Pioneering Radio Telescopes
From merry-go-rounds to weather controllers, the early radio telescopes certainly had their interesting histories. We are fortunate to have several of the United States' critical instruments on display for visitors.
The discovery of radio waves coming from space, the birth of radio astronomy, has its humble beginnings in a 440-acre field in Holmdel, New Jersey.
Bell Laboratories, the forerunners of wireless communications in the early part of the 20th century, put a recent hire, Karl Guthe Jansky, to work hunting sources of radio interference that plagued their new transatlantic communications.
In 1931, the 28-year old Jansky built a large Bruce antenna for his hunt. For his chosen wavelength of 14.5 meter, the antenna needed 400 feet of 7/8th inch brass piping supported on a bunch of wooden 2x4s. The Bruce design requires that the antenna be cut or bent into rectangular shapes to create a set of reflections for the waves hitting them. The 90-degree reflections help to magnify the signal.
Jansky mounted his antenna on four Model T Ford rubber wheels that ran along a 360-degree concrete track. With a motor, he could drive the antenna to map the sky once every 20 minutes.
With his so-called Merry-go-round, Jansky famously discovered the radio hiss he would later determine to be coming from the center of our Galaxy behind the constellation of Sagittarius.
Bell Labs created a replica of Jansky’s antenna and gave it to us for permanent display. It continues to baffle and inspire visitors to Green Bank.
Reber's DishA keen amateur radio aficionado, also in his twenties, Grote Reber was inspired by Jansky’s discovery of radio waves coming from the center of the Milky Way Galaxy. Using the laws of physics and optics, Reber set a goal of building a giant dish antenna to scoop up higher frequency waves that should better define the radio source Jansky had found.
From June to September of 1937, he built a two-ton, 31-foot parabola dish antenna in his backyard in Wheaton, Illinois. The antenna is mostly made from standard wooden 2x4s that Reber personally cut and fit. Behind the 31-foot dish radially span a 72-rafter wooden backup structure (BUS) whose long sides were shaved to form a parabolic crater.
The dish surface is 26-guage galvanized iron sheet metal. The dish was formed with 45 pie pieces of this sheet -- the outer ring uses thirty-six pieces, and the center requires nine. The sheets were then hand-fastened to the wood BUS, roughly a bolt every foot, around the surface to drape a parabola over the rafters, leaving a hole in the center for future retrofitting.
Children used the antenna as a playground, and neighbors believed it was actually built to collect water by controlling the weather, because when it rained, hundreds of gallons of water poured off the dish and down its central hole.
Reber’s first receiver was a large, cylindrical waveguide in which he nestled a dipole antenna. Over his years using this dish antenna, he experimented with many different kinds of receivers, amplifiers, and other electronics.
In 1943, after much research and saving for high-grade equipment, he began his famous survey of the radio sky. During his observations, he discovered other radio sources besides the center of the Galaxy, including what we now call Cassiopeia A.
His design is the model on which all major radio telescopes are based, so he is considered the father of the radio telescope as we know it.
When Reber moved out of Illinois to pursue longer wavelength astronomy in Tasmania, his antenna was adopted by the National Bureau of Standards, his employer. He supervised the transfer and display of his antenna in Green Bank in 1959 where it still sits today.
Doc Ewen's 21-cm HornIn 1950, Harold Ewen was a physics graduate student at Harvard University building a receiver to detect the signal of neutral hydrogen in our Galaxy. Theorists had predicted that this simple atom should be spinning in space, end over end, so its moving electrons should be giving off radio waves, about 21-centimeters long.
Edward Purcell, Ewen’s supervisor, asked for, and received, a grant of $500 from the Rumford Fund of the American Academy of Arts and Sciences for materials costs.
Ewen proceeded to design the horn antenna and the mixer and receiver, consulting with experts in these fields, including Sam Silver on antenna design and Bob Pound on mixers. The receiver used a frequency switching technique to cancel out background noise, a novel technique for astronomy at the time.
The horn antenna is 56 inches (142 cm) across by 43 inches (109 cm) high by a little over 127 inches (323 cm) long. The size of the horn was set by the geometric constraints of the fourth floor parapet at Lyman Lab at Harvard where "Doc" Ewen installed it.
The waveguide came in through the window to the receiver and recorder. In heavy rains, the horn antenna funneled water into the lab. During the winter, passing students found the horn a tempting target for snowballs.
They finally detected the 21-cm line of neutral hydrogen on March 25, 1951. In the following years, numerous astronomers, primarily Dutch, American, and Australian made increasingly accurate observations of the neutral hydrogen line.
The Harvard technique proved to be successful in mapping the large-scale distribution of matter in the galaxy. Observations of the 21-cm line remain a critical branch of radio astronomy to this day, resulting in ever more detailed understanding of the interstellar medium in our Galaxy, and in studies of external galaxies.
This famous little antenna is now on display at our observatory in Green Bank.
Bracewell's SpectroheliographFrom 1961 until 1980, Stanford ran a unique 32-antenna spectroheliograph, a cross-shaped radio telescope that continuously monitored the changing radio emission from the Sun.
Designed and operated by astronomer Ronald Bracewell and his graduate student, Govind Swarup, the 10-ft diameter dishes observed at a wavelength of 9 cm, and the 400-foot wide array made one complete image of the Sun (containing about 100 pixels) every day. Never before had a radio telescope been dedicated to continuous solar observing to this level of detail.
Besides increased knowledge of the Sun, these data served the very practical purpose of warning NASA and the military of impending problems associated with solar bursts, such as damaging earth satellites and endangering moon-bound Apollo astronauts of that era.
Each dish in the array was mounted on a 5-ft-tall concrete pier. Bracewell invited all distinguished visitors to his radio observatory to wield a chisel and hammer and inscribe their name on the side of a pier. Thus over a 20-year period he collected over 200 signatures in his unique "Guest Book."
Among the signatories were three Nobel Prize winners, observatory directors from around the world (including traditional optical observatories), and a large fraction of the pioneers who established the vibrant field of radio astronomy after World War II.
By 1980 the array with all of its historical signatures had become obsolete and was abandoned. In 2012, ten of the most interesting piers were cleared of brush, sawed off, and shipped to the Very Large Array.
Visitors to the site can borrow a guide listing all the names, with short descriptions of most of the signatories.
(Thanks to Woody Sullivan for the Bracewell text.)