Radar uses microwave radiation to detect the range, speed, and other characteristics of remote objects. Development of radar was accelerated during World War II due to its great military utility. Now radar is widely used for applications such as air traffic control, weather forecasting, navigation of ships, and speed limit enforcement.
Microwaves cannot be carried with usable efficiency in ordinary transmission lines but require waveguide,such as a metal pipe.
A Gunn diode oscillator and waveguide are used as a motion detector for automatic door openers.