Propagation Modes
To maintain good communications, there are several terms that need to be defined that pertain to the propagation of signals through the ionosphere.
Maximum Useable Frequency, (MUF)
The term Maximum Useable Frequency is defined as the maximum highest useable frequency that can be used for communication from one point to another. So for any given great circle distance there is a maximum useable frequency which is the highest frequency that will be reflected from a specific layer of the ionosphere and will return to the earth at the great circle distance location of the receiving station.
An increase in the distance between the transmitter and the receiver will require an increase in frequency. In other words, the greater the distance between points of a communications circuit, the higher the maximum useable frequency.
Lowest Useful Frequency, (LUF)
At certain frequencies the penetration of radio waves into the ionosphere is limited by absorption of the signal by some of the layers that make up the ionosphere. These layers are primarily located in the D region and in the lower part of the E region. For the most part, the higher the frequency up to the top of the MUF range, the less the absorption and the better the communications are.
Absorption is at a maximum in the range from approximately 500 KHz. to nearly 3 MHz, and decreasing from 3 MHz. up. To see this in real life all you have to do is to tune a general coverage receiver from 1.6 MHz upward in frequency to about 12 MHz. As you tune higher in frequency, signals start to come in from stations further and further away. The higher you tune, you will notice more signals from DX locations beginning to show up. This corresponds to decreasing absorption as the frequency increases.
As the frequency is increased, there will be one frequency where the received signal will just override the noise level at the receiver site This is called the Lowest Useable Frequency,(LUF). Frequencies lower than this will be absorbed too much for useful communication. The LUF also depends on the distance between points on the circuit and the transmitter power, as it is possible to use brute force to force a signal through to the receiver site. When it gets dark, the noise level in this frequency range increases as the frequency is lowered, and the LUF will change again, only this time it will decrease.
Optimum Working Frequency, (FOT)
Changes in the ionosphere occur on a daily basis, as well as an hourly basis. MUF predictions are made by averaging long-range observations and day to day variations are not taken in to account. So when picking a frequency or band to operate on, you must choose one that is just below the MUF. This will insure the band staying open a little longer than if you were right on the upper edge of the MUF. Of course this is a little optimistic, as most DX chasers are going to be right on the edge of the top part of the band that is nearest to the MUF. But for good communications the use of a lower frequency would be a lot better. This frequency range is also referred to as the Frequency of Optimum Traffic(FOT).
For instance, if 10 meters is really hot, but all you would like to do is ragchew with a few stations across the country for a couple of hours, then either 14,18,21 or 24 MHz would be a better choice, as they are below the MUF. This is an example that is using the principal daytime frequencies as an example, but this holds true for all frequencies. A good rule of thumb is to use a frequency that is 80% less than the MUF, so if 10 meters is really open, then 80% of 28 MHz is 22.4, so 15 meters is fine.
For those of you who have MS-DOS computers, there are several programs that will enable you to predict propagation from your QTH to other locations, by using the solar flux data that you can get from WWV. They are accurate enough for our purposes.
Ground Wave Propagation
Ground wave propagation refers to the types of radio communications that do not use ionospheric reflection to communicate from point to point. So the transmitted signal strength depends on other things to get from point A to point B. Transmitter power, transmitting antenna characteristic, frequency of operation. Signal diffraction around the curvature of the earth, electrical characteristics of the local terrain, such as conductivity and dielectric constant, characteristics of the circuit path, weather conditions. All of these factors have an effect on the received signal strength. The losses involved in ground wave transmissions are excessive sometimes, and ground wave communications are usually limited to several hundred miles at low frequencies and to exceptional high frequency applications.
Figure 1 shows the three types of ground wave communication propagation modes, and these will be discussed separately. These three modes are the direct wave, the ground wave, the surface wave and the troposphere wave.
Surface Wave Communications
The surface wave is that component of the ground wave that is affected primarily by the conductivity and dielectric constant of the earth and is able to follow the curvature of the earth. This type of propagation results in a very stable signal. If you are interested, VLF and LF use this type of propagation, almost exclusively. Loran C which is located on 100 kHz, relies on the consistent signal levels at the receiver end as any phase shift can throw off the digital data that is being transmitted. The low frequency of this signal and the very consistent levels of signal strength at distant locations allow excellent data transmission to take place. The Omega navigation system on 10 kHz also uses the VLF region for the same reason. The old LF navigation beacons for aircraft were in the 200-500 kHz frequency range for this reason also.
The old Loran A that was on 2 MHz suffered from all sorts of inaccuracies due to propagation delays, phase shift of the signal, and this is one of the reasons for shifting to the LF Loran C navigation system, to eliminate this type of problem. As an example of how constant the signal levels can be, Here in Corpus Christi, a signal strength reading was taken on one of the Loran stations that make up the chain that covers the Gulf of Mexico. A signal strength of 320 millivolts, (not microvolts) was measured with no day to day or hour to hour variation of any consequence. This was measured on an IFR FM/AM 1000S for those who are interested.
Ground Reflected Component.
The ground reflected component as its name states, is the portion of the radiated signal that reaches the receiving antenna after being reflected from either the ground or sea. For communications between points lower than a few thousand feet and separated by several miles, the ground reflected wave becomes important as a means of propagation. After reflection from the earth’s surface a phase reversal of 180 degrees takes place. Since the reflected wave takes a longer time to get to the antenna than the direct wave, a phase cancellation will result and the signal will cancel out the direct wave. This can be a problem at times.
Tropospheric Wave Component
The tropospheric is that component of the entire wave front which is refracted, NOT reflected in the lower atmosphere by relatively rapid gradients in atmospheric humidity and sometimes by the same type of changes in atmospheric density and temperature. A common cause of this is temperature inversions, which can give some rather long range communications on vhf at times. The amount of refraction increases as frequency increases, so the tropo as is called is more pronounced at vhf and uhf. The other components of ground wave communication are not as important at vhf or uhf.
Of the types of ground wave propagation, the surface wave is the only one that is of any importance to us. Since the direct wave and the ground reflected wave tend to cancel out each other, the surface wave is the primary one for communications purposes. The tropo mode of communications is too variable to be a reliable mode of communications for most applications.
How Frequency Affects Ground Wave Communications.
The frequency of operation primarily determines the particular component of the wave that will be the primary communications mode. When the ground conductivity is high and the wave frequency is below 10 MHz, the surface wave is the dominant mode of propagation. At frequencies greater than 10 MHz but less than 30 MHz, the electrical characteristics of the terrain will determine the characteristics of the surface wave. Above 30 MHz losses are excessive and this mode is not practical.
Ground wave communication on the different frequencies can best be defined in this manner:
VLF/LF
0.01 to 0.3 MHz is used for moderate to long distance communication and the losses to a vertically polarized signal are small and the wave is able to follow the curvature of the earth for several hundred or even thousands of miles and further, depending on transmitter power and antenna efficiency.
Medium frequency band
0.3 to 3.0 MHz is used for moderate range communications over land and for DX communications over water.
High frequency bands
3.0 to 30 MHz is for very short range ground wave communications as the absorption of the signal by the earth is quite high at the higher frequencies. Now keep in mind that we are talking only about ground wave communications, nothing else. That is for the next part of this tale.
The Ionosphere and Other Mysteries
The ionosphere is the next player in this game of electronic ping pong as it is the place where all of the action is, as far as hf communications is concerned. So a discussion of what makes up the ionosphere is now in order.
Construction of the Ionosphere
The ionosphere is made up of several layers of ionized gases that are ionized by the ultraviolet rays of the sun. The different layers of the ionosphere will be discussed briefly as to what each layer does and how it affects communications.
The D Layer
This area of the ionosphere is not always present, but when it is, it is only found in the daytime. The altitude of the D layer is between 50-90 km above the surface of the earth. The region is so highly ionized that no reflection occurs and all skywaves in the lower parts of the rf spectrum are completely absorbed.
E Layer
This layer is a daytime only layer and it is between 90 to 140 km above the earth’s surface. It is totally dependent on the UV radiation from the sun for its existence and is most dense directly under the sun. Seasonal variations occur as the sun’s zenith angle changes with the seasonal variations in the earth’s attitude in relation to the sun. The E layer usually vanishes very rapidly after the sun sets.
F1 Layer
This layer is like the E layer in that it follows the sun. The only difference is that it rises to merge with the next higher layer, the F2 layer
F2 Layer
The F2 layer is the highest and the most useful layer for skywave transmission as it exists during daylight and nighttime hours. This layer is between 150 and 250 km above the earth during the night for all of the seasons. Daytime altitudes in the summer vary from 250-300 km and this is due to the effect of solar heat on the layer which causes it to rise and fall with the rising and setting of the sun.
This layer is also affected by particle radiation from the sun, and this is evidenced by the effect that the magnetic field of the earth has on the distribution of the F2 layer. Since the magnetic field is not evenly distributed over the earth there are longitudinal variations exist in the F2 layer for points of equal latitude at the same local time.
Now that we have gotten a very basic description of the normal layers that make up the ionosphere, it is time to see what these layers do for us in our day to day adventures in the hf bands, be they only SWL DX chasing or looking for the elusive UV201A DX station on 20 meters.
Sky Wave Transmission
When we use skywave propagation, the signal leaves the antenna at an oblique angle and heads for an ionic layer. The signal is refracted, not reflected. However, reflecting is an easier method to explain than refracting. But so as to make things a little easier understood, a brief explanation of refraction is in order.
Refraction occurs when a signal either audio or electromagnetic,(to include light) passes from a medium of one density to a medium of another density. The signal then bends as it crosses the barrier between the mediums. A practical example of this is a spoon in a glass of water. The part of the spoon above the water appears to be in a different location than the part below the surface. Anyone who has tried to spear a fish also knows about this.
So now we have a radio wave that is being bent back towards the earth by the ionosphere. When that refracted signal hits the earth, it then bounces back to the ionosphere. This process will continue until all of the transmitted energy is absorbed by the earth and the ionosphere. Now, as has been stated many times before, the number of times the signal is bounced and refracted determines the amount of signal loss. So it should be apparent that the lower the angle of radiation, the fewer bounces that the signal has to take to get to point B from point A. It sounds simple and is, sort of. The actual process of ionization and generation of multiple layers in the ionosphere is a little more complicated.
Because of different energy levels at different wavelengths, the depth of penetration of the solar ultraviolet varies, which will cause the formation of the different layers that make up the ionosphere. This is a very abbreviated explanation, but it will have to do as we have more important things to dig into. The only layer that raises a lot of questions is the F2 layer and it apparently is created by some other method in addition to being affected by the sun.
However, that is not for us to be too concerned about, we have to go on to more important things, such as sky wave propagation. So, onward to the skywave.
DX and The Sky Wave
If you were to imagine a very long skinny curved pool table that resembles a curved very narrow road, with fences that are made of several rows of flexible rods along one side. Each row of rods is separated by a few inches. Now if each row of rods had different spacing between the rods that make up the fence, the first row consisting of rods separated by two inches, the next row of rods are spaced a little closer, say one and a half inches. The third row is made up of rods placed 1 inch apart and the forth row consists of a row of rods one half inch apart. To make things more interesting, there is no outside bank so if you choose the wrong bank angle, your ball will go right off the edge of the table and vanish, never to be seen again.
Now to make things real fun, suppose you have a big bag of pool balls of varying sizes, none of which is marked as to size. So if you intend to sink a ball in the pocket that you cannot see, due to the curvature of the pool table, you must choose the right size ball to get through the first row of rods and the right angle to bank that ball off of the outside row of rods and back through the other rows of rods. This has to be done in such a manner to enable the ball to make enough bounces, without loosing all of its energy bouncing around and still be able to get to the pocket and trip the switch that dumps a pile of gold (tax free) in your pocket.
To keep you on your toes, let’s make everything the same color, and cover one eye so as to eliminate depth perception and any sort of accurate guessing as to spacing between rods. If you scrooch down to see where you should make your initial shot, the rods all seem alike, and tend to blend into each other. You know if you use too big a ball it will not make it past the first row of rods and they will bounce it back and after several bounces between the rods and the bank, the ball will use all of its energy and stop, far short of the pocket. This is what the D layer does to lower frequency signals in the daytime, absorbing all of the rf energy in the same manner.
So the next ball you use is smaller, and it gets past the first row but misses all of the rods in all of the rows and flies off of the table, gone forever. Seems that you choose the wrong angle to try to bank your shot. This is what happens when your radiated signal is at an angle from the horizontal that is greater than the critical angle. What is the critical angle you say? This is the angle (usually greater than 45 deg.) that allows your signal to penetrate the ionosphere and head for the Andromeda Galaxy.(” What is that strange collection of signals on 7.213 MHz GorBix? I cannot understand it.” “Must be some sort of new stellar emission, Narikz. It isn’t very coherent.”)
Ok, wrong angle, wrong size ball, let’s keep trying until we get it right. After a few more bad shots, the ball finally just barely rolls into the pocket, but does not have enough energy to trip the switch that dumps the gold into your pocket. Good, you just found the LUF. So you have to keep changing things until you finally get a ball into the pocket with enough energy to trip the switch. Congrats! You have found MUF, (small ball) and a low angle of radiation.Now to keep tripping the switch, you need to keep the shot angle the same and the balls the same size. However, the spacing between rows is being changed by leprechauns and so is the spacing between rods. New ball game, new everything else too. That is what happens when the propagation changes. So in order to keep the gold flowing (DX), you have to keep changing your shot angle and the size of the ball to correct for the changing conditions. Are we having fun yet?
Since we can’t change the angle of radiation of our normal antennas, we have to change the size of the balls, in other words, change bands. So now you have some idea how skywave propagation works. Throw in an occasional ionospheric disturbance, add noise from normal terrestrial sources such as lightning storms, and stir well, add a dash of hi line noise and Bingo! We are having FUN.
This is a very simple explanation, and as such, there is a lot that has not been covered, due to space and time. So now a discussion of real life happenings that occur on the ham bands.
If you want to observe the fluctuations of the different layers of the ionosphere, a spectrum analyzer is the thing to do it with. There have been several good articles on constructing a spectrum analyzer in the last few years, and they are really handy to have around. Building one is a cheap way to get a good piece of test equipment.
When you hook an antenna to a spectrum analyzer, you can see where the signals start to appear on end of the spectrum and where they start to disappear on the other end of the spectrum. It is a very interesting display.
Since most hams do not have a device like this, a more practical and easy to use method is to listen to 40 meters. If you pick the frequency of one of the European SWBC stations and start listening at noon, as the day wears on towards evening, you will first start to hear a very faint signal in the late afternoon that will start to build up in intensity as the sun starts to sink in the west. By the time dusk sets in, the SWBC station is very strong and within an hour or so, it will dominate the frequency for the rest on the night.
Then as the sun starts to come up in Europe, the signal will start to fade, and by 10 am time at the transmitter location, the signal at your station will have fallen off to almost nothing. Of course, this will depend on how far you live from the Atlantic coastline.
Usually anyone living in the midwest and western areas will be able to get a chance to find DX from the Far East before the sun comes up at their location. Anyone who has operated on 40 meters has observed this happening.
This is a very good example of how the different layers behave as the sun moves across the sky at your location.
Occasionally, another interesting phenomena occurs on the high end of 75 meters, and above 4 MHz. In December and January, very occasionally, after sunset, stations separated by 75-400 miles in all directions will start to loose contact with the most easterly stations, one at a time. After a period of time ranging from 30 minutes to an hour or so, the stations would start to be heard again. This phenomena would move across the country from East to West on a line running from North to South, following the sunset.
The explanation for this is simple, since the sun had not set at the altitudes of the ionosphere, no ionospheric changes had taken place to any extent. Only when the sun went down at the altitudes of the different layers, did the layers change to the nighttime configuration. Usually this would take place at a slower more gradual rate, but for some reason, on the occasions when this happened, the layers apparently changed more rapidly to the night time conditions and the signals were absorbed or reflected somewhere else for a while. This was noticed on the Navy Mars frequencies in the range from 4.0-4.025 MHz during nets.
Reviewing the information in this article should make you more aware of how important it is to pick the correct antenna for the type of operating that you do. Also, it is important that you realize what the capabilities are of the different frequency ranges that we use and pick the correct band for what you want to do. Local ragchewing in the 0-400 mile range in the middle of the day is impossible on 20 meters or higher under normal conditions, but not on 40 meters. Transcontinental operations at night on 10 or 15 meters is not going to take place under normal conditions, but does on 40 and 75 meters as well as 160 meters. So it is important to understand what each of the bands that we use or listen to, is capable of doing or not doing at any given time of the day.
The study of ionospheric propagation and all that it entails is a little beyond the scope of this article, but there are many books that are ideal for further studies. The many other things that affect the layers of the ionosphere locally and on a global basis are much too complicated for detailed discussion at this time. Just remember, a little understanding of how signals are affected by the ionosphere and how you can make your choice of an antenna work for you can make your operating more rewarding.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 13th March 2024