Questions arose:
"Why do field strengths suffer if the signal path is anywhere near (within 5000 km, or about 3000 mi) the geomagnetic poles?"
"How high is the reflective layer of the ionosphere for mediumwave at night? Does it vary?"
"What about multiple hops? Is there an additional loss penalty there?"
"Testing shows long paths over the sea result in increased strengths. Why is that?"
"Does the solar cycle have anything to do with mediumwave skywave propagation?"
And those weren't all.
Some mysterious thing was also going on at the signal reflection point high in the ionosphere. There were losses there which couldn't be accounted for.
Then there was the noticeable variability hour by hour throughout the day, every day. And certain peaks at the sunrise and sunset periods. Then someone noticed that signal strengths even varied by a few dB as the seasons progressed throughout the year.
The engineers and scientists had a mess on their hands to try to sort out. World War II ended, and work continued in earnest to quantify all the new data being accumulated. New ideas came forth. Throughout the latter half of the 20th century, formulas were either tweaked or abandoned.
The current plethera of formulas and tweaks available for mediumwave skywave field strength calculation is almost mind-boggling. Essentially, it all boils down to the overall path loss from transmitter to receiver. Once we have that, we can determine the expected received field strength. Luckily, we can attain high accuracy by breaking things down to basics, then tally the sum of the parts. The final path loss figure, in our familiar dB scale, is simply the addition and subtraction of the gains and losses of the individual pieces.
THE HEROES AND WHAT THEY FOUND
K.A. Norton and John C.H. Wang of the FCC are 20th Century heroes of the first order. Almost singlehandedly they led the charge in the quest to calculate expected field strengths in the longwave and mediumwave regions. Norton led the early efforts, and Wang the later.
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| John C.H. Wang 1934-2019 |
Let's talk about some of the formulas which emerged from all of this testing, and how they evolved. Or didn't!
Over the last half of the 20th Century, several countries and the ITU (International Telecommunication Union) contributed greatly to the basic formulas we use for skywave field strength prediction. This was years in the making. Each region of the earth has different requirements. North America, for instance, has its close proximity to the geomagnetic pole to deal with. Polarization coupling loss (the loss as the signal transitions the reflection point in the ionosphere) is more of a problem in the tropics but not at high latitudes. East Asia and Oceania seemingly have greater propagational signal strengths than other areas of the world. One basic formula would not suffice for all regions. It wasn't until the millennium that the dust had finally settled and it became clear which formulas worked best for which region.
THE CAIRO CURVES
The early Cairo curves of 1938 present field strength as a function of great circle distance only. The two Cairo curves and the FCC clear channel curve are similar for distances up to about 1400 km. At 3000 km the north-south curve is about 8 dB greater than the east-west curve. At 5000 km the difference is about 18 dB. The FCC clear channel curve falls between the two Cairo curves. The Cairo curves did not gain much recognition (in part, because of World War II) until 1975 when the LF/MF conference adopted the north-south curve for use in the Asian part of Region 3. The east-west curve, because it often underestimates field strength levels, has virtually been disregarded. The Cairo curves served their purpose well in the early years. Today it cannot be considered a candidate for worldwide applications.
THE OLD REGION 2 METHOD
The old Region 2 method started out as the FCC clear channel curves. It has a long history dating back to 1935. It presents field strength as a function of great circle distance only. It does not take into account effects of other factors such as latitude, frequency, sunspot numbers, etc.
North America has perhaps the most significant propagation anomalies to deal with. Increased positional geomagnetic loss (not auroral loss due to disturbed ionosphere) is strongest here in North America because of our closer proximity to the north geomagnetic pole than Europe at large. Wang, a long-time employee of the FCC, started to work on the skywave measurement problem by 1970. He soon was on the right track to a solution.
Wang, in 1985 and again in 1989, reported that the old Region 2 method offers reasonable accuracy when applied to temperate latitudes, but when applied to low-latitude areas (e.g., Puerto Rico), it displays a tendency to underestimate. However, when applied to high-latitude areas (e.g., northern United States and Canada), it displays a strong tendency to overestimate. Wang stated, "Clearly, this is due to the fact that it lacks a treatment of latitude. The [old] Region 2 method has served its purposes well and cannot handle today's heavy demands for frequencies. It is not a candidate for worldwide applications."
This antiquated method remained the recommendation of the ITU and the FCC for Region 2 well into the 1980s.
THE FCC METHOD
John C.H. Wang, the star engineer of the FCC, had a heavy influence in developing the FCC formula. The modernized FCC method of the 1990s combines Wang's ideas on absorption losses and geomagnetic influence of the signal path's mid-point with the old Region 2 method and the original Cairo curves. Elsewhere, Wang contributed greatly to the world's scientific community by offering and publishing his personal ideas and formulas outside of the realm of the FCC. Wang's FCC contribution as well as his personal formulas include an additional loss factor of 4.95 dB to attempt to compensate for sunspot activity and the extra North American geo-polar proximity absorption. Neither the FCC's nor Wang's formulas account for frequency. Finally, where Wang adds a factor for the antenna's array gain over a standard quarterwave monopole, the FCC formula does not. The two methods produce, perhaps unsurprisingly, nearly identical results.
THE USSR METHOD
The USSR method, authored by Udaltsov and Shlyuger and proposed in 1972, appeared to be very promising at first. For one thing, it included a sound treatment of latitudes. A previous study by Wang, et al., in 1993, using data collected in Region 1 only, had mixed comments about this method. The findings were as follows [Wang]: "(1) When applied to single-hop paths within Europe, reasonably accurate results have been obtained. (2) When applied to long paths terminating in Region 1, calculated results are typically 10 to 20 dB lower than the measured values. The current study (late 1990s), which uses a much larger data bank, has strengthened these findings. Furthermore, the frequency term in this method indicates that the higher the frequency is, the lower the field strength is. Although this is theoretically sound, measured data from Brazil, New Zealand, and the United States, however, does not corroborate this." Wang also suggested that it was something short of a true worldwide method. For a number of years this method was included in the ITU's reccomendation for worldwide application at frequencies between 150 and 1600 kHz.
THE ITU METHOD
The 1974-1975 ITU Geneva conference adopted the USSR method for official use in Region 1 and in the southern part of Region 3 with modifications. P. Knight's 1975 sea gain formula and the J.G. Phillips-Knight 1965 polarization coupling loss term were also included. The ITU has adapted and modified this formula for general worldwide use to this day. It has some shortcomings for North America, as we will soon see.
The ITU method makes predictions that depend on both frequency and geomagnetic latitude. The field strength values are not symmetrical about the geomagnetic latitude equal to 0 degrees. The field strength expression also predicts lower field strength values as the frequency is increased in the MF band, but measurements performed in the United States show that the field strengths are higher at the higher frequencies in the MF band when compared to those measurements at the lower frequencies. Because of this discrepancy, the ITU method has not found wide acceptance as a worldwide prediction method. Curiously, their bandaid-approach is recommending a fixed frequency of 1000 kHz to represent the entire MF band.
THE WANG METHOD (1999)
The brilliant engineer John C.H. Wang started with the FCC in about 1970, and stayed for 40 years, continuing K.A. Norton's early work. Wang had made tremendous inroads by 1977, and after examining all of the available MF methods, developed a new MF skywave field strength prediction method for North America. Like the Udaltsov and Shlyuger method, the Wang method also contains a latitude term. The original FCC curves have a hump at roughly 100 km which Wang concluded was due to groundwave interference present in the 1935 data. The curves become smoother and better behaved after removal of these data points. Furthermore, this new method essentially linked the Cairo and the FCC clear channel curves together mathematically. The special case corresponding to a geomagnetic latitude of 35 degrees north in the Wang method is extremely close to the Cairo curve; the difference is within a fraction of a decibel. The special case corresponding to 45 degrees north is very similar to the old FCC curve. More importantly, it works well for long and short paths alike. Wang further improved on this method in 1979, modifying the ITU's basic loss factor (Kr) for North America and also tweaking the solar activity dependence factor (bsa). The formula was further tweaked again and published in 1985.
In 1986 the Region 2 conference which tackled the expanded band adopted Wang's method for calculating interregional interference. In 1990 this method became part of the FCC rules and regulations replacing the old clear channel curves (actually, the old Region 2 method) for domestic applications. In 1994 this method was adopted by the ITU for calculating field strengths between 1600 and 1700 kHz. This method has several other convenient features that should not be overlooked. It is simple and easy to use; a handheld calculator would suffice. The calculation procedures and required input information are similar to the Udaltsov and Shlyuger method, the method being used by Region 1 countries. Wang continued with improvements throughout the rest of the 1980s and into the millennium.
OTHER METHODS
There are other prediction methods, some obscure or archaic. Namely, these are: the Norton Method (1965), the EBU Method (1962, reaching its final form in 1978), the Barghausen Method (1966), the E. Oliver Method (1971), and the P. Knight Method (1973). The Knight's method eventually was simplified, evolving into the UK Method.
The Cairo curves, the Norton, the EBU, and original ITU-sanctioned 1974 USSR methods still used actual overland great circle distances in their formulas. The modified USSR method (1978) uses the slant distance. Wang of the FCC was using slant distance by 1977.
Over the last half of the 20th Century, several countries and the ITU (International Telecommunication Union) contributed greatly to the basic formulas we use for skywave field strength prediction. This was years in the making. Each region of the earth has different requirements. North America, for instance, has its close proximity to the geomagnetic pole to deal with. Polarization coupling loss (the loss as the signal transitions the reflection point in the ionosphere) is more of a problem in the tropics but not at high latitudes. East Asia and Oceania seemingly have greater propagational signal strengths than other areas of the world. One basic formula would not suffice for all regions. It wasn't until the millennium that the dust had finally settled and it became clear which formulas worked best for which region.
THE CAIRO CURVES
The early Cairo curves of 1938 present field strength as a function of great circle distance only. The two Cairo curves and the FCC clear channel curve are similar for distances up to about 1400 km. At 3000 km the north-south curve is about 8 dB greater than the east-west curve. At 5000 km the difference is about 18 dB. The FCC clear channel curve falls between the two Cairo curves. The Cairo curves did not gain much recognition (in part, because of World War II) until 1975 when the LF/MF conference adopted the north-south curve for use in the Asian part of Region 3. The east-west curve, because it often underestimates field strength levels, has virtually been disregarded. The Cairo curves served their purpose well in the early years. Today it cannot be considered a candidate for worldwide applications.
THE OLD REGION 2 METHOD
The old Region 2 method started out as the FCC clear channel curves. It has a long history dating back to 1935. It presents field strength as a function of great circle distance only. It does not take into account effects of other factors such as latitude, frequency, sunspot numbers, etc.
North America has perhaps the most significant propagation anomalies to deal with. Increased positional geomagnetic loss (not auroral loss due to disturbed ionosphere) is strongest here in North America because of our closer proximity to the north geomagnetic pole than Europe at large. Wang, a long-time employee of the FCC, started to work on the skywave measurement problem by 1970. He soon was on the right track to a solution.
Wang, in 1985 and again in 1989, reported that the old Region 2 method offers reasonable accuracy when applied to temperate latitudes, but when applied to low-latitude areas (e.g., Puerto Rico), it displays a tendency to underestimate. However, when applied to high-latitude areas (e.g., northern United States and Canada), it displays a strong tendency to overestimate. Wang stated, "Clearly, this is due to the fact that it lacks a treatment of latitude. The [old] Region 2 method has served its purposes well and cannot handle today's heavy demands for frequencies. It is not a candidate for worldwide applications."
This antiquated method remained the recommendation of the ITU and the FCC for Region 2 well into the 1980s.
THE FCC METHOD
John C.H. Wang, the star engineer of the FCC, had a heavy influence in developing the FCC formula. The modernized FCC method of the 1990s combines Wang's ideas on absorption losses and geomagnetic influence of the signal path's mid-point with the old Region 2 method and the original Cairo curves. Elsewhere, Wang contributed greatly to the world's scientific community by offering and publishing his personal ideas and formulas outside of the realm of the FCC. Wang's FCC contribution as well as his personal formulas include an additional loss factor of 4.95 dB to attempt to compensate for sunspot activity and the extra North American geo-polar proximity absorption. Neither the FCC's nor Wang's formulas account for frequency. Finally, where Wang adds a factor for the antenna's array gain over a standard quarterwave monopole, the FCC formula does not. The two methods produce, perhaps unsurprisingly, nearly identical results.
THE USSR METHOD
The USSR method, authored by Udaltsov and Shlyuger and proposed in 1972, appeared to be very promising at first. For one thing, it included a sound treatment of latitudes. A previous study by Wang, et al., in 1993, using data collected in Region 1 only, had mixed comments about this method. The findings were as follows [Wang]: "(1) When applied to single-hop paths within Europe, reasonably accurate results have been obtained. (2) When applied to long paths terminating in Region 1, calculated results are typically 10 to 20 dB lower than the measured values. The current study (late 1990s), which uses a much larger data bank, has strengthened these findings. Furthermore, the frequency term in this method indicates that the higher the frequency is, the lower the field strength is. Although this is theoretically sound, measured data from Brazil, New Zealand, and the United States, however, does not corroborate this." Wang also suggested that it was something short of a true worldwide method. For a number of years this method was included in the ITU's reccomendation for worldwide application at frequencies between 150 and 1600 kHz.
THE ITU METHOD
The 1974-1975 ITU Geneva conference adopted the USSR method for official use in Region 1 and in the southern part of Region 3 with modifications. P. Knight's 1975 sea gain formula and the J.G. Phillips-Knight 1965 polarization coupling loss term were also included. The ITU has adapted and modified this formula for general worldwide use to this day. It has some shortcomings for North America, as we will soon see.
The ITU method makes predictions that depend on both frequency and geomagnetic latitude. The field strength values are not symmetrical about the geomagnetic latitude equal to 0 degrees. The field strength expression also predicts lower field strength values as the frequency is increased in the MF band, but measurements performed in the United States show that the field strengths are higher at the higher frequencies in the MF band when compared to those measurements at the lower frequencies. Because of this discrepancy, the ITU method has not found wide acceptance as a worldwide prediction method. Curiously, their bandaid-approach is recommending a fixed frequency of 1000 kHz to represent the entire MF band.
THE WANG METHOD (1999)
The brilliant engineer John C.H. Wang started with the FCC in about 1970, and stayed for 40 years, continuing K.A. Norton's early work. Wang had made tremendous inroads by 1977, and after examining all of the available MF methods, developed a new MF skywave field strength prediction method for North America. Like the Udaltsov and Shlyuger method, the Wang method also contains a latitude term. The original FCC curves have a hump at roughly 100 km which Wang concluded was due to groundwave interference present in the 1935 data. The curves become smoother and better behaved after removal of these data points. Furthermore, this new method essentially linked the Cairo and the FCC clear channel curves together mathematically. The special case corresponding to a geomagnetic latitude of 35 degrees north in the Wang method is extremely close to the Cairo curve; the difference is within a fraction of a decibel. The special case corresponding to 45 degrees north is very similar to the old FCC curve. More importantly, it works well for long and short paths alike. Wang further improved on this method in 1979, modifying the ITU's basic loss factor (Kr) for North America and also tweaking the solar activity dependence factor (bsa). The formula was further tweaked again and published in 1985.
In 1986 the Region 2 conference which tackled the expanded band adopted Wang's method for calculating interregional interference. In 1990 this method became part of the FCC rules and regulations replacing the old clear channel curves (actually, the old Region 2 method) for domestic applications. In 1994 this method was adopted by the ITU for calculating field strengths between 1600 and 1700 kHz. This method has several other convenient features that should not be overlooked. It is simple and easy to use; a handheld calculator would suffice. The calculation procedures and required input information are similar to the Udaltsov and Shlyuger method, the method being used by Region 1 countries. Wang continued with improvements throughout the rest of the 1980s and into the millennium.
OTHER METHODS
There are other prediction methods, some obscure or archaic. Namely, these are: the Norton Method (1965), the EBU Method (1962, reaching its final form in 1978), the Barghausen Method (1966), the E. Oliver Method (1971), and the P. Knight Method (1973). The Knight's method eventually was simplified, evolving into the UK Method.
The Cairo curves, the Norton, the EBU, and original ITU-sanctioned 1974 USSR methods still used actual overland great circle distances in their formulas. The modified USSR method (1978) uses the slant distance. Wang of the FCC was using slant distance by 1977.
In the next article we'll introduce the formulas.
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