Short answers, pulled from the story.
Christian Hülsmeyer became the first person to use radio waves to detect distant metallic objects in 1904. He obtained a patent for his detection device in April 1904 and secured another patent for estimating distance to ships on the 23rd of September 1904.
The Daventry Experiment took place on the 26th of February 1935 using a powerful BBC shortwave transmitter as the source. Hugh Dowding became very impressed with the system's potential after operators detected a bomber, leading to immediate funding for further operational development.
Radar systems called Chain Home stretched across the entire UK including Northern Ireland by 1940 to provide vital advance information. This technology helped the Royal Air Force win the Battle of Britain without requiring significant numbers of fighter aircraft to be constantly airborne.
If an object moves toward or away from the transmitter there will be a slight change in frequency due to the Doppler effect. This produces information about target velocity allowing small objects to be detected in environments containing much larger nearby slow-moving objects.
Before World War II began, radar research was conducted in the United Kingdom, the United States, France, Germany, Italy, Japan, the Soviet Union, and the Netherlands. The Soviets produced their first mass production radars RUS-1 and RUS-2 Redut in 1939 while France installed aspects of its obstacle-locating apparatus on the ocean liner Normandie in 1935.