This article will provide the reader with some background information about high-frequency radio propagation, and the methods and information I use for predicting radio propagation conditions in my monthly column for 73 Amateur Radio Today.
I will discuss briefly radio wave propagation and the ionosphere which enable HF DX communication, but readers already familiar with the subject may wish to skip these fundamentals and move on to other material.
Some Definitions
“Hertz” (Hz) – Radio Frequency in cycles per second
“Kilohertz (kHz) – Radio Frequency in thousand of cycles per second
“Megahertz (MHz) – Radio Frequency in millions of cycles per second
“HF” – High Frequency – Radio Frequencies between about 3 and 30 MHz
“MF” – Medium Frequency – Radio Frequencies between about 1 and 3 MHz
“LF” – Low Frequency – radio frequencies below about 1 MHz
“Propagation” – transmission of radio signals using the ionosphere
“DX” – distant, as opposed to local, communication
“Ionosphere” – high regions of earth’s atmosphere where its gases may be ionized
“Magnetosphere” – the magnetic field enveloping the earth
“Sunspots” – visible regions of extreme magnetic activity on the sun’s surface
“Solar Flux” – output of radio-frequency energy from the sun
“Sunspot Cycle” – a period of approximately eleven years
“NOAA” – National Oceanic and Atmospheric Administration
“SEC” – Space Environment Center
“Layers” – Inaccurate but useful term for portions of the ionosphere above earth
“Wavelength” – the length of a radio wave expressed in meters
“Short-wave” – wavelengths between about 80 and 10 meters
“A” and “K” – indices of magnetic field variations
Note: The relationship between wavelength and frequency: Wavelength (in meters) equals 300,000 divided by frequency in Kilohertz (one thousand cycles per second). The number 300,000 is the speed of light (also radio waves) in kilometers per second.
The Atmosphere
Earth’s atmosphere surrounds the earth from its surface to a height of about 100 miles and consists of oxygen, nitrogen, carbon dioxide and other gases. The atmosphere at the earth’s surface is most dense and it exerts a pressure of about 14.7 pounds per square inch. However, density and pressure decrease with height until molecules of the various gases are few in number and far apart.
The Ionosphere
As atmosphere density decreases and the gas molecules become separated, ultraviolet light from the sun contains sufficient energy to separate electrons from the molecular atoms and create ions. Atoms of the various gases are ordinarily electrically neutral and possess no charge, but when they are separated into electrons and ions by ultraviolet light, the ions (electron-deficient atoms) become positively charged and electrically conducting. These charged particles follow the earth’s magnetic lines of force and become concentrated near the magnetic poles where the lines of force are closer together.
The sun’s energy output is not constant, but varies hourly, daily, seasonally, yearly and geographically. Therefore, the intensity of ultraviolet light falling on the upper levels of atmosphere is variable, altering the intensity of ionization..
The height of the ionosphere is considered to extend from about 30 to 300 miles above the earth’s surface and its electron density (hence conductivity) also varies with height. While it is commonly thought that the ionosphere reflects radio energy, it actually refract (bends) the waves. Longer radio wavelengths (such as AM radio at frequencies between about 550 and 1650 kHz) are refracted by lower layers of the ionosphere and shorter, higher-frequency wavelengths are refracted by higher levels. As ultraviolet light penetrates the atmosphere, its energy becomes ‘used up’ by producing ionization, until it reaches a level where it is no longer sufficiently energetic to produce ionization, and refraction of radio waves ceases. Below this level, radio propagation is considered to be by ‘ground wave’ rather than ‘sky wave’ and travels shorter distances.
For the sake of study and convention the ionosphere is considered to exist in ‘layers’ defined by alphabet letters from D (lowest above earth’s surface) to F (highest above the surface). Short-wave propagation primarily takes place in the E and F layers, the latter being subdivided into F1 (lower) and F2 (higher) portions.
You may be interested in The NEW Short-wave Propagation Handbook by George Jacobs, W3ASK; Theodore J. Cohen, N4XX; and Robert B. Rose, K6GKU and published by CQ Communications, Inc., 76 North Broadway, Hicksville, N.Y. 11801 USA. This excellent book in soft cover provides a comprehensive easy-to-read and easy-to-understand explanation of short-wave radio propagation.
The Sun
The source of all life on earth is our sun – a G-class star – at a distance of about 93 million miles from earth. The sun is a nuclear furnace which converts hydrogen to helium in a process known as “the carbon cycle” and releases vast quantities of energy into space. Only a very small part of the sun’s energy output is received by earth and other planets in our solar system.
The sun rotates on its axis approximately once every 27 days, so we can expect a particular area of the sun’s surface to rotate into view once again nearly a month later.
Sunspots are areas of intense magnetic activity that appear on the sun’s surface and can be seen from earth, often without any optical magnification, due to their large size…frequently many times larger than the earth. Astronomers have observed sunspots and noted their regular appearance and disappearance for over 300 years. They occur in ‘cycles’, each averaging 11years from beginning to end, but a cycle can be as short as 9 years and as long as 13 years.
Interestingly, sunspots in each new cycle have a reversed magnetic polarity compared with spots of the previous cycle, and they also appear in the sun’s opposite hemisphere. New sunspots usually appear closer to the poles and gradually move toward the solar equator as the cycle progresses. Between 1645 and 1715, no sunspots were recorded. In 1890 a researcher of sunspot cycles discovered the period of missing sunspots and it was later named for him as the “Maunder Minimum”. Recent research has discovered long periods of time throughout history when a profusion or a dearth of sunspots occurred, but the reason is still not known.
It is hypothesized that the sun may have several cycles of different lengths, including a 22-year cycle and a 55-year cycle. Current theory suggests that a ”sunspot cycle” of approximately 11 years may be only one-half of a cycle, and that a full cycle is 22-23 years, taking into account the reversed polarity and opposite hemisphere appearance. This is an attractive idea because it relates to our experience with other cyclic behavior. For example, a complete sine wave, from beginning to end, passes through positive and negative peaks as well as a zero or minimum condition.
The sun’s radiant energy occupies a vast frequency spectrum, including visible and invisible light, heat, and radio frequencies. Scientists have found it convenient to measure ‘radio’ output of the sun at a wavelength of 10.7 centimeters (2800 MHz) although regular measurements at many other frequencies are made each day at different locations around the world.
Values of radio energy measured at 10.7 cm are referred to as the ‘solar flux’ and reported daily at 18 minutes after each hour by WWV.
Solar flux varies widely during each sunspot cycle and can have minimum values below 40 at the beginning or end of an old cycle to a maximum of almost 300 at the cycle’s peak. As I write, we are at the beginning of sunspot cycle 23, and solar flux is generally well below 100, averaging around 85.
Those who depend on communicating by HF radio are at the mercy of solar flux, sunspots, and the sunspot cycle. At sunspot minima the ionization intensity of earth’s ionosphere is also low, and the maximum dependable or useful HF communication frequencies are likewise low. At sunspot maxima, maximum useful frequencies are very high…but communication can be fraught with other difficulties.
Radio ‘blackouts’ caused by solar flares, auroral displays from intense ionization, disruption of electrical power grids from fluctuating magnetic fields, and many other problems occur at times of sunspot maxima and high values of solar flux. ‘Magnetic ‘storms’ on the sun create storms in earth’s magnetic field and ionosphere and greatly upset radio communication. Even earth-orbiting satellites are affected by solar events..
Energetic particles (electrons and protons) from the sun, which impinge on earth’s atmosphere at these times, result in ionospheric and magnetic storms which affect radio communication by signal fading, noise and path changes…even disappearance of signal paths near the poles. The “a” index (measured at 3-hour intervals at various observatories around the world) is a measure of magnetic field changes caused by variations in the intensity of solar particle flux, and the value ranges from below 10 (quiet ionosphere) to about 30 (ionospheric storms). Each station reports its measurements every 3 hours, and these are averaged to provide a planetary index “a-sub-p”. When these values are averaged over a 24-hour period, they are reported as the “A” index.
It is useful to be able to express by a single digit large variations in the “a” index as measured at a single observatory, such as at Boulder, Colorado. This is referred to as the “K” index or “Boulder K” index and is obtained by factoring the reported “a” indices and results in values between zero and 5. “K” values of zero and 1 represent a quiet ionosphere and magnetic field with little or no signal fading. “K” values of 3 and above indicate a disturbed ionosphere and magnetic storms. Solar particle effects are greatest near the poles (auroral zones) and signal paths through these areas are affected most.
Radio Propagation and Forecasting
Many methods are used to forecast radio propagation ‘conditions’. Most depend upon continual and daily measurements of solar phenomena. In the United States, NOAA-SEC and WWV assume responsibility for making measurements, and collecting, recording and reporting the data. Their reports are available through printed publications, radio broadcasts, and the Internet.
Because of the sun’s rotation, many events (solar prominences, disappearing filaments, sunspot groups, flares, and similar violent solar events) which greatly affect radio communication, are often repeated in the same area of the solar disc (although with some variation) each 27 days. Sometimes a spot group, a flare or disappearing filament will be sustained during the entire solar ‘month’, while at other times these events fade quickly and disappear before the next full rotation. Nevertheless reasonably accurate short-term forecasts can be made on a reliable basis by carefully observation. Satellite-borne instruments for measuring solar energy at many frequencies are now used to supplement earth-based measurements, and yield a more complete ‘picture’ of solar behavior, thus improving the reliability and predictability of radio propagation forecasts. Computer programs such as “IONCAP” (TM) and a dozen or so others are readily available and can be used to forecast useful frequencies for DX communication, but require data input such as may be obtained from WWV..
Military and commercial HF radio transmissions depend on reliable assessment of propagation conditions, and these assessments may be required weeks and months – or even years – in advance of actual use… for planning communication strategies.. While short-term reliability of conventional methods has become accurate and reliable, long-term forecasting by present methods can become increasingly unreliable and speculative.
The Nelson System of Forecasting
John Nelson developed a system based on sun-planet alignments which allowed him to accurately predict future propagation ‘conditions’ and to forecast the occurrence of solar flares and other solar events which would be likely to disrupt radio communications. He was able to recommend optimum selection of communication frequencies, times and paths, by taking into account these predictions of potential trouble.
Nelson discovered that when planets of our solar system align themselves at 0-, 60-, 90-, or 180-degree angles with each other and the sun, they have an effect on earth’s magnetic field and ionosphere. The astronomical alignments he used are heliocentric (sun-centered) and not earth-centered (geocentric).
Planetary alignment data is published in the Nautical Almanac and the Astronomical Almanac which are available from the United States Government Printing Office, and the Internet provides information about the source of these documents. The almanacs are published a year or more in advance, and thus provide information about future alignments…necessary for using Nelson’s prediction system.
Interestingly, the Nelson system can be used ‘in reverse’ by correlating past alignments with recorded propagation anomalies and solar disturbances. Such historical ‘hindsight’ lends credence to the accuracy of future forecasts and provides some assurance of their dependability.
Using the Nelson System
Annual editions of the Astronomical Almanac and Nautical Almanac provide “Heliocentric” data pages for the various planets. If you would like to ‘try your hand’ at forecasting, select the column that lists “Heliographic Longitude” and note the angle for the first and last days of the month for each planet of interest.
On quadrille paper, draw four horizontal lines above each other near the bottom of the paper, and number these lines with the following angles in ten-degree steps: first line 0-90; second line 90-180; third line 180-270; and fourth line 270-360, thus covering all 360 degrees. Each quarter-inch interval along the line represents two degrees, and it is simple to interpolate angles as small as half a degree. On the same sheet, draw a vertical line along the left-hand edge and number each interval with the day of the month. Note that the intersection of the left-hand end of the first horizontal line (0 degrees)and the bottom of the vertical (day of the month) should be marked day one and not zero, since there is no ‘zero’ day.
Heliocentric Positions of the Planets for April 1998
Planets First of Month End of Month
Mercury 178.13′ 267.13′
Venus 237.50′ 283.50′
Earth 191.51′ 220.14′
Mars 028.19′ 045.08′
Jupiter 337.54′ 340.30′
Saturn 022.56′ 023.57′
Uranus 309.24′ 309.43′
Neptune 300.05′ 300.15′
Pluto omitted omitted
Now, plotting one planet at a time, place a pen or pencil mark to indicate the heliographic longitude (angle in degrees) for the FIRST day of the month along one of the horizontal lines. Then, place a pen or pencil mark to indicate the heliographic longitude (angle in degrees) for the LAST day of the month near the top of your paper. Draw a line connecting the marks representing the heliographic longitude for the first and last days. You should now have a slanting line from bottom to top of the paper, and note that this represents the number of total degrees that the planet moves in its orbit around the sun for that month.
As you draw the angle lines for each planet, note that some lines are more slanted than others, which illustrates that the inner planets (Mercury, Venus, Earth and Mars) have ‘faster’ orbital velocity compared with the outer planets (Jupiter, Saturn, Uranus, Neptune and Pluto). Mercury travels so quickly, that you will have to continue its lines on to the next 90-degree segment of your chart. However, the lines representing the paths for Uranus and Neptune are almost vertical, meaning that their orbital travel in a month of earth time is negligible on the scale of the chart you’re using.
Very quickly, you will see that the lines representing the position of the planets during the month often cross each other, and in the case of Mercury, there will be several crossings. These points of crossing will represent two angles (a specific angle for each planet) and a day of the month. Subtract the two angles and note the difference is 0, 90, or 180 degrees. Mark that difference near the point of crossing, and in the same way indicate the crossing point angle differences for all the planets plotted.
If you make a second chart with six horizontal lines representing 60-degree spacing, left to right in ten-degree increments (0-60, 60-120, 120-180, etc…360) you will be able to use the planetary ‘crossing’ angles as you did for the 90-degree chart. Nelson also used these smaller angles in preparing his forecasts. In fact, Nelson’s charts were prepared as polar (circular) projections, unlike my rectangular projections, allowing him to actually ‘see’ the planets’ orbital positions.
This, of course, requires drawing many more charts …a luxury I have not had the time to employ.
Nelson found that a 90-degree difference in crossing angles usually coincides with major disturbances in the ionosphere caused by solar events such as flares, while the oppositions (180-degree differences) and conjunctions (0-degree differences) usually coincide with lesser solar events. It is interesting to observe that these angular differences usually coincide with a peak in solar flux values, most noticeable in times of greater solar activity near the peak of each 11-year sunspot cycle.
Ions, which travel at velocities slower than the speed of light, require almost two days to reach earth’s atmosphere, whereas electrons which travel at the speed of light require about eight minutes.
You will also see on your chart that several planets may have crossings on or near the same date, which indicate an even more powerful effect on the ionosphere.
It seems possible, even probable, that planetary alignments affect the sun more by their electromagnetic influence than by their gravitational influence, although the latter is not completely disregarded.
If you use your alignment chart to plot daily changes in the solar flux and the planetary A and K values as reported by WWV, you may note some interesting “coincidences”. For example, peaks in the value of solar flux and the A and K indices often coincide (within a day or two) with specific planetary alignments and/or combinations of alignments.
I have made charts which indicate planetary alignments, solar flux values, noon-time temperatures at my location, A and K indices, solar flux values, geophysical disturbances, and other items of interest on the same chart for each day of the month. It becomes apparent when these charts are pasted together end to end for a year, that significant ionospheric disruptions, solar flares and other events coincide with specific alignments of the planets…not once or twice…but nearly always and with few exceptions. The charts also show declining activity as the sunspot cycle progresses toward a minimum.
Another interesting observation relates other types of geophysical phenomena, such as hurricanes, volcanic eruptions and earthquakes, with peaks in the solar flux, but – more particularly – ‘spikes’ (peak values) in the A and K indices. During the past 20 years of using the Nelson system, my successive charts justify my conclusion that Nelson discovered, utilized, and sufficiently proved a powerful relationship between planetary alignments and ionospheric propagation conditions.
Although I have observed a clear and consistent relationship between violent weather, volcanism and earthquakes, planetary alignments and solar activity, I have not been able to pinpoint the location on earth where disastrous geophysical phenomena may occur. It would be extraordinarily useful to be able to predict the time and place of natural disasters, but that capability will require refinement of the Nelson system beyond my present understanding and capability. It remains for others to use the system and confirm my findings, possibly by use of computer techniques rather than my own crude hand-made charts.
My friend Jim Lawyer, W8KRQ, recently sent me an interactive computer program* which displays, in color and motion, geocentric and heliocentric planetary positions for any desired time and date. Of particular interest to my own work, it also gives their positions in degrees of heliocentric longitude for any hour, day, month and year.
In addition to the almanacs mentioned earlier, computer ephemerides are available. They can be located through the Internet if you are interested in carrying on and furthering this most interesting work.
As always, I am interested in feedback from readers whose interests in propagation forecasting parallels my own. You are invited to correspond with me by e-mail at <jimpeg@netzone.com> or to the standard mail address shown.
Footnotes:
(1) Out of print and no longer available
* Planetarium V6.1 (copyright) by Christian Nuesch, Haldenstrasse 12, CH-8320 Fehraltorf, Switzerland FZX:++41 1955 13 93
Originally posted on the AntennaX Online Magazine by Jim Gray, W1XU
Last Updated : 26th April 2024