In 1937, Grote Reber, a radio engineer, became interested in Janksy's work and he build a parabolic reflector antenna (9.5 meter in diameter) in his backyard. After a few years of negative results, and many modifications with the receiver, Reber made the first systematic survey of the sky at a frequency of 160 MHz. The first radio maps were published in 1944. The maps show that the maximum radiation comes from a region in the constellation of Sagittarius. Minor maxima appear in Cygnus, Cassiopeia, Canis Major, and Puppis. The lowest minimum is in Perseus. Reber also reported radiation from the Sun. The period after the Second World War showed an enormous interest in radio astronomy. After the prediction of the 21 cm Hydrogen line by H.C van de Hulst in 1944 and the detection of the line in emission in 1951 the potential of radio astronomy was easily recognized, since Hydrogen is the most abundant element in the Universe. One of the most spectacular results has been the mapping of the structure of our Galaxy.
By the middle 1950s the number of identified sources was not yet sufficient for significant studies. Therefore much work was done in order to observe a large number of sources and to obtain accurate positions and sizes to get reliable identifications. Special instruments were constructed for this purpose, like the Mills cross near Sydney and a large interferometer in Cambridge. The sky survey made with the first instrument produced a source list of over 2200 sources. When this list was compared with the source list from Cambridge in the overlapping regions there was not much agreement. It turned out that the Cambridge list was not very reliable for source comparison due to source confusion. When two or more sources are simultaneously present within the antenna beams they produce fringe patterns with slightly different frequencies, resulting from the differences in their declinations. The combined fringe patterns contain amplitude variations analogous to beat tones in acoustics, and the maxima in the resulting output do not correspond to individual sources (Thompson et al., 1986).
This event led to the practical criterion that on average the source density should not exceed one in about twenty times the solid angle of the antenna beams. The frequency from the Cambridge telescope was increased from 81.5 MHz to 159 MHz, thereby reducing the solid angles of the beams by a factor of 4, and a new list of 471 sources was made. This was the famous 3C survey.