Antenna, Rebecca, AT-2/APN-2
This WWII-era "Rebecca" antenna is a AT-2/APN-2, but a version that has the brown (resin or masonite) stabilizing 'wings' between the mast and the blades. Two were typically mounted on each side of the aircraft forward of the wing, receiving navigational signals from the "Eureka" transmitting station and indicating those signals to the crewman on a BC-929-A receiver (not included). See photo of a similar antenna mounted to the fuselage of a B-24 bomber.
According to the Graphic Survey of Radio And Radar Equipment Used By The Army Air Forces dated July 1945:
“Radio Set AN/ APN-2 is an airborne radar interrogator / responder of the "Rebecca" type that will enable an aircraft to home on ground radar beacons of the "Eureka" type such as AN/PPN-1 and AN/PPN-2 and the heavier beacons, AN/ TPN-1 and AN/TPN-2. It was used in conjunction with suitable beacons for night landing of parachute troops, landing of gliders and maintaining airborne supply operation to isolated positions.
In operation the Rebecca (AN/APN-2) emits a pulse, "triggering" the Eureka (AN/PPN-1) and causing it to return a pulse. This returned pulse is received by the Rebecca in the aircraft and appears as a signal pip on the scope thereby indicating range and direction of the beacon.
A form of communication between Rebecca and Eureka is also provided. The Eureka operator, while listening for triggering through his earphones, can send the Rebecca operator a coded message by depressing a key provided for that purpose. Such messages are read in the Rebecca indicator as a blinking of the signal pip. This type of communication, though slow, can be used to notify Rebecca planes of any last minute changes caused, for example by change of wind direction or surprise enemy action.
Chief difference between Rebecca - Eureka and other beacon systems lies in the size and weight of the units. Rebecca is designed solely to interrogate an Eureka; it serves no other function and requires no other radar in the aircraft. Hence Rebecca can be installed in a troop carrier airplane at a cost in weight of only 85 pounds compared to an installed weight of 400-500 pounds for an AST radar.”