Ionospheric Radio Propagation

In the early days of radio communications, it was thought to be impossible to cover distances well beyond the horizon because of the limited reach of the ground wave propagation. When trans-atlantic communication was accomplished it was concluded, that the Earth was surrounded by an electrified layer that caused reflection of radio waves. Later experiments showed that more than one such layer existed. They are build up from ionized particles created by the Sun's ultra-violet radiation. Also cosmic radiation contributes to the building of these ionized layers. The part of the atmosphere where these ionized layers appear is called ionosphere, which roughly stretches from 100 km to 500 km above the Earth's surface.

Because of the reflective nature of the ionosphere, radio propagation is affected not only by the daily changes of water vapour in the troposphere (which is related to radio wave absorption) but also by the ionization activity in the upper layers of the atmosphere. Ionospheric radio propagation deals with sky wave propagation and is the key for all long-range radio communications. The ground wave reaches as far as the radio horizon, but under certain conditions, the sky wave may be reflected from ionized layers in the upper atmosphere back to the surface of the Earth, enabling word-wide radio skips.
Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for international short-wave communications, to radio navigation and to operation of radar systems.

The electrical behaviour of the atmosphere is characterized by the density of electrically active particles (electrons and ionized molecules) in the different layers. Ionization depends primarily on the Sun and its activity. The amount of ionization in the ionosphere varies greatly with the amount of radiation received from the Sun, which varies from day to night and also over the seasons. The local winter hemisphere is tipped away from the Sun, thus there is less received solar radiation. The Sun itself also has a varying activity, associated with the 11-year sunspot cycle, with more radiation occurring with more sunspots. Also the radiation received in the polar regions is different from the radiation in the mid-latitudes, or equatorial regions.


While propagating through the atmosphere and the ionosphere, there are three different processes that affect radio waves:

• Absorption: while travelling through the ionosphere, the electro magnetic radio wave gives up some of its energy by setting the ionized molecules in motion. Through this, some part of the energy of the radio wave is transferred to the atmosphere. This absorption is greater for lower frequencies. It also increases with the intensity of ionization and with the density of the atmosphere in the ionized regions.

• Refraction: when radio waves travel through the atmosphere, they bent slightly towards the surface of the Earth. That means that the radio horizon reaches about 15% beyond the visible horizon. This refraction is due to the variation of density of the atmospheric layers and the variations of degree of ionisation in the ionosphere. Low-frequency waves are bent more than those with higher frequency. For this reason, the lower frequency bands are more reliable (ground wave) than higher frequencies.

• Reflection: when radio waves bounce at a low angle on conducting layers such as the surface of the Earth or highly ionized layers in the ionosphere, they may be reflected. In the upper layers of the ionosphere, these ionized layers are identified as D-layer, E-layer and F-layer (consisting of F1- and F2-layer).

Classification of the Layers in the Ionosphere

There are four ionized layers or regions in the ionosphere. They are called the D, E, F1 and F2 layers. All four layers are present during daytime. At night, the F1 and F2 layers thin out and tend to merge into one F layer. The D and E layers disappear at night. These layers have a lower degree of ionization. After the Sun sets, the intensity of the ultra-violet radiation decreases, recombination of the ionized particles occurs and the lower ionized layers will dissolve. Just before sunrise, ionization is at its lowest point.

D-Layer

The D-layer, which is present during the daytime only, has the least ionization and therefore has the least effect on radio propagation. However, it almost completely absorbs radio waves of lower frequencies (up to 10 MHz). The ionization is proportional to the angle of elevation of the Sun and is the greatest at noon.

E-Layer

The Kennelly-Heaviside layer is a layer of ionised gas occurring between roughly 90-150 km. It reflects medium-frequency radio waves (up to about 20 MHz), and because of this reflection, radio waves can be propagated beyond the horizon.
Propagation is affected by the time of day. During the daytime, the solar wind presses this layer closer to the Earth, thereby limiting how far it can reflect radio waves. On the night side of the Earth, the solar wind drags the ionosphere further away, thereby greatly increasing the range which radio waves can travel by reflection, called sky wave. The extent of the effect is further influenced by the season, and the amount of sunspot activity.
This is the lowest useful ionized layer, and normally dissolves after sunset. The maximum one hop range of the E layer is 2000 Km.

F-Layer

The F-layer, or Appleton layer, is the most important reflection layer. The F region contains ionized gases at a height of around 150-800 km above sea level. It has the highest concentration of free electrons and ions anywhere in the atmosphere. During daytime when solar radiation is much higher than during the night time, the F layer splits into two sub-layers: the F1 and F2 layer with average virtual heights of 200 and 500 Km. After sunset these two layers merge again into a single F region.

Some Terms and Definitions

Angle of Radiation

The angle between the direction of the wave propagation and the horizon (or tangent of the Earth) is called wave angle or angle of radiation. This angle is influenced by the characteristics of the transmission antenna - especially the physical length with respect to the wavelength of the radio wave - but also by the properties of the ground below the antenna structure.

Ground Wave

The horizontal waves from the transmit antenna travels a line-of-sight distance or a little bit more, parallel to the surface of the Earth. This wave is called ground wave.

Critical Angle

The wave at a somewhat lower angle is just capable of being returned by the ionosphere. This radiation is called critical angle. Waves transmitted with a higher angle will be absorbed in the ionized layer.
Waves with a lower angle will be reflected. This is called sky wave. The sky wave will be reflected back to the surface of the Earth. The distance the reflected wave will have travelled on the Earth depends on the radiation angle. The lower the angle the larger the distance.

Skip Distance and Skip Zone

When the wave angle is lower than the critical angle for a particular frequency and for a particular constellation of the ionosphere (time of the day), it is reflected and returned to the surface of the Earth some distance away from the location of the transmitter.

The shortest possible distance at which sky wave communication is possible is called skip distance. It is not possible to receive the transmitted signal by sky wave propagation in the area between transmitter and skip distance. The area between the end of the ground wave reception and the beginning of sky wave reception is called skip zone. The range of the skip zone depends on the frequency used and the constellation of the ionosphere (height and state of the ionized layers).

Multi-Hop Propagation

Not only the ionosphere, also the Earth itself can act as a reflector for radio waves. This can result in a transmission consisting of multiple hops.

A radio signal can be reflected from the reception point on the Earth back into the ionosphere, reaching the Earth a second time at an even more distant point. This effect is illustrated on the right. The wave identified as "Critical Angle" travels from the transmitter via the ionosphere to point A, in the center of the picture, where it is reflected upwards at the surface of the Earth and travels another hop to point B, at the right. This shows a two-hop transmission situation, but through multiple hops a complete circumvention of the Earth is possible.

As indicated in the picture, the distance at which a ray eventually reaches the Earth depends on the height of the active ionospheric layer and the elevation launch angle at the transmitting antenna.

Usable Frequency

In order to reach a certain point on the Earth, the appropriate frequency and radiation angle must be chosen. This highly depends on the constellation of the ionosphere (sunspot cycle, time of the day, height of the ionized layers) and the antenna used. Although world-wide communication is possible through multiple reflection on the ionospheric layers and the surface of the Earth, the best possible quality is obtained with as little hops as possible.
The maximum single-hop distance for the F2 layer is about 4000 Km and 2000 Km for the E layer.

Sunspot Activity

Everything that happens in radio propagation on the Earth is highly affected by the atmospheric radiation from the Sun. The variable nature of radio propagation on the Earth reflects the ever-changing intensity of ultraviolet and X-ray radiation, which are the primary ionizing sources of solar energy. The total power radiated by the Sun is estimated at 4.0E23 kW. At its surface, the Sun emits about 60 megawatts per square meter and a significant part of that energy is radiated in the radio frequency spectrum by solar flares. That solar transmitter will interfere with any free space electromagnetic-based communication on Earth.

Also the ionisation effect of the Sun's radiation will strongly affect the condition of the ionosphere on the Earth. The atmospheric propagation of HF radio waves on Earth depends on the 11-year sunspot cycle activity. Higher sunspot activity leads to more ionisation in the ionosphere of the Earth. So the maximum sunspot periods (around 2014, 2025, ..) will give the best conditions for HF communications. The critical frequencies are higher during these sunspot maximum period, whereas during the periods of minimum sunspot activity (around 2010, 2021), the lower frequencies (160m, 80m and 40m) are the only bands that can be used at night.

Propagation in the HF Bands

Sky wave propagation offers long range communication with low transmit power. The most difficult question regarding sky wave propagation is what frequency to use. The radio waves in the HF Bands (3-30 MHz) use the ionospheric reflection most effectively.
Propagation conditions of radio waves in the HF bands are determined by the ionization conditions in the atmosphere. The sunspot activity as well as the time of the day will determine the maximum range that can be covered.

160-m band

The 160-m band offers reliable working conditions over a range of up to 50 Km during daytime and up to several thousand Km during winter nights.

80-m band

During the daytime, the 80-m band covers up to about 300 km. The band is more useful during the night with a range of several thousand Km and regular transoceanic communication during winter nights.

40-m band

The 40-m band has more or less the same characteristics as the 80-m band but offers even more skip range. Daytime distances of up to 1500 Km and world-wide communication during winter nights.

20-m band

The best amateur band for DX work is the 20-m band. During the high portion of the sunspot activity cycle, it is open to some part of the world practically throughout the 24 hours of the day. During sunspot minima it is generally useful only during twilight hours. There is practically always a skip zone on the band.

15-m band

The 15-m band shows highly variable conditions over the sunspot cycle. During sunspot maxima it is useful for long distance work during a large part of the day, but in years of low sunspot activity, it is even unusable during daytime.

10-m band

The 10-m band is generally considered to be a DX-band during the winter daylight hours and good for local work during the hours of darkness, for about half the sunspot cycle. At the sunspot minimum the band is usually dead.