A Brief History of Radio Astronomy

EXPERIMENTAL STATIONS

It was once again in the hallowed halls of Bell Laboratories that the field of radio astronomy was born. Karl Jansky first detected radio waves emanating from the Milky Way in August of 1931. His discovery was a happy accident, one of those serendipitous coincidences born out of pure research and playful investigation. Yet Jansky had been around radio and playing with radio long before he showed up at Bell.

Born in what was then still the Territory of Oklahoma, his father Cyril M. Jansky was the dean of the College of Engineering at the University of Oklahoma. Cyril was passionate about physics, and named his son after Dr. Karl Eugen Guthe, a physicist and professor at the University of Michigan who’d been a mentor to Cyril. Cyril had been born of Czech immigrants in Wisconsin, and later returned to his home state where he retired as a professor of Electrical Engineering from the University of Wisconsin.

Karl had a brother ten years older, Cyril Jr., a man who helped lift the United States into the radio age by helping to build some of the earliest transmitters in the country. His handiwork was on the early radio stations 9XM in Wisconsin and the 9XI in neighboring Minnesota, now stations WHA and KUOM respectively.

During WWI there had been a ban on civilian radio stations. In October of 1919 the ban was lifted and the Universities of Wisconsin and Minnesota applied for "War Department Training and Rehabilitation School" station licenses which they received. The “X” in both call signs designated them as experimental stations. The operation of 9XI was under the oversight of Cyril Jansky Jr. who was an electrical engineering professor at Minnesota.

In 1920 a one-kilowatt spark gap transmitter was installed at 9XI. Students used it to communicate with other amateur stations and university stations, such as their neighbors in Wisconsin where the set-up had also been overseen by Cyril Jr. As a service both stations provided weather forecast and market bulletins using Morse code. When the vacuum tube came along in 1921 they were able to start making audio broadcasts.

In the following years as the radio service became more codified, various types of licenses emerged with assigned wavelengths corresponding to whether the station provided entertainment or news and information. These changes were also reflected in the assigned call letters. The 9XI station had become WLB. The radio service was a major asset to the community when in 1922 a major snowstorm knocked out newswire services in the region. The Minneapolis Tribune asked the station's operators to help retrieve the day's news through a roundabout series of amateur radio relays, one station passing the news on to the next until it reached its destination. This type of radio relaying became a major tradition for ham radio operators. In the United States, the American Radio Relay League, the major organization and advocate for US hams, takes its name from just that tradition.

Karl Jansky grew up in this milieu and it’s no wonder he followed in the family footsteps to also become a physicist and radio engineer. He was attending the University of Wisconsin during the years when it had changed from operating as 9XM to WHA and it is very likely he was familiar with the equipment being used at the station. He graduated in 1927 with his BS in physics.

SIGNALS IN THE STATIC

Janksy quickly landed a job at Bell Labs and relocated himself to their site in Holmdel, New Jersey. Of the many things Bell Labs was interested in was the investigation of the properties of the atmosphere and ionosphere with the shortwave of the radio spectrum for use in trans-Atlantic radio telephone service.

It was early days yet in the study of propagation, noise, and everything that could affect a signal being transmitted to a distant place. Jansky’s job was to listen to the static that interfered with communication; in studying it he found signals where others may have only heard noise.

In 1933, five years before Orson Wells historic War of the Worlds broadcast, Jansky was able to pinpoint “Electrical disturbances apparently of extraterrestrial origin.” This was in fact the name of the paper he wrote on his findings. It all started out with an antenna he built.

Radio astronomy is like fishing, only instead of a pole you have an antenna to reel in the catch of distant transmissions. To do his research on atmospheric noise Karl built an antenna that was dubbed “Jansky’s Merry-Go-Round.” Mounted on a “turntable” of four Model-T Ford tires it could be rotated to determine the strength or weakness of a signal and thereby pinpoint it. It was designed to receive radio waves at a frequency of 20.5 MHz (wavelength about 14.6 meters). Next to this antenna there was a small shack that housed equipment including an analog pen-and-paper recording system that plotted the findings of the antenna.
For several months he recorded signals from all different directions and eventually he was able to categorize them. Within the noise he was able to detect thunderstorms. Close thunderstorms exhibited one set of characteristics and those far away exhibited another. Then there was a third sound he picked up, a faint steady hiss whose origins were unknown. This signal had a location of maximum intensity, rising and falling every day.

Initially he thought the hiss was from solar radiation, but he revised this initial theory after further investigation. He discussed the anomalous hiss with his friend Albert Melvin Skellett, an astrophysicist who also worked at Bell and whom later wrote a paper on the “Ionizing Effect of Meteors” and whose name appeared on many patents. Skellet looked at the data and noted that the time between the signal peaks was on an exact cycle: it restarted every 23 hours and 56 minutes. This time frame is a sidereal day, a time scale used by astronomers based on the rate of Earth’s rotation relative to the fixed stars. Armed with this knowledge he compared his observations with optical astronomy maps. He noted that the signal peaked when his antenna was pointed to the densest region of the Milky Way galaxy, in the Sagittarius constellation. Knowing that the sun was not a huge source of radio noise, he concluded that the cosmic hiss was being created by “gas and dust” in that far corner of the galaxy.

Jansky wrote up his findings in a 1933 paper titled “Electrical disturbances of apparently extraterrestrial origin.” His findings were also publicized by a New York Times article on May 5th of that year. Jansky called the sounds from space “star noise” and it was something he wanted to investigate further but he found little help. Radio was a completely new tool when applied to astronomy, and the astronomers of the time on the one hand didn’t see the ramification of its many potential uses. In the 1930s and 40s they were also hampered by the financial constraints of the Great Depression and following that, the war effort.

Meanwhile those overseeing Jansky’s work at Bell Labs didn’t see the point in his further investigation of “star noise”. They were looking for solutions to the problems affecting trans-Atlantic communication and didn’t want to sink further funds into something they couldn’t be sure would prove useful to their goal.

 A small number of scientists and astronomers were interested in his research, but Jansky didn’t live long enough to see his contributions really take off. He died at age 44 in 1950 due to a heart condition. He was later honored by having his name appended to the unit used by radio astronomers for the strength (or flux density) of radio sources. Jansky noise was also named after him and refers to high frequency static disturbances originating deep within the cosmos. These are just a few of the ways his work has been remembered.

As for the emissions coming from the center of the Milky Way, in the 1950’s astronomers and astrophysicists thought it was made by electrons in a powerful magnetic field. Today the thinking is that the radio emissions are caused by ions in orbit around a Black Hole at the center of the galaxy called Sagittarius A*.  Jansky, having pointed his antenna towards galactic center, also pointed others towards the possibilities of a field combining radio and astronomy.  

THE HAM WHO MAPPED THE RADIO STARS

Grote Reber was an amateur radio operator (W9GFZ) and amateur astronomer who followed Jansky’s lead and combined his two hobbies to make great discoveries about the cosmos we inhabit. Having heard of Jansky’s work he applied for a job at Bell Labs because he realized this new field was the one for him but the Depression still had the countries resources drained and they didn’t have anything for him. So without waiting for grants or asking anyone else’s permission he built a parabolic receiving dish in his backyard in Wheaton, Illinois, and set out to do the work on his own.

The antenna or radio telescope he built was more advanced than even what Jansky had built with the funds from Bell Labs. It was made of sheet metal and shaped into a nine meter in diameter parabolic dish focused to a receiver eight meters above the dish, all connected to his radio gear. It was on a stand that could be tilted to various parts of the sky, but unlike Jansky’s it wasn’t on a turntable. Perhaps he should have hit up Ford for some spare tires. Reber completed his build in September 1937, and was able to keep radio astronomy alive during those fraught and lean years.

 It took Reber three attempts before he detected a signal which confirmed the discovery of Jansky. The first time he was looking on the 3300 MHz frequency, and the second time at 900 MHz. Finally in 1938 he was successful in detecting signals from outer space on 160 MHz. In 1940 he made his first professional publication in the Astrophysical Journal and was contacted by Yerkes Observatory who offered him a position. He turned them down and kept walking his own path. He decided to make a radiofrequency sky map and was the first to do so. This was published in 1941 and expanded in 1943.

Reber continued to trawl the megahertz fishing for signals from the stars and he hauled in quite a catch. He researched, wrote, published, rinsed and repeated, the lone radio astronomer. Yet the body of work he created in the new field became a big bang for radio astronomy that exploded after WWII. A lot of the folks getting out of the service had been trained in radio, radar, and electronic communications in one way or the other, and many of these folks went on to pursue careers in some aspect of electronics. Some of them came home and built on the foundation of radio astronomy whose waters were first explored by Jansky and Reber.
  
Reber continued his quest to explore the mysteries of the stars and the spectrum. One mystery he tinkered with had to do with a standard theory surrounding radio emissions from beyond Earth which claimed they were caused by black-body radiation, or the thermal electromagnetic radiation, including light (of which radio is an invisible form), given off by all hot bodies. According to this line of thinking scientists of the time expected there to be a greater quantity of high-energy light than low-energy due to stars and other hot bodies in the cosmos. Reber dispelled this notion by showing that there was a vast amount of low-energy radio signals able to be detected with his radio telescope system. Later in the 1950s the idea of synchrotron radiation was used as an explanation for his mysterious measurements.

Reber was a man who liked to go his own way. As the field of radio astronomy grew some areas of research were growing crowded, so he decided to study a band of frequencies that weren’t getting much attention. He looked at the medium frequency range of signals around the AM broadcast band, those in the 0.5–3 MHz range. All those frequencies below 30 MHz bounce off the ionosphere, part of the reason they are able to be picked up in distant locations. To really listen for distant signals coming in from outside he needed to go somewhere that let those signals in.

He found such a place in Tasmania, where he moved after a brief stint surfing the spectrum in Hawaii, when he received some funding from the Research Corporation for Science Advancement. There in the southernmost state of Australia in the southern hemisphere, on the long winter nights when the sun barely shows his face, the pesky layer reflecting the radiowaves would go de-ionize, allowing the long waves from the stars to be caught by his radio telescope. Tasmania was also low in manmade electrical interference and RF. This allowed his equipment to receive like a dream and detect faint signals that elsewhere might have been obscured by noise.

Just as hams and shortwave listeners go to quiet out of the way spots that have low levels of manmade RF for DXpeditions, Reber’s love of radio and astronomy took him to exotic places, all in the continuing search for the ultimate DX signals –those trans-plutonian transmissions  from outside of our solar system, and perhaps even galaxy.
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For the rest of his career and life Reber lived in Tasmania searching for signals from the stars.

RADARS PUZZLING EVIDENCE

​During the war years there were some other explorations of radio astronomy happening below the radar, often being worked on by people involved in the field of radar. Radar had been shown to be a possibility for detecting objects as far back as Heinrich Hertz in 1886 when he showed that radio waves could be reflected off objects. The Russian physicist Alexander Popov developed a device for detecting distant lightning strikes in 1895. Ten years later the German inventor Christian Hülsmeyer demonstrated the use of radio to detect the “presence of distant metallic objects”, specifically ships at sea in distant fog. It was an invention that would have many practical uses. Many other radio experiments in direction finding and detection took place after this by excited investigators. 


During WWII several nations were working on the problem of radar independently though not yet called as such as part of their search for tools and effective strategies against their enemies. James Stanley Hey was a British physicist who joined the Army Operational Research Group (AORG) after a 6-week course at the Army Radio School to support his country during the fight against the Axis powers. 

Hey was tasked with one of those great traditions in radio: jamming, or rather ant-jamming in his case. Radio jamming is the intentional blocking or disrupting of a radio signal, often with another stronger interfering signal and is distinguished from natural sources of interference, and unintentional interference. It really got going as a method of miscommunication in WWII. Ground operators realized they could mislead the pilots of opposing forces by speaking in their language and leading them off in the wrong direction. Radar jamming uses the same principle, but the jammer sends out RF signals designed to interfere with those of other radar operators, saturating the enemy receiver with noise. Claude Shannon might have looked at it in terms of information theory: by increasing the noise in a system, the user has to work harder to lock on to a true signal. Jamming has also been extensively used in broadcast radio by oppressive regimes who don’t want the shortwave transmissions of other countries, such as when the United State’s station the Voice of America was jammed by the Soviet Union to stop their citizens from being able to listen. Broadcast jamming continues at the time of this writing in countries such as North Korea and China who want to keep outside transmissions, and outside messages, from entering their country.

Hey tackled the problem of German radar jamming and it led to discoveries relevant to radio astronomy. The Germans had been clever in their jams of Allied radar signals, leading to the escape of three German warships from the English Channel. Their signals had come off the French coast and interfered with those of the Allies. In February of 1942 Hey received reports of anti-aircraft radars being jammed in the 4-8 meter range of the spectrum. He found that the direction of maximum interference seemed to follow the path of the Sun. Following this lead he contacted the Royal Observatory and learned there was a very active sunspot. This led him to conclude that sunspots, which were already believed to emit streams of ions and electrons in magnetic fields of approximately 100 gauss, could also emit radio wave emissions in the meter-wavelength bands. After the war Hey continued his research in radio astronomy, working for the Royal Radar Establishment at Malvern.

Around the same time G. C. Southworth of the United States also found radio noise associated with the Sun in the centimeter portion of the spectrum. Southworth was a radio engineer who worked for AT&T starting in 1923 and eventually finished his tenure with them at Bell Labs where he retired in 1955. He is mostly remembered for his development of waveguides, but he was interested in all different aspects of radio and worked on other things such as ultrashort waves, the dielectric properties of water at ultrahigh frequencies, shortwave propagation, antenna arrays, earth currents, and radio astronomy. In 1950 he published his 675-page doorstopper of a tome, Principles and Applications of Waveguide Transmission. It was his nitty-gritty exploration of microwave techniques and it was while studying that range of the spectrum that he stumbled across signals from the sun.  

Back in England, J.A. Ratcliffe was another man working on the radar problem during the war years who encountered emissions from the sun. After graduating from Sidney Sussex College, Cambridge in natural sciences in 1924 he started researching propagation under Edward Victor Appleton, a pioneer of radiophysics. Appleton was an assistant demonstrator in experimental physics at the Cavendish Laboratory, part of the Department of Physics at the University of Cambridge. Under Appleton’s tutelage Ratcliffe and M. A. F. Barnett figured out ways to understand the mystery of ‘fading’ radio signals. They investigated why transmissions faded from fixed stations that often happens at sunset. A few years later Ratcliffe became the head of a group at Cavendish whose purpose was to study how radio waves get reflected off the ionized layer in the upper atmosphere and the nature of that layer of atmosphere.

As part of Britain’s defense and signals intelligence they had built a network of anti-aircraft radar stations known as Chain Home that covered the eastern and southern coasts of the country. Various physicists and scientific types were assigned to spend a month at these stations. Ratcliffe was sent to one of these near Dover. Next he was made part of the Telecommunications Research Establishment (TRE) who sent him to work at a Chain Home Low site. The “Low” sites were designated as such to detect planes flying below the altitudes the regular Chain Home stations were able to pick up. This work took him all around to various sites during the war.
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As the bitter years of war ended Ratcliffe was able to go back to Cavendish. The group had grown, and others soon joined in, including Martin Ryle from the TRE. Ryle ended up forming a section devoted to radio astronomy. Ryle and his colleagues developed further techniques for radio astronomy. The group went on to found the Mullard Radio Astronomy Observatory in the 1950s. 

Soon the techniques of interferometry were added into the mix. Optical interferometry had been used already by astronomers to get the resolution of a large telescope when using multiple smaller telescopes. The electromagnetic radiation collected at each of a number of separate small telescopes is combined to re-create the image that would have been obtained with the large telescope. This process is called “aperture synthesis”. The same principle can be used for any kind of wave be it light, sound or radio.

The first radio interferometer used for astronomical observation happened in Australia in 1946 by Jospeh Lade Pawsey, Ruby Payne-Scott and Lindsay McCready who used a single converted broadside array radar antenna at 200 MHz near Sydney. They had the idea to use radio waves reflected off the sea to produce an interference pattern. This specific technique became known as sea or sea-cliff interferometry. A radio detecting antenna was placed on top of a cliff to detect electromagnetic waves coming directly from the source and waves reflected off the surface of the water.  The two sets of waves are combined to create an interference pattern such as that produced by two separate antennas.

Numerous radar users in WWII had noticed “interference fringes” or the way radar radiation returned and reflected off the sea from incoming aircraft. They exploited this to observe the sun at dawn with interference arising from its reflections off the ocean. Using a baseline of 200 meters they determined that solar radiation during a burst phase was much smaller than the solar disk itself and came from a region known to be associated with a large grouping of sunspots. From this work the group was able to lay out the principles of aperture synthesis and published their results in a 1947 paper. 

A typical radio interferometry set up involves two or more separate antennas receiving radio waves from the same astronomical object and are joined to the same receiver. The antennas can be close together or spread very far apart. A variable delay device is used to compensate for the different times the waves come into the antennas. Another way interference patterns are created is by spacing the antennas in an attempt to make the waves interfere. The distance between them for interference depends on the wavelength and on the diameter of the source of the waves. 

Back in Cambridge Martin Ryle was also working on radio interferometry. With Antony Hewish and others in the Cavendish group he developed the technique of Earth-rotation aperture synthesis at radio wavelengths. He and Hewish received a Nobel prize for this work and their other contributions to the field.

Later in the 60s and 70s computers became part of the equation and their number crunching power was applied to some of the complex math, often involving Fourier transformations, used in radio astronomy.

All of this research branched out into observing a plethora of celestial radio sources. New discoveries were made adding to humanities cosmological knowledge. Specifically a number of new classes of objects unobservable by optical telescopes including pulsars, quasars and radio galaxies were received out of the aether enabling astrophysicists, cosmologists, and others of their ilk to refine their knowledge. The Cosmic Microwave Background Radiation was first detected using radio astronomy. 
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Meanwhile further developments in radar allowed it to be used to map our neighboring planets, and the whole toolkit of radio astronomy has been used to study everything from space weather and further observations of the sun. All of this has been used as fuel for the imagination of a number of musicians who continue to hear the music of the spheres.

Read the rest of the Radiophonic Laboratory series.
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