Sporadic-E Opens New Horizons
October 1961 Radio-Electronics

October 1961 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

Prior to studying for my amateur radio license back in 2010, I was not overly familiar with the specific way various layers of the Earth's atmosphere affected radio signals (electromagnetic waves). Terms like maximum usable frequency (MUF) and the various ionospheric regions (D, E, F1, F2) occur regularly in discussions of long distance communications. However, sporadic-E (E_s) was new to me. As the term implies, it refers to the irregular conductivity properties of the E-layer (aka the Kennelly–Heaviside layer) of the ionosphere, which is defined as the region between 90 and 150 km (56 to 93 mi). Sporadic-E conditions exist when solar activity, usually coinciding with the summer and winter solstices, causes excess ionization that reflects radio waves into the VHF spectrum (30-300 MHz), beyond the normal MUF of around 15 MHz. Lots of Hams exploit E_s in order to make long-distance (DX) contacts not usually possible otherwise. BTW, there is also a C region that interact with cosmic rays, but I cannot find anything about A or B regions (anybody know?).

Sporadic-E Opens New Horizons

By Stanley Leinwoll*

During the past several months Radio Free Europe schedules have included frequency assignments specifically selected to make use of sporadic-E propagation to reach RFE targets behind the Iron Curtain. These transmissions, initially undertaken on an experimental basis, are expected to have a significant impact on high-frequency communications for the next 5 to 10 years.

Although sporadic-E (E_s) propagation has been known for many years, this is the first time any international broadcast operation has used it in a regular schedule, with antennas specifically suited to exploit this mode of propagation.

This technique has made it possible to schedule frequencies well beyond those ordinarily considered the normal operating range. This effectively expands available spectrum space, and provides a valuable tool in RFE's continuing effort to combat jamming.

Late last May, Radio Free Europe included a 15-mc frequency in its schedule of Hungarian broadcasts from Biblis, Germany. This was during daylight hours, when the normal maximum usable frequency over this path is of the order of 9 mc.

The maximum usable frequency (MUF) is the highest frequency that is reflected by the regular F-layers of the ionosphere at any given time over a particular circuit. It is generally the highest frequency assigned to a circuit over which some degree of reliability is required. Scheduling a frequency 75% above the MUF as we did in the Biblis tests would result in extremely poor effectiveness figures with satisfactory reception unlikely more than 5% of the time.

Fig. 1 is an MUF curve for August, 1961. It shows how maximum usable frequency normally varies with time over the Biblis/Hungary path. The 15-mc signals from Biblis were propagated off the E-layer of the ionosphere, making use of a summertime anomaly occurring in the E-layer which is referred to as sporadic-E. While this anomaly has been known for some time, it is not well understood.

Fig. 2 shows the layers of the ionosphere on a typical summer day. These layers, made up of ionized gases of differing densities, can return obliquely incident radio waves to earth, thus making long-distance high-frequency radio communication possible.

Sporadic-E occurs as clouds or patches of high ionization density which form for no apparent reason, exist for up to several hours at a time, then disappear just as mysteriously. Sporadic-E clouds reflect radio waves of considerably higher frequency than those reflected by the normal F-layers of the ionosphere.

In mid-latitudes, E_s is most prevalent during the summer months, occurring over 50% of the time at mid-day. Many scientists have agreed that sporadic-E is a very important effect, but up to now methods for taking its effects into account have been poor.

 Sporadic-E is most evident during years of low sunspot activity. Since MUF's vary with sunspot number, E_s activity is most in evidence when MUF's are low. During years of high solar activity, MUF's are high, normally approaching the upper limit of the short-wave spectrum over many paths. At such times E_s effects are scarcely noticeable in the high-frequency bands.

Fig. 3 shows sunspot activity during the past 7 years. You can see that sunspot numbers have been declining steadily since 1958. The broken line is a projection of the sunspot cycle and indicates that, if the present trend continues, the cycle minimum will occur sometime in 1965. The decrease in sunspot number and the accompanying decrease in MUF will result in a corresponding decrease in the amount of propagationally useful high-frequency radio spectrum. In addition, world-wide demands for additional spectrum space in the high-frequency broadcast bands are expected to double in the coming decade.

The combination of decreasing spectrum space brought about by a decline in sunspot activity and increased demands for spectrum space resulting from an increase in short-wave broadcasts throughout the world has been of great concern to broadcasters, who anticipate that the amount of useful spectrum space will fall far short of what is needed.

About RFE Tests

As part of an extensive program to determine what countermeasures can be taken to deal with the problem of the dwindling spectrum, RFE has undertaken a series of tests to find out whether consistent broadcast operation above the MUF is possible and practical. Fig. 4 shows how these tests are being conducted.

 Since the E-layer of the ionosphere is considerably lower than the F-layers, an antenna with a relatively low radiation angle is required to bounce a signal off the E-layer.

RFE antennas are designed for normal F-layer propagation. Consequently, radiation angles for normal F-layer operation to targets in nearby Central Europe are relatively high. The problem of choosing a proper antenna was solved by scheduling broadcasts to Hungary on an antenna designed for beaming broadcasts to Rumania.

Fig. 4 shows that the vertical radiation angle of the Rumanian antenna is optimum for E-layer propagation of programs beamed to Hungary.

Results thus far have been excellent, with reception comparable to that observed on lower frequencies via normal F-layer propagation. At present, plans for incorporating regular E, transmissions into our schedules are under way.

Although the E-layer of the ionosphere exists principally during daylight hours, E_s activity is observed around the clock during the summer. Tests are under way to determine whether nighttime E_s transmissions above the F-layer MUF are practical.

Irrespective of the results of these nighttime tests, the success of the daytime E_s tests is a significant step toward solving a serious problem for broadcasters. In effect, it expands the available spectrum space at a time when additional channels are a valuable commodity.

Although E_s is observed about 50% of the days during the summer months, the RFE transmissions on 15-mc indicate that the signal is propagated considerably more than 50% of the time leading to the conclusion that, in addition to E_s, some other E-layer propagation mode is also a factor. It has been suggested that the phenomenon be referred to as anomalous E-layer propagation, rather than the misleading E_s.

In addition, using frequencies above the MUF has proved valuable in countering the effects of jamming.

Jamming is defined as the creation and transmission of various kinds of radio interference for the purpose of preventing normal reception of radio broadcasts. The Communists have been jamming Radio Free Europe broadcasts since 1951.

Since most of the jamming transmitters in Central Europe are designed for relatively high-angle operation, we have found that jamming from the satellites during the E_s tests has been relatively ineffective, and that it has been necessary for jammers at more distant locations, principally the Soviet Union, to be called into operation.

In addition to putting an additional strain on the Communist jamming system, the strength of the delivered jamming signal is weaker than it would be were the transmitter located closer to the target areas.

* Radio-frequency and propagation manager, Radio Free Europe.

Posted July 29, 2024