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Heinrich Hertz, who first demonstrated the basic principle of radar, might be called the first radar scientist. A fellow German, Christian Hulsmeyer, can be called the first radar engineer. In the early 1900s, Hulsmeyer built, patented, demonstrated, and tried to sell to various shipping companies and navies a shipborne radar, based on Hertz's apparatus, for avoiding collisions of ships with one another. He had no success in selling his radar. His work faded from memory, even in Germany where it had to be reinvented when the need for it finally arose. Although the basic principle of radar had been demonstrated in the early part of the 20th century, there was no further activity for over thirty years. The occurrence that, led to modern radar was when the World War I fabric-coated, externally braced biplane having fixed landing gear and open cockpit was replaced by the all-metal single-wing monoplane with its retractable landing gear, enclosed cockpit, and high horsepower engines that allowed it to fly long distances at high altitude carrying a large bomb-load. In the thirties, attack by bomber aircraft was a serious concern and military leaders were pessimistic about being able to defend against it. It was said at that time that "the bomber will always get through." In the 1930s, however, the development of VHF technology was rapidly flourishing in many countries, and it wasn't long before it was noticed that an aircraft, ship, or person passing through the propagation path of a two-way communications link produced a disturbance at the receiver. Radar was thus reborn. The early history of radar has been told in detail in many places, so it need not be repeated here. Suffice to say that at the start of World War II, in September 1939, eight countries had independently and almost simultaneously discovered and explored radar, and most had operational systems available by the time they entered the war. These countries were the US, UK, Germany, France, the Soviet Union- Japan, Italy, and the Netherlands.