blip on the radar display grew larger and stretched out. The same blip would be produced if the radar was tracking a group of targets in formation, so the transponder also had a motorized switch that turned the signal on and off, causing the blip to oscillate on the display. Mark I was used only experimentally, with about 50 sets completed in total.

The problem with Mark I was that it operated only on the 23 MHz Chain Home frequency. By 1939 there were already several other radars being introduced