In 1860 the Scottish physicist James Clerk Maxwell used some very clever calculations to show that light was made of intertwined waves of electric and magnetic fields - electromagnetic waves.   He was also able to predict that there were waves longer than the infrared waves found by the astronomer William Herschel.

James Clerk Maxwell

The German physicist Heinrich Hertz set out to create Maxwell's predicted electromagnetic waves.   In 1888  he managed to send a signal across his lab.      Basically what he did was to make a machine that created an electric spark which switched on and off many times a second.   On the other side of his lab he had a simple coil of wire with a tiny gap in it.     When his spark machine was switched on, the coil on the other side of the lab also had a tiny spark in its gap.   Something invisible was jumping across the room and transferring energy.   Hertz had made and received radio waves.

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Heinrich Hertz

In 1932 a young American radio engineer called Karl Jansky was trying to find the source of radio interference.   He built a big directional aerial and looked at wavelengths around 14m.   He identified many sources like thunderstorms but one signal was very odd.   It rose and

 fell every day, repeating itself every 23 hours and 56 minutes.   Jansky recognised this as the sidereal period - the time it takes for the Earth to rotate relative to the stars.   That meant that the source was extraterrestrial!  Jansky turned his aerial until he discovered the part of the sky that was creating the radio waves.   It was in Sagittarius.   We now know that it is created at the heart of our galaxy where it is thought huge clouds of gas and dust orbit a giant black hole.   To honour his discovery, radio emission strength is measured in Janskies. 

 Karl Jansky

Hearing of Jansky's discovery, another American radio engineer, Grote Reber, designed and built an aerial in his back garden in 1937, to pick up the signals for himself.   He decided to use a parabolic dish of wire netting to collect the waves and reflect them onto an aerial.   He knew that the signal was weak ,so he made his dish  nearly 10m in diameter.   It was the first radio telescope dish.   Goodness knows what his neighbours thought.  By 1941 Reber had produced the first radio map of the sky.

Reber stands next to the dish he built in his back garden.