Every aspect of amateur radio comes down to propagation. In fact, any radio system or network takes propagation into consideration. Will the signal reach, or propagate to the intended recipient? There are really two types of propagation for radio waves, line of sight, and reflected. Your favourite radio station relies on line of sight communications although sometimes if the conditions are right, the radio signal can bounce off layers in the atmosphere back to earth far beyond the range of a line of sight signal.
Radio waves reaching the ionosphere layers are called sky waves. Only these ones allow long-distance propagation (DX). To understand how they can be affected by the sun activity, we must, first of all, define characteristics of the ionosphere and its properties. Then we will focus on solar radiations, geomagnetic and ionospheric disturbances to close in dealing with solar and geomagnetic indices and their effects on critical frequencies like MUF, LUF and other FOT.
Several different types of propagation are used in practical radio. Line-of-sight propagation means radio waves which travel in a straight line from the transmitting antenna to the receiving antenna. Line of sight transmission is used for radio transmissions such as mobile phones, cordless phones, walkie-talkies, wireless networks, FM radio and television broadcasting, and satellite communication. Line-of-sight transmission on the surface of the Earth is limited to the distance to the visual horizon, which depends on the height of transmitting and receiving antennas. It is the only propagation method possible at microwave frequencies and above.
Most large-scale behaviour of radio waves is exactly the same as light except for the difference in scale. VHF radio waves are defined as having wavelengths between 10m and 1m (30–300MHz). In contrast, the wavelength of visible light extends from about 350 to 650 nanometres (1nm = 10 –9m), several million times shorter. If the earth was reduced in size by the same ratio, it would become a sphere of only a few metres diameter.
Radio waves and light waves also share a property which is unique across the entire Elecro Magnetic spectrum: the atmosphere is almost completely transparent to them. That is why both radio and light waves are used for communication.
This diagram illustrates some of the features and phenomena found in the various layers of the Earth’s atmosphere. Starting from ground level, the layers include the troposphere, stratosphere, mesosphere and thermosphere. The exosphere, which is above the thermosphere, is not shown in the diagram. Source: ACAR
Radio waves can propagate from transmitter to receiver in four ways
Ground waves exist only for vertical polarization, produced by vertical antennas when the transmitting and receiving antennas are close to the surface of the earth. The transmitted radiation induces currents in the earth, and the waves travel over the earth’s surface, being attenuated according to the energy absorbed by the conducting earth. The reason that horizontal antennas are not effective for ground wave propagation is that the horizontal electric field that they create is short-circuited by the earth.
Ground wave propagation is dominant only at relatively low frequencies, up to a few MHz. Skywave propagation is dependent on reflection from the ionosphere, a region of rarified air high above the earth’s surface that is ionized by sunlight (primarily ultraviolet radiation).
The ionosphere is responsible for long-distance communication in the high-frequency bands between 3 and 30 MHz. It is very dependent on the time of day, season, longitude on the earth, and the multiyear cyclic production of sunspots on the sun. It makes possible long-range communication using very low power transmitters.
There are a number of categories into which different types of RF propagation can be placed. These relate to the effects of the media through which the signals propagate.
Free space propagation
Here the radio waves travel in free space, or away from other objects which influence the way in which they travel. It is only the distance from the source which affects the way in which the signal strength reduces. This type of radio propagation is encountered with radio communications systems including satellites where the signals travel up to the satellite from the ground and back down again. Typically there is little influence from elements such as the atmosphere.
Ground wave propagation
When signals travel via the ground wave they are modified by the ground or terrain over which they travel. They also tend to follow the Earth’s curvature. Signals heard on the medium wave band during the day use this form of RF propagation.
Here the radio signals are modified and influenced by a region high in the earth’s atmosphere known as the ionosphere. This form of radio propagation is used by radio communications systems that transmit on the HF or shortwave bands. Using this form of propagation, stations may be heard from the other side of the globe dependent upon many factors including the radio frequencies used, the time of day, and a variety of other factors.
Here the signals are influenced by the variations of refractive index in the troposphere just above the earth’s surface. Tropospheric radio propagation is often the means by which signals at VHF and above are heard over extended distances.
In addition to these main categories, radio signals may also be affected in slightly different ways. Sometimes these may be considered as sub-categories, or they may be quite interesting on their own.
Our atmosphere consists of 5 levels. The troposphere, stratosphere, mesosphere, thermosphere and exosphere.
The troposphere is the lowest layer of our atmosphere. Starting at ground level, it extends upward to about 10 km above sea level. We humans live in the troposphere, and nearly all weather occurs in this lowest layer. Most clouds appear here, mainly because 99% of the water vapour in the atmosphere is found in the troposphere. Air pressure drops, and temperatures get colder, as you climb higher in the troposphere.
The troposphere is the lowest major atmospheric layer, extending from the Earth’s surface up to the bottom of the stratosphere. The troposphere is where all of Earth’s weather occurs. It contains approximately 80% of the total mass of the atmosphere.
The troposphere is characterized by decreasing temperature with height at an average rate of 6.5 degrees Celcius per kilometre). In contrast, the stratosphere has either constant or slowly increasing temperature with height.
The boundary between the troposphere and the stratosphere is called the “tropopause”, located at an altitude of around 5 miles in the winter, to around 8 miles high in the summer, and as high as 11 or 12 miles in the deep tropics.
When you see the top of a thunderstorm flatten out into an anvil cloud, it is usually because the updrafts in the storm have reached the tropopause, where the environmental air is warmer than the cloudy air in the storm, and so the cloudy air stops rising.
The next layer up is called the stratosphere. The stratosphere extends from the top of the troposphere to about 50 km above the ground. The infamous ozone layer is found within the stratosphere.
Ozone molecules in this layer absorb high-energy ultraviolet (UV) light from the Sun, converting the UV energy into heat. Unlike the troposphere, the stratosphere actually gets warmer the higher you go! That trend of rising temperatures with altitude means that air in the stratosphere lacks the turbulence and updrafts of the troposphere beneath. Commercial passenger jets fly in the lower stratosphere, partly because this less-turbulent layer provides a smoother ride.
The jet stream flows near the border between the troposphere and the stratosphere.
Above the stratosphere is the mesosphere. It extends upward to a height of about 85 km above our planet.
Most meteors burn up in the mesosphere. Unlike the stratosphere, temperatures once again grow colder as you rise up through the mesosphere. The coldest temperatures in Earth’s atmosphere, about -90° C (-130° F), are found near the top of this layer. The air in the mesosphere is far too thin to breathe; air pressure at the bottom of the layer is well below 1% of the pressure at sea level and continues dropping as you go higher.
The layer of very rare air above the mesosphere is called the thermosphere. High-energy X-rays and UV radiation from the Sun are absorbed in the thermosphere, raising its temperature to hundreds or at times thousands of degrees. However, the air in this layer is so thin that it would feel freezing cold to us!
In many ways, the thermosphere is more like outer space than a part of the atmosphere. Many satellites actually orbit Earth within the thermosphere! Variations in the amount of energy coming from the Sun exert a powerful influence on both the height of the top of this layer and the temperature within it. Because of this, the top of the thermosphere can be found anywhere between 500 and 1,000 km above the ground. Temperatures in the upper thermosphere can range from about 500° C (932° F) to 2,000° C (3,632° F) or higher.
The aurora, the Northern Lights and Southern Lights, occur in the thermosphere.
Although some experts consider the thermosphere to be the uppermost layer of our atmosphere, others consider the exosphere to be the actual “final frontier” of Earth’s gaseous envelope.
As you might imagine, the “air” in the exosphere is very, very, very thin, making this layer even more space-like than the thermosphere. In fact, air in the exosphere is constantly – though very gradually – “leaking” out of Earth’s atmosphere into outer space.
There is no clear-cut upper boundary where the exosphere finally fades away into space. Different definitions place the top of the exosphere somewhere between 100,000 km and 190,000 km above the surface of Earth. The latter value is about halfway to the Moon!
The ionosphere is not a distinct layer like the others mentioned above. Instead, the ionosphere is a series of regions in parts of the mesosphere and thermosphere where high-energy radiation from the Sun has knocked electrons loose from their parent atoms and molecules.
The electrically charged atoms and molecules that are formed in this way are called ions, giving the ionosphere its name and endowing this region with some special properties.
There are three main regions of the ionosphere, called the D layer, the E layer, and the F layer. These regions do not have sharp boundaries, and the altitudes at which they occur vary during the course of a day and from season to season. The D region is the lowest, starting about 60 or 70 km above the ground and extending upward to about 90 km. Next higher is the E region, starting at about 90 or 100 km up and extending to 120 or 150 km. The uppermost part of the ionosphere, the F region, starts about 150 km and extends far upward, sometimes as high as 500 km above the surface of our home planet.