Hertz, Hülsmeyer and a thirty-year pause (1886–1934)
In 1886 Heinrich Hertz proved that radio waves reflect off solid surfaces just like light. In 1904 a German engineer named Christian Hülsmeyer patented the Telemobiloskop — a brass-and-glass machine that warned ships of nearby vessels in fog. It worked at 3 kilometres. Nobody bought it.
The idea sat almost untouched for three decades. Radio was busy becoming broadcasting. Reflecting waves off ships and aircraft seemed like a parlour trick until military planners in the 1930s realised that an aircraft moving at 300 km/h could not be heard, seen or stopped without some way to detect it beyond the horizon.
Eight countries, one idea (1934–1939)
Between 1934 and 1939, scientists in the United Kingdom, United States, Germany, France, Italy, the Soviet Union, the Netherlands and Japan independently built working radar prototypes. They did not collaborate. Most did not know the others existed.
Robert Watson-Watt's 1935 Daventry experiment in the UK is usually credited as the first practical air-defence radar, because it led directly to the Chain Home network that ringed Britain by 1939. Rudolf Kühnhold in Germany demonstrated ship detection in 1934. Robert Page at the US Naval Research Lab built the first pulsed radar the same year.
Why pulsed radar changed everything
Early systems sent a continuous wave and listened for Doppler shifts. They could tell you something was moving, but not where it was. Pulsed radar — invented independently in the US, UK and Germany around 1934 — sent short bursts, then measured the time delay of the echo. Time-of-flight gives you exact range. Range plus antenna direction gives you a point on a map. That single change is what turned radar from a curiosity into a weapon.
The cavity magnetron — the most important radar invention
In February 1940 two physicists at Birmingham University, John Randall and Harry Boot, built the first cavity magnetron. It produced microwave-frequency radio at kilowatt power levels from a device the size of a fist. Before the magnetron, radar antennas were the size of a tennis court. After it, radar fit in a fighter's nose cone.
Britain handed the magnetron design to the United States as part of the Tizard Mission in late 1940 — historians often call it 'the most valuable cargo ever brought to American shores.' Every microwave oven on Earth today is a direct descendant.
Why this still matters
The radar sweep you see in every air-traffic tower, every weather forecast, every arcade radar game including Signal//Lock, is the same Plan Position Indicator (PPI) display invented in 1944. The physics has not changed: send a pulse, wait for the echo, plot the dot. Everything else is engineering on top of that one idea.