Antenna, AN-69A, Pair, WWII Radio Altimeter SCR-518 System

This pair of AN-69A Aircraft Antenna were components of the RC-128A Antenna Assembly, which was part of the early WWII SCR-518 Radar Altimeter System( known as AYA in the USN) used by aircraft of the US Army Air Force, such as the B-17 Flying Fortress bomber (see image of equipment installation), and the US Navy .  The SCR-518 system was a 100 lb airborne, frequency modulated (i.e., pulsed) radar altimeter designed for installation in aircraft to provide direct indication of altitude above terrain (or water) during flight.  It was developed by RCA and was in procurement prior to US entry into WWII with the order of nearly 12,000 units in procurement.  Procurement was eventually reduced in favor of the lighter SCR-618 system and eventually the SCR-718 system.  SCR-518 used either one pair or two pair of antenna (see images from its technical documentation). One (or one pair of) antenna acts as the transmitting antenna, emitting a downward modulated radio frequency. The earth's surface r