Map of tropospheric ducting QSO on 144 MHZ on 12/jul/2021
Map of marine tropo QSO above 2000 Km. made from D4 on 144 MHz in 2021.
This is one of the questions that every beginner on the VHF bands has ever asked himself and one of the questions that creates more confusion and errors when reporting a contact in the DX-Cluster, even among the most expert.
Getting to know the propagation mode with complete certainty is not always an
easy task and in some cases it is literally impossible to determine it just by
listening to an isolated signal. In addition, in some cases it is necessary to
listen to different signals from the same area during a certain period of time
in order to reach a conclusion, an isolated contact is not enough. In other
cases it is necessary to take into account the date and time, since certain
modes of propagation do not occur on any date or at any time.
Below I summarize the main characteristics of the most common propagation modes
used by radio amateurs and listeners, to facilitate the task of their
identification. Some audio recordings of actual signals are also included.
This article does not pretend to be exhaustive or to detail all the existing
modes and submodes of propagation. Dozens of articles could be written about each
particular mode and sub-mode.
Commonly known as a "tropo". It is produced by alterations in the atmosphere closer to the earth's surface, usually linked to the appearance of zones of thermal inversion that cause the signals to travel in a kind of natural waveguide. Some distinguish between "marine tropo" (AKA "maritime tropo") and "terrestrial tropo" depending on whether the signal travels above the surface of the sea or the land, but there is no evidence that there is any difference between them other than that.
The most common and spectacular due to the distances at which it allows two stations to be connected is the one that occurs over the sea, since there are no obstacles (mountains, etc.), the signal suffers very little attenuation during its travel. Obviously, "marine" and "terrestrial" tropo conditions can occur at the same time, what further increases the possible distance of the contacts when both are combined.
Affected bands: All VHF, UHF and SHF bands. It is very common in the bands of 2m and above, and much less prevalent in the 4m and 6m.
Dates / times: The "marine tropo" is very frequent in the summer months and very rare in the winter, while the "terrestrial" often occurs outside the summer season. Usually the strongest signals occur around dusk, but not in all cases. A few hours after dark the signals usually weaken.
Duration: From several hours to several days in a row. It never occurs for just a few minutes.
Signal strength: Variable, but extremely strong signals are not uncommon, often leading neophytes in the 2m band to confuse it with Sporadic E. Regardless of the signal level, it is normally very stable, with little variation of its intensity.
Fading (QSB) of the signal: Normally the signals are very stable, with little or no variation in their intensity for long periods of time. This is one of the main characteristics that distinguish this mode from Sporadic E, along with the duration of the openings.
Signal alterations: The signals propagated by this means do not suffer noticeable alterations or distortions, so communications in SSB, CW, digital and even FM are possible.
Distance covered: The usual distances range from 500 to 1000 km, but contacts several thousand km away in the case of the "marine trope" are not uncommon, provided that the signal travels at all times above from sea.
Predictability: It can be predicted to a certain extent by analyzing atmospheric conditions. In this sense, William Hepburn has an excellent page with forecast maps worldwide.
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Map of marine tropo QSO above 2000 Km. made from D4 on 144 MHz in 2021. |
It is also known as a "tropo" although the signal propagation mechanism is totally different from that of tropospheric ducting. In this case the signal is scattered in an area of the troposphere that is visible by both antennas.
The "Troposcatter" consists of the scattering of radio waves in the troposphere, caused by irregularities in the atmosphere. The troposphere is the part of the atmosphere that goes from the surface of the earth to the tropopause, which is at an altitude of approximately 10 km. Above the tropopause the temperature is constant, there is very little humidity and the air is calm, therefore there are very few irregularities that can scatter radio signals.
Affected bands: All VHF, UHF and SHF bands. Tropospheric Scattering is independent of the frequency from 144 MHz to above 10 GHz. At lower frequencies the wavelength becomes larger compared to the typical scattering cell, so its effect is less. Above 10 GHz, dispersion also takes place, but the absorption of oxygen and water vapor must be taken into account. However, even at 10 GHz this absorption does not represent more than 5 dB of losses per 1,000 km.
Dates / times: The height of the troposphere varies from summer to winter and also according to latitude, which influences the possibilities of DX. The tropopause is the upper limit of the troposphere, which contains most of the cells that scatter our radio signals. The higher the tropopause, the higher the cells will be and consequently the greater the distance at which we can work and the stronger the signals will be.
One of the reasons that conditions are better in summer is because the tropopause is higher. The other is that the refractive index is higher, which extends the radius horizon.
Duration: It is always present to a greater or lesser extent, every day of the year and at any time.
Signal strength: In general, signals will never be very strong and require antennas with a certain gain and a medium or high power level.
Fading (QSB) of the signal: The slow QSB is produced by general changes in the refractive conditions of the atmosphere, while the fast QSB is due to the movements of the small irregularities responsible for the dispersion process.
The frequency of the mid fading of the signal increases proportionally with distance and frequency. On VHF it is a few fades per minute, on UHF a few fades per second and in 10 GHz it is about twenty per second, which produces shaky audio. The frequency of this "fading" is inversely proportional to the dispersion conditions. The better the conditions, the lower the QSB.
Signals are generally relatively stable, with a QSB of about 13 to 20 dB at distances less than 300 km and only a few dB at distances greater than 500 km.
Signal alterations: A scattering is made up of multiple simultaneous reflections from many small objects (scattering cells). If all these resulting signals arrive in phase at the receiver, the dispersion is said to be coherent and the signal quality is perfect. If these signals arrive with different phases, the dispersion is incoherent and the signal sounds distorted. Forward tropospheric scatter is nearly consistent, so signal quality is good when using SSB, CW, or digital modes.
In long distance contacts the signal suffers a slight distortion because the angle of dispersion increases with the distance (In a QSO at 700 km the angle of dispersion is about 5 degrees). Side scatter or back scatter contacts are also possible for better equipped stations. In these cases the antennas are not pointing towards each other but rather both are pointing to the area of the atmosphere where the scattering occurs. The back scatter signals sound distorted, similar to that of Aurora or FAI.
Distance covered: Between 100 and 700 km. As the distance between the stations increases, the dispersion takes place at greater heights of the troposphere. For the most distant DX, the lower part of the “common volume” (volume of the troposphere visible by both antennas, taking into account their radiation lobes) is several km high. One effect of this is that a tall mountain midway between two very distant stations has no influence on the signal since the scattering occurs above the mountain.
Predictability: It is always present to a greater or lesser extent, every day of the year and at any time.
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Propagation by Sporadic E (also called "Sporadic", "E-Sporadic" or "ES") occurs due to an unusual ionization of regions of the E layer of the ionosphere, between 90 and 120 km high. These regions are commonly called "sporadic clouds" and their size, thickness and level of ionization determine the characteristics of the particular opening.
In order to make contacts, it is necessary to have a line of sight with the "cloud" and to be at a certain distance from it, a distance that varies according to its level of ionization. Contrary to what would seem logical, a higher level of ionization is necessary to be able to make contacts at shorter distances ("short skip").
Affected bands: All VHF bands up to 300 MHz and even the 10m and 12m HF bands. The duration and intensity of the opening is greater as lower is the frequency. Sporadic openings are common on 28 and 50 MHz, less common on 70 MHz, much less common on 144 MHz and extremely rare on 220 MHz. No Sporadic E has ever been detected that would allow contacts at frequencies above 300 MHz.
A remarkable characteristic is that the maximum usable frequency (MUF) is directly proportional to the ionization level of the sporadic cloud. In other words, if the sporadic has reached a MUF of 100 MHz, communications can be made at any frequency lower than 100. MHz, but not at higher frequencies. For lovers of VHF bands, part of the hobby is to follow the evolution of the MUF, in order to know in which bands to pay attention for possible contacts.
Dates / hours: The openings of Sporadic E occur mainly in the summer months of each hemisphere, although there is also a much less intense sporadic season in the winter months. Most of them take place in daylight hours, being very rare several hours before dawn or several hours after dusk. For the 28 and 50 MHz bands in the northern hemisphere, they usually occur from mid-April to mid-September and in 144 MHz from mid-May to mid-August.
Duration: From a few seconds to several hours. In the case of openings of a few seconds it can be difficult to distinguish it from meteor reflections, being necessary to evaluate the general conditions of the band in other locations. For example, if we are in Europe and there is a sporadic cloud over Europe, at a suitable distance, it is most likely that the signal has been reflected in the cloud, whose MUF has risen momentarily.
The duration of the openings of Sporadic E is inversely proportional to the frequency, that is to say that the same opening can last hours in 50 MHz, slightly less in 70 MHz and only a few minutes in 144 MHz, in case it reaches the MUF required.
Signal Strength: Sporadic E signals vary rapidly and constantly in intensity, but it is common to hear huge signals, as if they were from a local station.
Fading (QSB) of the signal: Usually a very fast and pronounced QSB is produced. The signals go from being extremely loud to totally disappearing in seconds, and vice versa. This is one of the main characteristics that allows it to be distinguished from tropospheric or "tropo" propagation. The QSB is more pronounced as higher is the frequency, that is, the signals are more stable at 50 MHz than at 144 MHz, for example.
Signal alterations: The signals propagated by this means do not suffer noticeable alterations or distortions, so communications in SSB, CW, digital and even FM are possible.
Distance covered: The jump distance depends on the specific height of the cloud in the E layer of the ionosphere and its ionization level. About 600-2500 km for frequencies below 80 MHz and 800-2500 km for 144 MHz. In the 50 MHz band and below it is quite common that sporadic multi-hop conditions take place, intercontinental contacts are not rare at great distances thanks to 3, 4 or even more consecutive and aligned jumps. At 144 MHz double hop contacts also occasionally occur, but none have ever been made for three hops or more. Back scatter conditions can also occur at 50 MHz.
Predictability: The cause of the exaggerated ionization of the E layer that causes sporadic-E openings is unknown, therefore it is totally unpredictable in the medium term. There are various theories about its cause, from the effect of the disintegration of meteorites to others that relate it to a compression of the plasma due to the ascending atmospheric conditions of lower layers. The truth is that none of these theories explains the phenomenon 100%, so it is most likely due to a "sporadic" combination of various factors. In the short term it is possible to follow the evolution of the MUF and when it rises approaching the working frequency, it will be the time to pay attention to the band of interest, always pointing the antenna towards the cloud.
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Map of some QSO by sporadic-E double hop on 144 MHz |
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Map of some QSO by multi-hop sporadic-E on 50 MHz in 2021 |
The propagation by Meteoric Scatter (or MS) occurs due to the ionization produced by meteorites when they fall on the earth and disintegrate in the E layer of the ionosphere, between 90 and 120 km of height. In the same way that meteorites leave a light trail (shooting star) they also leave an ionized trail that allows the momentary reflection of VHF signals.
As the duration of the reflections is very short, normally less than a second, it requires special techniques to be able to complete a contact between two stations, with the exception of the most important meteor showers in which even very brief contacts are possible in SSB.
Affected bands: All VHF bands and even eventually UHF contacts have been made between stations with large antennas and good power. Reflections are more frequent, of longer duration and intensity in the lower bands (50 and 70 MHz), decreasing as the frequency increases. At 144 MHz it is still relatively easy to make contacts by this mode, but at higher frequencies like 432 MHz it is only possible between stations with large antennas and a lot of power, being difficult even so.
Dates / hours: Two types of meteorites are distinguished: those associated with a shower (The Perseids in August are the best known and most important) and sporadic ones, the latter being present any day of the year at any time, since the earth is impacted constantly by small meteorites no bigger than a grain of rice.
For a meteor shower to generate trails, it is logically necessary that its radiant (an imaginary point in the sky from which meteorites appear to radiate) is above the horizon. The main sources of information on dates and best times of the main rains is the website of the International Meteorite Organization (IMO).
On the other hand, sporadic meteorites, despite falling 24 hours a day, produce more intense signals in the early hours of the morning, from a few hours before sunrise to a few hours after it.
Duration: With the exception of the meteorites of the most important showers, which can generate large trails that maintain sufficient ionization for several seconds (even several minutes), 99.9% of the reflections have a duration of less than one second, in many cases a few tens of milliseconds, so they are only useful for the purpose of making contacts using specialized techniques, which currently rely on the use of high-speed digital modes, mainly the MSK144.
The duration of the reflections is longer in the lower frequencies (50 MHz) and shorter as it increases.
Signal strength: The signals are normally of medium intensity, but depending on the level of ionization they can become very intense, almost as if it was Sporadic E. The degree of ionization depends on the mass of the meteorite and its speed when passing through the ionosphere.
Fading (QSB) of the signal: Being an intermittent mode and normally with very short reflections, it does not make much sense to talk about QSB, although it is true that in reflections of several seconds, during main showers, a very pronounced QSB is noticeable during the reflection, in a similar way to Sporadic-E.
Signal alterations: The signals propagated by this system can be affected by the Doppler effect, depending on the relative direction and speed of the meteorite.
Distance covered: Normally between 800 and 2300 km for "normal" contacts in which the meteor trail is approximately halfway between the two antennas. However, it is also possible to make oblique contacts (side scatter) and even backwards (back scatter) for distances below 800 km. On very rare occasions and only between stations with large antennas and high powers, double jump contacts have been achieved, taking several hours to complete the QSO.
Predictability: In the case of meteor showers, it is possible to calculate the days and hours with the best conditions (see IMO website). One of the most important parameters to take into account is the ZHR which indicates the expected number of meteorites per hour.
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Map of some QSO by MS in the USA on 50 MHz in January 2021 |
This is a fairly "exotic" mode of propagation that is produced by alterations in the magnetic field at certain specific points on the planet that cause the signals to bend along the lines of the magnetic field. The specific mechanisms by which these alterations occur are unknown.
In Europe the most important point and of which I personally know is located in the western part of the Alps and the stations always pointing their antennas towards that point can contact each other. From the Spanish east, for example, you can contact French stations, northern Italy, Croatia, Slovenia, Hungary, Austria, etc. It is necessary to emphasize that all stations must beam to the point where the magnetic field alteration occurs, not to each other.
Affected bands: It is a relatively common phenomenon in the 144 MHz band, with no evidence that it affects other bands. It is possible that the fact that it requires high gain antennas prevents it from being detected in lower bands such as 50 MHz, where groups of 4 or 8 phased antennas are not common.
Dates / hours: FAI openings normally take place in the late afternoon or early evening, during the months of May to August in the case of the northern hemisphere.
Duration: From a few minutes to several hours.
Signal strength: Signals are normally very weak, requiring high gain antennas and enough power to make contacts.
Fading (QSB) of the signal: The signals are quite stable, with little QSB.
Signal alterations: The signals propagated by this system suffer from a marked distortion similar to the Aurora signals, the use of SSB is impossible in most cases and therefore practically restricting its use to CW. It is therefore not possible to use digital modes, so since the mass adoption of modes such as the JT65 or the FT8 the number of QSO by FAI has been reduced to its minimum expression, but in the 1980s and 1990s it was a propagation mode frequently used by many stations in CW and even in SSB.
Distance covered: Normally between 800 and 2300 km.
Predictability: It cannot be predicted in the medium term, but FAI openings frequently occur at the end of a sporadic-E opening.
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We all have in mind the spectacular curtains of the Northern Lights (Northern Hemisphere) and the Southern Lights (Southern Hemisphere). In a similar way, electrically charged particles from the solar wind are trapped by the earth's magnetic field and redirected towards its magnetic poles, where they produce plasma instability at an altitude of 80-120 km. As a consequence, this region of the E layer of the Ionosphere causes the known blocking of HF signals, but at the same time it can cause the deviation of VHF signals, allowing communication over long distances.
It is necessary to emphasize that all stations must point their antennas to the point where the alteration of the E layer occurs, not to each other.
On the other hand, due to these alterations occurring near the earth's magnetic poles, it is a propagation mode normally only usable by the stations closest to said poles. In the case of Europe, the stations of the northern countries (United Kingdom, Denmark, Sweden, Finland, Norway, etc.) can contact each other relatively often when the Aurora opening occurs, but the stations further south will rarely be able to contact each other or take advantage of them. Under very, very exceptional conditions (maybe once every eleven years or so) there have been stations in the south of France, and even Spain and Italy, that have made some contact by Aurora.
Affected bands: All VHF bands, but the signals are better when the frequency is lower.
Dates / hours: Aurora openings are dependent on solar storms, but tend to be more frequent on dates close to the equinoxes, mainly in the afternoons and nights.
Duration: From a few minutes to several hours.
Signal strength: Signals are normally very weak, requiring high gain antennas and enough power to make contacts.
Fading (QSB) of the signal: The signals are quite stable, with little QSB.
Signal alterations: The signals propagated by this system suffer from a marked distortion, being difficult to use SSB in most cases and therefore practically restricting its use to CW. It is therefore not possible to use digital modes.
Distance covered: Normally between 800 and 2300 km.
Predictability: It can be predicted to some extend following the conditions of the Sun and specifically the so-called solar storms. In this sense, the DXMAPS propagation maps have the possibility of selecting an "Aurora Prediction" layer that overlays the prediction data generated by NOAA on the maps.
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Map of QSO by Aurora on 144 MHz in April 2021. QSO lines are from station to station and do no correspond to the real path followed by signals after being bent by the Aurora |
As the name suggests, this mode uses the Moon to reflect signals back to Earth. It is a very specialized mode that, due to the high losses of the Earth-Moon-Earth circuit, generally requires large groups of antennas and high power to perform QSO on a regular basis. However, since the advent of the JT65 mode and its much higher sensitivity than CW, there are many stations with modest installations (a single antenna and a few hundred watts) that eventually make Moon Bounce contacts.
Ideally, the ability to raise the antenna is required to follow the Moon, but even stations that do not have elevation can make contacts at the rising and setting of our satellite. The vertical radiation lobes of a Yagi allow EME to work while the Moon is less than 15 degrees above the horizon, providing approximately 3 hours of operation on each "pass" of the Moon.
Affected bands: All VHF, UHF and SHF bands. Each band has its particular challenges since the attenuation of the circuit, Doppler effect, cosmic noise, etc. depend on the frequency.
At 50 MHz the activity is relatively low due to the prohibitive size of the
antennas and in practically all cases the JT65A digital mode is used.
The band with the most activity by far is 144 MHz, where the JT65B mode is mainly
used, there is still a residual activity in CW.
At 432 MHz there is also activity, although less, mainly in the JT65C mode.
On 1296 MHz and microwaves the activity is much lower, also in JT65C mode and
to a lesser extent in CW and SSB, because due to the possibility of installing
high gain parabolic antennas the signals can be stronger.
Currently (summer 2021) they are experimenting with a new Q65 digital mode, which could replace the JT65 in the medium term.
Dates / times: Obviously the essential requirement to make a Moon Bounce contact is that both stations have the Moon visible. However, there are several factors that affect the signal strength, such as the distance between the Earth and the Moon and the position of the latter in relation to the cosmic background (greater cosmic background noise in some regions of the sky).
Duration: Contacts are possible while the Moon is above the horizon.
Signal strength: Signals are normally extremely weak, requiring high gain antennas and enough power to be able to make regular contacts.
Fading (QSB) of the signal: Signals when crossing the ionosphere undergo random changes of unpredictable duration in their polarization, which is constantly rotating (Faraday rotation). When using linearly polarized antennas, the signal can vary by up to 20 dB if the polarization of the signal is 90 degrees out of phase with the polarization of the antenna. To minimize this effect, it is common for EME stations to use dual polarization antennas (horizontal and vertical) in order to be able to choose the most convenient at a certain time and some even use dual reception systems (sometimes called "stereo"), which allow to receive both polarizations at the same time.
Signal alterations: The signals propagated by this system suffer from the Doppler effect caused by the movement of the Moon. The variation of the frequency due to the Doppler increases with the frequency, being little important in the lower frequencies (50 and 144 MHz) but it can become a problem in higher frequencies, especially in microwaves. The Doppler at a given time can be calculated by specific Moon tracking programs.
Distance covered: It is possible to contact any place on Earth, since the two antennas at one time or another will always have the Moon visible simultaneously, which is called a "common window". The more remote the stations are, the shorter the duration of said "common window" will be.
Predictability: EME conditions have a predictable part and an unpredictable part:
The distance between the Earth and the Moon and the position of the Moon on the noisy cosmic background are predictable and can be calculated by specific Moon tracking programs that calculate the degradation in dB (DGRD) from optimal conditions, which occur when the Moon is as close to Earth as possible and is also in the region of the sky with less cosmic noise. The lower the DGRD, the better the conditions.
The Faraday rotation, which is the polarization rotation that the signal undergoes as it passes through the ionosphere, is unpredictable.
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Ionospheric scatter ("Iono scatter") is due to alterations of the D layer of the ionosphere induced by solar radiation.
Affected bands: Better at low VHF frequencies such as 50 MHz. At 144 MHz it is possible but unusual.
Dates / times: The best openings take place in summer. The best hours are around noon and the worst after dark.
Duration: Several hours.
Signal strength: Signals are normally very weak, requiring high gain antennas and enough power to make contacts. At 50 MHz the losses in the circuit are estimated at about 225 dB.
Fading (QSB) of the signal: Signals are quite stable, with little QSB.
Signal alterations: There are no significant signal disturbances, but due to weak signals it is only possible to use CW or weak signal digital modes (JT65, FT8, Q65 ....)
Distance covered: Normally between 900 and 2000 km.
Predictability: They cannot be predicted in the medium term.
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Trans-equatorial propagation allows stations in the northern hemisphere to contact stations in the southern hemisphere (and vice versa) that are approximately at the same distance from the magnetic equator, as long as the signal passes through it approximately perpendicularly.
There are two types of Trans-equatorial:
Affected bands: All VHF and UHF bands. In 50 MHz and 70 MHz it is quite common, while in 144 MHz it is only common in the American sector.
In the Euro-African sector, quite a few openings of Type II TEP were detected on 144 MHz between 1978 and 1990, no more having been reported since then, for unknown reasons. In the same way, between 1978 and 1999, quite a few openings were reported on 144 MHz between Japan and Australia, neither having been reported any more later.
Dates / times: TEP occurs on dates around the equinoxes, especially in the autumn, approximately between 2 p.m. and 7 p.m. for type I TEP and between 8 p.m. and 11 p.m. for type II TEP (local hours). It is highly influenced by solar activity, so good TEP openings (at least Type I) occur in the years of maximum activity in the solar cycle. In the case of Type II TEP, this relationship is not so clear.
Duration: From a few minutes to several hours.
Signal strength: Signals are usually quite strong, although not always.
Fading (QSB) of the signal: Signals have some QSB, sometimes quite pronounced.
Signal alterations: The signal can present minor Doppler variations and in many cases a significant distortion that makes it difficult or even prevents the use of digital modes.
Distance covered: Normally between 6000 and 9000 km in the case of Type I and between 3000 and 8000 km for Type II.
Predictability: They cannot be predicted other than by their occurrence on the dates and times mentioned above.
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Map of TEP QSO on 144 MHz between JA & VK |
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Map of TEP QSO on 144 MHz in the American sector |
Map of some TEP QSO on 50 MHz |
Rain scatter is due to the scattering that SHF signals undergo when colliding with raindrops. It allows communications between two stations that simultaneously see the rain zone.
Affected bands: Only in SHF bands, mainly used in 10 GHz.
Dates / hours: At any time since it only requires that there is a common rain zone to the correspondents.
Duration: As long as the rain lasts.
Signal strength: Signals are normally very weak, requiring high gain antennas and enough power to make contacts.
Fading (QSB) of the signal: I do not have data regarding this issue.
Signal alterations: I do not have data regarding this issue.
Distance covered: I do not have data regarding this issue.
Predictability: That associated with weather forecasts.
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As its name indicates, this mode of propagation is the result of ionization of the F2 layer of the ionosphere. It is the same propagation mode that allows most contacts on the HF bands.
Affected bands: Only the 50 MHz band, apart from HF. The maximum possible frequency is about 60 MHz.
Dates / hours: As it requires a very strong ionization of the ionosphere, it can only occur during the years of the maximum solar cycle and only if the solar flux (SFI) is extraordinarily high. As in the case of the 28 MHz band, it usually only takes place during daylight hours and until shortly after dark.
The last 50 MHz "pure" F2 opening that I am aware of occurred in March 2002 during the peak of Solar Cycle 23, as Solar Cycle 24 was too weak to allow 50 MHz F2 contacts.
Duration: From a few minutes to several hours
Signal strength: The signals are normally very strong, with modest antennas and little power being enough to make the contacts.
Fading (QSB) of the signal: It usually follows a fast-slow-fast pattern.
Signal alterations: The signals propagated by this means do not suffer noticeable alterations or distortions, so communications in SSB, CW, digital and even FM are possible.
Distance covered: Between 3000 and 12000 Km.
Predictability: Hardly predictable, except for the maintenance of the Solar Flux Index (SFI) at exceptionally high values for several days.
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