<PSC> Propagation explaned

Propagation of HF radiowaves like 11m depend on a certain factors. On this page you can learn about the basics of HF propagation and become an even more skilled DX'er, with all the knowledge you can find here:

1. Radiowaves:

Radiowaves are electromagnetic waves that propagate with a speed near 300000km/s. Electromagnetic waves have a frequency and wavelength. There are different type of waves, with a very high or very low frequency. Even visible light is an electromagnetic wave, that has a very short wavelength. Your own eye is antenna, and you probably did not know!

Frequency or wavelength can be easily calculated with these two formulas:

Frequency (MHz) = 300 / Wavelength (m). For example a wavelength of 11m would have the following frequency: 300 / 11 = 27.272 MHz.

Wavelength (m) = 300 / Frequency (MHz). For example a frequency 27.555 MHz would have the following wavelength: 300 / 27.555 = 10,887m

In the world of communication the different wavelengths or frequencies have been divided into:

LF (Low Frequency)                    = 0,03 - 0,3 MHz (1000m to 10000m band)

MF (Middle Frequency)               = 0,3 - 3,0 MHz (100m to 1000m band)

HF (High Frequency)                   = 3,0 - 30 MHZ (10m to 100m band)

VHF (Very High Frequency )        = 30 - 300 MHz (1m to 10m band)

UHF (Ultra High Frequency)         = 300 - 3000 MHz (1m to 10cm band)

SHF (Super High Frequency)       = 3000 - 30000 MHz (10cm to 1cm band)

There are also differences in properties for these wavelengths. LF waves easily penetrates through dense materials like concrete, soil, etc. They can also follow the curve of the Earth, which makes LF ideal for long distance groundwave communication over more than 1000 kms. Submarines even use VLF (Very Low Frequency) radiowaves, because they can even travel through the Earth's core!

The higher the frequency becomes, the less they bend along the Earth's curve, and the less they can penetrate dense materials. 11m waves follow the Earth's curve only little.

2. Earth's atmosphere:

Before we explain the physics of HF propagation, it is good to know how the Earth's atmosphere is of great influence on propagation. Let us look at how Earth's atmosphere looks like:

Fig.1 The Atmosphere

2a. Troposphere: The lowest part of the atmosphere is the troposphere. This part of the atmosphere produces the weather as we know. The troposphere ends at roughly 14km under a small layer, the so called tropopause. Only the tops of large thunderstorm clouds (so called Cumulonimbus Incus) occasionally reach to 14km, and some even push up the tropopause. The troposphere doe not have great influence on HF propagation, but can sometimes extend normal  groundwave propagation, especially on 11m.

2b. Stratosphere: This part of the atmosphere doe not influence HF propagation. The upper part of the stratosphere holds the ozone layer, which filters harmful ultraviolet (UV) radiation. The only clouds you find here are so called Noctilucent iceclouds which are visible around midsummer evenings.

2c. Mesosphere: The mesosphere holds the so called D-layer. The most lower part of the ionosphere. The D-layer has an absorbing effect on HF radiowaves, especially up to 10MHz.

2c. Ionosphere: The ionosphere is very important for propagation of HF radiowaves. It holds the so called E-layer, F1-layer and F2-layer. These layers appear under influence of solar radiation.

3. Three types of propagation:

We divide propagation in three types:

3a. Groundwave: groundwaves are waves that propagate along the ground. Your favourite station on FM for example uses groundwaves. How far groundwaves can travel depends on the height of the antenna. That is why commercial broadcast stations on FM use large towers or are located on the highest mountains. 11m radiowaves usually travel around 30-50km on groundwave, with the antenna at an average antenna height of 10m. The less obstacles the radiowave encounters, the stronger the signal will be. Groundwaves across large surfaces of water travel much further than groundwaves in mountainous areas. Once the groundwave cannot follow the Earth's curve anymore, it travels into the sky and into space!

3b. Tropospheric: sometimes groundwaves travel further than theoretically possible. Distances up to 100km and even more are possible. This type of propagation is called Troposheric or "Tropo". In the troposhere, there are different layers of air, with different temperatures and different humidities. When it's windy, these layers of air are mixed together. But when it is not windy these different layers of air can exist. When you are in the center of a high pressure area, weather is very quiet. In the morning, the air layer close to the ground is relatively cool and moist (sometimes it produces fog or mist!), while the area above it is relatively warm and dry. The change of temperature can be easily 10°C over 100m, and is called an inversion. Now the cold layer of air is more dense than the warm layer of air. The sharp transition between cold and warm air, a temperature inversion, refracts radiowaves in the VHF and UHF bands. On some occasions there can be multiple inversions. Once a radiosignal has been caught between two inversions, it can travel in between like travelling through a kind tunnel. This propagation mode is called tropospheric ducting.Ducting has only been reported on VHF and UHF.

An inversion effect VHF and UHF bands most, but the higher HF bands like 10m and 11m also very well. There is QSL of contacts over 350km on a day with paths across stationary high pressure areas, and no reports of any ionospheric propagation at all on the same day.

3c. Ionospheric: radiowaves (skywaves) can travel far distances because they can be reflected to the Earth's ionosphere. They call such a reflection a "hop". The radiowave that is being reflected by the ionosphere can travel back to Earth, bounce of the Earths surface back up again to the ionosphere where it can be reflected down and up again (multihop propagation).

4. The Ionosphere:

The ionosphere is a thin layer of air. It is called ionosphere because it is formed by ions. Ions are charged particles that appear under the influence of solar radiation (ultraviolet and X-rays). These ions have the capability to bend or reflect a radiowave. That capability depends on the density of ions, the more ions the stronger the reflection. The maximum frequency that the ionosphere can reflect is called the MUF or Maximum Usable Frequency.

We read earlier that the ionosphere consists out of 4 layers:

D-layer: only absorbs radiowaves, especially under 4-5MHz. Appears very fast after sunrise, and disappears almost immediately during sunset.

E-Layer: reflects radiowaves upto 5MHz, radiowaves above 5Mhz are absorbed, but less than in the D-layer. Appears shortly after sunrise, and disappears shortly after sunset.

F1-layer: reflects radiowaves upto 10MHz. Appears shortly after sunrise and after sunset it merges with the F2-layer to become the F-layer.

F2-layer: reflects radiowaves upto 50MHz (occasionally MUF's of 70MHz have been reported). Appears after sunrise and disappears shortly and after sunset it merges with the F2-layer to become the F-layer. Is stronger in the winter than in the summer, due to seasonal effects.

As you can see the F2-layer is the most important one for us, it reflects our 27MHz radiowaves, and at nighttime the F-layer does the same.

5. Solar Activity:

You now know that the ionosphere appears under the influence of solar radiation, mainly Ultraviolet (UV) and X-ray. This solar radiation varies under the influence of:

5a. Sunspots and Solar Flux: sunspots are dark spots on the sun's surface, and can be compared with the crater of an active volcanoe. They produce the intense radiation which causes ionization of the ionosphere. The index for this radiation is called the Solar Flux and is measured at 2800MHz. The higher this Solar Flux, the higher the level of ionization. The lowest possible Solar Flux is 64 (no sunspot regions), and the highest numbers go well into 200. Conditions on 11m really start to become interesting on all latitudes, when the solar flux goes above 100. You can find the solar flux in our daily <SOLAR ACTIVITY INDEX>.

Sunspots are classified by their magnetic complexity. The more complex the magnetic configuration, the more active they are producing lots of radiation and all kind of other events like solar flares and CME's. Magnetic configuration is classified as:

You can also download a plug-in for Firefox web browser which shows you the actual Solar Flux at http://www.n0hr.com/Propfire.htm

In the picture below you can see many sunspots, and just above the middle, the largest sunspot ever recorded in modern history. These sunspots are clustered together in so called sunspot regions. The active regions are given a number when they appear on the sun's surface. You can find these numbers of active regions in our daily <SOLAR ACTIVITY INDEX>.

5b. Solar Wind: the solar wind is a constant stream of charged particles which flows form the sun into our solar system. Solar wind can reach speeds up to 1000km per second.

5c. EGF (Earth's Geomagnetic Field): our planet has a core that consists mainly of iron. The funny thing about iron is when you rotate its, it produces a magnetic field like in the picture below. This magnetic field protects the Earth from the charged particles from the solar wind. When solar wind speed is very low, the EGF is quiet, but when the solar wind's speed is very high, the EGF becomes unsettled to active, and in some occasions we even talk about a solar wind storm. The EGF is very important for the production of a stable ionosphere. A quiet EGF means a stable ionosphere, with relative high MUF's. An active or stormy EGF means unstable propagation with a relative low MUF.

The EGF is strongest around the equator and weakest on the north pole and south pole, as you can see in the picture below.

 The conditions of the EGF can be found on our Current Conditions page, as the K-index.

5d. Solar Flares: when groups of sunspots are active, they are likely to produces solar flares. These solar flares are like vulcanic eruptions with large flames shooting millions of kilometers into space, like in the pictures below. These solar flares produce a lot of radiation, like X-ray which causes the D-layer to grow stronger. Usually after a large solar flare, propagation blacks out, because of very high absorption of the D-layer. Solar Flares also cause and ejection of large masses of charged particles, which is called a Coronal Mass Ejection (CME). The strength of a solar flare is measured from C-class followed by a number, up to M-class and X-class. M-class and X-class flares are likely to produce a radio blackout. A-level means very low levels of X-ray radiation. The X-ray radiation level can be seen on our Current Conditions page, as the Solar X-ray Background Level.

Now the chance for a solar flare, depends on the magnetic configuration of the sunspot group:

The magnetic configuration is very important. For example, a sunspot group or region with 5 spots in Delta-class can produce much more and bigger solar flares, than a 30 spot group in Beta-class!

Solar Flares can even be heard on your own radio, especially the larger X-class flares, but also C-class flares that spit out lots of radiation. You can hear the level of background static rise for a short period, as the radiation reaches Earth. Click here for a recording of an X-class solar flare on 22MHz.

You can also find info on solar flares in our daily <SOLAR ACTIVITY INDEX>.

Click here to view a movie of a large series of solar flares. The image was filtered through an X-ray filter, so sunspot regions are visible as bright spots. The bright flashes are solar flares.

5e. Coronal Holes: that is a a hole through the sun's outer shell (the corona). Coronal holes are always there, and they always produces a stream of charged particles. When a coronal hole faces Earth, it's stream is likely to hit the EGF within 1-5 days, and push solar wind speeds up to 600-700 km/s, bringing the K-index to active or storm levels. The black area in the picture below is a very large coronal hole. You can find info on coronal holes in our daily <SOLAR ACTIVITY INDEX>.

5f. CME's (Coronal Mass Ejections): with every solar flare the sun spits out a stream of charged particles, called a coronal mass ejections, which can speed up the solar wind up to 1000km/s. Coronal mass ejections are large streams of charged particles, which travel very fast. They follow a solar flare within 72 hours after the eruption (like lava which flows form a volcanic eruption), but very fast moving CME's travel to Earth within 24 hours. A CME is likely to hit Earth when the sunspot region which produced the CME, is also directed to Earth. When in reports they speak of a full halo CME, it is directed fully to Earth. A partial halo means it is directed partial to Earth. You can find info on CME's in our daily <SOLAR ACTIVITY INDEX>.

Click here to view a movie of an extreme large CME, following the largest ever recorded solar flare. Watch closely around 2003/10/28 when a series large full halo CME's knock out the SOHO satellites instruments. On 2003/11/04 (19:54 hours) you can see the largest CME ever recorded, being ejected from the right side of the sun, a partial halo.

5g. The 27-day cycle: while Earth needs 1 day to rotate around, the Sun needs 27 days. That means that active sunspots that appear on certain days, are likely to return there 27 days later. It can be that the sunspots developed into more active regions in those 27 days, but it could also be that they decayed fully. But 27 days is a cycle that counts.

5h. The sunspot cycle: now the funny thing about the sun is that it's solar activity is not on the same level constantly. It follows an 11 years cycle, called the sunspot cycle. On the peak of this cycle, the number of sunspots can reach well over 100, with solar fluxes reaching over 200. In between those peaks, solar activity can be very low, with not a single sunspot for months and Solar Fluxes under 70.

In April 2000 the last Cycle (number 23) peaked with a smoothed sunspot number of 120. New Cycle 24 started late 2007, probably peaking somewhere in 2011. Other organizations predicting space weather, expect a peak between 140 and 160 sunspots.

5i. The Earth's seasons: as normal weather changes by the season propagation also does. Like with normal weather, temperatures at the equator remain at the same level during the year, but temperature differences between summer and winter increase as you go northwards or southwards. Same happens with propagation, but the other way around! The MUF of the F2 layer is higher in wintertime than in summertime. One cause is that more intense and longer sunshine in the summer give the D-layer more absorbing strength. Due to other complex atmospheric influences the ions in the F2 layer, tend do dissolve more quickly in the summer then in the winter, allowing winter MUF to be much higher then summer MUF.

6. Solar activity and its effects on Earth:

We have seen in Chapter 5 that X-ray and UV make the ionosphere, and charged particles influence the EGF. But these events strongly affect the ionization process and so propagation:

Solar flares: produce large amounts of X-ray radiation, causing radio blackouts, but can also intensify ionization in the F2-layer for a short period, with unstable and fast and deep QSB (fading).

Coronal holes: produce streams of energize particles which "presses" the EGF down. The EGF is weakest around the poles, so stormy conditions are most noticeable around the polar areas. Large streams can enter Earth's atmosphere on the poles, and collision with gases like oxygen, nitrogen, etc., produces the Northern Light or Aurora.

CME: same as Coronal Holes only with a much bigger effect. CME's have causes satellites and certain systems on Earth to go dead because of the intense bombardments with charged particles. Back in the 90's a giant CME caused a part of the North American power grid to go down. During such a CME the currents that flow through the atmosphere at high and polar latitudes exceed 1,000,000 (one million) Ampère!

7. Extraordinary propagation:

Sporadic-E: every year around mid summer (May-August) and mid winter (December-January) short skip propagation turns up with distances form 500-1800km. Remarkable about this propagation is that it can turn up in only 15 minutes, and disappear just as fast. Also signal levels are very high. On the CB band, you can hear small stations from other areas between 500-1800km with only 4 watts in FM with loud signals. QSO's with handhelds on the CB band have been made over more than 1000km (actual videos of that can be found on http://www.youtube.com).

The layer in the ionosphere responsible for this is called the Es-layer (Es = sporadic-E). This layer is at the same height of the normal E-layer, somewhere between 80-150km. Scientists still do not know what makes the Es-layer to appear and disappear, but it is sure it is not influenced by solar radiation. We do know that Es is at it's best during a sunspot minimum, because it turns op more regular with a quiet EGF. Studies on this type of propagation learned us, that Es like to turn in the late morning and early evening, but can show up at any time, even at night! The MUF of the Es-layer can even go into the VHF band, with recorded QSO's signals at 200MHz! Amateurs in the 6m (50Mhz) 4m (70Mhz) and 2m (144Mhz) band use Es to cover distances well over 2000km. In multihop occasions, even double that! I remember a summer holiday in Portugal, when we received Dutch FM stations in the 88-108MHz VHF band!

Backscatter: normally you would expect radiosignals to be reflected forwards in the ionosphere, but there is a propagation mode where signals are being reflected back from the surface after a first hop. The modulation of the station you are hearing via backscatter, sounds hollow like they talk through a tube, sometimes even a short echo can be heard. Backscatter signals are not very strong, usually not more than an S1 to S4. More than 100 Watts and a directional antenna is needed to produce a readable backscatter signal. Most favourable directions for backscatter are where the radiowave is being reflected froms surfaces likes seas and oceans.

It mostly occurs when the MUF of the F2-layer is far above 27MHz, and the reflection from the ionosphere is strong. Working via backscatter allows you to work stations within the so called blind zone (the area which is too far away for groundwaves and to nearby for ionospheric waves, usually between ± 50-500km). To work via backscatter both stations need to point there antenna to more or less the same point about 2000-4000 km away. If for example a station from Belgium wants to work a station from England, they could point there antenna's both in the direction of the Azores, or any other direction that produces the loudest signals.

Aurora: when a CME or Coronal Hole stream hits Earth's atmosphere it cannot penetrate on the equatorial regions, lower and middle latitudes. The magnetic fieldlines of the Earth are strongest there, and push they energizes particles to the polar regions where the magnetic field is weakest. When they enter the polar atmosphere, they collide with the different gases there, which can be seen as Northern Lights or Aurora (like neon gas in a tube is lighted up by bombarding the neon gas with charged particles). Now these Aurora clouds are so strongly ionized that they reflect radiowaves up to the UHF band. These Aurora clouds do not lie horizontally like the normal D-, E-, and F-layers, but are hanging more like curtains (like in the picture below). This allows you to work just like backscatter, but because Aurora only appears around the polar regions, you need to direct your antenna to the North- or Southpole (whatever is nearest). The distance you can work via Aurora ranges up to 2000km. Working Aurora needs a good set of ears. The very fast and strong QSB make the signal almost unreadable! Like someone is talking with a soar throat, or talking behind a blowing fan. Audio recordings of Aurora propagation QSO's can be found on on our X-files page.

You do need to be on the higher latitudes to work Aurora, as you can see in the picture of the Southpole below. The higher latitude you are, the more often you can work Aurora. Aurora appears when the K-index hits 3-4, but for locations like London in the UK and Berlin in Germany, the K-index needs to be 9 for Aurora to be visible. But when you see is, the sky seems like it is on fire! You can find the K-index on our Current Conditions page. Information on Aurora events can be found on our Solar Images page.

Meteorscatter: a remarkable propagation mode is meteorscatter. Meteors are small rocks, or dust particles that enter Earth's atmosphere at a high altitude (±100km) at very high speeds (>5000m/s). Because of these very high speeds, these meteors burn up because of the friction from Earth's atmosphere. When they burn up, the heat is so intense, that ionization of the surrounding air takes place. Now this ionization can be so intense, that the ionized air can reflect radiowaves up to 500MHz, but also 27MHz. In the picture below you can see a trail of a meteor at high altitude, this trail probably reflects radiowaves well into the VHF range. The distances for meteorscatter can range from 400 - 1800km for 27MHz, and the openings can last from a second up to a few minutes, with very strong QSB. An actual recording of a QSO via meteorscatter can be found on our X-files page

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Meteorscatter takes place in the periods of meteorshowers. All meteorshowers have a name, here are some important dates for meteorshowers which appear annually:

Meteorshowers tend to peak in the late winter evening into the night and early summer evening into the early morning.

TEP:  the Earth's magnetic poles are not at the same location as the exact north- and southpole, but are located about 1000km form the real poles. Therefore the magnetic equator doe not run straight like the normal equator.

On each side of the magnetic equator the intense sunshine and the Earth magnetic fieldlines magnetic cause extreme ionization there, with the F2-layer extending upward in the atmosphere near the magnetic equator up to 500km and higher. Because of this very high altitude and the high level of ionization, a TEP signal is reflected twice against the ionosphere and can take a signal with a single hop over 6000km, into the VHF range, and in a single occasion even up to 500MHz! You can see how the TEP mode can exist in the picture below.

TEP usually peaks between late afternoon and late evening. Signals tend to become stronger during the evening, but with more and sometimes extreme QSB.

FAI: Field Aligned Irregularities is the most unpredictable propagation mode. It appears at the same high as the E-layer, when ions are being driven together by the EGF, into a small cloud of ions (from several meters in length and width, up to several kilometers). At a certain point the density is so strong that the MUF of an FAI cloud can reach over 200MHz. It can appear on every time of the days, and is noticeable as very short openings, with mostly strong signals. Usually the signals are heavily subjected to strong QSB. These openings can last seconds up to minutes.

There is much more to explain about propagation, but it is our goal to keep things not too technical. If you want to know more, the internet is full of various information.

If you think we can improve this page, please do not hesitate to contact us via the contactform.

Good DX!

19DX072 Jean-Paul