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 reflects some of the signal which is then received by the second antenna (or second pair). During the time that the signal traveled to earth and returned to the aircraft, the transmitted frequency will have changed.  This difference is translated by the system into aircraft altitude.  The B-17F flight manual dated Dec 1942 states that the system was operable up to 20,000 ft and will work satisfactorily up to 30,000 ft before the readings become erroneous.  According to its Erection and Maintenance Manual, a pair of antenna were located on the bottom surface of each wing.  The SCR-518 system was principally operated by the aircraft's radio operator or bombardier. 

Each antenna measures ~8 inches wide and ~7 inches tall, with a base of ~2 inches by 3 inches.  They are in good condition.  This listing is for the pair.