HIGH FREQUENCY PROPAGATION
October 8, 2008 – Educational Radio Net, Session 20
Jim Hadlock K7WA
One of the things that got me interested in radio was hearing stations from far away places. At first I used my clock-radio in the AM broadcast band. I discovered that at night I could hear stations from Los Angeles, Salt Lake City, and even Mexico! Later I built a Knight Kit shortwave radio and began listening to broadcasts from South America, Russia and Japan. As a ham I've enjoyed the DX (long distance communication) aspect of our hobby for nearly fifty years, but the idea of a small radio signal propagating to and from far away places still intrigues me. Last year when I was in the Caribbean I made a 2-way contact with Paul, NG7Z, in Bothell on 40 meter CW – we were both running 5 watts of power. That, to me, is an example of the miracle of radio propagation – a very small signal covering a great distance.
The subject for tonight is High Frequency Propagation. We will discuss some of the factors that determine how a radio signal travels to far away places and resources for analyzing and predicting propagation conditions. If you have spent much time listening or operating in the high frequency bands between 160 meters and 10 meters you know that propagation is highly variable. How far you can communicate depends on many factors.
Lets begin with frequency. As I discovered with my clock-radio, far away signals on the AM broadcast band come in better at night. This characteristic applies to signals in the 160 meter, 80 meter, and 40 meter amateur bands as well. During daylight these bands may provide local coverage, but at night they can support world-wide communications. The higher amateur bands, 20 meters, 15 meters, and 10 meters are usually open during the daytime and quiet at night. These daily effects are due to the sun's radiation ionizing atoms and molecules in the earth's upper atmosphere, and the different layers of ionized material either absorbing, bending, or passing through radio signals of different frequencies. During daylight ionization in what's called the D-Layer, about 50 miles high, tends to absorb radio signals. This absorption is greater at low frequencies than high frequencies. If the signals pass through the D-Layer they can be refracted, or bent back to earth, by ionization in the F-Layer, 150 to 300 miles high. If the frequency is too high to be bent the radio signal will pass through the F-Layer and continue out into space. A few weeks ago Bob, K9PQ, discussed the terms Maximum Useable Frequency (MUF) and Lowest Useable Frequency (LUF) which are defined by this absorption/refraction process. After the sun sets the D-Layer starts breaking down due to the absence of solar radiation and propagation improves on the lower frequency bands. Although the F-Layer also breaks down in the absence of solar radiation, it often supports some propagation through the night. At sunrise solar radiation begins to build the D-Layer and F-Layer again.
In addition to the daily propagation cycle, seasonal effects vary greatly. Spring and Fall are similar, but Winter and Summer are very different. Due to the tilt of the earth's axis, radiation from the sun is weaker in the winter and stronger in the summer. The long winter nights make for very good low band propagation, while during the short summer nights the higher bands may remain open 24 hours a day.
The solar radiation which ionizes atoms and molecules in the earth's atmosphere is not constant. One of the best indications of strong radio propagation is the presence of sunspots on the surface of the sun. Sunspots are areas on the sun associated with ultraviolet radiation which ionizes the upper atmosphere. Sunspots can appear and disappear quickly or remain for several solar rotations (the sun rotates on its axis every 27.5 earth-days). Sunspots have been observed since Galileo invented the telescope in the early seventeenth century. Sunspots have been counted and recorded as long as they have been observed with records going all the way back to 1610. Currently there are two official sunspot numbers in common use, the daily "Boulder Sunspot Number," computed by the NOAA Space Environment Center, and the "International Sunspot Number" recorded in Europe. Both numbers use a method devised by Rudolph Wolf in 1848, which combines a count of groups of sunspots and a count of individual sunspots:
R=k(10g+s), where "R" is the sunspot number, "g" is the number of sunspot groups, "s" is the total number of individual sunspots in all the groups, and "k" is a variable scaling factor (usually <1) href="http://www.spaceweather.com/glossary/sunspotnumber.html">www.spaceweather.com/glossary/sunspotnumber.html
Understanding Solar Indices – www.arrl.org/tis/info/pdf/0209038.pdf
The Sun, the Earth, the Ionosphere: What the Numbers Mean and Propagation
Predictions – www.arrl.org/tis/info/k9la-prop.html
W1AW Propagation Bulletin – www.arrl.org/w1aw/prop
WWV - www.swpc.noaa.gov/ftpdir/latest/wwv.txt
NCDXF Beacons – www.ncdxf.org/Beacon/BeaconSchedule.html
Contributed by Wr5J, Curt Black
Big Bear Solar Observatory - http://www.bbso.njit.edu/
Showing posts with label Maximum Usable Frequency. Show all posts
Showing posts with label Maximum Usable Frequency. Show all posts
Monday, October 6, 2008
Wednesday, September 3, 2008
MUF and LUF, Bob, Week 15
Today's topic covers a set of General Class questions regarding MUF and LUF. For those that have the General Class Exam Question Pool, they are questions G3B01-G3B12.
The reason I am using the abbreviations at first is that two of the questions ask you to identify what MUF and LUF stand for. MUF stands for Maximum Usable Frequency and LUF stands for Lowest Usable Frequency. More specifically, they define the maximum (highest) and lowest frequencies that can be used to communicate between two stations. These terms refer to atmospheric conditions so it is assumed that we are talking about communications requiring skip. We don't talk about MUF or LUF for direct point to point communications. In the process of discussing MUF and LUF we are going to sneak in a bit of general radio wave propagation theory so hang on to your hats. Here we go.
When we talk about skip or skywave communication you will often hear that the radio wave was reflected back to earth or "bounced" back. These are handy terms to use but are not exactly accurate. What really happens is that the radio wave is refracted, which is to say bent or curved. To use an analogy with beams of light, the atmosphere does not act like a mirror, but instead acts like a lens or a prism. This is important to know for MUF because this bending of the radio transmission changes with its frequency. That is, the higher the frequency, the less the radio beam is bent. The MUF can be thought of as the frequency that just barely gets curved enough to make it back to earth. If the frequency is a little higher the transmission would still be bent but would not quite be bent enough to come back down and would go off at an angle that would just miss the surface of the earth and keep going into space.
The LUF has less to do with the bending of the signal as it does the absorption of the signal. This is not such a hard limit as the MUF since what you are measuring is how much you can have the signal be absorbed and still be readable above the noise floor. Mode of operation figures into this as well since, as we know, some modes like CW can be understood much further down in the noise than others like SSB. This absorption is a fairly continuous thing, that is to say, the lower the frequency the more the absorption. So the best, least absorbed, transmission occurs just below the MUF.
Since we want to be as close as we can to the MUF without going over, a bit like the game of 21, this is the frequency most often reported. So let's talk a bit about what affects this frequency and how it is measured. The MUF is determined by the characteristics of the ionosphere and by the location of the two stations. The ionosphere is not a simple static layer. It is constantly changing although much of the time the changes are fairly predictable and not great over small periods of time. The sun has the biggest effect on the ionosphere. At sunrise the ionosphere changes such that the MUF goes up significantly and quickly and stays high throughout the daylight hours. The MUF then gradually declines throughout the night, reaching it's lowest point just before dawn. Anything that affects the ionosphere, affects the MUF, so solar flares can significantly change your operating characteristics.
In order to be sure you aren't losing some of your transmission by straying over the MUF as it changes, you actually want to be a bit below the MUF for reliable working conditions. In one reference I found they defined the Optimum Working Frequency (OWF) as 85% to 90% of the MUF. In the same reference they also differentiate between the Operational MUF and the Basic MUF. The Basic MUF is more of a theoretical value while the Operational MUF takes into account the antennas, operating mode, power, etc.
Let's return to one of the points I made above, namely that the MUF depends on the locations of the two points. While this is true, this statement refers to changes in the MUF over great distances. You can be pretty confident, for example that the MUF between Seattle and Chicago is about the same as between Olympia and Chicago. But going around the globe the MUF varies by quite a bit and also, of course varies over time. In one of the references below is a near real-time map of the MUF around the world. But ultimately the MUF for you to communicate with another station is specific to your particular circumstances.
Finally, how do you find out what the MUF is? Until very recently, the best way to tell was to listen to beacons at various frequencies and locations to see if you could hear them. This is still a perfectly good way to do it but now there are many online sources to get up-to-the-minute values for MUF and there are also many programs for predicting it. The only web sites I know about are the ones I used to research this lesson and I haven't looked into any of the programs at all. Maybe there are others on frequency that can share some of the web sites or programs they use.
A near real-time MUF map of the world.
OWF and Operational vs. Basic MUF
The reason I am using the abbreviations at first is that two of the questions ask you to identify what MUF and LUF stand for. MUF stands for Maximum Usable Frequency and LUF stands for Lowest Usable Frequency. More specifically, they define the maximum (highest) and lowest frequencies that can be used to communicate between two stations. These terms refer to atmospheric conditions so it is assumed that we are talking about communications requiring skip. We don't talk about MUF or LUF for direct point to point communications. In the process of discussing MUF and LUF we are going to sneak in a bit of general radio wave propagation theory so hang on to your hats. Here we go.
When we talk about skip or skywave communication you will often hear that the radio wave was reflected back to earth or "bounced" back. These are handy terms to use but are not exactly accurate. What really happens is that the radio wave is refracted, which is to say bent or curved. To use an analogy with beams of light, the atmosphere does not act like a mirror, but instead acts like a lens or a prism. This is important to know for MUF because this bending of the radio transmission changes with its frequency. That is, the higher the frequency, the less the radio beam is bent. The MUF can be thought of as the frequency that just barely gets curved enough to make it back to earth. If the frequency is a little higher the transmission would still be bent but would not quite be bent enough to come back down and would go off at an angle that would just miss the surface of the earth and keep going into space.
The LUF has less to do with the bending of the signal as it does the absorption of the signal. This is not such a hard limit as the MUF since what you are measuring is how much you can have the signal be absorbed and still be readable above the noise floor. Mode of operation figures into this as well since, as we know, some modes like CW can be understood much further down in the noise than others like SSB. This absorption is a fairly continuous thing, that is to say, the lower the frequency the more the absorption. So the best, least absorbed, transmission occurs just below the MUF.
Since we want to be as close as we can to the MUF without going over, a bit like the game of 21, this is the frequency most often reported. So let's talk a bit about what affects this frequency and how it is measured. The MUF is determined by the characteristics of the ionosphere and by the location of the two stations. The ionosphere is not a simple static layer. It is constantly changing although much of the time the changes are fairly predictable and not great over small periods of time. The sun has the biggest effect on the ionosphere. At sunrise the ionosphere changes such that the MUF goes up significantly and quickly and stays high throughout the daylight hours. The MUF then gradually declines throughout the night, reaching it's lowest point just before dawn. Anything that affects the ionosphere, affects the MUF, so solar flares can significantly change your operating characteristics.
In order to be sure you aren't losing some of your transmission by straying over the MUF as it changes, you actually want to be a bit below the MUF for reliable working conditions. In one reference I found they defined the Optimum Working Frequency (OWF) as 85% to 90% of the MUF. In the same reference they also differentiate between the Operational MUF and the Basic MUF. The Basic MUF is more of a theoretical value while the Operational MUF takes into account the antennas, operating mode, power, etc.
Let's return to one of the points I made above, namely that the MUF depends on the locations of the two points. While this is true, this statement refers to changes in the MUF over great distances. You can be pretty confident, for example that the MUF between Seattle and Chicago is about the same as between Olympia and Chicago. But going around the globe the MUF varies by quite a bit and also, of course varies over time. In one of the references below is a near real-time map of the MUF around the world. But ultimately the MUF for you to communicate with another station is specific to your particular circumstances.
Finally, how do you find out what the MUF is? Until very recently, the best way to tell was to listen to beacons at various frequencies and locations to see if you could hear them. This is still a perfectly good way to do it but now there are many online sources to get up-to-the-minute values for MUF and there are also many programs for predicting it. The only web sites I know about are the ones I used to research this lesson and I haven't looked into any of the programs at all. Maybe there are others on frequency that can share some of the web sites or programs they use.
A near real-time MUF map of the world.
OWF and Operational vs. Basic MUF
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