A Seven Year Study of
50 mHz
Sporadic-E Propagation

   Though several goals were in mind at the start of this study, only three of them are summarized in this report, namely the hour-to-hour variations and variation within the solar cycle.  Analyses of geographical distribution and drifts of sporadic-E clouds were not made, though the raw data is available.

   Observations were conducted from San Antonio, Texas from January, 1964 through early August, 1966 and again from November, 1969 through the end of December, 1970.  In the period between August, 1966 and November, 1969, observations were conducted from Austin, Texas, some 85 miles northeast of San Antonio.

   Although the equipment used during the study varied from time-to-time, it was all commonly available 50 mHz amateur gear.  Receivers used has sensitivities on the order of 1 µv, and antenna systems had gains in the 7-9 db. range.  Equipment outage caused the loss of data between September and November, 1966.

   There frequently forms in the vicinity of the normal E-layer of the ionosphere, clouds or patches of abnormally intense ionization, which are capable of reflecting radio waves of frequencies much higher than those reflected by the regular E or F layers.  These clouds usually cover a rather small geographical region, approximately 50 to 100 miles in diameter.  They occur more or less at random and are relatively short lived, usually dissipating within a few hours.  This sporadic ionization generally occurs about 60 miles above the earth's surface, at about the same height as the regular E layer.  For this reason it is called sporadic-E ionization, or Es.

   As a results of an intensely ionized sporadic-E cloud, it is at times possible to communicate over relatively long distances on the 50 mHz amateur band, and on some occasions on 144 mHz as well, (see fig. 1).

   The height at which sporadic-E ionization occurs limits one-hop propagation to a maximum distance of approximately 1400 miles.  During periods of widespread Es ionization, two-hop propagation may sometimes be possible up to distances of approximately 2500 miles.  Band openings due to Es are often referred to as short-skip openings for this reason.

   Reflection from sporadic-E clouds takes place with very little signal loss, resulting in exceptionally strong signal levels during most openings.  Quite often it is possible to maintain communications considerably off the great circle path between two stations by means of back and side scatter from a sporadic-E cloud.

   What causes sporadic-E ionization in not yet fully known.  Since it occurs more often during the hours of daylight, it seems that ultra-violet radiation might play some role in its formation.  Since it also occurs at night, especially during the winter months, auroras and meteor trails are often suggested possible sources of ionization.  More recent theories indicate that the ionization might be caused by shearing forces associated with rapid wind movements in the ionosphere.

   Since little is known about the ionizing sources for Es, its behavior cannot be predicted by positive means at the present time.  Statistical studies, of the type discussed in this report, are the only means by which the characteristics of sporadic-E propagation can be determined.

   For the purpose of this study only the occurrence of Es propagation on the 50 mHz band was noted.  Onset and dropout times were recorded, but openings were not rated according to strength, quality, distance, bearing, etc.

   An Es "opening" was considered to be any signal (except those identified as groundwave or tropo-ducting), including backscatter, received over a distance up to the one-hop limit of 1400 miles.  Signals received from beyond 1400 miles were classified as multi-hop Es only when the season of the year or time of day eliminated the possibility of F layer reflection.

   Onset and dropout times were recorded for all openings, with signals checked every five minutes.  A 30 minute fade-out period was defined as terminating a given opening.

   No attempt was made to separate openings according to the ionization patches that might have been involved, and during some occasions reflections from several different patches was evident.  A given opening may have lasted from 5 to 500 or more minutes, and may have had one or two signals present, or hundreds of signals.

   As the author was a high school and college student for the period of this study, the times available for monitoring would vary greatly in a given year.  The summer periods, however, can in many ways be considered near continuous and complete.

   Though a few days of data were lost incident to the station location changes, the only long-term outage of equipment was in the September-November 1966 period.

   The following conclusions concerning the behavior of Es propagation on the 50 mHz band can be drawn from the study.

   Figure 2 shows the total time, in minutes, that 50 mHz Es propagation was observed for each month, summed for the 1964-1970 period.  A summer maximum with a secondary winter peak are clearly noted.  Nearly 80% of the yearly total of Es propagation took place from May through August, with a statistical maximum occurring in June (although July of a given year may have exceeded June).  A secondary maximum is evident in the month of December, with a definite minimum occurring in March.  This agrees with the long-known seasonal trends of Es propagation.

Diurnal Variation

   The diurnal, or hour-to-hour variation in Es propagation is shown in fig. 3, for both winter and summer.  The "double-hump" diurnal characteristics of Es propagation can be seen clearly.  During the summer months, a peak occurs between 10 A.M. and noon, local time, and again from 6 to 8 P.M.  Es propagation is primarily a daytime phenomenon during the summer months, decreasing rapidly after local sundown.

   During December, while the peaks occur at about the same local time as they do during the summer months, the latter period is well beyond sundown and into the hours of darkness.  This December peak may be due, at least in part, to increased meteor activity associated with the Ursids shower which occurs during the middle of this month.

Solar Cycle Variation

   Figure 4 presents the year-to-year variation in Es for the 1964-1970 period.  Es was very high during 1965, at a time when solar activity was very low.  Es was very low during 1969, when solar activity was at its peak.  This might imply some sort of inverse relationship between Es and solar activity, but this is upset by the high level of Es which was observed during 1968, when solar activity was near maximum.  According to this study, there is no clear cut relationship between Es and the solar cycle.

DX Heard and Worked

   During the seven year study WA5IYX heard or worked 48 of the 50 states via 50 mHz Es propagation.  Only Delaware and Alaska were missed.  Hawaii was worked just once on what seemed to a 3 hop Es opening, although it could have been an F2 layer-Es combination.  The following foreign prefixes were also heard or worked during the study period:

   CO2, CO5, FG7, HI8, KP4, KV4, TG9, VE1, VE2, VE3, VE4, VE5, VE6, VP7,
   XE1, XE2 and ZF1.

   In covering so large a span of time it is difficult to pick out only a few highlights.  During June of 1965 there was a four day period of Es during which openings occurred on three days which lasted more than 800 minutes each - a level nowhere else found in this study.  One afternoon the rare spectacle of simultaneous reception of the states of Washington and Massachusetts was made - from Texas this was double hop to each coast!

   June 1968 saw the shortest Es opening, 280 miles, between Big Springs and Austin, Texas.  The longest single Es opening of the study came on May 30, 1970 with onset recorded at 7 A.M. and final fade out of signals at 1:45 A.M. on May 31, or nearly 19 continuous hours.  On November 8, 1970 the longest multihop Es opening was observed (though some claim it was a combination of F2 and Es), with KH6 signals at close to 4,000 miles roaring in.

Conclusions

   This seven year study of Es propagation brings up to date similar studies made previously in the 50 mHz amateur band by Ferrell, the Monroes, and others.  It further confirms the diurnal and seasonal behavior of Es, but still leaves unanswered the relationship between this mode of propagation and the solar cycle, if any.

   These is still need for further, perhaps more specialized long-range Es propagation studies.  Perhaps similar studies in the future will establish some sort of relationship with solar activity, or point more positively in the direction of those phenomena which might be responsible for producing this sporadic ionization.

   There is now general acceptance in the scientific community of the wind-shear theory of Es formation.⁴  However, as yet there remain several points which have to be resolved.  Long-range studies of signals in the 50 mHz band conducted by radio amateurs, particularly in cooperation with professional scientific investigation such as was carried out with ESSA a short time ago,⁵ may hasten the answers to presently unresolved questions concerning this theory.

   The author would like to express his gratitude for the valuable assistance, comments, and encouragement received from many fellow amateurs during this study - in particular: George Jacobs, W3ASK; Morgan, K7ALE, and Dorothy Monroe, K7ALF; Bob Cooper, W5KHT; Mel Wilson, W2BOC; Bill Smith, KØCER; and Lorn Matheson, WAØEKO.

____________________________________________________________________________

*  P.O. Box 27376, San Antonio, Texas 78227

1  Monroe, M., and Monroe, D., "50 Mc Propagation Effects",
   CQ, June, 1962, p. 37.

2  Monroe, M., and Monroe, D., "50 Mc Propagation Effects",
   CQ, Nov., 1964, p. 82.

3  This section provided by George Jacobs, W3ASK, CQ's Propagation Editor.
   See also Jacobs, G. and Leinwoll, S., "V.h.f. Ionospheric Propagation,"
   CQ, Nov. 1969, p. 37.

4  Wilson, M.S., "Midlatitude Intense Sporadic-E Propagation - Part I,"
   QST, Dec. 1970, p. 52.

5  "Radio Amateur Volunteers Needed for 6-Meter Propagation Research,"
   CQ, May, 1970, p. 31.

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