Radio Propagation

The Basics

There are three main types of propagation

Radio Wave Propagation is the process in which your radio signal gets from your antenna to the antenna of the amateur station that you're talking to.

Line-of-Sight

Line-of-sight propagation requires a path where both antennas are visible to one another and with no obstructions. A path normally used by VHF and UHF communication.

Ground-Wave

Ground-waves follow the surface of the earth, rapidly attenuating at higher frequencies. Normally used by lower frequency transmissions such as Medium wave to Long wave

SkyWave

As the name indicates sky-waves go skywards and are reflected by the Ionosphere, providing long-distance communication. Normally used by High Frequency transmissions.

Radio Waves

When you transmit through your antenna, it is energised and radiates electromagnetic waves. The waves spread outwards and travel in free space at 300,000Km per sec or 186,000 miles per sec, or the speed of light. When the waves enter an ionized area the speed is modified. The further the wave moves away from the antenna the less strength it will have. (Field strength).

Polarization

A Radio Wave is Polarized in the direction of the electromagnetic wave of the field of the wave. A Horizontal antenna would normally radiate Horizontally Polarized waves and a Vertical antenna Vertically Polarized waves. If you transmit waves from your HF antenna, part of the energy follows the surface of the ground (Ground Wave) and is rapidly absorbed by the earth, therefore the range is very short, but most of the energy leaves at an angle as a Sky Wave and would be lost in space if it were not for the presence of the Ionosphere, which with the right conditions will bend or reflect the wave back to earth often at great distances from the transmitting station. This can be repeated a number of times where it can cover even greater distances known as Hops.

The Ionosphere

The Ionosphere consists of a number of ionized regions called layers. All of these layers except the lowest can reflect the waves back to earth if the layers are sufficiently ionized.A layer is said to be ionized when ultra-violet rays from the sun have caused gas atoms to lose free electrons and the stronger the ultra-violet rays, the more dense the layer becomes (the dielectric constant). If the ionization of the layer is too low, or the frequency too high, the wave will continue through the layer on to the next layer or space. If ionization is just right, the wave will bend back to earth in one or more hops.

Ionosphere Layers or regions

D layer is the lowest of the layers at a height of 50-90km above the earth. It forms rapidly at sunrise and disperses rapidly at sunset particularly where HF and medium wave signals are concerned. The D layer attenuates these frequencies until sunset and is therefore has its greatest attenuation during midsummer.

E layer >is at a height of about 120km above earth and is the lowest reflecting layer.

F1 layer is about 200km above earth and is a weak layer present only during the day for part of the year. It is capable of reflecting signals further than the E-layer, because of its greater height.

F2 layer exists during the day at a height of 350km during the spring, summer and autumn days. This is the main layer for daytime DX'ing during these times.

F layer is a result of the F1 and F2 layers combining at a height of about 200km at night and during winter days. The layers expand due to exposure to the sun for long periods of time(F2) and contract at night or in winter when there is little sun.

So you can see that propagation is dependant upon a number of factors: The time of day, the time of year, your antenna system, your power output, (to a degree) and also the amount of ultra-violet rays that the ionosphere is exposed to. The latter is determined by the solar cycle which we will deal with on the Solar Page.

These are only the very basics of propagation that you will require to get a good understanding, but it is an immense subject and there are many websites which deal with the different aspects and levels, which I have listed on the Links Page.