On Friday, December 27, 1940, the Admiralty carried out its first full-scale test of the Prof's aerial mines, a new air-defense idea that used small bombs lifted by balloons. Nine hundred balloons were prepared for launch as German planes approached, but the test immediately went wrong when the release team did not receive the launch order for half an hour.

After the delayed start, the results were still poor. About a third of the inflated balloons were defective, while others exploded too early or drifted down in unintended places. Because no German bombers actually appeared, officials suspended the test two hours later without proving the weapon in combat conditions.

Despite the failure, Churchill and the Prof remained convinced that the mines were important and practical. Churchill ordered continued production and further trials, and the project received the official code name Egglayer. At the same time, the RAF kept trying to improve its ability to detect and jam the Luftwaffe's radio beams, but German engineers continued altering their systems. German pilots worried that the RAF might turn those beams against them, yet in reality Fighter Command was still largely unable to find and attack bombers effectively at night.