Sunday, July 2, 2023

Mediumwave Skywave Prediction #1 - A Measurement History

Skywave propagation at mediumwave is a fascinating subject, both from a historical and technical standpoint. Radio itself has been around well more than 100 years, and broadcast radio since about 1920. As more and more stations entered the airwaves, nighttime spectral chaos ensued. How was it all sorted out? Who took charge of all this? How did we arrive at the calculations necessary to ensure that the thousands of radio stations transmitting didn't interfere with each other? What exactly goes into calculating a nighttime skywave signal strength for a distant medium wave station?

Let's try to answer these questions in this series. We'll cover the history in the first couple of articles, then dive into the technical in subsequent articles. Throughout this series, the LF and MF abbreviations, when used, refer to the longwave frequency (LF) and mediumwave frequency (MF) bands. Note that these articles discuss mediumwave skywave prediction only.


At the end of World War I, a fierce battle ensued between the US government and the Department of the Navy over control of the airwaves. The Department of Commerce eventually won and became master of the air and the regulatory agency for commercial radio here in the US. They started by establishing two broadcast frequencies: 833 kHz (360 meters) and 619 kHz (485 meters). The Federal Radio Commission took charge in 1926, lasting until 1934 when the current Federal Communications Commission was formed. By 1930, broadcast radio was on its way. Nighttime signals traversed the continent from coast to coast.

Throughout the early years of radio, interest mounted to quantitatively determine the service area of broadcast stations. Early mathematical efforts focused mainly on finding an accurate calculation for groundwave coverage. K.A. Norton of the FCC would play a major role worldwide in that effort. The intricacies of skywave would be unveiled later. You might be surprised to know that serious study of longwave and mediumwave skywave propagation didn't commence until some 12 years after the first commercial AM radio station went on the air.


The earliest worldwide concerted efforts to study longwave and mediumwave skywave propagation began in 1932. The International Radio Consultative Committee (CCIR), an arm of the ITU, formed a task force in that year to study propagation at frequencies between 150 and 2000 kHz. Three measurement campaigns were carried out between 1934 and 1937 on 23 long-range propagation paths between North America and Europe, North America and South America, and Europe and South America. Measurements on 10 short paths within South America were also carried out under the administration of Argentina. Two skywave propagation curves (skywave field strength graphs ordered by frequency and distance) were drawn based on the results of these measurements. One of the curves is for paths far away from Earth’s magnetic poles (north-south curve), while the other curve is for paths which approach Earth’s magnetic poles (east-west curve). The two curves were formally adopted at the 1938 International Radio Conference in Cairo and are known as the Cairo curves. They have survived, with modification in one form or another, to this day.

Click any image for the bigger picture.

The Cairo Curve Measurement Campaign

The Federal Communications Commission (FCC) of the United States carried out a skywave field strength measurement program in the spring of 1935 to derive a new set of curves for North America. At that time, there were eight clear channel stations. Nighttime signals of these stations were monitored at 11 receiving sites located in different parts of the United States. The curve corresponding to the annual median value (the signal level expected to be exceeded at least 50% of the time) was used to determine a station's coverage area, while the curve corresponding to the upper decile value (the signal level expected to be exceeded at least 10% of the time) was used to calculate the interference levels among co-channel stations. Characteristically, the 10% level is the higher signal level. These curves became part of the rules and regulations of the FCC and were adopted by the 1950 North American Regional Broadcasting Agreement (NARBA) for official use in the North American Region, which comprised the following areas: Bahama Islands, Canada, Cuba, Dominican Republic, Haiti, Jamaica, Mexico, and the United States. This method was eventually adopted with minor modifications for applications in all of ITU Region 2. It would not survive the millennium.

The FCC, knowing the clear channel curves had certain limitations (the curves do not take into consideration the effect of latitude and the proximity to the geomagnetic pole), initiated a long-term large-scale measurement program in 1939 to collect measurements from more than 40 propagation paths. The measurement program lasted for about one full sunspot cycle; in four cases it lasted for two cycles and ended in 1958. Frequencies of these paths ranged from 540 to 1530 kHz. Path lengths ranged from 322 to 4176 km. Mid-point geomagnetic latitudes (the signal reflection point between transmitter and receiver relative to geomagnetic north) ranged from 45 degrees to 56 degrees north, a narrow range of 11 degrees, although some paths from lower latitudes were later added. More about geomagnetic latitude later in the series.


I'll side-track for a minute and tell you about the ITU, the International Telecommunication Union, and how regions are defined. Today the ITU is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on May 7, 1865 as the International Telegraph Union, making it the first international organization. The ITU has divided up the planet into three regions. Region 1 comprises Europe, Africa, the entire former USSR, Mongolia, and the Middle East west of the Persian Gulf, including Turkey and Iraq. Region 3 contains most of non-former USSR, Asia east of and including Iran, and most of Australasia. Region 2 covers the Americas including Greenland, and some of the eastern Pacific Islands.


Back to our history.

The Canadian Department of Transportation took path measurements in 1947, a year of maximum sunspot number and minimum field strengths.

The EBU, the European Broadcasting Union, carried out an extensive measurement campaign from from 1952 to 1960 for paths in western Europe. A controversial field strength prediction method was developed by Ebert in 1962. In this method, empirical relationships were derived for the effects of solar activity, the influence of magnetic field, frequency, and other factors. The Ebert method cannot be considered a success because it displayed a strong tendency to grossly underestimate field strength levels, sometimes by 30 dB. It was soon abandoned. Although the Ebert method was not a success, the importance of the EBU measurements cannot be overlooked. 

Three international organizations, the EBU among them, in 1963 and 1964 set up 7 receiving locations on the continent of Africa and did studies of propagation paths from two transmitters on Ascension Island. One phase of the project was to study polarization coupling loss and sea gain. Germany also conducted measurements at Tsumeb, southwest Africa. Altogether, the African measurement campaign involved 15 receiving sites, and data from 33 paths was documented. Frequencies ranged from 164 kHz to 1484 kHz. Distances ranged from 550 km to 7540 km. Mid-point geomagnetic latitudes ranged from 29 degrees south to 40.2 degrees north. Of these 33 paths, three were from Europe to Africa.

In the late 1960s and early 1970s a number of administrations and scientific organizations made valuable contributions. The EBU reactivated its efforts and collected data from more than 30 propagation paths; many of these are intercontinental paths. In Eastern Europe, the International Organization of Radio and Television (OIRT) contributed data from 12 short intra-European paths between 600 and 1400 km at frequencies between 164 and 1554 kHz. The former USSR also collected a significant amount of measurements. A summary of their results and a proposed new calculation method was published in 1972.


The big one, perhaps the biggest ever. The ITU's Regional Administrative LF/MF Broadcasting Conferences were held in Geneva, Switzerland for Regions 1 and 3. This was a major deal on several fronts. Channel spacing was to be decided on, worldwide. It was 1975!!! Also signal strength calculation standards were to be fixed and tailored by region and sub-region. Asian countries, particularly China, preferred the Cairo north-south curves. Australia and New Zealand believed neither method was adequate for their applications. They believed field strength levels in their part of the world are stronger than those observed in other places. Finally, a compromise was reached.

It was decided that the USSR method was to be used for Region 1. The Cairo north-south curve was to be used for the northern part of Region 3 (east Asia). For the southern part of Region 3 (Oceania) the modified USSR method was to be used with a correction factor of 2.7 dB added to the basic formula. Sea gain and polarization coupling loss terms were to be included whenever applicable. The propagation issue was a lesser concern compared to the channel-spacing issue. The conference was deadlocked for a number of weeks over two separate proposals: 8 kHz versus the traditional 10 kHz separation. Finally, a compromise of 9 kHz was adopted which became effective in November of 1978 for Regions 1 and 3.

In the meantime, the interference situation in South America was going from bad to worse, mainly because of the lack of any regional agreement, although some bilateral agreements were in existence. The situation in North America was somewhat better, thanks in part to the 1950 NARBA agreement.

After the ITU's LF/MF conference for Regions 1 and 3 was over, a number of administrations in South America petitioned the ITU to convene a regional conference involving all countries in ITU Region 2, the Americas. Consequently, two sessions took place. The first session dealt with technical matters and took place in 1980 in Buenos Aires. The second session dealt with the actual planning and took place in 1981 in Rio de Janeiro. The FCC clear channel curve was adopted for use in the entire region. It was also decided that sea gain and polarization coupling loss terms were not to be included in the calculations. At the first session, channel spacing was a very hot topic. The United States was in favor of 9 kHz (for all of South America), while Argentina and Canada were strongly against it. At the second session, the United States withdrew its proposal, and 10 kHz spacing was quickly agreed upon. It should be mentioned that in Region 2, longwave is not used for broadcasting. Therefore the 1980-1981 conference dealt with mediumwave only (535 kHz to 1605 kHz).


The CCIR Documents of the 1978 Kyoto Assembly further modified the 1974 sky-wave field strength prediction method for MF (150 to 1600 kHz) and recommended its provisional use worldwide. Several sky-wave field strength prediction methods proposed for various parts of the world also were described. 

They are:

1) Cairo North-South curve adopted for use in Asian part of Region 3 - mathematical approximation presented.

2) EBU method to be used in European Broadcasting Area with separate formula for distances less than 300 km.

3) USSR method - valid between 37° and 60° geomagnetic latitude for distances up to 6000 km and has no frequency dependence.

4) UK method - valid for all distances worldwide except for the auroral zones and has no frequency dependence.

5) Region 2 would use the FCC's method. The Wang 1977 method (Wang was a newly-hired and brilliant engineer at the FCC) was given as an alternative method for use in Region 2.

In 1979 Wang proposed a modification of the CCIR Kyoto 1978 worldwide method to improve accuracy in Region 2. Also in the same year, the Inter-American Conference on Telecommunications extended the FCC median signal level curve to distances beyond 4300 km using the Cairo North-South Curve and recommended its adoption for Region 2.

In response to Region 2 countries' request for more frequencies for broadcasting, the 1979 World Administrative Radio Conference (WARC-79, held in Geneva) of the ITU allocated the band 1605-1705 kHz for broadcasting in Region 2 only. Two sessions of regional conference took place in 1986 (Geneva) and 1988 (Rio de Janeiro) for the planning of the use of this expanded band in Region 2. It should be noted that this band is used by other services in Regions 1 and 3.

In preparation for the use of the expanded band and recognizing the need for additional data, particularly data from low and high latitude areas, the FCC initiated two separate projects in the early 1980s. In 1980, the FCC and the Institute for Telecommunication Sciences (ITS) of the Department of Commerce jointly began to collect low-latitude data at two receiving sites: Kingsville, Texas, and Cabo Rojo, Puerto Rico. The FCC-ITS efforts in low-latitude areas were supplemented by Brazil and Mexico; both administrations also collected a significant amount of data from low-latitude areas. In 1981, the FCC started a joint project with the Geophysical Institute, University of Alaska. The Alaskan project concentrated on high latitude data and lasted for five years, collecting data representing different levels of solar activity.

Administrations in the Region 3 area, Australasia, in cooperation with the Asian-Pacific Broadcasting Union, were equally active and productive in their path testing. In the northern part of this region, data from 84 paths had been documented by 1981. Australia and New Zealand jointly collected data from 85 paths. The Japanese administration had carried out a series of mobile experiments in the Pacific by 1987.

By the year 2000, measurements from more than 400 propagation paths had been documented. Great circle lengths of these paths ranged from 290 to 11,890 km. Signals of the few very short paths were verified to be skywaves. Frequencies ranged from 164 kHz to 1610 kHz. Control-point geomagnetic latitudes ranged from 46.2 south to 63.8 north geomagnetic latitude. A large amount of literature had been generated. By this time, largely the work of the ITU in setting standards and regulations for the longwave and mediumwave bands was finished. Fine tuning of the skywave calculation formulas was left to the scientists.

In the next part of this series, we'll wrap up the history and then go on to explore elementary skywave prediction and what is involved in solving it.

ITU Regions

Information for these articles has been gathered from the following resources:

An Objective Evaluation of Available LF/MF Skywave Propagation Models
John C.H. Wang
Radio Science, Volume 34, Number 3
May-June 1999

NTIA Report 99-368
Medium Frequency Propagation Prediction Techniques and Antenna Modeling 
for Intelligent Transportation Systems (ITS) Broadcast Applications
Nicholas DeMinco
August 1999

International Telecommunication Union Handbook
The Ionosphere and its Effects on Radiowave Propagation
Radio Communication Bureau 1998

Code of Federal Regulations Title 47
Radio Broadcast Services (FCC)
47 CFR Part 73

FCC Standard AM Broadcast Technical Standards
   ...notes and changes to 47 CFR Part 73
Broadcast Service Bureau
Filed January 20, 1987

Medium Frequency Propagation: a survey
P. Knight
BBC Research Department 1983/5
May 1983

Comparison of Available Methods for Predicting Medium Frequency 
Sky-Wave Field Strengths
Margo PoKempner
June 1980

LF AND MF SKY-WAVE PROPAGATION: the origin of the Cairo curves
P. Knight
BBC Research Department 1977/42
November 1977

1 comment:

plaws said...

I am amazed and saddened (given subsequent events) that the FCC et al were still making serious studies of MW propagation until the end of the 20th C.

The "Standard Broadcast" band seems like it's going to be wasted when KDKA finally signs off (I assume it will be the ceremonial last station standing ...) and that's unfortunate. But it's not like the industry has embraced any technological change (AMAX, CQUAM, HDRadio).