Grote Reber, who has died aged 90, was, for several years, the world's only radio astronomer. In 1937, he built a radio telescope in his backyard in Wheaton, Illinois, with which he produced the first preliminary radio maps of the sky. His discoveries were largely responsible for the postwar development of radio astronomy, and gave astronomers an entirely new window on the universe.
Born in Chicago, Reber graduated in radio engineering from what is now the Illinois Institute of Technology. An amateur astronomer and radio ham, he began to think about the possibilities of radio astronomy in 1931, while still a student, his imagination having been fired by the extraordinary experimental results achieved by the radio engineer Karl Jansky, of Bell Telephone Laboratories.
While investigating the sources of interference on short-wave radio transmissions used for ship-to-shore communications, Jansky had discovered there was a constant stream of radio waves emanating from the stars. At first, the stream of very weak signals seemed to be coming from the sun. But the direction of strongest reception slowly drifted away from the sun, and made a circuit of the sky. Jansky decided the strongest signals were coming from the Milky Way, particularly from the direction of Sagittarius, towards the centre of the galaxy.
The finding made front-page news, but Jansky's interests lay elsewhere and his results received little attention from other scientists; the discovery had too many drawbacks to explain away, and the recording instruments seemed capable of only producing incomprehensible squiggles and fuzzy patches.
It was left to Reber to combine his ham radio knowledge, and his fascination with astronomy, to determine whether the waves were coming from our own galaxy, the Milky Way, or from some other celestial objects. After applying unsuccessfully for jobs with Jansky, and with astronomical observatories, he decided to study radio astronomy on his own, though his first attempts to adapt his short-wave radio receiver to pick up interstellar radio waves failed.
Then he went to work in his own backyard, building a dish-shaped radio telescope that cost him the equivalent of six months' salary. The parabolic dish reflector, made of sheet metal, was 31.4ft in diameter. It was designed to collect faint signals from outer space, and focus them on a sensitive radio receiver 20ft above the dish, which amplified the signals a million times so that they were strong enough for a chart recorder to compile maps. Reber spent hours scanning the skies every night because there was too much interference from the sparks in car engines during the daytime.
His first receiver, designed to detect 3,300 megahertz signals from outer space, met with little success. It was the same story for the second design, tuned to 900 megahertz. Finally, in 1938, a third receiver at 160 megahertz (1.9 metres wavelength) picked up radio emissions from the Milky Way, confirming Jansky's discovery.
After two years of intense work, Reber published Cosmic Static, a series of articles in the Astrophysical Journal that many scientists regard as the birth of intentional radio astronomy. In 1944, he produced the first detailed contour radio map of the sky, with brighter areas indicating richer radio sources - the brightest was the centre of the Milky Way, and there were other bright sources in Cygnus and Cassiopeia.
Over the years, Reber's increasingly detailed measurements were published in many prestigious journals, including Nature and the Journal of Geophysical Research. His sky surveys ensured that radio astronomy became a major field of research, and paved the way for the discoveries of quasars, pulsars and the remnant glow left over from the Big Bang.
In 1944, he became the first scientist to detect radio emissions from the Andromeda galaxy and the sun. In 1951, having moved his radio telescope to Virginia, and tried unsuccessfully to get funding for a 220ft radio telescope, he went to Hawaii to help with the development of a radio astronomy observatory.
Ten years later, Reber settled in Tasmania, and began to explore a neglected area of research involving cosmic radio waves at very low frequencies (1-2 MHz, or 150-300 metres wavelength). Tasmania was one of the few places where waves of these frequencies could penetrate the earth's ionosphere at times of low solar activity.
Many of the major awards Reber won were usually reserved for professional astronomers. A lasting memorial to his achievements was established when he donated his original telescope to the US National Radio Astronomy Observatory at Green Bank, West Virginia. In supervising its assembly, he also oversaw the construction of a fullscale reproduction of the antenna Jansky used to discover the existence of cosmic radio waves.
· Grote Reber, radio astrono- mer, born December 22 1911; died December 20 2002