"This was one of the British technological crown jewels included in the Tizzard Mission to the US in late 1940. It included ideas & designs for the Photoelectric fuse & the Radio fuse. It also included the Cavity Magnetron which was quoted by the historian of the American OSRD as "the most valuable cargo to ever reach our shores' ".
TECHNICAL DESIGN POINT: VT fuzes had a small, especially-shaped antenna wrapped around the battery and oscillator radio source for transmission and reception. The antenna had to be configured as designed. However, when a glowing base tracer element was used to allow the gunners to see where the shells were going and adjust their aim, this burning tracer was ionized and electrically conductive, leaving a long invisible trail of electrified air behind the shell. This acted like an extension of the antenna in the fuze and short-circuited its operation. Thus, VT-equipped shells had to have their tracers removed, To fix the aiming vision problem, every so many shells would be an old powder or clockwork time fuze with a tracer installed. The smallest shells that could fit a VT fuze in WWII was a 3" (75mm) gun projectile, with 4-5.25" being used in the majority of US and British naval guns. Against German troops in Europe and Japanese-held islands, larger guns from 106mm through 203mm Army guns and up to 16" naval guns had HE shells with VT fuzes.
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