History(from google) Oliver Heaviside(1850-1925 I Heaviside and Kennelly, in 1902, predicted that there should be an ionised layer(电离层) in the upper atmosphere that would reflect radio waves. They pointed out that it would be useful for long distance communication, allowing radio signals to travel to distant parts of the earth by bouncing off the underside of this layer. The existence of the layer, now known as the heaviside layer or the ionosphere, was demonstrated in the 1920s If radio waves bounce off the inside of the ionosphere, then they must also bounce off the outside so any radio waves from outside the earth would not get through to the ground -- they would bounce back into space a Thus the predictions by Heaviside, combined with Planck's radiation theory, probably discouraged further attempts to detect radio waves from the Sun and other celestial objects. For whatever reason, there seem to have been no attempts for 30 years, until Jansky's unexpected discovery in 1932 Later it was learned that the reflection from the ionosphere is very dependent on the frequency (or wavelength) it reflects most of the radiation of frequency less than about 20 MHz. but the ionosphere is not a barrier to frequencies above about 50 MHZ Radio astronomy had to wait for the development of high frequency radio receivers 上海海学信息工程学院上海海事大学信息工程学院 History (from google) ◼ Oliver Heaviside (1850-1925) ◼ Heaviside and Kennelly, in 1902, predicted that there should be an ionised layer (电离层） in the upper atmosphere that would reflect radio waves. They pointed out that it would be useful for long distance communication, allowing radio signals to travel to distant parts of the earth by bouncing off the underside of this layer. The existence of the layer, now known as the Heaviside layer or the ionosphere, was demonstrated in the 1920s. ◼ If radio waves bounce off the inside of the ionosphere, then they must also bounce off the outside. So any radio waves from outside the earth would not get through to the ground -- they would bounce back into space. ◼ Thus the predictions by Heaviside, combined with Planck's radiation theory, probably discouraged further attempts to detect radio waves from the Sun and other celestial objects. For whatever reason, there seem to have been no attempts for 30 years, until Jansky's unexpected discovery in 1932. ◼ Later it was learned that the reflection from the ionosphere is very dependent on the frequency (or wavelength). It reflects most of the radiation of frequency less than about 20 MHz. But the ionosphere is not a barrier to frequencies above about 50 MHz. Radio astronomy had to wait for the development of high frequency radio receivers