Christian Hülsmeyer invented first radio wave detector 1904

The history of radar (where radar ... radio waves were reflected by metallic objects. This possibility was suggested in James Clerk Maxwell's seminal work on electromagnetism. However, it was not until the early 20th century that systems able to use these principles were becoming widely available, and it was German inventor Christian Hülsmeyer who first used them to build a simple ship detection device intended to help avoid collisions in fog (Reichspatent Nr. 165546 in 1904)...

The German inventor Christian Hülsmeyer was the first to use radio waves to detect "the presence of distant metallic objects". In 1904, he demonstrated the feasibility of detecting a ship in dense fog, but not its distance from the transmitter. He obtained a patent for his detection device ...

One of these closed the article ... this invention might have significance for future warfare." The building of the Telemobiloscope and its demonstrations had depleted the initial funds of the Telemobiloskop–Gesellschaft firm. On 12 August 1904, rights to the system were sold to Trading Company Z.H. Gumpel daselbst of Hannover. The sales agreement, to which Heinrich Mannheim was a signer, said that Hülsmeyer would be given ...

Other scientists and engineers, including Nikola Tesla and Guglielmo Marconi, had suggested that it might be possible to detect objects using reflected electromagnetic waves, but Hülsmeyer’s work was the first practical demonstration and the subject of the first patents.

Early radar history and how radar works is described. Many links to radar tech sources.

The first radar – Hülsmeyer, ... close to this site, Christian Hülsmeyer · demonstrated his Telemobiloskop: a spark gap transmitter, simple parabolic antennas, detector and an indicator....

However, his device, called the telemobiloscope, was unable to directly measure the distance to a target. Nonetheless, the telemobiloscope became the first patented device to detect the presence of distant objects using radio waves.

content is nearly similar to that of which Hülsmeyer had expressed, about eighteen years

Hülsmeyer contacted companies in the shipbuilding industry, the new radio industry, and the navy – but although ship collisions were common, they all kindly rejected the Telemobiloscope.