Pulse aquí para ver la traducción al Español de esta página

The first thing to say about propagation on the LF- and MF-bands is that it is very different from that experienced on the HF-bands. The major difference is that where HF-signals are refracted back towards the Earth's surface by the F-region of the Ionosphere, LF, VLF, and low MF cannot pass through the lower D- and E-regions and long distance propagation is dependent on these regions. Whereas the F-region exists at around 250 to 600 km, the D-region is at 50 to 90 km with the E-region above it.

The variations in propagation conditions that we see are due to variations in the ion density caused by variation in the radiation from the Sun.
 

Ground-wave

Signals radiated horizontally from the transmitting antenna will tend to follow the curve of the Earth and are often referred to as ground-waves. One would normally expect the wave to travel in straight lines like light, so gradually climbing, but the wave front which is travelling out from the antenna is slowed slightly near the ground due to the refractive index being higher than air. This has the effect of tilting the wave front forward so that the bottom stays in contact with the ground. This effect is not as prominent as is seen at lower frequencies like 136 kHz, because the attenuation inflicted by the ground losses is higher as frequencies increase. There is a further effect that that reduces the strength of the ground-wave from the average amateur antenna. This is due to the restrictions in the choice of location of an amateur antenna. A commercial military or broadcast station with usually be able to select a clear site with good ground parameters, and also be able to afford to install multiple ground wires in the form of a mat around one half wavelength in diameter. This configuration ensures that the signal is generated efficiently and stays close to the ground. In an amateur installation where the ground is poorer, and fewer shorter ground wires are all that can be installed there is a "cut-in" in the antenna radiation pattern. Instead of resembling a half doughnut the signal is weaker close to the ground, and the direction of take-off of the strongest signal is a few degrees above horizontal.

The ground-wave will spread out from the antenna and will form the main part of the signal received at relatively short distances of say 100 to 300 km. There are applications that will calculate the strength of the ground-wave over different types of ground, given the transmitter power and the range [1].
 

Ionospheric wave

The majority of the power radiated by the antenna travels upwards and, if it were not for the ionised belt that surrounds the Earth called the ionosphere would be lost to space. Most radio amateurs are aware of these layers returning HF-signals back to the Earth's surface enabling worldwide communication on the short-wave bands. The regions responsible for this which were "discovered" by radio amateurs in the 1920s and referred to as the F-layer at altitudes of 250 to 600 km. Below this there exists the E-layer (90 to 250 km) and the D-layer (50 to 90 km).
These are generally regarded as a nuisance because they absorb the HF-signals stopping them from reaching the F-layer, at frequencies below about 10 MHz.

The situation is different for frequencies below about 1 MHz. Here the D-layer does absorb the signal but it is sufficiently ionised at the mid levels to refract these lower frequencies back to the Earth with some attenuation. The D-layer is at its most intense during the daylight period and LF- and MF-signals are generally weaker in daytime that in the darkness period. This gives rise to the myth that daytime propagation is due to ground-wave only. This is disproved by plots of daytime signal strength of receiving stations around 400 to 500 km from the transmitter which show a gentle dome shape peaking at noon at mid-path. This shape follows closely the level of ionisation in the D-layer due to to the "height" of the Sun or the strength of the radiation during the day.
 

Fading

At some distance around 400 to 700 km depending the on the power of the transmitter and noise levels at the receiver the ground-wave and ionospheric wave arrive at the receiver at roughly the same strength. This leads to problems at short ranges on 630 m because the path distances of the two components of the received signal are different, and the ionospheric (often called sky-wave) path length varies as the ionisation level changes. This leads to severe "fading". When the sky-wave and ground-wave arrive in phase the signals add but when they are 180 degrees out of phase they cancel, and the resulting signal is equal to the difference in amplitudes over the two paths. Thus if the two paths produce signals at the same amplitude, the received signal with disappear completely as the phase difference passes though 180 degrees.
 

Daytime range

Daytime maximum range on 630 m for one traverse of the ionosphere can be estimated by a simple geometric calculation [2].  If we assume that there is mirror like reflection at an altitude we will call the "apparent reflection height", maximum one-hop range requires the "ray" to depart the transmitter at zero degrees with respect to the ground. It will reach the receiver at a similar elevation. The true range will be more uncertain as it will depend upon the transmitter power and the ground "roughness". Over good ground some of the sky-wave to be may travel a considerable distance before becoming disconnected from the ground parameter effects, and start to travel towards the ionosphere. If we assume an "apparent reflection height" of 50 km then the range works out at about 1000 km. At this range the sky-wave is stronger than the ground-wave so that fading is reduced from that seen at short ranges. However because amateur power is limited and signals are often close to local noise levels, where even a little fading can make reception difficult.
 

Nighttime range

The daytime ionisation which produces the D-layer is due to to hard Solar Ultra Violet radiation and soft X-rays. These are shadowed at sunset and the D-layer ionisation recombines rapidly. This results in the apparent reflection height increasing to the upper D-region or even the lower E-layer.

The E-region starts at about 100 km where the remaining atmosphere is extremely thin. The ionisation is due to cosmic rays and some charge from the solar wind, but because the density is low the recombination time is much longer and the E-layer persists for the whole of the darkness period. If we perform the same geometric calculation for a 100 km reflection we now see a maximum one-hop range of just over 2000 km. Thus the best long distance contacts are made when both the transmitting and receiving stations are in darkness. When talking about darkness it is of course the state of the ionosphere we must consider, not the ground shadow. So it is possible to make a good contact to the West up to an hour after ground sunrise. And signals may be received from the East about a hour before sunset.
 

Multi-hop propagation

After one hop the signal at maximum range approaches the ground at grazing incidence. At this point the wave may skip across the contact point and start rising towards the ionosphere again as the ground level falls away again due to the Earth's curvature. This is often referred to as a ground bounce, but in fact the signal does not need to contact the ground. Some that does may behave like ground-wave being bent by the permitivity of the ground, extending the range of the first hop slightly. There will be attenuation at this contact point, but this may well be more due to the roughness of the ground surface with respect to the wavelength as much as the ground parameters. This would also explain why there is less attenuation over sea-water, though the increased conductivity of salt water plays some part as well. Fresh water is not quite so effective.

If we consider the path from western Europe to to the east coast of the USA the path is around 6000 to 7000 km making this a 3-hop path at night on the basis of our simple geometric model. These can be thought of as different propagation modes by analogy with transmission of microwaves in waveguide. Some observers refer to waveguide modes at LF/MF which is incorrect because the high frequency limit of wave guide modes at LF is about 50 kHz, Above the frequency the Earth-Ionosphere waveguide is too large compared with the wavelengths for a wave guide mode to be sustained. Thus a "hop model" is more useful and easier to understand.

It is however slightly more complicated than that. A careful observation of the path from the UK to VO1NA in Newfoundland on 136 kHz, a 4500 km path from the UK showed that on some nights the path opened when the sunset shadow reached mid Atlantic at 100 km altitude which suggest that there was a weak single hop signal. The three hop path to the East coast US sometimes shows a similar effect with an opening starting when the shadow reached 2/3rds of the way across the Atlantic. These observations suggest that there can be a number of multi-hop paths viable at the same time. The phase differences due to the different distances travelled by the different modes leads again to fading effects, but the is generally no ground-wave signal of any significant strength reaching the receiver, at this range.

The ionosphere has been considered so far as a spherical shell of electrons surrounding the Earth like an onion skin. This is far from the real state of the ionosphere with is a swirling mass of ions and electrons under the influence of the Earth's magnetic field and containing "bubbles" of varying density, and supporting waves, usually called gravity waves (NOTE not gravitation waves which are a totally different effect) like the waves on the surface of a pond [3]. This means that even in "Solar quiet" conditions there are many factors affecting the strength of a signal at a distant receiver.
 

Solar Effects

The sun has the most effect on the ionosphere and radio wave propagation. We have described how the D-Region is formed by the daytime radiation of hard UV and soft X-rays. This happens on a normal quite day and there is a slow variation in the Solar radiation flux from season to season and throughout the well known 11 year cycle of solar activity. The peak years of the cycle produce worldwide signal paths at the higher HF frequencies (30 MHz) and often right up to 50 and 60 MHz in the low VHF region by their ionisation of the F-layer. High activity peaks are not always as good for the LF/MF bands, and in general some of the best results are gained In the solar minimum periods.
 

Solar Flares

The Sun is not a "solid" object but a vast ball of spinning plasma. The layers move faster at the solar equator than the at higher latitudes. This puts a tearing stress on the plasma which starts to move in whorls rather like the atmosphere on Earth. Moving charge in the plasma creates magnetic fields which interact with the Solar magnetic field. The fields in the vicinity of the whorls becomes very strained and energy is required to maintain the distorted fields. This is extracted from the hot surface plasma cooling it slightly so the whorls show up on the Solar surface as dark spots. These "Sun spots" have been observed by astronomers since about 1700, long before radio and the ionosphere were discovered. The same spots can reappear about 27 days after they move off the visible disc. As a result we have a record of Solar activity for over three hundred years. This is but a small period compared with the age of the Sun so although we have discovered an 11 year cycle of activity, we are not yet able to discern longer term variations accurately. So each cycle is an interesting event to anticipate.

As the Sun rotates and the stress built up in the sun-spot regions increases, it eventually reaches an unstable state where it snaps into a lower energy configuration. This is like one magnet spinning as another is made to approach it, north pole to north pole. When the twisted field in the sunspot region collapses the energy released produces a large flash of electromagnetic energy and also can eject massive quantities of plasma away from the Solar surface. The flash of EM energy is called a solar flare and can be detected optically, however it releases energy in a wide spectrum from radio waves to X-rays. The ultra-violet portion of this so-called Solar-flare affects the ionosphere on the daylight side of the Earth only. Its effect is to greatly increase the level of ionisation in the lowest D-region. This produces a strongly absorbing D-layer at HF and what is referred to as a Dillinger fade-out on the HF bands.

The effect on LF and MF is totally different. Remember that there is normally an absorbing layer beneath the higher ionisation at the "reflection height" but this burst of high intensity radiation ionises the D-region so intensely that it will return an LF/MF signal from its lowest extent with little attenuation. This produces a pulse of increased strength closely following the shape of the solar radiation flux. Most flares are relatively short events lasting a few minutes at most, though there are occasional long-tail flare that can persist for around an hour. Solar flares are short and unpredictable (we can tell when they are most likely but not the exact moment of release) so they are not really useful for enhancing the range of signals, though the increase in strength can be as much as 10dB. If the range is short so the ground-wave is strong the flare may result in a decrease in strength at the receiver. This will depend upon the phase difference between the sky-wave and ground-wave.
 

Geomagnetic storms

From time to time the magnetometers monitoring the Earth's magnetic field at various world wide observatories start to swing around violently. Many of these observatories were established as early as the 17th century as an aid to marine navigation. It may have been noticed that the "storms" analogous to stormy weather in the atmosphere often occurred about 2 days after a central Solar flare was observed.

Their effects also often included intense aurora spreading further south than usual, It was not until rocket and satellite observations became possible in the second half of the 20th century that the mechanism for these effects was discovered.

The rotating Sun throws of a stream of plasma from it outermost region called the corona, which may be thought of as the Solar atmosphere. This streams outwards though the Solar system and is referred to as the Solar Wind. The release of energy in the production of a Solar flare kicks a large bubble of plasma from the Sun's surface layer. This bubble, known as a "Coronal Mass Ejection" (CME), travels at high speed, between 300 and 900 km/second, and because it is a cloud of fast moving charged particles it carries with a strong magnetic field.

A Geomagnetic storm is created when this magnetic field of the plasma bubble collides with the Earth's field at the boundary of the Magnetosphere. The severity of a storm is best indicated by the K-index [4]. This is a logarithmic scale with 0 and 1 indicating very quiet conditions and 8 and 9 indicating a severe storm. The K-index values are calculated from observations at each magnetic observatory and are then processed to produces a planetary (average) K-index Kp.

Occasionally a region of the Solar "atmosphere" forms a "hole" which allows the coronal material to flow out in greater volumes that the normal Solar Wind, These are referred to as high speed streams and if they collide with Earth produce storming. The levels as usually around Kp=5 at their strongest, and their impact on LF/MF propagation is usually not as severe as that due to CME. There is a fascinating animation on the NOAA web site which predicts the position of the streams with respect to Earth [5].

Geomagnetic storms were observed to cause a decrease in LF signal strength of long paths when these bands were used for worldwide communications in the 1920s. It was also noticed that the signals did not recover immediately the storm had passed and the K-index returned to low values. It was discovered that the stronger the storm the longer the poor conditions would last. The causes of this were not discovered until rocket sounding of the near Earth environment was started in the 1960s. This began with the discovery of the Van Allen ionisation belts between 4 and 9 Earth radii out, in the region occupied by the geostationary communications and TV satellite orbits.
 

The Equatorial Ring Current

The cause of the long delayed recovery of LF/MF propagation after a geomagnetic storm was not solved until after the discovery and exploration of the Van Allen belts. They are located 4 to 9 Earth radii out into space, with positive ions rotating in one direction and electrons the other way. Whilst the ions rotate over the equator the lighter electrons circle the north-south lines of magnetic force being reflected onto a reverse path as the lines bunch together near to the Poles. This trapped charge is supplied by the Solar wind, and the state of the rings can be detected by the changes they induce on the normal geomagnetic field.

When a CME plasma cloud approaches the Earth and collides with the Earth's Magnetosphere (the sheath of force lines generated by the Earth's iron core) there are two possible outcomes. These depend upon the direction of the magnetic field carried by the CME. If the field is opposing the Earth's field (north pole to north pole) the plasma cloud "bounces off" the magnetosphere There may be small disturbances seen on the magnetometers but there is generally no effect on LF/MF propagation. If however the fields "connect" (north pole to south pole) then there are weak points in the magnetosphere at the poles where charged particles may enter the magnetosphere and even the atmosphere. In this latter case they produce strong auroral effects often far south of normal aurora sightings.

Some charged particles also enters the Ring Current [6]. The main part of the plasma is guided round the magnetosphere and becomes trapped in the magneto-tail on the side of Earth remote from the Sun. As the charge in the tail builds up the magnetic field there becomes very distorted like a wound up elastic band. Eventually the energy in the tail is released as the magnetic field snaps back to a lower energy state and like its start at the Solar surface. As a result, two masses of the trapped plasma are propelled in opposite directions one towards Earth, where because it is now inside the Magnetosphere enters the Ring Current, and the other onwards towards to outer solar system.
 

The Dst Index

The state of the Ring Current is measured by the "Disturbance storm-time" (Dst) index, which measures the magnetic field due to the circulating charge in the Ring Current. Field measurements are taken by a number of observatories and corrected to represent the equivalent field at the Earth's Equator.

The field due to the Ring Current varies between -20 nT in quiet conditions and -500 nT in a severe storm. This value has to be measured as a deviation against the Geomagnetic field which has a value of 50000 nT (50 μT). Earthbound magnetometers are notoriously difficult to use and generally Dst values are only quoted after significant averaging and processing to remove rogue values. Kyoto University [7] does however produce an hourly real time chart of the their estimate of the Dst index, which is continually massaged to remove rogue readings. Colorado University [8] publishes plots of their estimate also on a real-time basis, but their estimate is based on measurements derived from the ACE satellite Solar Wind data, and they plot this together with the Kyoto values. The Colorado University estimates are smoother and more consistent but between 10 and 20nT lower than Kyoto.

At this stage one might wonder where all this is leading, but a quick look at the Dst plots shows that they do in fact mirror the propagation on the LF/MF bands. There is a deep dip in the Dst after a CME impact, the Dst drops from -20 nT to more than -100 nT but then returns slowly following a logarithmic curve over the following days. This can be explained by considering the Ring Current as a reservoir of energetic (hot) electrons which enter the ionosphere at sunset and sunrise, when the magnetosphere is distorted by the solar wind pressure as parts of the earth move round to face the Sun. This distortion allows the electrons to "leak out" of their "magnetic bottle" and enter the ionosphere, slowly diffusing south from the polar regions.

These hot electrons have the same effect on radio propagation as the photo-dissociated electrons produces by EM radiation from the Sum in daytime. The difference is that they are "hotter" or moving very fast. This means that if they encounter an ion or atom of the atmosphere they will tend to bounce off losing little energy in the process. These so called "precipitated electrons" (those from the Ring Current) take much longer to decay, or be captured. Some plots of station strength after a CME impact show the signal recovering at the rate of about 5 dB per hour after dark, whereas the photo-dissociated electrons in the D-layer decay almost immediately the radiation is removed. This does not explain why after severe storms the propagation may be depressed for up to 30 days except, you may remember that injection occurs at the sunrise and sunset edges. The Dst index can be considered as a measure of the quantity of charge in the ring current, and the logarithmic recovery suggests a concentration driven diffusion process. Thus the recovery of the propagation conditions seems to closely follow the Dst or the quantity of charge in the Ring Current. We can use this to estimate the length of a period poor conditions. It also shows how the "poor" period is extended when the Ring Current charge is "topped up" by further CMEs.

LF/MF propagation conditions usually start to display "good" conditions when the Colorado University Dst estimate rises to around -20 nT. However this is not the only effect causing changes in propagation, and just because the Dst is -20 nT does not guarantee that there will be good propagation.

This is demonstrated by a plot of the signal strength from an LF military station, The Canadian Naval station whose call-sign CFH, is located at Newport Corner just outside Halifax in Nova Scotia and transmits on 137 kHz. During 2003 it was transmitting continuously with a steady radiated power of 20 kW. The signal as received on the East Coast of the UK was recorded with samples every minute. This data was re-analysed recently to plot the propagation against the Dst index in the preceding 12 hours. The "propagation quality" was defined as the percentage of the time the signal exceeded a given strength, as a proportion of the darkness hours for the path.

The plot shows that there is a sharp cut off around -60 nT at the selected minimum signal level as the Dst index is depressed. It should be noted that this path is around 4500 km so is a 2-hop path. Two passages through the ionosphere mean two doses of attenuation. The longer multi-hop paths are more sensitive to this attenuation that 1000 km paths around Europe, though these will show poor results for signals close to the signal-to-noise threshold.

The region of precipitation is around the magnetic poles and the hot electrons diffuse away from that point. Thus the effect of the attenuation is reduced for paths at lower latitudes. This was demonstrated by observations of apparently good propagation from Southern Europe to the French Island of La Reunion at the same times as North Atlantic paths from Newfoundland to Europe being heavily attenuated. Conversely high latitude paths such as to Alaska suffer immediate high attenuation plus effects due to the aurora curtain and Polar Cap Absorption (PCA), which is caused by Proton storms.

The above notes give a broad idea of the propagation features of the LF-bands. A basic understanding will hopefully assist in trying to select the right transmission modes for a particular path. In particular the fading period due to multipath effects varies with the band used and as the frequencies move higher so the fading gets to be more rapid. It is essential to select a transmission mode that is not made incomprehensible by long deep fades.

Propagation varies over the year and in winter it is often possible to hear Newfoundland stations well into daytime, and for European station to be heard well before sunset in North America. Very long paths such as Europe to Australia have been claimed and often the times indicate that the path does not have to be in darkness over complete signal travel.

Remember propagation prediction is a difficult job to do accurately, it can become a self-full-filling "prophecy", the forecast is bad, so its not worth transmitting, so nobody is heard, so propagation is bad !! It is always worth trying a call on a quiet band to catch those conditions that cannot be predicted. Very occasional QSOs have caught the lift of an unpredictable Solar flare for improved daylight range. Some of the early transatlantic crossings on 136 kHz were made due to coinciding with favourable fading peaks for long enough to complete the contact.
 

References

[1] The late Reg Edwards Software available via http://www.wireless.org.uk/g4fgq/

[2] 

[3] Peter Martinez,  Radcom

[4] National Oceanic and Atmospheric Administration (NOAA)

[5] Solar wind prediction

[6] The Terrestrial Ring Current: Origin, Formation, and Decay, Daglis, et al, Rev. Geophys.,37(4) 407-436, 1999

[7] Kyoto University Dst index page

[8] Colorado University Dst data

Further reading: The Solar Terrestrial Environment, J.R.Hargreaves, Cambridge University Press (1992)