VHF Tropo Ducting Index

(Click on image below for full 6 day preview)

Historical image shown courtesy of William Hepburn

Great example above of a Tropo Ducting path potentially open between Spain and South America on Tuesday 25th June 2013. That evening lots of 144 MHz DX being worked between mainland Spain and the Canary islands, mistakenly reported on the DXclusters as via Sporadic-E, as even on 50 MHz the Es had been extremely poor throughout Europe all day long.

VHF 144 MHz Tropo propagation openings map

(Click on historical image below for current map)

Above a VHF Tropo propagation map using 144 MHz APRS signals, by Jon Harder NG0E, shown on the morning of Wednesday 26th June 2013 indicating pathways. Interestingly the Tropo ducting paths are still open from Portugal and the Azores to the Canary Islands.

Historical image shown courtesy of John Harder NG0E

Sporadic Es and Ionospheric chart for Rome

Showing daily height of Sporadic-E clouds (h'Es) and MUF.

For the Northern hemisphere, between April to August MUF (3000)F2 frequencies greater than 15 MHz generally indicate potentially good 50 MHz or higher Sporadic-E (Es) conditions. It is possible for the MUF to exceed 30 MHz in Winter at the peak of the solar cycle every 11 years, but this does not guarantee any Es propagation.

Also shown, if present, is the height of the Sporadic-E reflecting atmospheric layer usually between 90-131 km (h'Es), with the higher the height the better, as it may indicate stronger ionised metal bearing layers which will support higher VHF frequencies and also last longer than Es layers at lower altitudes. The live data Ionogram shown below from Rome, Italy is a very good guide to whether or not Sporadic-E exists over central Europe on any given day, it is one of several Ionogram generating sites, the others can be found from the RSGB website here or the DK5YA VHF page.

It is interesting to observe that the height of the Sporadic-E layers is not uniform across all of the locations in Europe at the same time of measurement, for example at 08:00 UTC on 13.07.2018 the heights ranged from 108 km in Rome, 97 km in Athens, 108 km in Svalbard, 105 km in Dourbes, Belgium, 115 km in Juliusruh, Denmark, 103 km in Tromso, Norway, 113 km in Gibilmanna and 115 km in Pruhonice.

Also the Sporadic-E layer heights vary by time of day, with Rome descending from 108 km at 08:00 to 99 km by 11:00 UTC, it is thought all sporadic-E clouds descend over time after reaching their peak altitude.

Image above courtesy of the Rome Observatory of the Instituto Nazionale di Geofisica e Vulcanologia (INGV)

Sporadic-E and theories

Sporadic-E (abbreviation Es) enhanced VHF radio propagation is just that, sporadic yet present most days for several hours at a time during daylight hours on 50 MHz usually most years between May to August in the Northern hemisphere, peaking in June sometimes with rare short duration openings supporting radio signal reflections on frequencies as high as 144 MHz, and with daily timings over several years showing the greatest chance of Es being present  between 11:00-12:00 UTC and 16:00-18:00 UTC. Sporadic-E is observed on 144 MHz less than 10% as often as on 50 MHz.

For many years there was also recorded a much weaker and shorter Sporadic-E season around the Winter solstice (21st December) when the intensity of the sun's solar radiation is at its maximum over the winter months. I had only ever worked it once on 28th December 2018.

However as of 2020/2021 things have changed quite noticeably from previously recorded years, the usual Es season didn't end in August but carried on throughout September, October, November, December and now January 2021 with at least 3 large European wide Sporadic-E openings each month on 50 MHz workable even from here in the far North of England. I don't know if this is because many more Radio Amateurs are using the weak signal data mode FT8 and are able now to detect3and work Es openings in a way that wasn't previously possible with CW and SSB and/or is it because so many people are at home monitoring the VHF bands due to Covid-19 lockdowns. Or is some unexplained physical change in the atmosphere.

Excellent Sporadic-E 144 MHz events occurred in 1989, 2006, 2009, 2010, 2011, 2017 and 2020 which have been at both maximum and minimum points in the 11 year sun solar cycle, which demonstrates that is not a factor required for Sporadic-E to occur. For an excellent evaluation summary of 144 MHz Sporadic-E from 2001 to the present day I highly recommend viewing the mmmonvhf  page here which shows you all the data collated and broken down into time, day, month, year, quantity and Es cloud positions.

Another characteristic supported by my own live monitoring over 35 years and extensive collected data from DXcluster spots since 2001, in the Northern hemisphere for European propagation, is that the vast majority of Sporadic-E reflection areas or clouds seem to occur mostly over the Bay of Biscay, Switzerland and the Balkans. Here in the UK it is probable on most days in Summer to work Italy and Spain easily on 50 MHz, it is much rarer perhaps 5-10% of Es days for the reflecting Es cloud to be situated over the North Sea allowing communication from the UK to Scandinavia, however when this does occur it is in this very marked direction. Even rarer openings occur in the direction of Iceland from the UK, but many more multi hop Sporadic-Es clouds do reasonably often open paths from the UK to the USA.

Sporadic-E (Es) occurs in the Ionosphere at heights of between 90-131 km (110 km average) altitude and appears to concern strong areas of non-uniform and patchy plasma metallic ion and electron density irregularities that cause VHF radio waves to be reflected back to Earth by forward scatter. The metallic ions necessary are deposited in the Ionosphere by daily meteor activity (heating and ablation) even in the absence of major shower or meteor storm activity, the primary metal types being iron (Fe) and Magnesium (Mg). The total metal ion density determining whether or not the Es layer can support VHF signal forward scatter or not. The winds and electric fields at these altitudes act to compress the ions into thin layers of around 4 km in depth.

There are minimum distances for each band for propagation via Sporadic-E, lesser distances would be impossible by this mode as the required MUF would be simply too high and must therefore be by another mechanism such as Aircraft Scatter or Tropo scatter etc. The minimum distances by VHF band are shown below.

50 MHz minimum Es distance 400 km

70 MHz minimum Es distance 400 km

144 MHz minimum Es distance 1400 km

Research and published papers indicate that it is the daily ablation of thousands of metallic meteors from all directions that are required, rather than intense meteor showers from single radiants. During the Summer months there is approximately 3 times more metallic meteorite deposition than in the winter months.

There have been several theories proposed over the years for Sporadic-E formation, some are being substantiated and some have limited correlation to observed events, and the required ingredients required to produce Es layers, to support forward scatter at VHF frequencies, appear to differ between the 50 MHz and 144 MHz frequency bands.

Sporadic-E occurs most notably on the VHF 28 MHz, 50 MHz, 70 MHz and 144 MHz amateur radio bands where the ionized E layer of the atmosphere at around 110 km altitude reflects forward scatter VHF radio signals back to Earth, rather than them normally travelling straight through the atmosphere into space, with received radio signals being extremely strong. Low power levels of 3 Watts have allowed many QSOs between Europe and North America on the 50 MHz band via multiple reflections.

Monitor the VHF amateur radio bands and beacons and if Es signals are exceptionally strong on a lower band such as 50 MHz and at the lower end of the single hop distance range (500 km) this can be a good indicator that Es will be supporting even higher frequencies, so consider listening up on the 70 MHz or 144 MHz bands too. If travelling in your car away from your shack try monitoring 87.6 MHz FM on your analogue vehicle radio to see if you can hear Es broadcast station signals from outside the UK (Tip from Dave Edwards G7RAU, thanks!)

Long distances on 50 MHz can be worked via Sporadic-E layers from around 400-7000 km, with single hop 400-2350 km where the path is Earth-Cloud-Earth, double-hop up to 4700 km where the path is Earth-Cloud-Earth-Cloud-Earth and triple-hop up to 7000 km where the path is Earth-Cloud-Earth-Cloud-Earth-Cloud-Earth. With the advent of special weak signal data modes even these distances may be exceeded in the future.

An absolutely exceptional partial Sporadic-E opening occurred on the morning of 24th July 2018 between 0700-0800 UTC when the station of VK8AW (PH57) Darwin, Australia worked and was heard by stations in Europe on 50 MHz with the furthest station being G3TXF (IO71) in England at a distance of 14,118 km!

Previous theory and long established observations prior to 2018 stated there should be no F2 or TEP propagation except at solar maximum, but having communicated via e-mail with Gary Ashdown VK8AW it appears that his signals were reaching Southern Europe and the Middle East via Trans Equatorial Propagation (TEP) and were being further extended by widespread European Sporadic-E linking to it. Apparently 48-50 MHz signals are observed by VK8AW regularly from the Middle East and China and the new weak signal data mode FT8 is allowing two way radio communications, where they were previously unworkable or undetectable except at solar maximum.

This mechanism of Sporadic-E plus TEP propagation has been seen again on 50 MHz in Summer 2019 with many Japanese stations working Southern Europe and even UK stations on the South coast of England.

Also in June 2019 Sporadic-E plus marine Tropo Ducting allowed signals on the 144 MHz band to be received from D41CV in the Cape Verde Islands off the West Coast of Africa to be received by OE3NFC in Austria at a distance of 5107 km, the marine tropo ducting path being from Cape Verde to Southern Spain where it joined the Sporadic-E path between Southern Spain and Austria.

On 144 MHz the distances for single hop Es contacts appear to all be in the range of between 1400-2350 km with rarer multi hop exceeding this to a recorded maximum of 3600 km.

The exact multi hop mechanism for each signal may not be readily identifiable and may be signals reflecting off a single Es cloud layer or multiple separate Sporadic-E clouds and/or various mixtures of Cloud and the Earth with reflections from water offering significantly less attenuation than from land.

It is also theorised and pretty much correlated that the Es cloud layers are not uniform and flat, but may parts tilted at times at up to 45 degrees where they often have an irregular wavy surface, like corrugated metal sheeting (see image below).  This can slightly complicate understanding the precise Sporadic-E hop mechanism being observed on 50 MHz with the theory being of many instances where two cloud Es formations exist, the Es layers being tilted towards each other.

This two cloud Es formation appears to account for 50 MHz signals in the distance range of 2400-3200 km i.e. more than a single hop maximum (2350 km), but much less than for double-hop (4700 km). The signal path being Earth-Cloud-Cloud-Earth.

Noctilucent clouds at around 85km altitude - Photo by Jan Koeman, Kloetinge, the Netherlands, July 2009

In the two Es cloud formation path (not double-hop) live example shown below, captured at 18:12 UTC on 17th July 2018, I was hearing the station of SV1NZX in KM17 square via Sporadic-E on 50 MHz at a very strong signal level, my station G0ISW in IO84 square being 2741 km in distance away and therefore well beyond the maximum single hop distance of 2350 km.

Looking at the live MUF mapping software display there were two significant Es cloud areas in place (circled in red) on the path, both at a similar distance from our stations, therefore it appears that the signal was being reflected from these separate Es clouds at the same time, with both Es clouds layers probably being slightly tilted towards each other with wavy irregular cloud surfaces, the path being Earth-Cloud-Cloud-Earth.

The exact conditions necessary for the appearance of Sporadic-E layers have remained mostly a mystery until recent years where a gradual consensus is being formed, if likened to a recipe the key ingredient always required is the Sun strongly illuminating the ionosphere and causing significant and intense solar ionising radiation, with the peak ionisation being at local midday during the Summer months (May to August) in the Northern hemisphere. Additional ingredients required for the formation of Sporadic-E may include some of the following: High Meteoric deposition rates, Ionospheric Wind shear, thunder storms, Atmospheric Gravity Waves (AGS), Noctilucent Clouds and Solar cycle activity.

Below is a table of theorised Sporadic-E required ingredients and observed strong correlation in the Northern hemisphere.

Frustratingly however, sometimes the Sporadic-E on 144 MHz defies any correlation with AGW, wind shear and thunder storms. In the example below at 17:53 UTC on 23rd July 2018 there is a pronounced Es reflecting layer shown over the Balkans and their mountain ranges, but checks on live wind charts show no jet stream or other significant wind activity anywhere near and also no lightning whatsoever.

After sunset the ionised Es layer usually fades until it can no longer support VHF signals being reflected from it, probably due to no longer receiving solar radiation, however on rare occasions Sporadic-E VHF propagation has been known to occur even up to local midnight.

Image from US Naval Research Laboratory

From observations in Europe, over the past 30 years,  these are randomly generated signal reflecting areas, which vary in size depending on the frequency band they can support  i.e. 50 MHz Sporadic-E clouds are significantly larger and can often be around 500 x 500 km (5 x 5 grid squares) in size, whereas on 144 MHz they may often only be 50-100 km in size i.e. a single grid square, unless a very significant and much rarer large opening event. The usual small reflecting area on 144 MHz means fewer stations will be ideally located to benefit from using it.

In the images below white lines show the paths on 50 MHz, grey lines are 70 MHz and red lines are 144 MHz.

On 144 MHz the Sporadic-E layers in Summer have been observed to move slowly from East to West and sometimes correlate with intense thunder storms, but not always. Note the correlation between this very significant 144 MHz Sporadic-E opening reflection zone, shown below on 12.07.2018, and the live map of lightning strikes at the same time in the same area. The 144 MHz opening on the border of Poland with Ukraine shown below lasted just over 1 hour and despite much similar lightning visible in Norway no such Sporadic-E opening occurred there. However later in the week further extensive thunder storms were present and no Sporadic-E was observed anywhere in Europe, all the necessary combination of ingredients not being present.

In addition whilst 50 MHz Sporadic-E band openings can last most of the day, 144 MHz openings can be very short lived and occur much less frequently, only being available for a few days each year and for much less time, sometimes only as little as a few minutes or an hour.

From my own observations 144 MHz Sporadic-E reflecting areas do often coincide with intense thunder storms and due to the height of the Es reflecting layer being between 90-131 km I can only theorise a link with the little known electrical Sprite phenomenon perhaps, possibly generating Atmospheric Gravity Waves (AGW) and causing wind shear at 90 km altitude and higher. No such correlation with thunder storms and Sporadic-E is seen anywhere near as often on the other lower VHF bands.

Image credit https://en.wikipedia.org/w/index.php?curid=4262250

Abestrobi Own work CC BY-SA 3.0

The higher the frequency the more intense the Sporadic-E ionised layer has to be to support forward scatter propagation via it, at 144 MHz in particular it can be frustrating to see the small reflecting area favour radio amateurs in a particular geographic area reasonably close, but in your own location nothing is heard and vice versa. Sometimes the reflecting areas at the centre of the lines connecting the QSOs converge over a particular area each year, the Bay of Biscay near France is one such area. Often the reflecting layer/cloud moves over hours in a East to West direction. Using the Live-MUF mapping software it is very easy in real time to observe these events.

Sometimes on 144 MHz there may be correlation with Es reflecting areas and thunder storms generating extensive lightning activity (another theory), but by no means do they coincide every time. On the morning of 3rd May 2016 there was a clear 50 MHz Es reflecting layer centred over the Baltic states, but no wind shear or AGW from mountains, nor any thunder storm lightning activity found using live data, so the required ingredients were much simpler.

In June 1989, during a particularly intense radio Es event on 144 MHz, I observed the distant DX stations heard to slowly all shift SW over the course of two hours, in a direction not associated with the jet stream, but consistent with the Earth's rotation in relation to the Sun.

If we consider the 'wind shear' theory formulated in the 1960's by Whitehead (1961) and Axford (1963) in which vertical shears in the horizontal wind form thin layers (several kilometres) thick from metallic ionisation through ion-neutral collisional coupling and acting in the presence of the Earths magnetic field and through the Lorentz force. The original metallic ions thought to be present from meteoric origin.

With the wind shear effects being required at the Sporadic-E height of at least 80-90 km we should look first at evidence of observed wind effects or clouds near that altitude with the highest visible clouds in the Earth's atmosphere being the rarely seen Noctilucent clouds at 80-90 km in altitude, just below the Mesopause. These clouds are also seasonal May to August in the Northern hemisphere and are bluish-white in colour and are only visible just after sunset when still illuminated by the sun and have undulations, corrugations and rich veins. These clouds do prove that there are wind effects present at these very high altitudes and surprisingly also water vapour too, their non uniform composition can be likened to the theorised irregularity of Sporadic-E clouds. Apparently research 25 years ago found that these Noctilucent clouds exhibit good radio reflection properties due to metallic meteor deposition being attracted to and coating the tiny water droplets which have turned to ice.

In 2020 I have come to the conclusion that there is a definite link between Sporadic-E propagation and Noctilucent clouds at around 80-90 km altitude. These clouds only form in exactly the same Summer months as Sporadic-E and historic research has shown they exhibit good radio reflecting properties, they are thin and wispy, non regular and vary considerably all factors which would support the variability of Sporadic-E reflecting areas. Also in the Summer months the predominant wind direction at these altitudes is from East to West just the same as observed Sporadic-E reflection areas movement and the wind speeds can apparently reach up to 300 km/Hour.

The noctilucent clouds are formed from ice crystals being attracted to metallic meteorite particles and gathering around them acting as a reflector to radio waves if sufficiently dense.

Wind shear from the Jet Stream at altitudes of between 9-16 km above ground level over mountains has been suggested at 144 MHz to have an association with Sporadic Es, part of the so called 'wind shear' theory. Using the live wind chart below and setting the wind height altitude to 500 hPA  (11 km) should allow observations and comparisons to be conducted in real-time with Sporadic Es cluster maps to support or discount any such correlation, with wind effects.

(Click on historical image below for current live jet stream map)

For European Sporadic-E formed via AGW effects we need to look also at a map of European mountain ranges to see if there is any correlation with observed Es clouds, it does not however explain Es clouds on 50 MHz often seen formed over the Bay of Biscay, North Sea or the Atlantic Ocean. Sporadic-E has been observed to regularly form over the Balkans, Pyrenees and the Alps, does this coincide with Jet stream wind direction on the same days?

Noctilucent clouds do prove that there are wind effects present at these very high altitudes caused by Atmospheric Gravity Waves (AGW), their non uniform composition can be likened to the theorised irregularity of Sporadic-E clouds.

Noctilucent clouds at around 85km altitude - Photo by Jan Koeman, Kloetinge, the Netherlands, July 2009

How are there wind effects and potential 'wind shear' at 90 km altitude required for Sporadic-E? Well, Atmospheric Gravity Waves (AGW) required to form Noctilucent clouds and other wind effects at these high altitudes can be generated by either violent intense thunderstorms or wind flow over high mountain ranges, which cause vertical displacement of the air flow with the AGWs forming when buoyancy pushes air up and gravity later pulls it back down.

The AGW can be likened to a corrugated tin roof effect being placed on the original horizontal air flow and these uneven waves of air then travel to high altitudes and new waves form underneath so a sequence of waves will be formed. AGW are medium scale waves with the horizontal wavelength ranging from several tens to several thousands of kilometres and a vertical wavelength of several kilometres. Using radar measurement AGW have been observed to reach heights of around 85-88 km, the same height as Noctilucent clouds. The AGW can even penetrate up to the Ionosphere where they trigger ionospheric irregularities and add to the recipe for Sporadic-E likelihood.

The generally accepted height of the Sporadic Es reflecting layer is around 90-131 km, which is very significantly higher than the jet stream altitude of 11 km. Although annual the intensity of Es events varies by year and in recent years has been very poor compared with the 1980's and 1990's, despite peaks and troughs of the solar cycle. In 2018 it has been much better, than for some time.

There is a definite connection with the radiation intensity of the Sun due to the seasonal Summer nature of the event in the Northern hemisphere. Also the Es events tend to mostly occur during daylight hours at VHF. A 3-6 day cycle pattern of building up to a Es peak also seems quite prevalent, possibly an electric ion charge/discharge cycle period perhaps, it does seem strange to see very active Sporadic-E one day and absolutely nothing the next day despite weather and Solar conditions appearing unchanged.

The Earth has a Global Electric Circuit (click on image below for video explanation) with lots of variables daily impacting upon it at Ionospheric altitudes, many of these factors will contribute to whether or not VHF radio signals can be propagated over vastly longer distances than normal on any given day.

When I participated in the fantastic Leonids November 2002 meteor storm event, with over 700+ meteors per minute, the intensity was such that the whole 144 MHz band was wide open for many hours with the high signal strengths associated with Sporadic-E signals. I have also observed other meteor showers coinciding with a Sporadic-E like opening, as happened most recently in April 2017 during the peak of the Lyrids meteor shower, no other mention of Sporadic-E by stations until May, but the signal strengths I observed that day were very high and prolonged again, for an hour or more.

The long standing mystery of what exactly causes Sporadic Es propagation continues, since it was first observed in the 1930's.

Tropo and Air pressure charts

(Click on historical image below for current chart)

Above an Air Pressure chart from the UK Met Office for Wednesday 26th June 2013 showing the very large system of High pressure between Africa and South America leading to good Tropo ducting opportunities.

Historical image shown courtesy of UK Met Office

Auroral Oval

http://www.swpc.noaa.gov/images/aurora-forecast-northern-hemisphere.jpg

(Images provided by NOAA)

VHF/UHF QSOs real time maps

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VHF/UHF DX clusters

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VHF/UHF Propagation modes explained

Propagation type

Distances

Comments for European stations

Line of sight

0-50 Km

(Ground)

0-110 km

(Mountain)

0-390 Km

(Aircraft)

0-618 Km

(HA Balloon)

This is the mode by which most of your local 144/432 MHz FM simplex conversations will be made, either direct to stations or via repeaters.

Dependant upon antenna height above sea/ground level and visible radio horizon distance. Line of sight (LOS) distance can be increased with height or decreased by obstructions such as mountains, buildings etc.

Due to the curvature of the Earth there is a limit to how far VHF/UHF signals can travel before disappearing into space.

The formula for calculating your 'radio horizon' for an 'unobstructed path' or maximum line of sight distance is

Horizon Km = 3.569 x √ height in metres

Example 1: Person standing at ground level (sea level) holding a handheld radio

Horizon Km = 3.569 x √ 1.80 metres  = 4.78 Km

Example 2: My home amateur radio station, located 154m ASL and antenna a further 5m AGL

Horizon Km = 3.569 x √ 159 metres  = 45 Km

Example 3: Person standing on summit of Helvelyn mountain, holding a handheld radio

Horizon Km = 3.569 x √ 950 metres + 1.80 metres  = 110 Km

Click on this link for VHF/UHF Line of Sight range calculator.

Line of sight is the direct free-space path that exists between two points. Using binoculars on a clear day, it is easy to determine if visual line of sight exists between two points that are miles apart. To have a clear line of sight there must be no obstructions between the two locations. Often this means that the observation points must be high enough to allow the viewer to see over any ground-based obstructions.

The following obstructions might obscure a visual link:

• Topographic features, such as mountains

• The curvature of the Earth

• Buildings and other man-made objects

• Trees

If any of these obstructions rise high enough to block the view from end to end, there is no visual line of sight.

Obstructions that can interfere with visual line of sight can also interfere with radio line of sight. But one must also consider the Fresnel effect. If a hard object, such as a mountain ridge or building, is too close to the signal path, it can damage the radio signal or reduce its strength. This happens even though the obstacle does not obscure the direct, visual line of sight. The Fresnel zone for a radio beam is an elliptical area immediately surrounding the visual path. It varies in thickness depending on the length of the signal path and the frequency of the signal. 

As shown in the picture above, when a hard object protrudes into the signal path within the Fresnel zone, knife-edge diffraction can deflect part of the signal and cause it to reach the receiving antenna slightly later than the direct signal. Since these deflected signals are out of phase with the direct signal, they can reduce its power or cancel it out altogether. If trees or other 'soft' objects protrude into the Fresnel zone, they can attenuate (reduced the strength of) a passing signal. In short, the fact that you can see a location does not mean that you can establish a quality radio link to that location.

There are several options to establish or improve the line of sight:

·        Raise the antenna mounting point on the existing structure

·        Build a new structure, i.e. radio tower.

·        Increase the height of an existing tower

·        Locate a different mounting point, i.e. building or tower, for the antenna

·        Cut down problem trees

My own radio horizon is obstructed by nearby hills and mountains as you can see in the charts below, with the blue line being the profile of the mountains as viewed and the red line being horizon distances. Near 120 degrees I have an almost totally unobstructed path in the region of a maximum 45 Km, whereas immediately behind my house to the North I have a hill  that blocks my signals in that direction.

Click on this link for VHF/UHF Line of Sight range calculator.

Obviously if the transmitter is not located on the ground, but instead is in an aircraft or balloon the line of sight distances can be vastly increased.

Example 4: Light aircraft at 3,000 m altitude, carrying Amateur radio transmitter

Horizon Km = 3.569 x √ 3,000 metres  = 195 Km

Example 5: Commercial aircraft at 12,192 m altitude, carrying Amateur radio transmitter

Horizon Km = 3.569 x √ 12,192 metres  = 394 Km

Example 5: Helium filled Amateur Radio High Altitude Balloon (ARHAB) transmitter at altitude of 30,000 m

Horizon Km = 3.569 x √ 30,000 metres  = 618 Km

Knife edge diffraction

50-110 km

Your LOS signal, which can be blocked by high terrain can sometimes be diffracted or bent over the top of the obstruction, particularly in mountainous areas if the top of the obstruction is 'sharp', hence the term 'Knife-edge diffraction'.

More information and software calculator here

I live in a mountainous area and have experienced a few instances where contacts have been made with stations that should have been totally obstructed by high mountains in between. Single knife edge or rarer double knife edge diffraction observations have been made by me over the Pennine Mountains between Penrith and Hexham. The image below shows the cross section and the distinct knife edges.

Tropo Scatter

110-700 km

This propagation mode is available all the time and is the main one for longer contacts, particularly at 144 MHz on SSB within the UK or to mainland Europe. Slow fading of signals often apparent and reasonable signal strengths.

Where your 'line of sight' distance has been exceeded due to the curvature of the Earth or obstructions, this mode is the one most likely to be found by radio amateurs, but does require typically horizontal steerable antennas and SSB, rather than FM.

Particularly useful on the 144 MHz band where from the UK it is possible to work nearby stations in France and Belgium all the time. With high gain antennas and sensitive receivers Germany and Denmark also become within range.

However this propagation type doesn't favour 50 MHz so well and can be disappointing.

This propagation mode was used by NATO, from around 1956 to the late 1980's, as part of the ACE HIGH Troposcatter system on frequencies between 832 MHz and 959 MHz, in a chain of 49 stations running from Norway to Turkey. Transmitting power was around 10 KW and huge dish antennas were used!

I remember seeing the huge dishes at Cape Greco (JCGZ) in SE Cyprus in the late 1980's, but am struggling to find any photos of them apart from this one.

Looking at Google Earth imagery below, from 2003, it appears the dishes have now been removed.

Aircraft Scatter

(50/144 MHz)

Up to 850 km

(10 GHz)

Aircraft scatter propagation (ACS) has been regularly used successfully on frequencies of 50 MHz and above. It can be subject to rapid fading of signals at 144 MHz and higher frequencies and may not be particularly easy to catch or use. The higher the frequency used the better the results are likely to be.

Imagine bouncing your radio signals off the metal aircraft body, which will be travelling at between 500-600 mph, in the same way you would bounce light off a mirror. Due to the speed of aircraft transit, maximum 30 second transmit periods are recommended and data modes such as JT6M (30s periods) or ISCAT-B (30 or 15 second periods) will probably yield the best results.

I have often found using 50 MHz and JT6M data mode that identified Boeing 747 airliners are sufficiently large, with their 64m wingspan, to produce good aircraft scatter. The scatter period on 50 MHz can last up to around 1 minute if crossing the direct path between stations and significantly longer if flying along the direct path.

Due to the curvature of the Earth and VHF signals being line of sight there is a maximum distance limit as to how far Aircraft Scatter (ACS) propagation can be used. This maximum distance is approximately 758 km for Civilian commercial aircraft reflections.

Also this maximum theoretical distance using commercial airliners does not take into account any path attenuation.

Using the calculations seen before for VHF line of sight signals we find that for a signal from a commercial aircraft altitude to sea level, the theoretical maximum radio horizon is 379 km as shown in the calculation below.

Example: Commercial aircraft at normal maximum 11,276 m (37,000 feet) altitude, carrying Amateur radio transmitter

Horizon Km = 3.569 x √ 11,276 metres  = 379 km (235 miles)

However, from my own recorded results the very best distance line of sight to a Civilian commercial aircraft I have obtained has been 338 km (210 miles) due to nearby ground obstructions i.e. mountains.

This zone of no line of sight could be referred to as a Radar Shadow Area (RSA), see image below for a better understanding how closer aircraft can be hidden yet further away higher aircraft ADSB transmissions can be observed.

Some modest increase in theoretical distance will be exhibited by amateur radio stations being at an elevation above sea level. However even the top of mountains will only add about 110 km more so the distance could be extended to nearly 500 km

So for the two legs from ground stations at mountain tops to aircraft and scattered back to ground the maximum distance is 2 x(379+110) km = 978 km.

Do any aircraft ever fly higher than 11,276 m (37,000 feet)? 

Yes, historically the supersonic Concorde used to fly at a cruise altitude of 18,900 m (62,000 feet) and the US Air Force SR71 Blackbird reconnaissance aircraft set an altitude record in 1976 of 25,950m (85,135 feet) although it is likely it could fly higher, but that maximum remains classified.

Some smaller modern military jet fighters apparently have a service ceiling of 65,000 feet, but stealth radar absorbing materials used to avoid enemy detection by radar will also prevent amateur radio aircraft scatter.

The only aircraft I have observed at significantly higher altitudes have been rare U2 flights passing over the United Kingdom with a transmitted height of 60,000 feet, although they could have been at a different higher altitude apparently as anything over 60,000 feet is deliberately not shown.

There may be other classified experimental military aircraft operational today, but due to the limitations of having air-breathing engines they too are limited in maximum altitude.

If we use 25,950m (85,135 feet) as the maximum possible, but most unlikely, theoretical and practical altitude then the radio horizon would be:

Horizon Km = 3.569 x √ 25,950 metres  = 575 km and for ground to aircraft scatter and back to ground that distance would be doubled to 1150 km in theory!

Also the aircraft size is key to whether or not is offers enough surface area for the transmit frequency in use, at 50 MHz (6m) it appears an aircraft the size of a Boeing 747 with a 64m wingspan is required for good results.

Smaller identified aircraft such as Boeing 737, with 34m wingspan, have not been observed by me to have as much success on 50 MHz, surprisingly.

Realistically for all amateur radio purposes a practical maximum for aircraft scatter (ACS) propagation remains around 700-1000 km.

Any DX spots showing aircraft scatter (ACS) over this 1000 km distance can only be operator error and should be discounted, with another propagation mechanism such as MS or Es being the actual medium used.

RADAR (Radio Detection And Ranging) has used radio signals since before WW2 to determine the flight path of aircraft. Early German WW2 radar used frequencies near to the amateur 144 MHz band. Modern stealth aircraft such as the US Air Force F-117 were designed so that their shape would not easily reflect Radar signals back to the receiving station, by avoiding having any vertical angles.

Some early experimentation has been done by SM6FHZ and his website detailing how to work regularly via this mode, using flight timetables is here. Frequencies of 144 MHz, 432 MHz and 1296 MHz have all been used successfully by him. Some imagery and an explanation of how you can experiment to listen yourself can be found on the website of G3CWI here.

Also the website of PA0EHG provides a fascinating account of his experimentation at 1296 MHz a frequency particularly well suited to ACS propagation, as well as his use of SM7LCB online Path and Scatter maps.

Since 2013 a fabulous new piece of software called AirScout has been written by Frank DL2ALF especially for Aircraft Scatter propagation. You get moving aircraft over a map in real-time as well as a plot showing where your signal and the station you are trying to work have a mutual reflective scatter zone into which the aircraft can fly and their times predicted.

Additionally a path profile is generated which shows obstructions such as mountains. This software is a superb tool to assist others for ACS research and real-time working and of course fun!

In the Summer of 2013 I first experimented with this software and using WSJT JT6M data mode on 50 MHz SSB was able to take advantage of regular aircraft scatter (ACS) between the UK and Ireland at a distance of 350 Km. The 30 second transmission periods for JT6M fitted perfectly the 1 minute long observed reflections, with fairly stable strong signal strengths seen. Happy days!

Since then I have had many successful contacts on 50 MHz using aircraft scatter and have even been able to predict the reflections timed to the minute using AirScout software by DL2ALF.

Signal strengths have been often observed at 6-12 dB.

Aurora

250-1100km

Aurora favours Northern Europe. March is often a good month. You need to point your antenna between North and East and reflect your signal off the moving Auroral curtain.

Speak much slower than normal and compensate for the Doppler shift, which makes everyone sound like Daleks!

50 MHz is particularly good for this mode, 144 MHz is useable and 432 MHz is extremely difficult due to the high Doppler shift.

More information can be found here.

FAI

250-1100km

TEP

Trans-Equatorial Propagation (TEP)

Prior to 2018 during exceptional VHF openings some amateurs worked DX stations located 8000 km away crossing the Equator. Imagine:  From Southern Europe to South Africa on 50MHz or even 144MHz ! This phenomenon seemed to occur when both stations were located at equal distances North and South of the Equator and experiencing a high level of electron density in Autumn and Spring, usually during periods of solar cycle maximum activity and the equinoxes.

The stations located over 45° of latitude north (or south) are usually too far off the geomagnetic equator to make use of F-layer FAI. Sometimes however, these latitudes could be worked via an additional sporadic-E hop, even if signals are usually weak and typically exhibit the fluttery and hollow like sound of pure FAI.

It was observed prior to 2018 that there were two distinctly different types of TEP that could occur:

The first type occurred during the late afternoon and early evening hours and was generally limited to distances under 6000 km. Signals propagated by this mode were limited to the low VHF band (<60 MHz), were of high signal strength and suffered moderate distortion (due to multipath). Single sideband voice communications were possible with this mode.

The second type of TEP occurred from around 1900 to 2300 hours local time. Contacts were made at 144 MHz, and even very rarely on 432 MHz.

The signal strength was moderately high, but subject to intense rapid fading, making morse code (narrow band CW) the only possible communication mode. One amateur described the signal quality in the following words: "we tried SSB but there was so much distortion that not a single word could be identified. [this mode] has a lot of flutter and fading and ... even the morse comes through like a breathing noise, not a clear tone" (from the Dawn of Amateur Radio in the UK and Greece by Norman F Joly).

Events in 2018 as previously mentioned for the Australian station of VK8AW working stations in Europe on 50 MHz at solar cycle minimum via TEP and Sporadic-E have now thrown previously accepted observations and theory out of the  window. This appears due to the new weak signal data mode FT8 which is allowing two way communications via TEP to be successful even at solar cycle minimum, with the Middle East and China both being heard most days at his station near Darwin in Northern Australia.

It also appears that VK8AW being ideally situated just within the TEP zone at -40 degrees latitude and having a very high gain 50 MHz station, using weak signal data modes, has been able to regularly observe four (4) separate TEP waves each day and not just two as previously thought.

This has been observed again in June 2019 with EA8 stations working Japan on 50 MHz apparently using a combination of Es and TEP

The following vertical total electron content map from NASA may help to indicate whether or not propagation via TEP is more or less likely. Click on image below for live data.

Here below is an old image from 2012 clearly showing the very high Total Electron Content shown in red colour as two distinct areas equidistant North and South of the Magnetic Equator, which in all probability was very likely to have been a TEP path for VHF radio signals at that time.

Here is a further example from the European Space Agency

Tropo Ducting

200-1000km

Exceptionally up to 4700km over Sea paths

Signals can be quite strong. Look for periods of high air pressure over the UK and Europe. Often extensive fog can indicate the right conditions for this propagation mode. Once established paths can be open for many hours or days. Often you may hear far away 144 MHz/432 MHz repeaters that normally cannot be heard.

Sea paths possible exceptionally up to 4700 km on 144MHz SSB, paths between Scotland and the Canary Islands have been worked several times. The IARU Region 1 record for two-way communication on 144 MHz was set at 4163 km in 2018 between EI3KD and D4Z using CW on the Cape Verde islands, the maximum distance heard earlier in the day was as far North as GM around 4700 km which is astounding.

October often the best month. These Ducts form at heights between 450m to 3000m, but are blocked by higher mountains along the path. They require stable High pressure areas, fog can be a good indicator.

Click here for atmospheric temperature soundings.

Select Europe map and then click on site to view readings. Gif image to 700mB best. Look for temperature inversions, where the inversion thickness layer is wide enough to support ducting at 144 & 432MHz, using the table below.

Ionoscatter

Meteor Scatter

500-2350km

Most meteorites have a significant iron metal content and when they burn up in the atmosphere at heights between 85-90 km they leave behind metallic ionised trails which reflect VHF signals back to Earth, that would otherwise be lost in space. Signals are typically of very short duration, but can be strong typically from -2 to +13db. During the rarest and most intense meteor showers the duration of signal reflections can be several minutes or even hours and can be confused for Sporadic-E propagation.

Summer months are best for the major showers, but winter months are active too. Random meteors occur all the time, day or night, and there are far more meteors than can be seen visually. Can be a mode that can revolutionise 50/70/144 MHz SSB contacts using software such as WSJT or MSHV by LZ2HV (latest MSK144 mode with 15 second intervals, is very popular in 2017) for long distance contacts. My favourite propagation type!

The lowest distance 500 km MS contacts can be very difficult to complete due to the high angles required, fewer meteors trails being in just the right place and nearby radio signal obstructions such as mountains, mid distance MS contacts around 700-1200 km being far easier.

Whilst most stations use directional horizontal beams and 100W or more, success can be achieved with omni-directional antennas such as horizontal loops and surprisingly even with vertical colinears.

The DX record for MS is somewhat over 2350 km, however this must be by the use of at least two different propagation mechanisms, for example MS + Sporadic-E or Tropo Ducting, as the curvature of the Earth and meteor heights set physical limits for pure MS QSOs.

Reflections of radio signals can last from around 250 milliseconds (1/4 of a second) to 30 seconds plus, but the vast majority are extremely brief. It can take a long time to complete a QSO in the region of 30 minutes or an hour, unless there is a major Meteor shower. In November 2002 the Leonids storm was the best ever with over 700+ meteors each hour. So many meteors were striking the atmosphere that an almost continuous reflective layer was formed with amazing easily completed verbal QSO's, almost like Sporadic-E signals that lasted for many hours continuously.

For Meteor Scatter the 50 MHz band is by far the best, 144 MHz is usable too but more difficult and 432 MHz and higher almost unheard of.

To easily hear Meteor pings tune your transceiver to a distant strong VHF Band 1 TV station video carrier or VHF FM Radio station and you will hear nothing until the signal is reflected briefly by a passing meteor! Please note that during the Summer months Sporadic E (Es) may allow you to hear the TV or radio carrier continuously.

The Spanish TV Transmitter shown above, closed down in 2010, but in 2011 the TV Transmitter in Prague shown below was active. Sadly almost all Band 1 TV stations are closing down, replaced by UHF digital instead.

Unfortunately Band 1 analogue TV is being phased out in Europe and so the availability of these TV carriers is being much reduced for monitoring Meteor Scatter. There are some alternatives, such as the French GRAVES space surveillance radar system on 143.050 MHz CW.

It is also possible to carry out some interesting Radio Astronomy using the signals from Band 1 TV carriers and your Radio receiver to detect Meteor reflections and create imagery of the fireballs or meteors burning up in the atmosphere using SpectrumLab software.

Sporadic E (Es)

50MHz  

400-2350km

(Single hop)

800-4700km

(Double hop)

Up to 7000km

(Triple hop Sp-E or SSSP)

Sporadic E (Es) is an abnormal propagation mode at mid-latitudes which occurs during the Summer season, from May to August in the Northern hemisphere and from November to February in the Southern hemisphere. Very strong signal strengths are common, particularly in the peak month of June in Europe. There may be a much weaker and short lived Sporadic-E event around the Winter Soltice (December 21st) when the sun is directly overhead and at its most powerful and closest to Earth during the winter months.

Intense solar radiation and high metallic meteor deposition rates are required. The reflection takes place in a thin layer up to a maximum thickness of 4 km varying in altitude between 90 - 131 km above Earth, (often around 110 km) the higher the height of the Es cloud the greater the distances that can be worked as the angles of reflection are shallower for those stations furthest apart.

The Es Maximum Useable Frequency (MUF) varies from 20 MHz to at least 220 MHz with the primary limits for minimum and maximum distances for Es signals being the geometry of the Earth, electron density of Es clouds and their height. Maximum path distance will occur just below the MUF cutoff.

The ionisation clouds can drift westwards at speeds of few hundred km per hour. There is a weak periodicity noted during the season and typically Es is observed on 1 to 3 successive days and remains absent for a few days to reoccur again, a bit like charging up a battery and then depleting it.

The Sporadic-E cloud sizes vary those capable of reflecting radio signals at 50 MHz tend to be in the region of around 500 km x 500 km, whereas for 144 MHz they are often only 50-100 km in size. It is often found that nearby amateur radio stations to you can work stations you cannot hear and vice versa, it all depends on where that Es cloud is.

Es do not typically occur during the small hours, the events usually begin at dawn, there is a peak around 12:00 UTC and a second peak in the evening around 18:00 UTC. Es propagation is usually gone by local midnight.

For the UK 50 MHz Es propagation favours stations located furthest South, however openings for Northern based stations do occur, but interestingly often an hour or more later than for stations in the London area. This can result in big pileups with stations around London being worked by the DX station, only for an hour later stations in the North getting a look in! This can be most frustrating as the DX station may well go QRT by then or the pileup is so huge that the more Southerly stations with stronger signal strengths are the only ones heard.

50MHz 2,350km is max 'single' hop distance. 50MHz Sporadic E (Es) season is usually from May to August in the Northern Hemisphere, peaking in June. 'Double' or 'triple' hop often seen vastly increasing the distances worked.

Some distances worked when at solar minimum in 2007 or solar maximum in 2013 have been in the order of 6000km, is this triple hop Sporadic-E or something else such as Short-path Summer Solstice Propagation (SSSP)? On 20th June 2013 there was a 50 MHz Es opening from Europe to the Caribbean that vastly exceeds double hop distances. On 12/13th June 2018 triple hop Es to Brasil in South America was observed by me on 50 MHz here in IO84 square.

Some theories suggest these double hop Es occur from the Es layer clouds not being flat, but having an irregular or bumpy surface which can reflect the radio signals to other clouds before returning to Earth. Or some theories favour reflections from bodies of water on the ground back to the Es cloud layer. Sporadic E remains a mystery, since first observed in the 1930's.

144MHz

1400-2350km

(Single hop)

Around 3600km

(Double hop)

Rare double hop Sporadic-E up to around 3600km perhaps with ground reflections from large inland waterways such as lakes and rivers as one theory suggests.    Click on link for more information.

F2 layer

50MHz

>3200km

Only open on 50MHz at the peak of the 11 year solar cycle. For the Northern hemisphere in the Winter months open from October to December and possible from Europe to work all Continents, including Australia. Last peak was in 2013/2014. Get ready for the pileups in 11 years time!