Today we take it for granted, but it took the efforts of many to figure out what was happening. บทความโดย Robert H. Welsh, N3RW ใน QST ฉบับ December 2009
For decades, QST readers have studied articles describing the best methods for getting their signal from point A to point B. Past articles have taught us how to build or improve receivers, transmitters, filters and antennas (my favorite). From the 160 meter band through the HF spectrum to VHF, we are subject to one part of radio communications beyond our control. That part is the Earth’s ionosphere. Its discovery, aided in part by radio amateurs working with government and university scientists, makes an interesting story.
I suspect that most readers of QST recognize Guglielmo Marconi as one of the earliest radio “amateurs.” Marconi’s efforts using radio to cover long distances over water is well documented. There is still a question as to whether in 1901, he actually received the Morse code letter S transmitted from Poldu in Cornwall, England to St John’s, Newfoundland; nevertheless, it is accepted that his efforts led to radio as a viable means of long-distance communications.Keep in mind that prior to radio as a communications tool, wire telegraphy was the primary tool for long distance communications. Wires cannot easily span the surface of the world’s oceans, but radio waves can.
Laying the Ground Work
The physics community of the early 20th century investigated this new phenomenon.
Several theories were proposed by prominent physicists to explain how radio signals propagated over long distances. One of the several theories proposed was that the Earth’s upper atmosphere acted as some sort of a reflector. Another theory took into
account the optical phenomenon of superposition of waves. Another wave phenomenon was suggested — radio waves were diffracted just as light waves were diffracted, or bent, around an obstruction. Diffraction could not explain how these waves bent around the smooth curvature of the Earth.
Even the great Marconi attempted to explain how radio waves traveled. He observed the difference between day and night propagation of radio waves. His explanation was based on the presence or absence of sunlight; that is, during the day, the Sun’s rays falling on the antenna acted as a shield. Not a bad explanation for those amateurs active on 160 and 80 meters during daylight hours.
ในระหว่างที่มาร์โคนี่ทำการทดลองเรื่องการเดินทางของคลื่นวิทยุ เขาสังเกตเห็นความแตกต่างของการแพร่กระจายคลื่น ระหว่างกลางวันและกลางคืน การสังเกตของเขาใช้การมีและการไม่มีของแสงแดด เขาอธิบายว่าตอนกลางวันจะมีแสงแดด ซึ่งจะตกกระทบไปที่สายอากาศ มันทำหน้าที่ห่อหุ้มสายอากาศเอาไว้ เป็นการอธิบายที่ไม่เลวสำหรับความถี่ต่ำอย่างย่าน 160 และ 80 มิเตอร์ (1.8 MHz และ 3.5 MHz) ในเวลากลางวัน
According to the scientific method, a theory must be tested to confirm its validity. One of the equations developed to support a scientific theory proposed to show that as the wavelength decreased, the radio waves traveled over longer distances. This was the Austin-Cohen equation. Given this hypothesis, the shorter wavelengths were considered useless for long-distance radio communications and were relegated to those folks called radio amateurs. How fortuitous for us! The professional scientific community slowly began to take notice that radio amateurs using shorter wavelengths were having great success in communicating over long distances.
Figure 1 — Early pioneers in ionospheric research. Dr Merle Tuve and Dr Gregory Breit (at left) at the pulsed echo sounder for ionospheric research located at the Department of Terrestrial Magnetism, Carnegie Institution, Washington, DC. The photo is dated February 14, 1927.
รูปที่ 1 ช่วงแรกในการทดลอง ค้นคว้าเกี่ยวกับชั้นไอโอโนสเฟียร์ โดยดอกเตอร์ทั้งสองส่งสัญญาณเป็นจังหวะสั้น ๆ ไปยังชั้นไอโอโนสเฟียร์
An Answer Comes into View
In 1902, two scientists independently suggested that radio waves were bent by a conducting layer in the Earth’s atmosphere. Those scientists were Arthur Edwin Kennelly in the US and Oliver Heaviside in England. The reflecting layer that we
now call the ionosphere became known as the Kennelly-Heaviside layer. It took two decades of experimenting before this hypothesis was proven to be the correct explanation for longdistance high frequency radio communications. As stated in the previous paragraph, the Kennelly-Heaviside layer hypothesis required experimental testing before it could be accepted.
Lee De Forest is well-known as the inventor of the triode vacuum tube. By inserting a third element between the cathode and the anode, this third element (the control grid) is able to control the flow of charge from cathode to anode, thus producing amplification. Not as well-known is De Forest’s work in radio propagation. During the period 1912 to 1914, De Forest and Dr Leonard F. Miller of the Federal Telegraph Company made the first crude measurements of the Kennelly-Heaviside
layer height using a spark transmitter. The spark transmitter delivered 1200 kW (that’s 1.2 MW) to an antenna at currents approaching 750 A. The wavelength of this transmitter was 3260 meters (a frequency of 92 kHz). Signals from this transmitter located in Los Angeles were received 350 miles north in San Francisco and 300 miles east in Phoenix. De Forest published the results of these tests in the journal London Electrician in 1912.
De Forest hypothesized that the main wave was returned by a reflecting layer whose heights were 17, 27 and 37 miles above Earth.De Forest makes an interesting comment regarding his experiments: “I know nothing about what goes on up above, and that attempts at exact explanation are silly.” Perhaps not the best attitude for an experimenter.
Development Continues
During the period of the 1920s, radio grew exponentially through increased commercial broadcasting and the use of shorter wavelengths for long distance communications. During that same period, there was an increased interest in the physics of subatomic particles. Much of the particle physics research was based on an increased understanding of the nature of matter. By the 1930s, this led to that branch
of physics known as quantum mechanics. Pulse techniques were developed to accelerate charged particles. The offshoot of this research for radio physics was the
introduction of high-power pulse generators. Hams think of these as CW transmitters. The two scientists recognized as the discoverers of the pulse technique for confirming the existence of the ionosphere were Merle Tuve and Gregory Breit.]
Tuve was an active amateur during his undergraduate days at the University of Minnesota. He spent considerable time operating the club station, 9NB. It is interesting to note that one of Tuve’s friends in Amateur Radio was Ernest Orlando Lawrence, for whom the Lawrence- Berkeley Laboratory at the University of California is named.
Lawrence eventually went into the field of nuclear physics. He used the accelerated charged-particle concept to construct the atom-smashing machine known as a cyclowas tron, for which Lawrence earned the Nobel Prize in Physics. Tuve received his undergraduate degree in electrical engineering.
Some time after graduation, Tuve received a letter from Lawrence wherein Lawrence suggested that the two of them start a business selling and installing radios.It is important to note here how young Amateur Radio operators set the stage for great work later in life within the physics community.
Two years later, Tuve earned a master’sdegree in physics. Following graduation, he cyclowas employed by the Carnegie Institution of Washington in a program to measure the height of the radio conducting layer.
Tuve worked under Gregory Breit, who held the position of Mathematical Physicist in the Department of Terrestrial Magnetism at the Carnegie Institution. This experiment would lead to Tuve’s doctoral dissertation. Breit began ionospheric measurements in 1924. He assumed a layer height of 62 miles; the same height suggested in a 1913 publication by De Forest. In addition, Breit was aware of propagation experiments by Albert Hoyt Taylor of the Naval Research Laboratories.
The ARRL Joins In
Albert Hoyt Taylor and his colleagues at NRL, in cooperation
with John Reinartz (1XAM and 1QP) of the ARRL and other radio amateurs, had discovered that high frequency radio waves could be transmitted to a distant receiver while being imperceptible at many points in between. Taylor labeled the gaps skip distances and conducted an in-depth investigation to determine their characteristics. He published detailed experimental measurements of skip distances in early 1925, including with them estimates of the height of the conducting layer previously suggested by Kennelly and Heaviside.
By 1925, Breit and Tuve designed an experiment to measure the height of the conducting layer by transmitting pulsed signals, then receiving the echoes and measuring the time lapse from transmitted pulse to received echo from both the sky wave and the ground wave. Considering the speed of radio waves is the same as the speed of light — that is, constant at 3 × 108 meters per second — Breit and Tuve used basic physics to calculate the distance traveled by the pulse. During a meeting in Washington, DC during November 1924, Breit and Tuve met with several leading radio experts. A plan was conceived in which several powerful radio transmitters would provide the source signal and the received signal would be at the Carnegie Institution. The transmitters were station KDKA in Pittsburgh, Pennsylvania, the National Bureau of Standards station WWV, the Naval Research Laboratories station NKF and coastal station WSC operated by the Radio Corporation of America in New Jersey. (See Figure 1.)
Figure 2 — Ray-path drawing representation of that from Tuve and Breit’s March 1925 article. LL is the postulated refl ecting layer. Ant 1 is the transmitting antenna location. Ant 2 is the receiving antenna location. The distance along the surface is L and the height of the layer is H.
The best results were received from the NRL transmitter, one of the earliest crystal controlled transmitters in use. Breit and Tuve used the new technique of oscillograph recording to analyze the received pulses. On July 28, 1925, they received the first conclusive results of ionospheric reflection at a frequency of 4.2 MHz from a 10 kW transmitter sending 200 μs pulses.
One of the most interesting aspects of their results was that the height of the reflecting layer varied from day to night. Their calculations indicated that the height ranged from 55 miles (88 km) during the day but rose to 130 miles (208 km) at night. They were not yet aware that the layers varied as a result of solar ultraviolet and X-ray emissions, which did not excite the atmosphere when the Sun was not visible. Today, hams recognize that the ionosphere behaves as if it were in several distinct layers at different heights. These are referred to as the D layer at a height of about 30 miles (50 km), the E layer at about 60 miles (100 km) and the F1 and F2 at heights ranging from about w180 miles (300 km)
Figure 3 — Illustration of the operation of a Digisonde that has the capability of portable operation. The system can measure seven observable parameters of refl ected or refracted signals as described in the text.
The method used by Breit and Tuve
to directly measure the ionosphere’s height
was to:
- Use directional loop antennas for receiving.
- Record the received pulses from both the sky-wave signal and ground-wave signal.
- Apply the time difference between the sky-wave signal and the ground-wave signal.
- From the difference, use their derived equation to measure the layer height.
The derived equation was a slight modification of the Euclidean geometry statement about right triangles known to legions of high school students as the Pythagorean theorem. (See Figure 2.) From this equation and comparing the time for the sky wave to reach the receiver compared to the time for the ground wave to reach the receiver, Breit and Tuve calculated the height of the reflecting layer for 4.2 MHz transmission.
From Tuve and Breit’s 1925 paper, the equation suggested that the time of arrival of the sky wave is given by Tsky wave = (2×H/C)(1 + (L/{2×H}))1/2 where Tsky wave is the arrival time, C is the speed of light, H is the height and L is the length over the surface of the Earth. Whereas the time of arrival of the ground wave is given by:
Tground wave = L/C.
From the difference in the two arrival times, the experimenters could arrive at a virtual height for the reflecting layer. Their results obtained from recording several different transmitting stations suggested a reflecting layer height of about 80 miles (128 km).
From their initial experiments in the 1920s, sounding the Earth’s ionosphere has developed into a remarkable analytical tool. Prior to the WWII, there were few ionospheric sounders in operation — those in England; Washington, DC; Peru; Australia, and the Soviet Union. During the war, the number increased to about 50 stations. By the time of the International Geophysical Year (1957-58), the numbers had increased to about 150.
The techniques used today are a direct offshoot of Breit and Tuve’s 1925 experiments. One of the most common systems is a vertical incidence sounding, referred to as an ionosonde. The ionosonde is basically a pulsed radar operating at a frequency range of about 1 MHz to 40 MHz. The measurement is based on the equation:
H = 0.5 × C × T
where C = the speed of light, T = is
the travel time of the pulse, and H is the layer height (also known as the virtual height).
An example of current ionospheric studies comes from the University of Massachusetts Lowell Center for Atmospheric Research. They developed a low power (300 W) ionosonde named Digisonde that has the capability of portable operation.
The system has the following capabilities: the simultaneous measurement (see Figure 3) of seven observable parameters of reflected, or in oblique incidence, refracted signals received from the ionosphere including frequency, range (or height for vertical incidence measurements), amplitude, phase, Doppler shift and spread, angle of arrival and wave polarization.
This is just one example of the on going research to understand the Earth’s ionosphere. As amateurs who use the HF and VHF regions of the electromagnetic spectrum, we owe much to past and present experimenters. They provide us with an understanding of how our signals propagate around our planet. As either a DXer or ragchewer, we rely on this information to better communicate via radio.
I would like to thank Dr Nelson Klein, Professor of Physics at Bucks County Community College for his valuable discussions while I researched this article.
