Thread by @cocteautriplets

Today marks 80 years since the end of the Battle of Britain, in which radar played a critical role for the first time. Let's also recall that British radar development stemmed from an Air Ministry prize of £1000 to anyone who could build a ray that could kill a sheep at 100 yards

The idea of a "death ray" was not new, but the Ministry was alarmed in the mid-1930s by reports that the Germans were ahead in death ray development

The physicist Robert Watson-Watt was asked to investigate these reports. He turned the problem over to his assistant Arnold Watkins, who quickly demonstrated the theory was impossible, but also that the principle might be used to detect aircraft at a distance

Watson-Watt and Watkins worked quickly and within a matter of months built a prototype set that was used to detect a Handley-Page Heyford bomber at 7 miles

The transmitter used was simply an existing BBC shortwave radio broadcast station. They set their recievers up and detected the radio broadcasts echoing back off the target

Watkins made the inspired suggestion as he had been investigating commercial radio sets, and a model from the General Post Office mentioned in the user manual that end users could expect interference if aircraft were flying overhead

Watkins and Watson-Watt are credited with the British invention of radar and this first success test against an aircraft. A number of nations at this time were making independent leaps and bounds in "radar" so there's no single inventor credited

The "Chain Home" radar network was built in great secrecy and at great speed. Technically it was relatively simple but this diagram shows just how wide the coverage was in 1939 then 1940

So the moral of the story is don't try and kill sheep, don't believe the hype, and always read the instruction manual

Watson-Watt was also the father of "High Frequency Direction Finding" (or Huffduff), which he had first come up with to try and track lightning strikes.

HF/DF allowed radio transmissions to be accurately located and when miniaturised into a shipborne version was instrumental (alongside more advanced radar) in helping win the "Battle of the Atlantic"

The beauty of HF/DF is that it's totally passive - you don't need to transmit a signal yourself and therefore potentially give away your presence - and also that you don't need to know what's being transmitted so cryptanalysis is not required.

You basically just get a dial that says "there be dragons" and can guess from the characteristics of the signal an approximate range and even if the transmitting antenna is wet or not (i.e. is it a freshly surfaced submarine)

It was the work of an exiled Polish engineer, Wacław Struszyński, which made a miniaturised and accurate shipborne set possible. Previously the system used land-based receivers on a baseline of hundreds of miles form eachother.

Struszyński's development of a miniaturised antennae, and also a method to cancel out the disruption caused by the ships own superstructure is described as "a breakthrough of transcendent importance".

The German U-boat system was so terribly effective because it was centrally controlled. Central control relied on communications, which were by radio. Such was the confidence that accurate radio direction finding at sea was impossible that they never changed this

The Germans had plenty of photos of the HF/DF antennae on allied ships, they knew that allied ships and aircraft could quite reliably and conveniently find themselves in the right place at the right time, they just did not believe this could be down to passive direction finding

It is estimated that "without shipborne high frequency direction finding, Allied convoy losses in early 1943 would have been 25 to 50 percent higher, with U-boat kills being reduced by one-third"

Footnote and an apology. I spent most of that threat calling Arnold Wilkins "Arnold Watkins".