vhfer's view of Solar Cycle 20

A discussion of Solar Cycle 20, now on the wane, and its effects on long-distance 50-MHz propagation Pat Dyer, WA5IYX, 5315 Silvertip Drive, San Antonio, Texas 78228
As Solar Cycle 20 is finally, though somewhat unevenly, drawing to a close, it is appropriate to consider it and its effects on F2 propagation in the spectrum above 30 MHz (and more particularly 6 meters).  The following are the results of personal observations and data collection on both aspects.

   As far back as the peak of Solar Cycle 19 in 1957 I had casually noticed sunspots, but it was not until mid 1963 that regular plots were made and records kept.  The whole period since has involved daily plot-counts, weather and other factors permitting, using a 3-inch reflecting telescope (f/10) with a 60-power eyepiece to produce a 5-inch diameter projected image of the solar disk.  Only when sky conditions were deemed suitable would a record be made, thus avoiding the inherent inaccuracies of trying to observe the sun through even moderate cirrus clouds, etc.  The only large lack of data was the period from June through September, 1971.

   Fig. 1 shows the daily average sunspot count by month.  As some months may have involved as many as 25 or more plots and other as few as 10 or less, the smoothed averages show in fig. 2 are more meaningful.  These smoothed averages are made by taking sunspot data for six months before and after a given month and then averaging it.  The so-called Wolf numbers from my data (made by taking ten times the number of sunspot groups and adding to the spot count) show substantially the same features as those presented in figs. 1 and 2.

[Dyer raw daily 
sunspot count averages, 1963-1974
fig.  1.  Raw daily average sunspot counts, 1963-1974.

[Dyer smoothed sunspot count,
fig.  2.  Smoothed sunspot counts, 1964-1973.

   Unlike the official records made by the Swiss, this data shows a rather later peak of Cycle 20 in 1970 vs the late 1968 or 1969 peak often cited elsewhere.  A peak daily count occurred on November 17, 1970, with some 67 spots plotted in four groups.  The Wolf-count peak for a given day was on February 1, 1968, with 53 spots in ten groups (W = 153).  The results of this plot are shown in fig. 3.
[Feb 1, 1968 Dyer
sunspot plot]
Fig.  3.  Sunspot diagram for February, 1968.
  Diagram shows 53 spots in 10 groups (Wolfe number = 153).

   The later stages of Solar Cycle 19 in 1963 are very evident in the graphs.  This period was followed by a rather prolonged minimum running through 1964 and well into 1965.  During these lean years the sun was spotless for many days at a time. A rebirth of activity was dramatic in 1966.  In fact, the spotless sun of November, 3, 1966, was not duplicated until October 13, 1972.  However, during the past year the incidences of zero counts have been becoming more and more frequent.

   Lacking more sophisticated equipment, I was unable to view the other associated solar events such as prominences and flares.

   For those readers who are interested in conducting their own solar observations just about any astronomy book will provide the necessary information.  My method of projection viewing and plotting is the simplest and the most safe.  Photographic setups provide the most accurate records but the cost factor there can be limiting.  Regardless of the method you use, do not observe the sun directly without adequate filtering devices.  Both visible and infrared, as well as ultraviolet rays, must be reduced to safe levels to prevent permanent eye damage (which can occur quickly and painlessly).

vhf propagation

   The ionospheric effects of a solar cycle depend greatly on the location of the observer.  My interest in vhf propagation came about in the early 1960s first in the realm of TV-DXing and sporadic-E.  I became an amateur in late 1963, and the results of 50-MHz Es observations made during the period from 1964 to 1970 have appeared elsewhere. 1, 2

   Much to my regret, suitable equipment for monitoring 30- through 50-MHz spectrum was not available until the fall of 1967.  Even since then equipment has been on the simple side: a Radio Shack Patrolman, and, in 1970, an Allied A-2586.  Recently a Hallicrafters SX-62 has been revamped.  Simple random-wire or whip antenna systems have been the rule.

   On 50-MHz a low-power a-m transceiver and a five-element Yagi at 20 feet (6.1 meters) was used until the fall of 1968 when a higher power ssb rig was acquired.

   Detailed records of F2 MUFs in the 30- to 50-MHz region were not kept regularly until the fall of 1968.  The late 1969, early 1970 data has been cut due to various receiver-related problems.  Actually, the term MUF (maximum useable frequency) in these cases should be taken as MOF (maximum observed frequency) as no method (e.g., backscatter radar) was available to determine if the band was "open" higher than the highest incoming signal frequency.

   Figs. 4 and 5 show the number of days each month that F2 signals were observed in the contiguous United States and from Latin America at the indicated frequencies.  As most of the latter are unidentified signals, there is a possibility that Es propagation was inadvertently included at times.  However, as will be discussed in more detail later, Es often played a big role in providing link-ups with F2 openings that would have otherwise passed, undetected.  These graphs only include direct F2 modes, and thus do not consider backscatter or transequatorial scatter (TE) propagation.

[US F2 MUF's]
fig.  4.
F2 MUFs for the United States plotted here were determined by noting the highest frequency incoming signal within the 48 contiguous states.  This meant the skip for a given frequency was down to 3000 km or less.  When theoretical considerations are applied, the transcontinental MUF (e.g., W4-W6) was considerably higher than I could observe.  Using the ITS maps, the 4000 Km MUF (nominal maximum 1-hop F2) can be extrapolated knowing how short the skip is on 35 MHz, etc.

[Latin American F2 MUF's]
fig.  5.  Latin American F2 MUFs.

   There was always a large difference in MUF behavior in the United States and Latin America, with little apparent relationship to one another.  For instance, on many occasions South American signals were well above 45 MHz while in the United States even 10 meters was dead.  But, on other occasions the MUFs in the United States almost seemed to be keeping pace with those to Latin America.

   In figs. 4 and 5 the F2 "season" has been limited to September through April, although occasionally during the summer Latin American signals reached the 40-MHz region.  Both seasonal and solar epoch variations are easily found.  For U.S. MUFs the best months were October-December; this in striking contrast to the Latin American peaks of March and April.  Year-to-year changes, while not always smooth, show the decline of Solar Cycle 20.

   I should mention a word about the seemingly arbitrary frequency divisions used in figs. 4 and 5.  The selection is natural for the U.S. and FCC assignments produce large groups at certain frequencies (i.e., fire departments at 33 MHz; pagers and mobile phones at 35 MHz; and law enforcement at 37 and 39 MHz).  The Latin American situation is different as it is next to impossible to obtain station assignment information.  Thus, no simple groupings are known which could make a more meaningful frequency division system than that used in figs. 4 and 5.

six meters

   Since 50-MHz DX is of considerable interest to the vhf operator, it's worthwhile to take a more detailed look at Solar Cycle 20's F2 effects on six meters.  Table 1 gives a month-by-month summary of the numbers of days and minutes total open on 50 MHz by various modes.  The mode determination is a rather simple process of considering the distances, peak antenna headings, fade rates, etc.

                                                table 1.
                     Observed 50 MHz band openings (# days, total minutes)

                                     F2              F2 bs         TE
                1967 April         1   85               -            -
                     September     1   55               -            -
                     October          -             1   15           -
                1968 March         3   80               -            -
                     April        14  635           6  530       3   40
                     May           2   30               -        1   30
                     September        -                 -        3  255
                     October          -             1    5       1   80
                1969 February      1   40           1   30           -
                     March         1    5           3  410           -
                     April        11  290           6  655       4   60
                     September        -                 -        1   90
                1970 February         -                 -        3  150
                     March         3   45           1  135           -
                     April         9  340           6  265       3  200
                     May           2   40               -            -
                     November         -                 -        1   10
                1971 March         1   20           2   45           -
                1972 March         3   60           4  170           -
                     April         8  220           3  175           -
                     September     1   10           1   45           -
                1973 April         1    5           1   20           -
                     September     1   15           1   20           -
                     October       1   45               -            -
                1974 March         2   35               -            -
                     September     1   15               -            -
   Fig. 6 shows the time of day of F2 and TE openings on 50 MHz for the month of April summed over the period from 1967 to 1973.  The time to be on the air is clearly in the afternoon.  Almost without exception, I suspect that all the transequatorial scatter openings made it this far north with the help of an Es link.  The use of beacons by CE3QG and OA4C in those years was a priceless asset. 3  The lack of TE since 1970 is believed to be due, in large part, to the loss of activity from these two stations.
[Diurnal Graph of 50-MHz April F2 & TE, 1967-1973]
fig.  6.  Graph showing 50-MHz F2 and TE openings during the month of April, 1967-1973.

   Backscatter, although not plotted in fig. 6, has much the same shape with earlier onset and later fadeout points.  This is very consistent with the pattern of F2 backscatter from the southeast, followed by direct F2 from South America proper, ending with backscatter again from the South and Southwest.

   The 50-MHz F2 paths to South America's more remote end, namely Argentina, Uruguay, and Chile, are very likely the result of what are known as F2-F2 paths, shown in fig. 7.  These are sometimes called trapezoidal paths due to their shape, and they provide very strong signals since an intermediate ground reflection with signal loss is eliminated.  The geomagnetic equator, with its attendant "bulges" of F2 ionization on each side, is responsible for these tilted layers.

[Texas-Argentina F2-F2 path]
fig.  7.  Propagation over an F2-F2 path from Texas to Argentina (not to scale).

   The geometry of the F2-F2 path is likely a rather ticklish affair requiring several different conditions to coincide.  For example, if the ionization on the more northerly bulge of the path is not correct, the path is disrupted.  Too low a level will cause the 50-MHz signal to overshoot the second bulge to the south, while the level which is too high may cause undershooting.  This may explain the often observed oddity of six-meter stations from Argentina and Uruguay appearing when all the stations in Ecuador and Venezuela were at 44 to 46 MHz.

   Sporadic-E, often seen as a friend in linking up with an F2 or TE opening, can just as easily ruin, by topside reflection, what would otherwise be a good path as shown in fig. 8.  Since Es may be partially transparent, the effect is very likely quite variable.

[Sporadic-E 'shielding']
fig.  8.  Sporadic-E shielding of a 50-MHz F2 path (not to scale).

   Six-meter F2 backscatter can be either single or double-hop in nature (perhaps giving rise to a total path length of 9000 miles or more).  The best earth reflection regions are over the oceans, or to the south and southwest of my station.  This is ground backscatter and not direct backscatter from the ionized regions per se.  Es effects here are much the same as with the other two modes already discussed.

   The following is an expansion with comments of the 50-MHz effects summarized in table 1.  Suitable references are noted in the cases of major events.

The month of April brought me my first meeting of 50-MHz F2 DX.  It was more than five months before it was heard again.

With some openings in March, April proved to be the best month of the Cycle, helped along by vast amounts of early season Es, which aided the first TE openings noted here.  Es also kept the F2 season alive well into May. The fall, though providing plenty of Es-to-TE links, did not bring the huge F2 openings that were anticipated by many operators.

A very strong magnetic disturbance on February 2nd brought in backscatter here and several other modes elsewhere.4  An Es-to-F2 link later in the month provided some 40 minutes of the ZK1AA beacon.  March and April, in contrast to the previous spring, brought in much more backscatter than direct F2. The lack of F2 was probably due to the poor Es season.

Good Es in February permitted TE once again.  Overall, April was better than expected, with many instances of the Cook Island beacon.  However, the highlight of the year and the Cycle was on March 8th where, in one 90-minute period 50-MHz F2 backscatter was noted in some 16 states as far north as Illinois, with direct paths to Puerto Rico.  This was the largest magnetic storm of Cycle 20 and occurred during a time of year when it would do the F2 layer the most good.5  The last incidence of TE here occurred during November, in the midst of an Es opening to Guatemala.

50-MHz propagation might be best described as a recession, with only very scarce F2 effects in March.

A rather unexpected upturn in solar activity in the spring provided the best spring March-April F2 in two years.

Openings on 50 MHz, though very scarce, were amazing in that any occurred at all with such low solar levels.

   Close followers of six-meter DX have probably noted by now a conspicuous absence of details on January 1, 1968.6  While this location is very good for Latin American F2 (though not as good as Florida), when it comes to transcontinental F2 on 50 MHz it is just too close to each coast to get any.  On the date in question 46-MHz was in from the Pacific northwest region, while second-hand reports from the 10-meter nets raved about the coast-to-coast 6-meter opening in progress.

   Also, being this far south geomagnetically, no direct evidence of any of the many auroral events appeared.  The closest incident was June 5, 1967, when a magnetic storm created extremely fluttery Es (apparently) to Florida and perhaps either double-hop Es or single-hop F2 to Puerto Rico. With so much Es in June it is impossible to be sure of the modes without ionosonde evidence at hand.  The prior week (May 25th) produced what were likely 49-MHz Latin American F2 signals while Florida had both visual and radio aurora.7, 8

   Along other lines, solar activity introduced vhf noise bursts to me in July, 1967.  Although the event observed was nothing extraordinary, having the 50-MHz background noise rise by 40 or 50 dB for the first time was a memorable occurrence.  During the ensuing years, while monitoring 30-50 MHz, numerous solar noise bursts have been logged incidental to MUF observations.  A particular incident in April 1973, when a solar noise burst was noted simultaneously with an increase in an F2 backscatter signal level, was vivid evidence of the association of flares, noise bursts, and extra solar ionizing energy.

   To step out of vhf for a moment, another equally dramatic trait of high solar levels is the high-frequency blackout (caused by extra D-layer ionization and consequent increase in collisions and absorption) when you are positive that your receiver has stopped working.  Many of these blackouts were stumbled upon while attempting to get a WWV propagation forecast and the vhf Es and F2 openings went along virtually unaffected.


   While Solar Cycle 20 has not yet completely withered away, there is little doubt that it will be quite some time before the F2 effects on 50 MHz become as common as they were in 1968.  However, devoted 50-MHz DXers might still be able to catch a few of the freak openings still left in the Cycle.  For a better chance at catching the openings, the following suggestions are offered:

1.  If you don't already have a receiver that will tune 30 to 50 MHz, by all means get one that does.  While an SP-600 or one of its relatives is best, you can get by with a lot less.

2.  Become familiar with the DX signals that frequent your area on the band.  This can be helpful in looking for the more common Es openings that might affect 6 meters.9  When the conditions appear favorable, don't just listen, call CQ.  You may end up with a hoarse voice and not get a reply, but at least you tried.  For those fortunate enough to have beacons, it will be a lot easier.

3.  Obtain copies of the Telecommunications Research and Engineering Report 13.  This has ionospheric predictions for F2 median MUF (both at zero and 4000-km ranges) as well as normal E at various solar cycle levels (with interpolation methods) in the form of world maps at two-hour intervals for each month of the year.*  While not all that useful in predicting the 50-MHz openings, they have supplanted the monthly bulletins that ITS (formerly CRPL) used to publish and are valuable in determining in which directions from your location the MUF's are peaking at a given time of day, season, etc.  The F2-F2 path can be inferred from the maps, but its MUF will be somewhat greater than the 4000-km MUF given on the maps for each "bulge" near the equator.

4.  If you have high-frequency capabilities do all you can with contacts in regions where 50 MHz might be likely to stir their interest in at least listening on that band, if not actually setting up a station.  Innumerable openings have been lost due to lack of 50-MHz activity in Venezuela and other parts of northern South America - openings where all sorts of high-frequency harmonics were pouring through on or near 50 MHz.  In addition, the use of beacons on 50 MHz should be encouraged in these DX spots.

   The foregoing and suggestions elsewhere10 are a valid formula for getting into shape for the next solar cycle peak - you have plenty of time as it will not likely occur until the latter part of this decade.  I'm afraid that a lot of plans for Solar Cycle 20 got going too late to be of much benefit, particularly the set-up of some 50-MHz beacons.


   I hope that this article will serve as a stimulus to others to undertake similar observations and recording of their data.  This is only one of the ways amateurs can justify the portions of the spectrum we occupy - by contributing to the basic understanding of vhf propagation.

   Over the years I have been indebted to several fellow amateurs for their encouragement and advice.  Bob Cooper, W5KHT, deserves special acknowledgement for getting me to keep more accurate notes on the F2 DX conditions in the 30-50 MHz region.  I wish that I started earlier in the Cycle.

1. Pat Dyer, WA5IYX, "Fifty-Megahertz Es, 1964-1970," Radio Science, March 1972, page 351.
2. Pat Dyer, WA5IYX,
"A Seven-Year Study of 50-MHz Sporadic-E," CQ, August, 1972, page 66.
3. Robert Cooper, KV4FU/K6EDX, "An Automatic Two-Way DX beacon for VHF," ham radio, October, 1969, page 52.
4. Bill Smith, K4AYO, "World Above 50 MHz," QST, April, 1969, page 95.
5. Bill Smith, KØCER, "World Above 50 MHz," QST, June, 1970, page 94.
6. Bill Smith, WB4HIP, "World Above 50 MHz," QST, March, 1968, page 83.
7. Bill Smith, W1DVE, "World Above 50 MHz," QST, August, 1967, page 76.
8. Bill Smith, W1DVE, "World Above 50 MHz," QST, July, 1967, page 93.
9. Morrie Goldman, WA9RAQ, "Predicting Six-Meter Sporadic-E Openings," ham radio,, October, 1972, page 38.
10. Robert Cooper, K6EDX. "Interpreting 50-MHz MUF Tendencies in the Current Sunspot Cycle," QST, March, 1968, page 19.

*  Four volumes are available.  Volume 1 is the instruction manual ($.30), while Volumes 2, 3, and 4 are for smoothed sunspot levels of 10, 110, and 160, respectively ($3.00 each).  Order from the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402.

This is a "graphically-enhanced" version of my original article using things like Lotus Works and Paint Shop Pro to recreate some of the figures rather than just do a scan (and touch up) from the 1974 magazine illustrations.  There are also a few links to the numerical tables used to generate graphs as well as additional data for the sample sunspot plot figure.

Please note that this article was mostly written around October of 1973, with then some later updated data sent into ham radio thru part of 1974.  Considering the far-better solar flux levels produced in Cycles 21 and 22, some of this seems almost "quaint" now!  Unfortunately Cycle 23 is not, so far, looking to be as good as its immediate two predecessors.  If worldwide 6-m activity (in many Region 1 and 3 locales then forbidden due to their low-band VHF TV allocations) and coordination had been as good as it is now Cycle 20 would have likely been able to produce a lot more DX.  Actually, considering magnetic indices, the August 1972 magnetic storm was really more intense than the above-mentioned one in March 1970.
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