come and go very quickly.
2. Sometimes it can last for hours, sometimes just minutes.
3. Propagation tends to be highly directional and patchy. For example, someone just a few miles away from you might be working lots of strong stations a long way away, while you can’t hear any of them at all. At other times you might be the lucky one.
4. Long distance stations tend to be clustered into relatively small geographic areas on opposite sides of the area of enhancement. See Fig. 3, where the enhanced area of ionosphere is circled.
5. It is usually a daytime mode of propagation.
6. Sporadic E propagation spreads higher in frequency as the level of ionisation increases. 50MHz is the first VHF amateur band to be affected. 70MHz is next. The VHF broadcast band from 88 to 108MHz can be affected, but not as often as the bands below it in frequency.
The 144MHz band is affected least often and almost always for the shortest periods. Sporadic E propagation never reaches as high in frequency as the 432MHz band.
The maximum distance you can hear or contact a station by Sporadic E propagation is a bit over 2000km. This is governed by the height of the E Layer above the ground and the curvature of the Earth. When there’s a lot of Sporadic E
propagation about, a signal that has been refracted back to Earth can be reflected back up and couple into another patch of highly ionised E Layer, if one happens to be conveniently located. This can double the maximum distance that can be worked. Consider yourself lucky if you work a station somewhere like Cyprus or Israel from the UK on 50MHz.
It will almost certainly have been by double-hop Es if you do. Consider yourself very lucky indeed if you work a station somewhere like Cyprus or Israel from the UK on 144MHz.
For long distance communication at HF we don’t tend to think much about the E Layer, because the F1 and F2 Layers are higher and support communication at longer distances. We normally expect HF signals to pass through it and be refracted back to Earth by the F Layer. This is shown in Fig. 4, with the E Layer shown as a faint colour to indicate the weak ionisation. Now here’s the ‘but’.
From about April to August the E Layer can sometimes become more highly ionised, making it able to support radio communication at HF.
This leads us to an interesting aspect of propagation. If we transmit a High Frequency signal when the E Layer is enhanced, some of it will be refracted back to the ground by that layer, while some of it will pass through and hopefully be refracted back to the ground by one of the F Layers higher in the ionosphere.
Essentially you get two maximum distances, one from each layer of the ionosphere. See Fig. 5.
CrunchTime
Now I would like you to think about what happens when the ionisation of the E Layer becomes really strong. So strong in fact that none of an HF transmission can pass through it. Even if the F Layer is sufficiently highly ionised to refract the signals back to Earth at long distance, for a while you will be limited to working stations at much shorter distance. This is shown in Fig. 6, where I have made the E Layer a darker colour still, to indicate its strength. When we get this kind of HF propagation we call it Blanketing E.