Long delayed echo

Lore & Background

The first documented observation of long delayed echoes occurred in 1927 when civil engineer and amateur radio operator Jørgen Hals, near Oslo, Norway, repeatedly heard signals from the Dutch short-wave transmitting station PCJJ at Eindhoven. Alongside the usual echo that circled the Earth in about one-seventh of a second, Hals heard a weaker echo approximately three seconds later, with an amplitude between one-tenth and one-twentieth of the principal signal. He wrote to physicist Carl Størmer describing the event, and physicist Balthasar van der Pol assisted in investigating the echoes, but no suitable explanation was found due to their sporadic nature and variations in time delay.

Long delayed echoes have been heard sporadically from 1927 to the present day. Five hypotheses are considered most likely among the fifteen possible explanations listed by Shlionskiy, as summarized by Sverre Holm. These include ducting in the Earth's magnetosphere and ionosphere at low HF frequencies (1–4 MHz), signals traveling many times around the world (with one report of a 28 MHz signal delayed by up to about 9 seconds, implying up to 65 rounds around the Earth), mode conversion where signals couple to plasma waves in the upper ionosphere (with echoes recorded up to 40 seconds at 5–12 MHz), reflection from distant plasma clouds coming originally from the Sun (with several thousand echoes of delay up to 16 seconds at 9.9 MHz), and non-linearity in addition to mode conversion, which could explain amateur VHF/UHF echoes (including a 4.6-second delay at 1296 MHz and a 5.75-second delay at 432 MHz).

Reader's Guide

The significance of long delayed echoes lies in their persistent mystery and the range of scientific inquiry they have inspired. They have been investigated by physicists and radio amateurs for nearly a century, yet no single explanation has been universally accepted. The phenomenon has prompted research into magnetospheric ducting, plasma wave interactions, and solar plasma reflections, each supported by experimental observations but none fully accounting for all reported events. The possibility of hoaxes has been acknowledged, with Volker Grassmann noting that attempts at deception cannot be ruled out and that short transmissions on different frequencies can help exclude potential troublemakers; a worldwide logging system has been developed to reduce errors or hoaxes. Long delayed echoes have also entered discussions of extraterrestrial intelligence, as Ronald Bracewell proposed that automated probes might attract attention by returning our own signals, citing LDEs as a possible case—a concept expanded by Duncan Lunan. The echoes thus occupy a unique place in radio propagation studies, bridging atmospheric physics, space science, and speculative inquiry, while remaining an open observational puzzle.

Did You Know?

• The first long delayed echo was observed in 1927 by Jørgen Hals near Oslo, Norway, while listening to Dutch station PCJJ.
• Delays longer than 2.7 seconds are considered long delayed echoes.
• One experiment recorded echoes with delays up to 40 seconds at frequencies between 5 and 12 MHz.
• A 28 MHz signal was reported delayed by up to about 9 seconds, implying travel up to 65 times around the Earth.

Theorists Who Saw the Invisible

As far back as 1839, the German mathematician Carl Friedrich Gauss suggested that some electrically conducting region in the atmosphere might explain the observed fluctuations in Earth's magnetic field. That idea lay dormant for decades until December 12, 1901, when Guglielmo Marconi received three Morse-code dits in St. John's, Newfoundland, using a kite-supported antenna stretching 152.4 metres. The transmitting station at Poldhu, Cornwall, had fired a spark-gap signal at roughly 500 kilohertz with a power a hundred times greater than anything previously generated. For that signal to cross the Atlantic, it would have needed to reflect off the upper atmosphere twice—a claim later contested by Dr. Jack Belrose on theoretical and experimental grounds. Marconi nonetheless repeated transatlantic wireless the following year in Glace Bay, Nova Scotia. The theoretical groundwork had already been laid: in 1902, Oliver Heaviside proposed a conducting upper layer to explain how radio waves could follow the Earth's curvature, and Arthur Edwin Kennelly independently uncovered some of the region's radio-electrical properties. It was not until 1926 that Scottish physicist Robert Watson-Watt, in a letter not published in Nature until 1969, formally introduced the term ionosphere to complete the family of atmospheric layer names.

The Wave-Guide Revolution

Heaviside and Kennelly reframed long-range radio as a guided-wave problem. Rather than a signal simply bouncing, they envisioned the electrically conductive ocean floor and a hypothetical upper atmospheric layer acting together as a wave guide. Heaviside drew on his Telegrapher's equations and the known conductivity of seawater to argue that a sufficiently conducting layer aloft could trap and channel energy. Kennelly went further, calculating that at roughly 80 kilometres the air's conductivity exceeded that of seawater by a factor of twenty. In that model, radio energy propagated as cylindrical waves whose slower divergence allowed detection at great distances. In 1919, G. N. Watson refined the geometry, treating the Earth as a conducting sphere with a concentric spherical reflective shell, and used that framework to reproduce the Austin-Cohen formula and other long-distance propagation results. Four years later, Joseph Larmor published a quantitative theory of ionic refraction, demonstrating precisely how ionized air bent the direction of radio waves. Together, these theoretical advances transformed the ionosphere from a speculative curiosity into a calculable medium through which engineers could design transcontinental communication systems.

From Eclipse to Nobel

The ionosphere moved from theory to hard evidence through a series of clever experiments. In 1925, Dr. Alfred N. Goldsmith and his team in New York observed a solar eclipse and noted that short-wave radio signals weakened or vanished while long waves steadied, directly linking sunlight to the ionization that guided radio transmission. Between 1923 and 1925, Edward V. Appleton, alongside Miles Barnett, Reginald Smith-Rose, and R. H. Barfield, conducted radio sounding experiments that pinned the so-called E layer at between 80 and 90 kilometres altitude. By varying transmitter frequency and measuring the fading produced by interference between sky waves and ground waves, they showed that the ionized layer grew thicker and closer to the surface during daylight, when solar radiation was strongest. Appleton's 1927 confirmation of the ionosphere's existence earned him the 1947 Nobel Prize. Lloyd Berkner subsequently measured the layer's height and electron density, enabling the first complete theory of short-wave propagation. Maurice V. Wilkes and J. A. Ratcliffe extended the work to very long radio waves, while Vitaly Ginzburg developed a general theory of electromagnetic wave propagation in plasmas like the ionosphere.

Satellites and the Measured Sky

The practical stakes of the ionosphere became undeniable when engineers realized it shaped every long-range radio link and, later, every GPS fix. The U.S. Radio Act of 1912 restricted amateur operators to frequencies above 1.5 megahertz, dismissing lower bands as useless; that restriction inadvertently pushed researchers toward the high-frequency propagation the ionosphere made possible, recognized by 1923. In the early 1930s, test transmissions from Radio Luxembourg accidentally demonstrated the first known radio modification of the ionosphere, a phenomenon HAARP revisited in 2017 through the Luxembourg Effect. Satellite observation transformed the field: Alouette 1 launched in 1962, followed by Alouette 2 in 1965, the ISIS pair in 1969 and 1971, and AEROS-A and B in 1972 and 1975, all dedicated to ionospheric measurement. Syncom 2, the first operational geosynchronous satellite launched in July 1963, carried radio beacons enabling the first-ever measurement of total electron content along a beam from geostationary orbit to a ground receiver. Australian geophysicist Elizabeth Essex-Cohen applied this technique from 1969 onward to monitor the atmosphere above Australia and Antarctica. Today the ionosphere still deflects GPS paths and delays arrival, a reminder that this invisible shell remains a living variable in every navigation system.