Radio waves and Sounding the Ionosphere - Part 2
By Marcel H. De Canck, ON5AU
I
n the previous issue we learned how radio communication was invented and improved step by step.
However, new questions were raised about this mysterious wireless communication. Radio waves
seem to have the same propagation characteristics as light, but did not always exactly behave as
light. Especially, the line of sight property did not coincide. Otherwise trans-Atlantic communications
should be impossible. So new questions were raised and theories advanced, presenting a new
challenge for scientists to investigate and prove these theories. This research history of why radio
communications were possible much father beyond the line of sight distance is this issue’s subject.
Some History
Marconi surprising transatlantic radio communications called for new explanations. In principle
two possibilities existed: either the waves reached that distant region behind the earth curvature by
diffraction or they were reflected from a conducting layer somewhere high up in the atmosphere, Fig.
64.1. Karl Friedrich Gauss already suggested such a conducting layer in 1839. His opinion was that
the variation of the geomagnetic field was most probably caused by electric currents in the upper
atmosphere.
Johann Carl Friedrich Gauss (30 April 1777 – 23 February 1855) was a
German mathematician and scientist who contributed significantly to many
fields, including number theory, analysis, differential geometry, geodesy,
magnetism, astronomy, and optics.
In 1831 Gauss developed a fruitful collaboration with the physics
professor Wilhelm Weber; it led to new knowledge in the field of magnetism
(including finding a representation for the unit of magnetism in terms of
mass, length and time) and the discovery of Kirchhoff's circuit laws in
electricity. Gauss ordered a magnetic observatory to be built in the garden
of the observatory and with Weber founded the magnetischer Verein
("magnetic club"), which supported measurements of earth's magnetic field
in many regions of the world. He developed a method of measuring the
horizontal intensity of the magnetic field which has been in use well into the second half of the 20th
century and worked out the mathematical theory for separating the inner (core and crust) and outer
(Magnetospheric) sources of Earth's magnetic field.
In 1902, short after Marconi’s remarkable transatlantic radio communications, Kennelly and
Heaviside postulated independently an atmospheric conducting layer which would work as a reflector
of radio waves and allow long distance transmissions. It was soon suggested that the solar ultraviolet
radiation could create the necessary electrical conductivity by ionizing the neutral gas. But the
explanation was not generally accepted and the so-called Kennelly-Heaviside layer was taken more
as a myth than reality. Diffraction calculations showed that radio signals should be attenuated too
strongly to be detectable at the observed long distance, so the problem continued.
Also William Henry Eccles, an assistant to Guglielmo Marconi, supported the KennellyHeaviside theory and suggested in 1912 that solar radiation was responsible for the observed
differences in radio wave propagation during the day and night.
The Kennelly-Heaviside layer, also known as the E region or simply the Heaviside layer, is a
layer of ionized gas occurring at 90-150km above the ground — one of several layers in the Earth's
ionosphere. It reflects medium-frequency radio waves, and because of this reflection radio waves can
be propagated beyond the horizon. Its existence was predicted in 1902 independently and almost
simultaneously by the American electrical engineer Arthur Edwin Kennelly (1861-1939) and the
British physicist Oliver Heaviside (1850-1925).
antenneX Issue No. 123 – July 2007
Page 1
The diffraction theory and its calculations predict that long wavelengths have a longer range
than the short ones. Therefore the governments reserved the long waves for their use. The short
waves were left for the keen amateurs. Contrary to all preconceptions, these amateurs managed in
transatlantic communications in the early 1920’s, Fig. 64.2. A two-way transatlantic communication
between USA and France was accomplished for the first time on November 17, 1923.
This fact renewed the interest in the Kennelly-Heaviside layer. Two research groups, one in
America and one in England, were practically simultaneously able to prove decisively the existence of
the Kennelly-Heaviside layer and even to measure the altitude.
Fig. 64.1. The two explanations to long distance communications further then the line of sight reach.
Fig. 64.2. QST article from January, 1923 announcing the first Transatlantic QSO by radio amateurs
More information about this first transatlantic radio amateur communication is found at the following
hyperlinks.
antenneX Issue No. 123 – July 2007
Page 2
http://hamgallery.com/qsl/country/France/1mo.htm
http://www.eht.com/oldradio/awa/events/transatlantic/1sttransatlantic.htm
Oliver Heaviside (May 18, 1850 – February 3, 1925) was a self-taught
English electrical engineer, mathematician, and physicist.
In 1902, Heaviside proposed the existence of the KennellyHeaviside Layer of the ionosphere which bears his name. Heaviside's
proposal included means by which radio signals are transmitted around
the earth's curvature. The existence of the ionosphere was confirmed
in 1923.
Arthur Edwin Kennelly (December 17, 1861 - June 18, 1939) was an
American engineer in electricity.
When Kennelly considered the results of Guglielmo Marconi's
reception, in Newfoundland in 1901, of radio signals transmitted from
England he found they were received far better than was predicted by
existing radio-wave theory. Up to this time, little was known about the
physical properties of the Earth's upper atmosphere. The following year
Kennelly deduced that the reason Marconi's radio waves were able to
propagate across the distance of the Atlantic Ocean was that the
signals were being reflected back to Earth from an ionized layer in the
upper atmosphere acting like a radio mirror or roof. He showed that this
would be achieved by an electrically conducting stratum in the rarefied
atmosphere at a height of about fifty miles and with a high conductivity.
William Henry Eccles (August 23, 1875 - April 29, 1966) was a British
physicist and a pioneer in the development of radio communication.
Following graduation from the Royal College of Science, London, in
1898, he became an assistant to Guglielmo Marconi, the Italian radio
entrepreneur. In 1901 he received his doctorate from the Royal College
of Science. Eccles was an advocate of Oliver Heaviside's theory that a
conducting layer of the upper atmosphere could reflect radio waves
around the curvature of the Earth, thus enabling their transmission over
long distances. Originally known as the Kennelly-Heaviside layer, this
region of the Earth's atmosphere became known as the Ionosphere. In
1912 Eccles suggested that solar radiation was responsible for the
observed differences in radio wave propagation during the day and
night. He carried out experiments into atmospheric disturbances of radio
waves and used wave detectors and amplifiers in his work.
Following World War I helped in the design of the first long wave radio
station, and became involved in the early work of the British
Broadcasting Company (later the BBC) following its establishment in 1922.
antenneX Issue No. 123 – July 2007
Page 3
The early probing of the ionosphere
In America, (1925, 1926) G. Breit and M. A. Tuve devised a technique for determining the
height of the reflecting region. Radio waves travel with the speed of light, thus the height of the
reflecting region can be calculated if one can measure the time taken by the transmitted radio waves
back to the earth.
Breit and Tuve had access to a pulsed transmitter. By measuring the delays of vertically
reflected pulses by means of an oscilloscope it was simple to calculate the reflection height from the
delay time and the speed of light. This experiment worked as a model for the future ionosondes and
also eventually contributed to the development of the radar.
Gregory Breit (July 14, 1899 – September 11, 1981) was a Russian-born
American physicist, professor at universities in New York, Wisconsin, Yale,
and Buffalo.
Merle Antony Tuve (27 June 1901 - 20 May 1982) was a leading American
scientist and physicist. Merle graduated in physics at the University of
Minnesota in 1922 and obtained a master’s degree in 1923. While at
Minnesota Merle developed a close friendship with Breit, a theoretical
physicist. Merle went to the Johns Hopkins University to work for his
doctorate.
After Tuve’s arrival at Johns Hopkins, Breit sought his collaboration
in a possible effort to study the Kennelly-Heaviside layer. This study and
experiments lead to the theoretical fundaments of the development of radar.
At the time, the electronics equipment available was primitive and relatively insensitive. To
demonstrate the existence of the ionosphere it would be necessary to find evidence that radio signals
arrived over at least two paths, a ground wave and a sky wave. To take an example: if a receiver were
set up 10 miles from a radio transmitter, and if the Kennelly-Heaviside layer were 100 miles above the
receiver, two pulses should arrive, a direct pulse and then, a millisecond later, a reflected pulse. If the
height of the ionized or reflecting layer were increased or decreased, then the difference in time of
arrival of the two pulses would change correspondingly.
A method for a test of the existence of ionization in the upper atmosphere has been
developed, and a definite proof of the existence of echoes from the upper regions has been obtained.
The echoes are present for 70-meter waves with an 8-mile base near Washington, D. C. Tuve devised
the necessary detecting equipment and were able to use a Naval Research Laboratory oscillator for
their source of radiation. They observed delayed pulses but could not eliminate the possibility that
these were reflections from the Blue Ridge Mountains. However, one evening they found that after
sunset the reflecting layer moved upward from a height of about 60 miles to a height of more than 115
miles as the delayed pulses began to arrive at longer intervals. The experiment was a success. Breit
persuaded Johns Hopkins University to accept the work as the basis for Tuve's Ph.D. thesis, and the
degree was granted in 1926. Verification of the existence of the ionosphere opened an important field
of research and suggested the practicability of radar.
antenneX Issue No. 123 – July 2007
Page 4
Finally, the effective height of the layer was found to be between 50 and 100 miles. At times
multiple reflections were present. Radio fading was shown to be not only an effect of interference
between the ground and the reflected waves, but also to a large extent an effect of the presence or
absence of reflected waves. A seasonal variation in the effective height between summer and fall
seems to exist. A smaller diurnal effect was also suspected. The height seems greater in the fall than
in the summer and greater in the afternoon than in the morning. The effects of wavelength and of
location have also been studied.
The presence of a conducting layer in the upper atmosphere that made long-distance radio
communication possible was hypothesized in 1902. But experimental proof of the existence of the
"Kennelly-Heaviside layer" (now called the ionosphere) was not forthcoming for more than two
decades. Beginning in the summer of 1925, DTM researchers Gregory Breit and Merle Tuve
developed a method of bouncing pulsed radio signals off the ionized layer and observing the echoes.
Through cooperative experiments with the Naval Research Laboratory, ionosphere heights of 50 to
100 miles were demonstrated. Their technique paved the way for the worldwide study of radio
transmission and laid the groundwork for the later development of radar.
In England, (1926) Appleton and Barnett applied two different methods based on continuous
transmission. In the first method, the elevation angle of the signal arriving at the receiver was
measured. When the distance between the transmitter and the receiver was known, the altitude of the
reflecting layer could be calculated. With the second method the receiver was close to the transmitter
and changes in the interference pattern of the ground wave and a nearly vertically reflected wave were
observed when the transmitting frequency was slowly varied.
In the course of his investigation with radio waves of shorter wavelengths, Appleton
discovered another reflecting layer at the height of roughly 200 to 400 km. This layer was called the
Appleton layer. The names of these two layers were changed by Appleton as E-layer (KennellyHeaviside layer) and F-layer (Appleton layer). This is the nomenclature now universally followed.
Subsequently another layer, a layer below the E layer, was discovered. This layer was called D-layer.
The D-layer, which is located between 50 and 90 km altitude, disappears during the dark hours. The
F-layer splits into two different regions, namely F1 and F2. The F1 region, which exists only in
daytime, has a peak density around 200km. In the F2 region, the altitude of the peak density occurs at
antenneX Issue No. 123 – July 2007
Page 5
about 300km in daytime and it shifts to higher altitude in the night. The E-region is between 90 and
160 km and the F-region (sometimes called the Appleton layer) is from 160 km up to about 400 km.
A quantitative discussion of the results enables one to eliminate too gradual distributions of
electron density. The measured retardation is shown to correspond to a height greater than the actual
by amounts differing for various polarizations of the refracted waves.
Miles Aylmer Fulton Barnett was born in Dunedin on 30 April 1901.
After attending Christ’s College, Christchurch, New Zealand, he went
on to the University of Otago, where he gained the Beverly Scholarship
in physics and higher mathematics (1921). His thesis analyzed the
operation of some of the equipment used by Professor Robert Jack in
his experimental radio broadcasts of 1921 and 1922.
Barnett entered Clare College, University of Cambridge, in
1924 to study in the Cavendish Laboratory. He was assigned by Sir
Ernest Rutherford to an investigation of the propagation of radio waves
under the supervision of E. V. Appleton. The existence of an electrically
conducting layer in the upper atmosphere that could reflect radio waves
beyond the curve of the earth had earlier been postulated by A. E.
Kennelly in the United States and Oliver Heaviside in Britain. The
experiments at Cambridge confirmed this theory, as well as showing that at times the reflections could
come from a second, higher layer. These two ionized layers – the Kennelly–Heaviside and the higher
Appleton–Barnett – are now known as the E and F layers respectively. For his study of radio
propagation via the ionosphere Barnett was awarded a PhD in 1927 and elected a fellow of the
Institute of Physics in 1929.
Edward Appleton was born in 1892 in Bradford. When he was 18 he
earned a scholarship to Cambridge University to study the natural
sciences. The outbreak of the First World War in 1914 led him to join the
Royal Engineers where he was first introduced to radio.
After the war he returned to Cambridge as a student of the
renowned scientist J. J. Thomson (famous for discovering the electron).
Appleton’s own research was focused on radio waves and how they
interact with the atmosphere.
After proving the existence of the ionosphere, Appleton worked
with Robert Watson-Watt in developing technology that would lead to the
invention of RADAR, a vital component of the UK 's defense during the
second world war.
Appleton was knighted in 1941 for his contributions to British military research and received
the 1947 Nobel Prize for physics for his 'Investigations into the upper layer of the atmosphere'. He died
in 1965.
The existence of the Kennelly-Heaviside layer was now firmly established. Both groups had
found that the altitude of the layer was about 100 km in the daytime. However, at night essentially
greater reflection heights were observed. It was also found that reflections at lower heights
occasionally took place. It was accepted that the Kennelly-Heaviside layer actually consisted of
stratified structures, which were named as D, E and F layers. These names were given by E.V.
Appleton, who described the origin of the nomenclature as follows:
The story of how I came to give the names D, E and F is really a simple one. In the early work
with broadcasting wavelength, I obtained reflections from the Kennelly-Heaviside layer and I used on
my diagrams the letter E for the electric vector of the down coming wave. When I found in winter 1925
that I could get reflections from a higher and completely different layer, I used the latter F for the
antenneX Issue No. 123 – July 2007
Page 6
electric vector of the down coming wave. Then about the same time I got occasionally reflections from
a very low height and so naturally used the letter D for the electric vector of the returning waves.
Then I suddenly realized that I must name these discrete layers and being rather fearful of
assuming any finality about measurements, I felt I ought not to call these layers A, B and C, since
there might be undiscovered layers, both below and above them. I therefore felt that the original
designation for the electric field vector D, E and F might be used for the layers themselves.
It had thus turned out that the Kennelly-Heaviside layer was actually greatly structured and its
name was misleading to some extent at least: there was more than one layer. In 1926 Robert A.
Watson-Watt suggested that it should be given the name “Ionosphere,” which then gradually replaced
the older term.
These early experiments carried out by Appleton and his co-workers attracted the attention of
Sisir Kumar Mitra and he decided to conduct similar investigations in his newly established
laboratory. He could motivate a small team of young and enthusiastic scientists to take up this
challenging work.
The first experimental evidence of E-region of the ionosphere was obtained in 1930. Mitra also
gave a theory of the D-layer, which was first reported by Appleton in 1928. D-layer is an absorbing
layer formed during the daytime just below the E-layer, The echo from this layer is only occasionally
observed. Mitra and his co-workers conclusively established the existence of this layer. Mitra’s group
detected also echoes from as low as 30 km. The very low level reflections were believed to come from
a hitherto unsuspected layer. Mitra called this layer C-layer. One of the most important works of Mitra
was his explanation for the Appleton ionization anomaly.
Mitra’s group could measure the heights of the different layers of the ionosphere by an
instrument designed and built indigenously. The investigations carried out by Mitra’s laboratory
provided the first general picture of the ionospheric condition in a sub-tropical region of low latitude like
Calcutta. The equatorial anomaly, the two tropical highly ionized crests, was discovered.
Sisir Kumar Mitra (Bengali:] (October 24, 1890–August 13, 1963) was an
Indian physicist.
Among his accomplishments were his investigations into the
ionosphere. Dr. Mitra proposed that ultraviolet light from the Sun created the
middle, or E layer of the ionosphere. He also determined that ions in the
ionosphere's F layer were what caused luminescence of the night sky, giving
it a dusty hue rather than pitch black. In 1947 he published a reference
treatise titled "The Upper Atmosphere" on atmospheric research.
Today, the existence of the various ionospheric layers with their different properties and
influences to the radio wave propagations and attenuations is common knowledge. At least hourly we
have access to the ionosphere’s behaviour and structure worldwide. Hundreds of ionosondes of
different kind are sounding continuously the ionosphere. In the next episode I shall explain these
ionosondes and the resulting ionograms. So stay tuned.
-30antenneX Online Issue No. 123 — July 2007
Send mail to [email protected] with questions or comments.
Copyright © 1988-2007 All rights reserved - antenneX©
antenneX Issue No. 123 – July 2007
Page 7