VHF/UHF Propagation modes explained

Propagation type

Distances

Comments for European stations

Line of sight

0-50 Km

(Ground)

0-100 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

For a Radio Path Study calculating your RF path line of sight anywhere in the World using Google Maps, click on the image below

Knife edge diffraction

50-100 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'.

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

100-800 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 800 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 theoretical maximum for aircraft scatter (ACS) propagation remains around 700-1000 km for frequencies in the GHz microwave bands

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

This Propagation mode seems to occur when both stations are located at equal distances North and South of the Magnetic 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/s, 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 for the Australian station of VK8AW working stations in Europe on 50 MHz at solar cycle minimum via TEP combined with 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 TEP + ES

The following vertical total electron content map from NASA may help to indicate whether or not propagation via TEP is more or less likely.

Right Click on image below and select 'open link in new tab' 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 long 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

400-2400km

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.

Extremely rare, once or twice in a lifetime, events can have so many meteors hours burning up that the E layer permits reflections lasting for several hours, just like Sporadic-E propagation. I have only ever witnessed this once with the Leonids shower in November 2002, with 700+ meteors per hour being recorded.

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 may 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 2300+ 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 with Sporadic-E signals that lasted for many hours continuously on even 144 MHz.

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 has been phased out in Western 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.

Sporadic E (Es)

50MHz / 70MHz 

400-2400km

(Single hop)

2400-4800km

(x2 ES)

4800-7200km

(x3 ES)

7200-9600km

(x4 ES)

Sporadic E (Es) is an abnormal propagation mode at mid-latitudes which occurs mainly 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 few further Sporadic-E events at other times of year, especially during larger Meteor Showers

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 - 130 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 sometimes be observed to 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, some recorded as only 5km away from 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 darkness hours, the events usually begin at dawn, there is a peak around 12:00 UTC and a second peak in the evening around 16:00 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 2,400km 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, via triple hop Sporadic-E. 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 Brazil 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.

On 50 MHz rare x4 Es hops have been observed between Europe and Japan, these have been seen in July 2023 over an entirely daylight path in the early morning UK time, before sunset in Japan.

Sporadic E remains a mystery, since first observed in the 1930's.

144MHz

1400-2400km

(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 peaks of the 11 year solar cycle. For the Northern hemisphere in the Winter months open from October to January and possible from Europe to work all Continents, including Australia. Last peaks I observed F2 propagation at 50 MHz was in 2024 and 2013