The HF, or shortwave, band is generally considered to spread from 3 to 30MHz. At these frequencies it is possible to communicate over the horizon by effectively reflecting radio waves off the ionosphere. About 50 years ago, HF was extensively used for long distance communication. Large networks of relay stations were constructed to communicate this traffic. The ionosphere is, however, a complex and constantly changing phenomenon. Simpler and more reliable methods such as satellites and cable replaced these networks. But now digital technologies are giving the HF band a new lease of life. Techniques for automatically selecting and maintaining a suitable transmission frequency have been developed. Equalisation, coding and interleaving enables data to be reliably transmitted over constantly changing HF channels.
The ionosphere is a region of the earth's atmosphere where charged particles exist. The ionosphere starts at altitudes above 80km (50miles). At this height the atmosphere is thin enough that free electrons can exist for short periods before they are captured by a nearby positive ion. The ionosphere has properties of both a gas and a plasma. Solar radiations dislodges electrons from gas atoms or molecules by UV and higher frequency solar radiation. Ionization is balanced by recombination where free electrons are captured by positive ions. The degree of ionization varies with altitude. At the highest levels the solar radiation is the most intense but the number of atoms is small so there are few charged particles. As the altitude decreases the number of gas atoms increases, increasing the ionization process and also the recombination process as the chance of a free electron encountering an ion increases. Also the solar radiation intensity decreases as more photons are absorbed at higher levels.
The earth's atmosphere is composed of a number of different gasses. Up to about 100km the gasses are well mixed due to turbulence. Above this height the composition of the gasses varies according to their molecular weight. In the lower ionosphere molecular oxygen and nitrogen are abundant. Above 200km atomic oxygen predominates. The differing capability of the sun to ionize the different gasses and differences in the recombination processes results in several distinct ionization peaks called the D, E, F1 and F2 layers.
This is the lowest layer occurring around 50 to 90km. As the lowest layer, the atmospheric density is the highest and recombination of ionized particles occurs rapidly. Ionization is created by short wave X rays and cosmic rays. Longer wavelength radiation (100nm plus) ionizes nitrous oxide. Constant solar energy is needed to sustain ionization. Consequently this layer is only present during daylight hours. Noon electron densities reach 10 8 to 10 9 electrons/m 3 . As the sun sets this layer recombines and disappears. There is also a strong seasonal variation with a maximum in summer.
At HF the layer absorbs rather than refracts (VLF and ELF is reflected). As energy from the electromagnetic wave sets electrons in motion, there is a high probability that the energy will be absorbed in a collision with a neutral particle. The electromagnetic energy is turned into kinetic energy (heat) and, as far as radio propagation is concerned, lost.
This occurs around from 90 to 130km. Radio waves interacting with this layer undergo refraction back towards the earth. This can be visualized as reflection off the ionosphere at some virtual height. Molecular ions are formed by X and ultra-violet solar radiation. This layer is still low enough for recombination to rapidly occur. The layer consequently only exists during daylight hours. Maximum density occurs around midday (10 11 electrons/m 3 at the maximum which occurs about 110km) and it disappears after sunset. A seasonal maximum also occurs in summer.
This occurs in the least dense portion of the atmosphere. During the night it consists of a single layer at about 300km. This layer remains ionized throughout the night with the ionization density falling to a minimum just before sunrise. The F layer is ionized by solar radiation with wavelengths between about 15 to 80nm.
During the day two separate layers occur called F1 and F2 in the regions 130 to 210km and 250 to 400km respectively. Why is this? In the F layer the peak of ion production occurs at around 160km but the ion loss process decreases even faster resulting in a peak in electron density at around 250 to 400km (where plasma diffusion limits the density). In the F layer the loss process is ion-atom exchange where an ion interacts with an atom to produce a molecular ion which then captures an electron. Below this height dissociative recombination predominates where molecular ions exist which capture electrons. If the changeover height for these two loss processes is higher than the peak of ion production then the electron density peaks at the peak height of production, producing the F1 layer. The peak is around 2x10 11 electrons/m 3 . The density then decreases with increasing height until loss through ion-atom exchange predominates and the density starts to increase again. The F2 layer peaks at about 10 12 electrons/m 3 during the day and 5x10 10 electrons/m 3 during the night. The F1 layer is occasionally the reflecting layer but usually waves that penetrate the E layer also penetrate the F1 layer, just suffering some additional absorption.
The F layer is probably the most useful layer as it exists through the night and is has the highest electron density. This means that it will 'reflect' signals that pass through the other layers. Each layer has its maximum useable frequency (MUF) above which it will not 'reflect' signals back towards the earth. The F layer, as it has the highest electron density, has the highest MUF. The optimum transmission frequency (frequence optimum de travail FOT) is typically 70 to 80% of the F2 layer MUF during the daytime. Below this, even though the signals may pass through the D and E layers, they will suffer increasing absorption.
The F layer is strongly influenced by winds, diffusion and other dynamic effects making it hard to model. Height and ionization varies over the day, seasonally and with sunspot cycle It does not simply follow the sun's zenith angle in a simple way since, with such low collision rates, solar energy can be stored for many hours. Near the equator the ionization varies strongly with latitude. At high latitudes there is a region of strongly depressed electron density.
This is enhanced ionization that occurs at E region heights resulting in much greater critical frequencies. It can cause long distance propagation at frequencies much higher than HF, for example causing TV signals to interfere. As well as enhancing E layer propagation it can disrupt F layer propagation by reflecting signals that would normally pass through up to the F layer. Its occurrence is latitude dependent. In central Europe, for example, it occurs more often in summer and during the day. In high latitudes it mostly occurs at night. In low latitudes it predominantly occurs during the day.
Propagation near the Polar Regions
In the auroral regions the ionosphere may carry a current approaching or exceeding a million amps. This may effect the propagation conditions in these regions resulting in, for example, large Doppler shifts.
Given the complex layer structure of the ionosphere, signals may propagate by a number of paths from the transmitter to the receiver. They may 'reflect' directly off the E layer (1E) in a single hop. They may reflect off the E layer back to earth, back up to the E layer again before arriving at the receiver i.e. 2 hop (2E). Three or more hops can occur. The signals may propagate off the F layers in one or more hops. They may arrive at the receiver via both or a combination of the different layers. Multiple hops are less visible during the daytime where the absorption of the lower layers prevents their propagation.
The different propagation modes can result in multiple versions of the signal arriving at the receiver with a delay spread up to typically 10ms.
Solar flares may disrupt HF communications either by causing increased D layer absorption or by depressing F2 layer electron densities. They do this by causing increased X-ray and ultra violet radiation leading to a 'Sudden Ionospheric Disturbance', releasing high energy protons causing polar cap absorption and streams of charged particles causing ionospheric storms.
Sudden Ionospheric Disturbance (SID)
A solar flare transmits UV and X-ray radiation that rapidly reaches the earth (this takes about 8 minutes). This produces abnormally high ionization in the D region causing increased absorption of MF, HF and VHF frequencies and also increased reflection of LF and VLF. It can cause a complete and sudden loss of HF propagation. This can only occur on the sunlit side of the earth and is most frequent at the maximum of the sun spot cycle.
These may last for several days and are caused by streams of charged particles (protons and electrons). They may take 1 or 2 days to reach the earth and are deflected by the earth's magnetic field towards the auroral zones. They cause increased ionization in the D region and an expansion and diffusion of the F2 layer, causing decreased critical frequencies and higher heights. Again ionospheric storms are most severe at solar maximum but are, perhaps, more significant at solar minimum.
Magnetic storms and auroral effects also occur with ionospheric storms. Magnetic storms are disturbances of the earth's magnetic field where the earth's field fluctuates over much wider limits than normally occurs. They can last from a few hours to several days.
Polar Cap Absorption (PCA)
There are infrequent but major disturbances that occur throughout the polar regions. They are caused by high energy protons that are guided by the earth's magnetic field towards the polar regions. These may take from 15minutes to 3 hours to reach the earth from the sun. These are called polar cap absorption events or solar proton events (SPE). They cause a considerable increase in D layer ionization resulting in strong HF and VHF absorption, blacking out HF communication in the polar regions for up to a day. The SPE itself may last for up to a week or more. They are almost always preceded by a major flare and occur most often at a sunspot maximum.
When a radio wave travels through the ionosphere its electric field imparts an oscillatory motion on the electrons. These re-radiate modifying the velocity of the radio wave and, if the electron concentration is changing, refracting the wave back towards the earth if its frequency is not too high. The earth's magnetic field modifies the oscillatory motion of the electrons causing them to move in complicated orbits. Their re-radiation is not, generally, in the same polarisation. The polarisation changes continuously as the wave travels through the ionosphere. It becomes split into two components; the ordinary and extraordinary waves. The ordinary wave behaves practically the same as if the magnetic field was not present. This effect is most apparent for waves that have traveled in the upper F region. The layer appears to split as the ordinary and extraordinary waves propagate with slightly different delays.
A Prediction Example
The above figure gives the predicted MUFs for each layer for the propagation between Berlin and London. This was run for June 2000 with a relatively high sun spot number of 100. The clear daytime variation of the E and F1 layers is apparent while the MUF of the F2 layer remains relatively constant throughout the day.
This shows how the predicted virtual heights and mode strengths (in decibels) varies through the day at 6MHz, again for the Berlin-London link. The F2 layer is always present. The three night time heights are for the 1-hop, 2-hop and 3-hop propagation modes. E, sporadic E and F1 modes are predicted to occur during the day. The strongest modes occur during the night. The lowest signal strengths occur at midday when D layer absorption is at its peak.
A Measured Example
This shows a measured evening ionogram. The D layer has dissipated revealing a number of different propagation paths. Just after 1ms is a strong E layer reflection extending to almost 4MHz. At about 1.6ms is smaller F1 mode extending to 3MHz. The F2 reflection starts at 2ms and is present until almost 10MHz. The splitting into the ordinary and extraordinary components can be seen at the higher frequencies. At 2.5ms a small two hop mode off the E layer can be seen. At longer delays multiple hop reflections off the F2 layer are apparent.
The following graph, taken at a different time, shows how a channel can vary over time. This plots the channel impulse response for a multiple hop F2 channel.
Last updated 16 July, 2000
© 2000 Michael Wells