Radio Frequencies

Modern radio astronomy started with a serendipitous discovery by Karl Guthe Jansky, an engineer with Bell Telephone Laboratories, in the early 1930s. Jansky was investigating static that interfered with short wave voice transmissions using a turntable mounted 100 ft. by 20 ft. directional antenna working at a frequency of 20.5 MHz (wavelength about 14.6 meters). By rotating the antenna the direction of a received "static" could be pinpointed. A small shed to the side of the antenna housed an analog pen-and-paper recording system. After sorting out signals from nearby and distant thunderstorms, Jansky continued to investigate a faint steady hiss of unknown origin. Jansky finally determined that the signal repeated on a cycle of 23 hours and 56 minutes. This four-minute lag is typical of an astronomical source "fixed" on the celestial sphere rotating in sync with sidereal time. By comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the Milky Way and was strongest in the direction of the center the galaxy, in the constellation of Sagittarius. Grote Reber helped pioneer radio astronomy when he built the first parabolic "dish" radio telescope (9m in diameter) in 1937. He was instrumental in repeating Karl Guthe Jansky's pioneering but somewhat simple work, and went on to conduct the first sky survey in the radio frequencies. On February 27, 1942, J.S. Hey, a British Army research officer, helped progress radio astronomy further, when he discovered that the sun emitted radio waves. After World War II, substantial improvements in radio astronomy technology were made by astronomers in Europe, Australia and the United States, and the field of radio astronomy began to blossom. One of the most notable developments came in 1946 with the introduction of the technique called astronomical interferometry where many radio telescopes are combined in a large array to achieve much higher resolutions. Martin Ryle's group in Cambridge obtained a Nobel Prize for this and later aperture synthesis work. The Lloyd's mirror interferometer was also developed independently in 1946 by Joseph Pawsey's group at the CSIR, (later CSIRO) in Sydney. In the early 1950s the Cambridge Interferometer mapped the radio sky to produce the famous 2C and 3C surveys of radio sources. Two issues, one astronomical and one technical, dominated the research in Cambridge, from the late 1940's for more than thirty years. What was the nature of the discrete radio sources, or `radio stars'? Where were they, what were they, what were their properties, how many were there, how did they work and what was their significance in the Universe? Of parallel importance was the puzzle of how to devise new kinds of radio telescope which would elucidate these astronomical questions. Earliest observation? While investigating atmospheric electricity in 1900, Nikola Tesla noted repetitive signals that he deduced must be coming from a non-terrestrial source. Although Tesla mistook this to be radio communication from intelligent beings living on Venus or Mars it may have been the earliest observation of an astronomical radio source (A 1996 analysis indicated Tesla may have been observing Jovian plasma torus signals).