When James Maxwell predicted mathematically the existence of wavelengths on either side of the optical portion of the spectrum, it was not proven. In 1887, Heinrich Hertz discovered radio waves and proved Maxwell’s theory. In 1902, Arthur Edwin Kennelly (1861-1931), encouraged by Thomas Alva Edison (1847-1931) suggested to the Lick Observatory in California that radiations of longer wavelength might be emanating from the Sun and they might be audible. He also predicted the presence of the ionosphere that bounced back man-made radio waves. The opening of the radio window led to other windows of observation in the electromagnetic spectrum.
The discovery of the radio window resulted from a rather simple test of a radio telephone. A young radio engineer, hired by Bell Telephone Lab was probing the quality of transatlantic radio-telephone in 1928. He had a strange antenna, 20.5 m long and 4 m wide, mounted on turn-table nick-named ‘merry-go-round’. He was working on the frequency of 20.5 MHz. He heard a steady signal, though there was no thunderstorm locally or in the region. Moreover, he found that the signal reached its highest point a few minutes earlier each day! Not being an astronomer, he was confused, but persisted in his probe. He was Karl Jansky (1905-1950), now regarded as the pioneer of radio astronomy.
Jansky, after consulting his friend, Albert M. Skellent, continued his observations for full one year. He soon found the reason for the daily delay in the radio sound. The source of the radio waves was a star. The accuracy of celestial objects was limited in the early days of radio astronomy.
The Earth takes 23 hours and 56 minutes to rotate with respect stars, and so the Earth has to take another four minutes to complete the rotation with respect to the Sun, as it moves in its orbit around the Sun. In 1933 Jansky published his estimate of the source’s location. It was, he said, far away in the direction of Sagittarius. In 1935, he suggested that the radio signals came from the Milky Way galaxy and that they originated from interstellar dust and gas. He proposed a 30-m dish antenna to study the signals. His employers did not encourage him, though he made front-page news in New York Times!
Fortunately, one person understood Jansky’s idea. He was Grote Reber (1911-2002), an American radio engineer, who became the first radio astronomer. A parabolic reflecting antenna, 10 m in diameter, was constructed and as the Earth rotated, it scanned the sky. The radio window opened. Soon, astronomers found out that the Universe looked different in radio wavelengths. The radio window covers about four decades of the spectrum as against less than one decade of the optical window.
The atmosphere is almost transparent from about 10 mega Hertz to tens of giga Hertz. The reflection from the ionosphere, however, sets the lower frequency limit for the radio window. The attenuation is too high to allow observations from the ground of millimeter and submillimetre waves. Large steerable radio telescopes are used to gather radio waves from large areas, thought reflectors much larger than 100 m are not practical, because of the Earth’s gravitation.