[Nick Beale]

The way the story is usually told, Watson-Watt was actually asked to investigate the possibilty of using radio waves to damage or destroy aircraft (was a "death ray" feasible, in fact) and in responding to that enquiry from the government, it came up that radio waves were disturbed by aircraft. After that, the issue of detection was followed up.

IIRC from pritchard's book, Hülsmeyer's device was intended to give a proximity alert to ships in fog by sounding a klaxon. It wasn't, in 1904–05 giving ranges and bearings. Quite an achievement nonetheless.

I think that the cathode ray tube was invented around the same time, so sooner or later someone was going to bring the two ideas together, I guess.