AN INTRODUCTION TO HF PROPAGATION

AN INTRODUCTION TO HF PROPAGATION

This article is designed to be an introduction to the terms and basic mechanics of propagation methods that are to be found on the HF and VHF bands. The descriptions of the terms quite basic in order to keep them understandable, so they may appear to be a little vague or not technically 100% correct. A complete description would take many pages and require much research, which is beyond the scope of this article. The descriptions given are just an insight into the most important area of our hobby for without the propagation of our signals, there could be no radio communication! If you have an area of interest, I would recommend using the mass of freely available information to further your knowledge of the particular subject. I have arranged the article in a ‘Question and Answer’ format as this is far easier to digest than a single narrative. Hopefully the range of questions posed and accompanying answers will cover most of the queries raised when trying to understand propagation.

WHAT IS PROPAGATION?
The term PROPAGATION is used to describe how the transmitted signal reaches the receiving station or target area. The way signals reach the receiving station are governed mainly by distance and frequency. That is an over simplification, but this article is designed to be an introduction, rather than an in depth explanation of every mode and variation of propagation phenomena. Other major players affecting MF/HF propagation are the 11 year solar cycle, time of year, day-to-day solar activity and time of day. For VHF (i.e. frequencies above 30MHz) the picture is somewhat different with the weather conditions and the state of the lower atmosphere playing a major role in determining if a station can be contacted. Other things affecting longer distance propagation on VHF are solar activity and location of both transmitting and receiving stations. Best distance under normal conditions can be obtained if there is an unobstructed salt water path between both stations (i.e. between Cornwall and the Canary Islands, which has been achieved on 2m several times).

WHAT ARE THE LAYERS OF THE ATMOSPHERE?
The ATMOSPHERE, the envelope of gases that surround the earth is comprised of many different layers, each having an effect on the propagation of radio waves. The atmosphere varies in density, being most dense near the earth’s surface and becoming less and less dense as height above the earth increases. Temperature in the atmosphere also varies with height with the rate of change is fairly constant. The portion of the atmosphere where the temperature reduction is fairly constant with height is called the TROPOSPHERE. The troposphere extends from the surface of the earth up to a height of 10km or so. This is the part of the atmosphere that contains the right mixture of gases to support life and is also responsible for our weather. The point at which the temperature stabilizes is called the TROPOPAUSE and is to be found at around 10-20km above the earth’s surface.
From the tropopause to about 60km up we find the STRATOSPHERE. This is the part of the atmosphere where aircraft fly as the air is thinner and hence there is less friction and drag, allowing the aircraft to fly faster and use less fuel. Temperature is fairly constant but does tend to actually increase as the top of the stratosphere is reached (the STRATOPAUSE). The next layers form the IONOSPHERE, perhaps the amateurs greatest asset, perhaps also his worst enemy!

WHAT IS THE IONOSPHERE?
The IONOSPHERE is comprised of several layers of gas located from 70 to 650km (approx) above the earths surface. These layers can combine, split or change their characteristics which makes forecasting conditions a bit of a hit and miss affair. Some properties of the ionosphere are well known and can be predicted with a fair degree of accuracy, whereas others are totally unpredictable. As I said, the ionosphere is split into several layers, each of which has been assigned an identifying letter to, hopefully, ease confusion. Starting at the lowest level, we have the D layer/region, next is the E layer, then the F layers (which are split into F1 and F2). All of these layers/regions have unique properties and each either aids or hinders our efforts to propagate a signal.

WHAT IS THE D REGION?
The D REGION is the lowest layer of the ionosphere, extending from about 60km to 100km above the earth’s surface. The reason that this area is called a ‘region’ instead of a ‘layer’ is that it is only really present during daylight (the time that the sun is illuminating the ionosphere, which is considerably longer than terrestrial daylight). At night the D region virtually disappears allowing signals to pass through to the higher levels of the ionosphere where they will be refracted back to earth (often the term used is reflected, but refracted is the correct description). The D region affects the lower frequencies and is the reason why signals on MW and 160m are confined to a few 10’s to maybe a couple of hundred km during daylight hours. While the sun is illuminating the ionosphere, the D region is highly ionised and acts like an RF ‘sponge’ to MF signals. As the sun sets, the D region becomes less and less ionised and almost disappears, the remaining ionised molecules are spread so thinly as to make little difference to the signal passing through.

WHAT IS THE MYSTERIOUS ‘E’ LAYER?
The E LAYER of the ionosphere is the stuff of radio legends, fantastic feats and has puzzled scientists for as long as they have known about it’s existence! Way back in 1902 (just after the ‘birth’ of radio) 2 physicists, one based either side of the Atlantic, first put forward the idea of there being an ionised layer in the atmosphere. The two were called Arthur Kennelly (USA) and Oliver Heaviside (UK), hence the old term for the E layer, the “Kennelly-Heaviside Layer”. Edward Appleton carried out experiments during 1924 to confirm not only the existence of the Kennelly-Heaviside layer, but that in fact there was at least one other layer above it. Appleton was the first person to refer to the Kennelly-Heaviside layer by it’s now common name, the E-layer. He named it the E layer because, he considered, it reflected the “electric” field of the radio wave. The layer above this he named the F-layer. The F-layer is sometimes still referred to as the “Appleton Layer”. Subsequently another layer, below the E layer, was discovered by Robert Watson-Wyatt, and designated the D layer (or region, for the reasons described previously). The E layer is located around 100-130km above the earth’s surface and the air density is still quite high, allowing the recombination of molecules (ionisation) to take place easily. As the sun illuminating the ionosphere causes ionisation, the ionosphere is most ionised at local midday. After sunset, ionisation rapidly falls off until it has all but ceased.

WHAT ARE THE ‘F’ LAYERS?
As Appleton had discovered, there is another layer above the E layer which he named the F-layer. What he didn’t realise is that the story is not quite that simple and the F-layer is in fact comprised of 2 layers that recombine at night (so can one assume he did his experiments at night?). To differentiate these layers, they were identified as F1 and F2. The F1 is the lower of the two and is to be found at heights of 150-250km. The higher F2 layer extends from about 300-500km or so. As stated, these heights are only valid during daylight. At night, the layers combine and form a single layer at a height of about 250-350km. These heights vary according to time of year, the solar cycle and latitude. Because the F layers are the only ones that are present in some form 24 hours a day, they are considered to be the most important part of the ionosphere as far as radio communications are concerned.

WHAT DOES THE TERM “CRITICAL FREQUENCY” MEAN?
The CRITICAL FREQUENCY is the highest frequency that will be returned to earth when the wave is radiated vertically (this is very different to being VERTICALLY POLARISED) towards the ionosphere. Once the critical frequency is passed, the radio wave will not be returned to earth and will travel out into space. When listening to propagation forecasts, the term “F2 DAYTIME CRITICAL FREQUENCY” will be heard. What this means is that the frequency stated is the highest frequency that the F2 layer will return to earth during daylight (as the F2 layer ceases to exist during darkness). When written, the symbol for critical frequency is ‘f0’. Sometimes the critical frequency is measured against the E layer instead of the usual F layer. The symbol used to denote an E layer critical frequency is ‘f0E’. The f0E is usually around 2-4MHz, depending on time of day, year and solar cycle.

WHAT DOES THE ABBREVIATION “MUF” STAND FOR?
The term “MUF” is used a lot in radio communications and stands for “MAXIMUM USABLE FREQUENCY”. As we have just seen, when a radio wave is transmitted vertically, it will not be reflected back to earth above a certain frequency. If the angle of the radiated wave is changed from vertical to something nearer horizontal, the frequency at which it will be reflected increases significantly. To calculate roughly the MUF, multiply the F2 critical frequency by 2.15. The MUF, however, varies considerably dependent on the location of the transmitting station and the path attempted. The path with the highest MUF from the UK tends to be towards South America, which is why South Americans are often the last stations to fade out on the HF bands before they close for the night. It is an extremely useful thing to know the MUF of a given path as the maximum chance of working a station is obtained from using the band that is closest in frequency to the MUF (check beacons in the part of the world you are interested in to see the highest band they can be received on). A strange thing is that you may be able to hear loud signals from the desired location but cannot make contact. If this happens (and it will!), try switching to the next band up. Signals may be much weaker, but you will be surprised that more often than not you will be able to make the contact without problem. As signals are weaker, this is a sure sign that you are near the edge of the MUF and exploiting this fact will provide many more contacts with far off places than using the lower band, despite the stronger signal levels on the lower band. This is using the MUF to your advantage. If you are interested in working DX, it is advisable to obtain one of the excellent propagation prediction programs that available. Most are freeware or shareware and provide good results. A word of warning though, don’t take the results from these programs as gospel, as day to day variations can throw a spanner in the works. If you need to input solar data (as some require), try to obtain monthly average figures as they will give a much more accurate result. Making best use of the MUF can give surprising results and if you are restricted to low power (say 10w, as the foundation licensees are) or poor antennas, this is often the only way to compete with the big guns. Using this, and other, aids is how I have managed to work over 160 countries using just 3 watts and simple antennas. Operating skill plays a part, but using the available resources such as knowing the MUF and how the bands react during the course of the day, month, year etc. will be even more valuable.

WHAT IS THE ANGLE OF RADIATION AND WHY IS IT IMPORTANT?
As discussed earlier, if a radio wave leaves the transmitter at an angle less than vertical, it will be reflected back to earth at a higher frequency. Also it will be reflected back at a distance from the transmitter. This is obviously a vital fact if we are wanting to contact someone who is some distance from the transmitting site. At high angles of radiation, signals will be reflected back down at a short distance from the transmitter. As this angle is decreased towards the horizontal, so the distance covered by the reflected wave increases. The angle at which the radio wave leaves the transmitting antenna is known as the ‘ANGLE OF RADIATION’. As the radiation angle is reduced, so the distance covered by each reflection increases, hence the need for an antenna with a low radiation angle when trying to work long distances on the HF bands. The angle that the transmitted wave hits the ionosphere is known as the ANGLE OF INCIDENCE, and can be different to the angle of radiation (due to anomalies in the atmosphere before it reaches the ionosphere). If you are interested in working long distances, then you would equip your station with a low angle antenna (such as a well grounded vertical or a high multi element beam type antenna). If, however, your interests lie in shorter distances (for example, UK nets on 80m), then a low dipole would be a better option as it will have a much higher radiation angle than a beam, vertical or high dipole. I am talking of heights with respect to wavelength rather than physical height in metres. A low dipole on 80 would be anything less than about 60 feet, whereas a yagi for 10m would be considered to be high if it were located at the same height. If you are looking to work long distances on 80m (and 40m for that matter), very few people are able to erect a dipole at sufficient height (120 feet would be a minimum to negate effects from the ground), so a vertical is a much better alternative. It should be borne in mind that in order for a vertical to have a low angle of radiation an extensive radial system needs to be employed.

WHAT ARE N.V.I.S., GROUNDWAVE AND SKYWAVE?
N.V.I.S. is the acronym for Near Vertical Incidence Skywave (or ‘cloudwarming’ as it has become known amongst us radio folk). NVIS is the art of sending a nearly vertical radio wave up into the ionosphere, so that it may be reflected back at a short distance from the transmitter. It is particularly useful at low frequencies to cover the “SKIP ZONE”, the area that that is longer than the distance covered by GROUNDWAVE, but shorter than the first normal reflection (or hop) from the ionosphere. The groundwave is the part of the signal that travels along the surface of the earth to the receiver. The strength of the groundwave is inversely proportional to frequency, being strongest at low frequencies. As frequency is increased, the distance covered by the groundwave decreases. Local MW stations rely on the properties of groundwave propagation to reach the target audience. As the range of a MW groundwave signal is limited to a 100 or so km during daylight, stations that are geographically separated by 200km or so can operate on the same frequency without causing interference to each other (in theory!). The SKYWAVE is the part of the signal that is sent skywards and is reflected from the troposphere with the SPACEWAVE (not a term in common use these days) being the part that is reflected from the ionosphere. The Skywave is now taken to mean any signal other than the groundwave. As the D layer disappears at night, MW signals from the continent and further afield can be heard as they are being propagated by skywave. Normally skywave signals on MW would not be heard during daylight as the D region would absorb the signals like an RF sponge. stations that are within the skip zone cannot be heard as there is no method of getting the signal to the receiver. NVIS is a relatively new technique to try and get signals into the skip zone. By sending a nearly vertical wave to the ionosphere and near, but not above, the critical frequency, it should be possible to get a signal to reflect into the skip zone.

WHAT IS MEANT BY ‘MULTI HOP’
As we have seen, signals can be reflected off the ionosphere back to earth at a distance governed by the angles of radiation and incidence and by the frequency in use. The maximum distance that be covered in a single ‘hop’ as it is known is about 4500km. So how can we hear signals from 15000km? The answer lies in the ionosphere (as usual), each time a signal is reflected back to earth, some of the signal is bounced back to the ionosphere. The type of terrain the signal meets when it reaches earth determines how much will be reflected back to the ionosphere. Salt water is one of the best reflectors as far as this is concerned, so a signal that hits salt water (i.e. the sea or ocean) will suffer much less attenuation than a signal that bouncing from a dry desert. Each time a signal bounces it loses some of it’s strength and eventually can become so weak that it is no longer detectable. When the signal enters the ionosphere it also suffers attenuation due to the friction (and hence heat) of the radio wave vibrating ionised molecules. Other strange effects can take place in the ionosphere and the signal may travel a considerable distance within the ionosphere itself before returning to earth, so predicting where a radio wave will return is a bit hit and miss! The term ‘MULTIHOP’ describes the action of a radio wave bouncing back into the ionosphere and subsequently being returned to earth more than once. To traverse the distance between here and Australia would take 4-5 hops, or more if the signal was being propagated the other way ‘LONG PATH’. The diagram below illustrates multihop propagation. You can see that it is possible to contact a station by using a higher angle of radiation and taking more hops, but this means the signal will be weaker than the lower angle choice that uses less hops (the fewer the hops, the less the signal is attenuated). Another way signals can be propagated is via ‘CHORDAL HOP’, which is where the signal bounces from the upper ionosphere without sufficient strength to return to earth. The signal becomes ‘trapped’ between the lower and upper levels of the ionosphere and bounces, or hops, within the ionosphere. Eventually, the signal strikes the upper layers at an angle sufficient to allow it penetrate the lower levels of the ionosphere and return to earth. The advantage of chordal hopping is that the signal suffers much less attenuation than if it were to return to earth each time before bouncing to the higher ionospheric levels.
WHAT WHAT IS ‘SHORT PATH’ AND ‘LONG PATH’?
When a radio wave is propagated to a distant place, it travels in a theoretically straight line (the shortest distance between two points is a straight line). Now with the earth being a sphere, normal maps (with the Mercator projection) will give you false information as to how the radio wave reaches it’s target. To get the true picture you need to look at a globe or a special type of map called a ‘GREAT CIRCLE’, which is a representation of the globe if it were flattened onto a piece of paper.
This picture shows how you would expect a radio wave to travel from England to New Zealand, as viewed on a Mercator projection map.
The actual path taken by the England to New Zealand signal viewed on the same map.
The England – New Zealand path as shown on a great circle map (notice that it is a straight line and crosses over the North Pole). If an aircraft were to fly direct from the UK to New Zealand, this would be the preferred route as it is the shortest or SHORT PATH. Most radio communication on the HF bands is conducted via Short Path.
This is the LONG PATH from UK to New Zealand, which is the exact opposite of the most direct route. The signals are propagated to the desired location by beaming your signals 180 degrees from the normal heading. The signals can be stronger at certain times of the day when propagated via long path. Sometimes long path is the only path that has an MUF high enough to support communication to the desired location. When using an omni directional antenna, such as a vertical or low dipole, signals can be received from both short and long path simultaneously, which can lead to confusion at the receiver as there will be a noticeable echo on the signal (the long path signal has travelled considerably further so takes longer to reach the receiver). This echo is only a few 10’s of milliseconds but can play havoc with CW reception as the spaces between characters can be filled in with characters from the echo signal rendering the signal almost unreadable. The echo length varies in proportion to the distance travelled. Listening to a New Zealand station on both paths would not cause to many problems as the difference in path length is minimal but listening to a West Coast USA station via long and short path simultaneously causes real headaches as there is a considerable difference in path lengths which produces a delayed echo. Weak CW from California in the late afternoon on 14MHz with a vertical can be almost impossible as both long and short path signal strengths are almost identical, rendering the signal into a garbled mass of dots and dashes. A directional antenna, with the ability to rotate would make life a lot easier!!

WHAT IS THE ‘GREYLINE’?
The GREYLINE terminator is the imaginary line drawn on a map to show daylight and darkness. The greyline is very important for radio communications and is another thing in our weaponry that we can use to get our signal to the desired location. As darkness falls, we enter the twilight zone, where it is not really light, but it isn’t completely dark either. When the stations at both ends of the path are near darkness, signal levels often increase for the period that both stations are in twilight. Often this is the ONLY time contact can be made between stations on a given band. Typically this affects lower bands more (160-40m) but there is increasing evidence that using the greyline can be advantageous on all HF bands. This ‘window of opportunity’ only exists for a short time, especially on the lowest frequencies before absorption increases to such a level that the signal cannot penetrate the D layer (which is now building in strength as the path comes into daylight). On 160m (1.8MHz), the window can be as little as 20 mins or even less between UK and New Zealand. If you are looking to work the other side of the world on 160m, you need to be prepared as a few minutes either way can mean the path being closed. Greyline paths are not open every day, as you need suitable propagation conditions to the intended target. The best times to try this path is around the equinoxes (March and September) and at dawn or dusk.

WHAT IS THE S.F.I.?
SFI stands for SOLAR FLUX INDEX. The SFI is a figure derived from the number of active sunspots visible on the face of the sun based on observations of thermal and radio noise received at a wavelength of 10.7cm. (2.8GHz) at an observatory in British Columbia, Canada. The thermal noise is related directly to the amount of plasma trapped in the magnetic field that overlie the active regions of the sun, which in turn is related to the amount of magnetic flux (the lines of force generated by a magnet) generated by these active regions.

WHAT ARE SUNSPOTS?
A SUNSPOT is an area of intense magnetic activity on the surface of the sun which is cooler than the rest of the sun’s surface. Because it is cooler, a sunspot is darker and therefore visible against the hotter, brighter background. Some sunspot clusters are so large they can be seen without magnification. The next sentence is extremely important and must be adhered to at all times when observing the sun. NEVER LOOK DIRECTLY AT THE SUN, ALWAYS PROJECT THE SOLAR IMAGE ONTO A PIECE OF CARD WHEN MAKING OBSERVATIONS. FAILURE TO DO THIS CAN RESULT IN PERMANENT BLINDNESS. There are filters on the market that supposedly make it safe to look at the sun directly - don’t trust them! If you value your eyesight, you will never look at the sun directly. Another advantage of projecting the sun onto a card is that the image can be magnified, making the identification of sunspots easy. Sunspots are a vital part of long distance radio communications as they tied to the ionisation level of the F layer. The more active sunspot groups, the higher the magnetic and ultraviolet radiation from the sun and the higher the SFI. If a graph were plotted of daily sunspot numbers, it would be rather spiky and difficult to interpret over time, so a method of averaging is used, called ‘smoothing’, which gives us the SMOOTHED SUNSPOT NUMBER or SSN. In the years where there are few or no sunspots, radio propagation on the higher bands can be non existent. However all is not lost as the lower frequency bands tend to improve as solar activity declines.

WHAT IS THE SOLAR CYCLE?
Every 11 years or so the sun goes through a complete cycle of events. At the beginning of a new cycle, the first sunspots are noticed on the otherwise blank solar disc. This first batch of sunspots heralds the gradual improvement in the higher HF bands. At the very bottom of the cycle, where few or no sunspots are visible, the bands from 21MHz up can be dead for weeks on end. Except for some localised ionisation during the summer which supports inter-Europe contacts, virtually no signals will be heard. Trying to work long distances during sunspot minima on 15 or 10m is a thankless task! Over the next 4 or 5 years, more and more sunspot groups become active. As more groups are active, so geomagnetic levels in the sun rise and the ionosphere becomes increasingly ionised, leading to an increase in the MUF and hence better conditions on the higher bands. The years 2000-2001 saw the peak of the current 11 year solar activity cycle where many groups were active, and remained active for long periods. This solar cycle was predicted to be the best for many years, but those who have been active for a few cycles will know that it did not live up to expectations! An unusual feature of this cycle was that it appeared to have two peaks. The first occurred in the middle of 2000, with a tail off around the beginning of 2001. The second peak was around late 2001, with conditions beginning to trail off during 2002. We can expect conditions on the higher bands to fall off over the next 3 or 4 years with the minima in 2007-2008. This ‘11 year’ cycle is an average and can be anywhere from 9 upto about 15 years between maxima. The term solar activity refers to a combination of the amount of sunspots and also the effect the solar geomagnetic activity has on the earth’s magnetic field. At sunspot maxima, solar activity is generally high (or active) due to the increase in sunspot activity and at sunspot minima, solar activity tends to be at low (or quiet) levels.

WHAT ARE THE A AND K INDEX?
The explanation of the solar cycle is based on ‘smoothed’ figures, that is the average figures for each month are used rather than using daily figures. The reason for this is to give a more accurate picture than using daily figures would. Day to day solar activity can vary dramatically, so using these figures for propagation prediction can lead to unsatisfactory results. As well as variations in the geomagnetic flux, there are two other measurements of the magnetic field activity (caused by interactions of the solar wind the magnetosphere and the ionosphere) that are used in predictions. These are the A AND K INDICES. The K index is a 3 hourly reading of the magnetic activity compared to a standard ‘quiet’ day taken at various observatories. This index was first introduced by J. Bartels back in 1938 and consists of a single number in the range 0-9. The planetary K index (Kp) is the mean or average index of readings from 13 observatories located between 44 and 60 degrees latitude each side of the equator. The planetary index measures solar particle radiation by it’s magnetic effects. The A index relates to geomagnetic stability and is derived from the K indices of the previous day. The Planetary A index (Ap), is derived from the A indices of the world wide network of magnetometers.

WHAT IS A SOLAR FLARE/GEOMAGNETIC STORM?
As mentioned above, the A and K indices give us a good indicator to propagation conditions. As the index increases so geomagnetic activity increases until a level is reached that has been classified as storm level. A solar storm occurs when nuclear fission within the sun reaches such a high level that matter is ejected outwards from the sun, towards the earth. The most common of these is the SOLAR FLARE, many of which occur during the course of a year. These flares range in intensity from mild to severe. The strength of a solar flare (X-ray flux) is measured by the GOES spacecraft and is denoted by a letter and number system. The strength ranges from the weakest, A, through B, C and M to the most severe, X. Within these classes, a further subdivision using a numeric system is made ranging up to 9, so the most severe flare would be classed as an X9 event. Thankfully there are few of these during an 11 year cycle. A flare is considered significant if it rises above M levels. To put a solar flare into context, large flares can emit 10 million times the energy that is released by a volcanic eruption. Such an intense release of energy not only disrupts earthly communications but it can also cause damage to satellite control systems as they do not have the benefit of an atmosphere do dampen the force of the shock wave. A solar flare travels at such speed that within 10 minutes or so of eruption it can reach the earth’s atmosphere causing short-lived radio blackouts on the sunlit side of the earth.

WHAT IS A CME?
Another common result of excess nuclear activity within the sun is the CME or CORONAL MASS EJECTION. A CME is an explosion within the sun that causes large amounts of solar matter to be ejected into space. This ejection causes a ‘shock wave’ of magnetic disturbance to head towards earth. When the shock wave hits the ionosphere it disturbs the earths magnetic field to such an extent that some of the electrons in the field can be lost to the shockwave. This disturbance to the magnetic field shows up as a rapid increase in the A and K indices. With electrons missing, the ionosphere loses ionisation, the MUF falls and absorption can increase to such a level that virtually all HF communications are wiped out. Once the CME has passed, it can be several hours or even days before the earths magnetic field returns to normal and the missing electrons are replaced, allowing higher levels of ionisation to once again occur. A CME travels much slower than a flare and can take a couple of days to reach a distance that will cause disruption to communications.