Fig. 3. Sunspot diagram for February, 1968.
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
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
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)
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.
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.
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.
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.
The lack of TE since 1970 is believed to be due, in large part, to the loss
of activity from these two stations.
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.
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.
fig. 8. Sporadic-E shielding of a 50-MHz F2 path (not
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
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
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
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
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
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.